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Aerospace and Defense

October 1, 2006

GPS: Launches of Satellites and Institutional Initiatives

Successful launch of the second modernized Block IIR satellite, IIR-15(M2), on September 25 and scheduling of another IIR-M launch on November 14 underlines recent progress in the GPS program.

IIR-15(M2), also identified by its space vehicle number (SVN58) and pseudorandom code number (PRN31), will be placed into orbital plane A, slot 2. The U.S. Air Force has designated the satellite to be launched in November as GPS IIR-16/M3, PRN15/SVN55.

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By Inside GNSS

Correlators for L2C

The term “Correlator” is often used in discussions of GPS and GNSS receiver design. It has been used to describe devices as simple as a single exclusive OR gate through to complete “baseband” chips that include a microprocessor.

Most usually, and in this article, the term describes the hardware or software that produces all of the required correlation data for a single signal from a specific GNSS satellite signal. This is also sometimes termed a “channel.”

The term “Correlator” is often used in discussions of GPS and GNSS receiver design. It has been used to describe devices as simple as a single exclusive OR gate through to complete “baseband” chips that include a microprocessor.

Most usually, and in this article, the term describes the hardware or software that produces all of the required correlation data for a single signal from a specific GNSS satellite signal. This is also sometimes termed a “channel.”

With the open GPS civil signal at the L2 frequency (L2C) now becoming available on Block IIR-M satellites, receiver designers have the opportunity to work with a markedly different GNSS signal resource. The first IIR-M spacecraft (designated SVN53/PRN17) was launched September 25, 2005, and the second is scheduled to go into orbit on September 14, 2006. (IIR-Ms also transmit the new GPS L1/L2 military (M-code) signal, but we will not treat this issue here.)

Against that historic backdrop, then, this article examines some of the novel elements of the L2C signal and its implications for GNSS receiver correlators. Our focus will be on a technically challenging aspect of receiver operation: initial acquisition of the signal and its processing by the correlator.

But first we will review some of the key functional aspects of GNSS correlators and some of the signal parameters that affect their operation.

. . . We can more easily explain the role of a correlator if we examine its two functions separately. In a GNSS receiver, correlation is used in two distinct activities:

Acquisition. Before the receiver knows whether it can receive a certain satellite’s signal, it must “search” for it using the correlation in an ordered but relatively indiscriminate way. Effectively, many correlation trials are run for each of many code delays and Doppler frequency offsets.

Tracking. Once acquired, the receiver must still despread the received signal in order to receive data and measure pseudoranges. Several correlators are usually used to keep the local code as closely aligned to the received code as possible. To do this, a “delay-locked loop” is used, with the correlators operating within the loops, some typically ahead of the received code (“early”) and some behind (“late”).

In other words, correlation is used both to “get” the signal and to “keep” it. These actions should be considered quite separately. In this article, we concentrate on the acquisition process.

. . . Various signal parameters affect correlation. These are listed for the three GPS signals in Table 1. Both L2C and L5 have dataless sub-signals which are time-multiplexed (CM and CL) and in quadrature (I5 and Q5), respectively. Both use longer codes than L1, while L5 has a higher chipping rate. L5 also has the added complication of Neuman-Hoffman codes, which will not be further discussed here.

. . . Correlation Signal “Shape” – the L1 civilian C/A-code signal is a single BPSK modulation. Despite the fact that the L2C signal is also BPSK, it introduces another layer of complexity by having the data-carrying and dataless signals multiplexed in time. Typically, the shorter (20-millisecond) data-carrying CM code will be acquired first, then the receiver would hand over to the longer (1.5-second) dataless CL code for tracking.

. . . The beauty of the exploitation of the circular convolution is that the code in the data does not need to be well aligned with the stored data— the whole point of acquisition, after all, is to perform this alignment. So, if the CM code was chopped up into pieces smaller than 20 milliseconds, this circular convolution would no longer reflect the time domain correlation, and a receiver would need to have many stored codes.

. . . Many GPS chipsets are optimized for operation in mobile telephone handsets and, as such, are aimed at minimizing the drain on handset batteries. For such applications, the large L2C acquisition overhead presents a serious problem . .

In this article, we have primarily examined the implications of signal acquisition for the new L2C signal. In a typical L2C-only receiver, significantly more effort is required to acquire the signal than is the case for L1 C/A code: more than 200 times higher in the hardware case and more than 500 times higher in the software case. However, because the long CL code does not carry any data, it can be used for the long integrations required for acquisition in a weak-signal environment.

For the complete story, including figures, graphs, and images, please download the PDF of the article, above.

The author wishes to acknowledge the useful suggestions made by Eamonn Glennon of Signav Pty Ltd.

July 1, 2006

The L2C Study: Benefits of the New GPS Civil Signal

GPS has had enormous benefits to the economy and society that go well beyond military and civil aviation applications – that is becoming ever more widely understood. What has been more open to discussion are the civilian non-aviation benefits of further U.S. efforts at GPS modernization, particularly the introduction of additional signals.

GPS has had enormous benefits to the economy and society that go well beyond military and civil aviation applications – that is becoming ever more widely understood. What has been more open to discussion are the civilian non-aviation benefits of further U.S. efforts at GPS modernization, particularly the introduction of additional signals.

In an effort to define and measure civilian benefits, the U.S. departments of commerce and transportation commissioned some economic analyses of civil signal modernization. Particular emphasis was placed on the value of the L2C signal centered at 1227.60 MHZ, which recently began broadcasting from the first modernized GPS Block IIR-M satellite. This article is an outgrowth of that effort.

The analysis focused on the value of signals at more than one frequency for precision non-aviation use by business and government. It considered how utilization of the second civilian signal and its benefits would evolve in the coming decades as the L2C constellation expands and as additional signals become available from GPS and other GNSSes.

In the study, projections were developed under four scenarios — with the “moderate benefit”scenario seeming most likely — that reflect combinations of developments, including the strength of markets, the timing of L2C signal availability, the timing of Galileo availability, and complementary and competitive relationships with augmentations.

The main findings of the study are:

  • The projected number of U.S. high precision users of any signal nearly doubles from 39,000 to 75,000 from 2004 to 2008, and reaches 146,000 in 2012 and 333,000 in 2017.
  • Under a “moderate benefits” scenario, the number of L2C users reaches 64,000 by 2017, of which 35,000 are dual frequency users and 29,000 use three or more frequencies.
  • Civilian benefits of L2C net of user costs range from $1.4-$9.6 billion under alternative scenarios and civilian net benefits are about $5.8 billion under the moderate benefits scenario.
  • Results are robust.
  • Positive present values of benefits net of user costs are obtained in all tests.
  • The ratio of benefits to user costs ranges from 8 to 20 in all tests.

In addition to the domestic benefits examined, L2C will undoubtedly have important international benefits.
This article presents in more detail how we defined the problem, approached the study, and arrived at those conclusions.

The L2C Evolution
L2C, together with the present L1 C/A-code signal and the future modernized civil signal L1C, will provide an alternative to augmented single frequency GPS for precision users. Separate investigations have outlined the incremental benefits of L1C (See sidebar, “The L1C Studies,” at the end of this article)

L2C signals can be used for both horizontal and vertical measurement and positioning along with L1 C/A as satellites become available over more areas and in more times of the day. The first satellite can be used for improved timing. L2C also can be used in configurations of three or more frequencies in combination with the forthcoming GPS L5 signal and with signals from Galileo and GLONASS.

At various times in each signal’s deployment and development of markets, other signals will, to varying degrees, provide complements to L2C and competitors to it. L2C has its greatest potential to generate benefits for dual frequency applications until alternative signals are widely utilized, and for long-term use in applications taking advantage of three or more frequencies.

The L2 signal is currently being widely used for augmentations, and the new signals can be used in that way along with the existing constellation. However, L5’s use as a competitor to L2C and as a partner to L2C in multiple frequency implementations primarily depends on the launch timeline for satellites carrying the L5 signal since L5, centered at the 1176.45 MHz frequency, is not currently in service. Plans call for its implementation on the GPS Block IIF satellites, with the first IIF now expected to be launched in 2008.

L2C deployment requires a commitment to operational capability. Decisions will be required as to launch dates and signal activation for each successive satellite containing the signal. The L2C benefits study is intended to contribute to decisions about L2C deployment with consideration of alternative scenarios informed by quantitative and qualitative analysis. 

To explore the implications of L2C evolution, we make projections about the numbers of U.S. precision users, incremental benefits, and user costs, based on examination of applications and available evidence on value of benefits, and consider how these can unfold over the period 2006–2030.

The analysis focuses on precision users of L2C who use two or more frequencies, although we do include estimates for supplementary multiple-frequency users and single-frequency users. However, the estimates of these types of use are more conjectural and do not contribute much to the overall value of benefits.

Benefits net of user costs are measured according to the widely accepted economic productivity approach, which includes productivity gains and cost savings. This comprehensive approach is more appropriate than one that measures benefits simply by expenditures on equipment and services.

Incremental benefits and user costs are defined to include all differences in outcomes from what would be expected in the absence of L2C. 

Signal Advantages and Availability
The L2C signal, scheduled to be the first of the modernized civil GPS signals, is intended for civilian purposes other than aviation and safety-of-life. It will provide greater accuracy and robustness and faster signal acquisition than the current L1 C/A-code signal.

Higher signal power and forward error correction will improve GPS mobile, indoor, and other uses.
The L5 signal that will arrive within a few years will be in a protected aeronautical radionavigation system (ARNS) band intended for aviation and other safety-of-life uses and will have broader applications.

Multiple signals will allow many users to obtain greater precision and availability at lower cost than achievable with proprietary augmentation systems. However, signal combinations combined with public and private augmentations for even greater precision and reliability will support applications with some of the greatest potential benefits.

Combined use of L2C with L1 C/A and L5 will also enable some precision users to achieve even greater reliability and accuracy. Although available simulations differ on the size of benefits of three signals over two, many professionals expect important advantages from such “tri-laning” techniques.

The U.S. Air Force launched first satellite containing the L2C frequency on September 25, 2005, and the signal became available on December 16. Going forward, two to four Block IIR-M satellites are expected to be launched each year. With six to eight satellites anticipated to be available by about December 2007, users will be able to access at least one single satellite with L2C at almost all times. Eighteen L2C-capable satellites  (including the Block IIF generation) will be available by about 2011 and 24 L2C signals, around 2012. (These statements are based on official 2005 launch schedules and are subject to revision.)

The first L5 launch is scheduled for March 2008. L5 does not have a GPS signal in use at its frequency, so it will not be usable to any great extent until a large part of its constellation is available. In contrast, L2 is in place to transmit the military P(Y) code and the carrier signals of the satellites are currently being used along with L1 C/A for higher-accuracy applications.

Consequently, the L2C signal can be used immediately as a second frequency. The GPS signal L1C, which is being planned now for implementation on the GPS III satellites scheduled for launch beginning in 2013, will be able to be used immediately, even for single frequency use, without augmentation because it is at the same frequency as the L1 C/A-code.

Using Multiple Frequency GPS
Many private and government precision applications could potentially benefit from multiple frequency GPS.
For example:

  • Centimeter accuracy is important to many land and marine surveying applications including planning, zoning, and land management; cadastral surveying, harbor and port mapping, aids to navigation, coastal resources management, mapping, and surveys of sensitive habitats.
  • Machine control applications using high precision GPS have grown rapidly in a number of sectors, including agriculture and forestry, mining, construction, energy, transportation, structural monitoring and positioning for mapping and geographic modeling..
  • Civil applications that rely on precise timing will benefit from increased GPS signal availability and elimination of atmospheric effects possible using dual-frequency techniques.  Beneficiary industries include those operating cellular telephone, power, and financial information networks.

Scope of Benefits and Costs
Incremental benefits — those that arise because of the availability of L2C— include far more than the comparison of multiple frequency with augmented single frequency use. Companies adopting GPS in the future may even skip single-frequency options and instead choose multiple-frequency equipment (incorporating L2C) over non-GPS alternatives. Large candidate markets include construction, agriculture, and other applications where technological alternatives exist.

In some organizations, dual-frequency GPS will be the catalyst for extensive changes in systems that will occur earlier than if dual frequency GPS had not been adopted.

In the L2C study, benefits are measured according to the “economic productivity approach,” which is superior to the expenditure/economic impact approach because:

  • Productivity gains and cost savings, which this approach emphasizes, are the main purpose of much of GPS deployment and can be much larger than expenditures.
  • Benefits may accrue to a large number of customers of the purchaser, as occurs with use of GPS timing in communications, financial services, and electric power and in use of GPS positioning for mapping, structural monitoring, and weather.
  • The more common approach (economic impact) gauges benefits by added GPS spending without deducting the loss of benefits of non-GPS expenditures that are replaced.

L2C benefits can take both market and non-market forms, including increases in the productivity of business and government operations, user cost savings, benefits to the public through provision of public services and saving lives, and through improved health and environment.

Net benefits are benefits minus user costs. Incremental user costs include all additional costs that are expected with the availability of L2C, not simply the difference in costs between single- and dual-frequency receivers. These can take the forms of enhancements and accessories purchased when adding L2C capability (e.g.  better displays, controllers and software) or costs associated with users upgrading to multiple frequency GPS from less sophisticated single-frequency GPS systems or non-GPS systems.

However, incremental user cost is net of savings from use of receivers with less proprietary technology and any reduced use of private augmentation subscription services.

Expenditures to develop the GPS system infrastructure (satellites and ground segment) are not included, however, because most represent nonrecurring, sunk costs. Moreover, if we added them to our L2C analysis, we would need to include benefits to aviation and military users as well as their associated equipment costs.

The analysis takes into account alternative conditions of timing and impact of alternatives through the use of scenarios. Projections of signal use and value of benefits are developed through the year 2030 under four scenarios: High Opportunity, Moderate Benefits, Diluted Benefits, and Opportunity lost.

These scenarios reflect combinations of developments, including the strength of markets, the timing of L2C signal availability, the timing of Galileo availability, and complementary and competitive relationships with augmentations. (See the sidebar, “L2C Benefit Scenarios” at the end of this article for details of assumptions behind each.)

Probabilities are not given for the scenarios because the likelihood of alternative Galileo delays cannot be evaluated quantitatively. Moreover, the diluted benefits and opportunity lost scenarios are significantly affected by U.S. GPS policy, which is also not predicted.

Estimates of GPS Users
The L2C study projections shown in Figure 1 are based on assumed rates of decline in prices for user equipment and services and increases in the number of users in response to price changes. Projections reflect assessments of market sizes and patterns of market penetration under each scenario. Allowance also is made for effects of economic growth on market size. Table 1 (to view tables and figures, please download the PDF of this article using the link above) provides a detailed breakdown of results by scenario.

Within each scenario, projections are made for precision L2C users of three or more frequencies, dual frequency precision users, multiple frequency supplementary users, and single frequency users of L2C.

The starting point for determining the number of high precision users is a widely relied–upon estimate of 50,000 high precision users worldwide in 2000. We assumed that the United States had 40 percent of precision users in that year.

The study further assumes that the number of U.S. high-precision GPS users will grow by 18 percent per year from 2000 to 2030. This projection is based on a rate of price decline for user equipment of 15 percent per year and a corresponding a 1 percent increase in users for each 1 percent decline in price. Finally, we include an assumption of general growth in the economy (i.e., independent of GPS receiver price) that adds 3 percent per year.

These assumptions and calculations produce a projection of U.S. high precision GPS users — those using augmentations, of 38,776 in 2004. The estimated number of U.S. high precision users of any signal or combination nearly doubles to 75,177 from 2004 to 2008 and reaches 145,752 in 2012 and 333,445 in 2017.

We computed the numbers of multi-frequency GPS users by applying an estimated percentage to the number of high-precision users for each scenario. The number of multi-frequency precision users adopting dual versus three or more frequencies was then calculated using projected values for the percent of each category. Finally, the number of L2C users was calculated based on projections of the percent of multiple frequency users that use L2C, constructed to reflect the dynamics of each of the scenarios. 

Rapid growth is projected in the numbers of U.S. precision multiple-frequency L2C users. In the moderate benefits scenario, the number of L2C users reaches 64,000 by 2017, of which 35,000 are dual frequency users and 29,000 use three or more frequencies. The numbers of L2C users vary widely among scenarios.

Average Net Benefits per User
The study defines average incremental net value of benefits per L2C user as the incremental value of benefits per L2C user above the incremental user cost of equipment and services. Benefits largely reflect productivity gains and/or cost savings. Estimates reflect a review of available evidence ranging from formal studies to case histories and expert opinion across a wide range of applications.

Our research suggests that average annual incremental benefit per precision L2C user net of costs could reach the range of $8,000–$16,000 per year. This includes benefits across systems that are not attributable to specific numbers of users and non-market benefits, such as safety and environmental advantages, as well as market benefits associated with the value of goods and services transactions. Market benefits attributable to numbers of users are estimated at 60 percent of all incremental net benefits.

These are peak values after benefits have had an opportunity to rise with experience using the new signal. The values decline from their peaks as new users with lower benefits are attracted by declining costs and some high benefit users move to alternatives.

In considering the plausibility of these figures, consider that:

  • If a worker saved one hour a week by avoiding rescheduling due to signal unavailability, slow signal acquisition, loss of lock and additional work due to phase ambiguities, and further assuming labor costs of $80 per hour (including salary, fringe benefits, equipment, support staff and other overheads), — the saving would total $4,000 per year. Improvements in the organization’s processes with better work flow could make the savings even greater.
  • If the telecommunications, electricity generation, and financial industries together had system benefits that together were valued at $20 per customer over 20 million customers, the benefits would be $400 million per year. Market benefits of $400 million per year, if divided by 100,000 dual frequency users, for example, would amount to an average of $4,000 per user per year.
  • $400 million in non-market benefits over 100,000 precision users would equal an additional $4,000 per user per year.

(This could result, for example, from avoiding 100 deaths due to industrial accidents or environmental impacts at a value of $4 million per incident.)

The present values of incremental user costs range among scenarios from $175 million to $514 million in year 2005 purchasing power.

Costs represent one eighth or less of the total value of benefits in each scenario.

Value of Benefits
Civilian net benefits per user are incremental, net of incremental costs, and derive from prospects for major areas of application. The patterns incorporate some high-value initial use, assume that higher benefit users switch earlier to newer signals, factor in a buildup of productivity gains with experience, and project lower values for late-entry users attracted by lower equipment prices as well as later increases in higher benefit users switching to alternative signals.

We calculate the value of civilian net benefits of L2C through multiplying civilian net benefits per user by the number of L2C users for the user type and scenario. Higher net benefit scenarios result from higher benefits per user and larger numbers of users.

At a 7 percent real (above inflation) discount rate, present values of total net civilian market benefits range from $9.6 billion to $1.4 billion dollars. Benefits under the moderate benefits scenario have a present value of $5.8 billion and those under the high opportunity scenario $9.6 billion. (Values are discounted using annual data to calendar year 2006. That essentially places the values at the middle of 2006.)

Nearly all of the incremental benefits of L2C stem from precision use of two or more frequencies. That is both because of moderate numbers of other types of users in these and their low benefits per user.

The timeframe in which other signals become available after L2C plays an important role in the size of estimated benefits. In the high opportunity scenario, for example, dual-frequency net benefits appear higher than benefits from use of three or more frequencies because the latter applications start later as additional frequencies become available.

In the other scenarios, benefits from applications using three or more signals are higher than dual-frequency benefits because the benefits of dual frequency remain as strong when competing frequencies become available.

New spending can encourage greater long run economic growth, especially when it is associated with new technology for widely usable infrastructure. The spending may induce others to innovate, invest in greater capacity, take risks and/or provide financing. While direct estimates of the size of long run economic multipliers are not readily available, analyses of determinants of growth suggest that effects are modest, perhaps adding 20% to market benefits. Because of the uncertainty surrounding such estimates, no allowance is made for growth multiplier effects in the estimates shown.

Cost-Benefit Analysis
The ratio of incremental civilian benefits to user costs is calculated by dividing the present discounted value of total incremental benefits (including net benefits and costs) by the present value of incremental costs. These are shown with a 7% real (above inflation) discount rate.

The ratios of benefits to costs range from a multiple of 20 in the high opportunity scenario to 9 in the opportunity lost scenario. It would be surprising if benefit/cost ratios were not high because only direct user expenses (and not system costs) are included to get a picture of incremental costs of each set of outcomes. 

The moderate benefits scenario, which has a ratio of 20, is considered more likely than the others. Because of the interest in obtaining the greatest benefits, focusing on the present value of net benefits is appropriate for policy rather than using the benefit/cost ratio when all ratios are high.

As mentioned, changes in various factors could substantially affect the outcomes of L2C benefits and produce either an overstatement or an understatement of these. See the “Benefit Variables” sidebar at the end of this article for a listing of the most important factors.

Rapid growth is projected in the numbers of U.S. precision GPS users and in most scenarios for the numbers of high-precision multiple frequency L2C users. Substantial L2C benefits can occur along with availability of other signals and constellations, augmentations, and alternative technologies. While Galileo will compete with L2C, Galileo signals also can increase precision L2C use in multiple frequency applications, an alternative that will become increasingly affordable.

The economic productivity approach offers a means of considering benefits in a comprehensive way. Benefits and costs are incremental. They are defined to include all changes that occur as a result of the existence of L2C.

Defined comprehensively, benefits can encompass results from more extensive changes in equipment and systems and include both benefits that are attributable to specific numbers of users and those that may be incorporated in systems and spread over a broad population. They include both market and non-market benefits — those that are not bought and sold in markets, such as benefits to life, health, security and the environment.

User costs also are incremental, including all changes that occur with the availability of L2C, and are net of savings from moving to less sophisticated and less proprietary equipment.

Sidebar: The L1C Studies
Before the L2C study, important progress had already been made in understanding the benefits of additional GPS signals. These activities included the discussion of civilian applications in the report of the Defense Science Board Task Force on GPS, released last December, and the L1C Study undertaken by the Interagency GPS Executive Board in 2004. (See the “Additional Resources” section at the end of this article to find out how to obtain these studies on line.)

Upper limits of total benefits of L1C for the single year 2005 — including those obtained by single- and multiple-frequency users in private households, businesses, governments — were estimated at approximately $2 billion: $640 million for mobile and wireless location services, $62.5 million for information/data services, $990 million for “commercial GPS,” and $490 million for in-vehicle information and navigation services (telematics).

The L1C study approximated a “rough order of magnitude” dollar value of L1C applications based on 2005 spending by applying a “team consensus” for an assumed incremental benefit as a percentage of market value (revenue) for each of 13 user categories. Spending in user group categories was based on a compilation of trade estimates.

Sidebar: L2C Benefit Scenarios
The four scenarios developed to support the L2C benefits study, along with the assumptions underling each, include the following:

High Opportunity

  • Timely signal availability
  • Larger than expected markets
  • High complementarity with L5
  • Success of High-Accuracy Nationwide Differential GPS augmentation
  • Full Galileo deployment in 2012 with less than complete technical performance

Moderate Benefits

  • Timely L2C availability
  • Large potential markets
  • Benefits moderated by competition from other signals and augmentations
  • Full Galileo deployment in 2011

Diluted Benefits

  • Large potential markets
  • Gradual L2C deployment and uncertainty about schedules slows investment in innovation and market development
  • Many users wait for L5 and for Galileo, which is expected in 2010
  • Improvements in public and private augmentations make single signal use more attractive

Opportunity Lost

  • Late signal initiation and protracted pace of L2C deployment
  • Slow introduction and adoption of user equipment
  • Some users wait for Galileo
  • Moderately large potential market size, moderate effects of availability of other signals and delay in Galileo FOC to 2011
  • Attractiveness of augmentations

Sidebar: Benefit Variables
Overstatement could result from competition from other signals, from augmentations and from other technologies that is greater than anticipated. For example,

  • Greater attractiveness of other signals because of the availability of satellites from Galileo in addition to those from GPS at the L1 and L5 frequencies
  • Advances in augmentations that make single frequency use more attractive
  • Slower price declines for L2C user equipment
  • Less triple frequency use when additional satellites are available from Galileo and/or greater use of Galileo signals at frequencies that do not correspond with L1 and L5
  • More users waiting for L5 for non-aviation civilian dual frequency use than allowed for in the study.

Understatement could result from

  • More important and/or numerous applications than were allowed for in the calculations
  • Faster price declines for multiple frequency user equipment (e.g. if competition squeezes high end margins even more) and/or larger price sensitivity of demand
  • Non-market benefits greater than the 25% of market benefits assumed
  • Impacts of L2C on long run economic growth, which were not included in the calculations and perhaps could add perhaps 20% to benefits.

For figures, graphs, and images, please download the PDF of the article, above.

Steve Bayless, Tyler Duval, Jason Kim, Scott Pace, Mike Shaw, Tom Stansell, Dave Turner, Jack Wells, Rodney Weiher, and Avery Sen offered comments, guidance and assistance to the study. Many others contributed expertise through interviews.

Additional Resources
Kenneth W. Hudnut, and Bryan Titus, GPS L1 Civil Signal Modernization (L1C), Interagency GPS Executive Board, July 30, 2004, <>

U.S. Defense Science Board, The Future of the Global Positioning System, Washington, D.C.: Office of the Under Secretary of Defense For Acquisition, Technology, and Logistics, October 2005, <>



Europe and the United States are on the verge of a very important decision about their plans to implement a common civil signal waveform at the L1 frequency: Should that waveform be pure binary offset carrier — BOC(1,1) — or a mixture of 90.9 percent BOC(1,1) and 9.09 percent BOC(6,1), a combination called multiplexed BOC (MBOC). The desire for a common civil L1 signal is enshrined in a 2004 agreement on GNSS cooperation between the United States and the European Union (EU).

Europe and the United States are on the verge of a very important decision about their plans to implement a common civil signal waveform at the L1 frequency: Should that waveform be pure binary offset carrier — BOC(1,1) — or a mixture of 90.9 percent BOC(1,1) and 9.09 percent BOC(6,1), a combination called multiplexed BOC (MBOC). The desire for a common civil L1 signal is enshrined in a 2004 agreement on GNSS cooperation between the United States and the European Union (EU).

For the EU and the European Space Agency (ESA), that decision — and its consequences — will come sooner: with the Galileo L1 Open Service (OS) that will be transmitted from satellites to be launched beginning in the next few years. For the United States, the waveform decision will shape the design of the L1 civil signal (L1C) planned for the GPS III satellites scheduled to launch in 2013. For a background on the process that led to design of the GPS L1 civil signal and its relevance to the BOC/MBOC discussion, see the sidebar L1C, BOC, and MBOC.

The May/June issue of Inside GNSS contained a “Working Papers” column titled, “MBOC: The New Optimized Spreading Modulation Recommended for Galileo L1 OS and GPS L1C”. Authored by members of a technical working group set up under the U.S./EU agreement, the article discussed the anticipated MBOC benefits, primarily improved code tracking performance in multipath. The column also noted that, while lower-cost BOC(1,1) receivers would be able to use MBOC, it would come at the cost of a reduction in received signal power.

An article in the “360 Degrees” news section of the same issue of Inside GNSS noted that some GNSS receiver manufacturers believe MBOC is not best for their applications and perhaps should not have been recommended. (This point was noted on page 17 of the May/June issue under the subtitle “MBOC Doubters.”) See the sidebar “Other Observers” (below) for additional comments from companies with concerns about MBOC recommendation.

This article, therefore, continues the discussion of a common signal waveform by asking several companies with different product perspectives whether they consider the proposed MBOC waveform to be more or less desirable for their applications than the BOC(1,1). Currently, BOC (1,1) is the baseline defined in the June 26, 2004, document signed by the U.S. Secretary of State and the vice-president of the European Commission (the EU’s executive branch): “Agreement on the Promotion, Provision and Use of Galileo and GPS Satellite-Based Navigation Systems and Related Applications.”

Maximum benefit from MBOC will be obtained by receivers using recently invented technology that employs computationally intensive algorithms. Although such receivers clearly will provide benefits to their users because of the BOC(6,1) component of MBOC, the practical value of the benefits have not been quantified, which is one purpose of the questions raised in this article. For the moment, let’s call all these prospective MBOC users “Paul”.

Meanwhile, patents on the most widely used multipath mitigation technologies today, such as the “narrow correlator” and the more effective “double-delta” techniques, will expire about the time the new signals are fully available, making these techniques more widely available. Unfortunately, the double-delta technology cannot use the BOC(6,1) component of MBOC. In addition, narrowband receivers, which today dominate consumer products, also cannot use the BOC(6,1). Let’s call all these users “Peter”.

Therefore, the fundamental question raised by this article is whether we should rob Peter to pay Paul. If the amount taken is quite small and the benefits are large, then the answer should be “yes.” If the amount taken creates a burden to Peter, now and for decades to come, with little benefit to Paul, then the answer should be “no.” The in-between cases are more difficult. The purpose of this article is to explore the tradeoffs.

To address this issue, we invited engineers from companies building a range of GNSS receivers to take part in the discussion. We’ll introduce these participants a little later. But first, let’s take a look at the technical issues underlying the discussion.

BOC/MBOC Background

The RF spectrum of a GPS signal is primarily defined by the pseudorandom code that modulates its carrier and associated data. A pseudorandom code appears to be a completely random sequence of binary values, although the sequence actually repeats identically, over and over.

For the C/A code on the L1 frequency (1,575.42 MHz), the state of the code (either +1 or –1) may change at a clock rate of 1.023 MHz. We call this binary phase shift keying, or BPSK(1), meaning BPSK modulation with a pseudorandom code clocked at 1.023 MHz. Note that the bits of a pseudorandom code often are referred to as “chips,” and four BPSK chips are illustrated at the top of Figure 1. (To view any figures, tables or graphs for this story, please download the PDF version using the link at the top of this article.)

Among many other topics, the 2004 U.S./EU agreement settled on a common baseline modulation for the Galileo L1 OS and the GPS L1C signals: BOC(1,1). (The BOC(n,m) notation means a binary offset carrier with n being a 1.023 MHz square wave and m being a 1.023 MHz pseudorandom code.) Like BPSK(1), the BOC(1,1) waveform also is a BPSK modulation, meaning there are only two states, either a +1 or a –1. The timing relationships of the code and the square wave are illustrated by Figure 1.

Although the agreement defined BOC(1,1) as the baseline for both Galileo L1 OS and GPS L1C, it left the door open for a possible signal “optimization” within the overall framework of the agreement. As documented in the paper by G.W. Hein et al., “A candidate for the GALILEO L1 OS Optimized Signal” (cited in the “Additional Resources” section at the end of this article) and many other papers, the EC Signal Task Force (STF) after much study initially recommended a composite binary coded symbols (CBCS) waveform.

Because the agreement made it desirable for GPS L1C and Galileo L1 OS to have an identical signal spectrum and because GPS III implementation of CBCS would be difficult, a search was made by a joint EC/US working group to find an optimized signal that was acceptable for both GPS and Galileo. The result is MBOC (discussed in the May/June “Working Papers” column and the like-named IEEE/ION PLANS 2006 paper by G. W. Hein et al. cited in “Additional Resources.”).

Like all modernized GPS signals — including M-code, L2C, and L5 — L1C will have two components. One carries the message data and the other, with no message, serves as a pilot carrier. Whereas all prior modernized GPS signals have a 50/50 power split between the data component and the pilot carrier, L1C has 25 percent of its power in the data component and 75 percent in the pilot carrier.

The L1C MBOC implementation would modulate the entire data component and 29 of every 33 code chips of the pilot carrier with BOC(1,1). However, 4 of every 33 pilot carrier chips would be modulated with a BOC(6,1) waveform, as illustrated in Figure 2. The upper part of the figure shows 33 pilot carrier chips. Four of these are filled to show the ones with the BOC(6,1) modulation. Below the 33 chips is a magnified view of one BOC(1,1) chip and one BOC(6,1) chip.

The BOC(1,1) chip is exactly as illustrated in Figure 1 while the BOC(6,1) chip contains six cycles of a 6.138 MHz square wave. With this image in mind, we can easily calculate that the pilot carrier has 29/33 of its power in BOC(1,1) and 4/33 of its power in BOC(6,1). Because the pilot carrier contains 75 percent of the total L1C signal power, then the percent of total BOC(6,1) power is 75 × (4/33) or 9.0909+percent. Conversely, the data signal has 25 percent of the total L1C signal power; so, the calculation of BOC(1,1) power is 25 + 75 × (29/33) or 90.9090+ percent.

Because the Galileo OS signal has a 50/50 power split between data and pilot carrier, the implementation is somewhat different in order to achieve the same percentages of BOC(1,1) and BOC(6,1) power. For the most likely time division version of MBOC for Galileo, 2 of 11 chips in the pilot carrier would be BOC(6,1) with none in the data component. Thus, the percent of total BOC(6,1) power is 50 × (2/11) or 9.0909+ percent. Similarly, the percent of total BOC(1,1) power is 50 + 50 × (9/11) or 90.9090+ percent. This makes the spectrum of Galileo L1 OS the same as GPS L1C.

Code Transitions. The fundamental purpose of MBOC is to provide more code transitions than BOC(1,1) alone, as is evident in Figure 2. (A code loop tracks only the code transitions.) However, these extra transitions come on top of the increased number in BOC(1,1) compared to the L1 C/A signal.

Taking into account that the pilot carrier has either 75 percent of the signal power with GPS or 50 percent with Galileo, GPS with BOC(1,1) has 2.25 times more “power weighted code transitions” than C/A-code (a 3.5-dB increase). Galileo with BOC(1,1) has 1.5 times more (a 1.8-dB increase). MBOC on GPS would further increase the net transitions by another factor of 1.8 (2.6-dB increase), and the most aggressive version of MBOC on Galileo would increase the net transitions by a factor of 2.2 (3.4-dB increase).

Therefore, given the improvement of BOC(1,1) over C/A code, the question raised by this article is whether a further improvement in number of transitions is worth subtracting a small amount of signal power during all signal acquisitions, for all narrowband receivers, and for all receivers using the double-delta form of multipath mitigation.

A portion of Table 1 from the May/June “Working Papers” column is reproduced here, also as Table 1. Of the eight possible waveforms in the original table, only three are included here. These are representative of all the options, and they include the two versions of MBOC considered most likely for implementation in Galileo and the only version GPS would use.

Two new columns have been added in our abbreviated version of the table. The first is an index to identify the particular option, and the last identifies whether GPS or Galileo would use that option.

Receiver Implementations

Most GNSS receivers will acquire the signal and track the carrier and code using only the pilot carrier. For GPS L1C this decision is driven because 75 percent of the signal power is in the pilot carrier. Little added benefit comes from using the data component during acquisition and no benefit for code or carrier tracking, especially with weak signals.

For Galileo, the decision is driven by the data rate of 125 bits per second (bps) and the resulting symbol rate of 250 symbols per second (sps). This allows only 4 milliseconds of coherent integration on the Galileo data component (compared with 10 milliseconds on the GPS data component). Because coherent integration of the pilot carrier is not limited by data rate, it predominantly will be the signal used for acquisition as well as for carrier and code tracking.

Reflecting the reasons just stated, Figure 3 compares the spectral power density in the pilot carrier for each of the three signal options listed in Table 1. In each case the relevant BOC(1,1) spectrum is shown along with one of the three MBOC options. These plots show power spectral density on a linear scale rather than a logarithmic dB scale, which renders small differences more prominent.

The center panel shows the GPS case with either BOC(1,1) or TMBOC-75. (The BOC(1,1) peaks are arbitrarily scaled to reach 1.0 Watt per Hertz (W/Hz). The BOC(1,1) peaks of TMBOC-75 are lower by 12% (-0.6 dB) in order to put additional power into the BOC(6,1) component of TMBOC-75, primarily at ±6 MHz.

All three panels of Figure 3 have the same relative scaling. The reason the peaks of the BOC(1,1) components in panels 1 and 3 are at 0.67 W/Hz is that GPS L1C will transmit 75 percent of its total signal power in the pilot carrier whereas Galileo will transmit 50 percent. The difference is simply 0.5/0.75 = 0.67 (-1.8 dB).

The first panel of Figure 3 also shows the Galileo TMBOC-50 option in which the BOC(1,1) component peaks are lowered by 18 percent (-0.9 dB) in order to provide power for the BOC(6,1) component, primarily at ±6 MHz.

The third panel shows the same Galileo BOC(1,1) power density but with the CBOC-50 option. In this case the BOC(6,1) component exists in the data channel as well as the pilot carrier. That is why it is half the amplitude at ±6 MHz as in panels 1 and 2. That also is why less power is taken from the BOC(1,1) component for the BOC(6,1) component; in this case the reduction is 9 percent (-0.4 dB). This is not considered an advantage by those who want to track the BOC(6,1) component, and it also reduces the data channel power for narrowband receivers by the same 9 percent or 0.4 dB.

As stated before, the fundamental question raised by this article is whether we should rob Peter to pay Paul. As with all such top-level questions, the answers lie in the details and in the perceptions of those affected. Inside GNSS posed a series of questions to industry experts in order to explore their perspectives and preferences.

The Questions and Answers

Q: What segment of the GNSS market do your answers address? Describe your market, including typical products and the size of the market.

Fenton – High precision survey and mapping, agriculture/machine control, unmanned vehicles, scientific products, and SBAS ground infrastructure where centimeter accuracy is very important. NovAtel sells at the OEM level to software developers and system integrators and calculates its present total addressable market (TAM) at $300-$400 million USD, again at the OEM level.

Garin – We are focused on consumer electronics where very low cost and very low power are of critical importance, such as personal navigation devices (PNDs), cellular phones, and in general applications where the power consumption is at a premium. These objectives should be reached with little to no impact on the user experience. The loss of performance due to design tradeoffs is mitigated by assisted GPS (A-GPS).

Hatch /Knight – NavCom supplies high-precision, multi-frequency GNSS receivers that employ advanced multipath and signal processing techniques, augmented by differential corrections from our StarFire network. These receivers are widely used in the agriculture, forestry, construction, survey, and offshore oil exploration markets. Current market size is on the order of 100,000 units per year.

Sheynblat/Rowitch – Our answers address wireless products for the consumer, enterprise, and emergency services markets. There are over 150 million Qualcomm GPS enabled wireless handsets in the market today, and this large market penetration and heavy usage is primarily driven by low cost, low power, and high sensitivity. The vast majority of other GPS enabled consumer devices worldwide are also cost driven.

Stratton – Rockwell Collins is a leading provider of GPS receivers to the U.S. military and its allies, and we are also a major supplier of GNSS avionics to the civil aviation industry. The civil aviation applications demand high integrity and compatibility with augmentation systems, while the military requirements range from low-power, large-volume production to high-dynamic and highly jam-resistant architectures (as well as civil compatible receivers). Military receivers are impacted due to civil compatibility requirements. Our company has produced over a half million GPS receivers and has a majority market share in military and high-end civil aviation (air transport, business, and regional) markets.

Studenny – Our market is commercial aviation where continuity of operation and integrity are the most important performance parameters.

Weill – I and a colleague, Dr. Ben Fisher, of Comm Sciences Corporation, are the inventors of a new multipath mitigation approach which we call Multipath Mitigation Technology (MMT), so our primary product is technology for improved multipath mitigation. MMT is currently incorporated in several GPS receivers manufactured by NovAtel, Inc. Their implementation of MMT is called the Vision Correlator.

Q: Which signal environments are important for your products: open sky, indoor, urban canyon, etc.

Fenton – In general, most of our customers operate in open sky environments. However, a significant number are operating under or near tree canopy and in urban canyons.

Garin – Ninety percent of our applications are or will be indoors and in urban canyons.

Hatch /Knight – Our receivers are mostly used in open sky and under-foliage conditions.

Stratton – Our products use civil signals mainly in open sky conditions, although civil signals may be used to assist the acquisition of military signals in a broad variety of environments.

Studenny – Aircraft environments, with particular attention to safety-of-life. Also, ground-based augmentation system (GBAS) ground stations.

Weill – Any environment in which multipath is regarded as a problem, including precision survey, indoor (911) assisted GPS, and military and commercial aviation.

Q: Which design parameters are most critical for your products: power, cost, sensitivity, accuracy, time to fix, etc.

Fenton – In general, our products service the high end “commercial” markets. Our customers in general have priorities in the following order: a) accuracy, b) robust tracking, c) cost, d) power, e) time to first fix.

Garin – The most important criteria are, from the highest to the lowest: power, cost, sensitivity, time-to-first-fix, and finally, accuracy.

Hatch /Knight – Accuracy is most important.

Sheynblat/Rowitch – We have invested substantial engineering effort to achieve market-leading sensitivities (-160 dBm) while maintaining very low receiver cost. Engineering investment, focus on sensitivity, and close attention to cost models is probably also true for other vendors focused on mass market, AGPS enabled devices that have to work indoors. All of these GPS vendors go to great lengths to improve sensitivity for difficult indoor scenarios. Every dB counts and may make the difference between a successful or a failed fix, which is of particular concern for E-911 and other emergency situations.

Stratton – The tradeoff in relative importance of these parameters varies widely depending on the particular application, though life-cycle cost (including development and certification) arguably is most significant.

Studenny – Actually, all parameters are important. However, we focus on safety-of-life and the drivers are both continuity of operation and integrity (hazardously misleading information or HMI).

Specifically, we believe cross-correlation, false self-correlation, and the ability to resist RFI, as well as improving multipath performance, are signal properties of great interest to us. A well-selected coding scheme minimizes all of these and HMI in particular. Finally, HMI may become a legal issue for non-aviation commercial applications, especially if those applications involve chargeable services, implied safety-of-life, and other such services.

Weill – MMT is most effective in receivers that have high bandwidth and are receiving high-bandwidth signals. However, it can substantially improve multipath performance at lower bandwidths.

Q: Do you really care whether GPS and Galileo implement plain BOC(1,1) or MBOC? Why?

Fenton – Yes, we expect that the MBOC signals combined with the latest code tracking techniques will provide a majority of our customers a significant performance benefit for code and carrier tracking accuracy in applications where multipath interference is a problem.

Garin – I do not believe that MBOC will significantly benefit our short-term market. The MBOC expected multipath performance improvement will be meaningless in the urban context, where the dominant multipath is Non Line of Sight and where the majority of the mass market usage is concentrated. However we believe that a carrier phase higher accuracy mass market will emerge within a 5 year timeframe, with back-office processing capabilities, and wireless connected field GPS sensors. This will be the counterpart of the A-GPS architecture in cell phone business. MBOC would have an important role to play in this perspective. We envision this new market only in benign environments, and not geared towards the surveyors or GIS professionals.

Hatch /Knight – The MBOC signal will significantly improve the minimum code tracking signal to noise ratio where future multipath mitigation techniques are effective. The expected threshold improvements will be approximately equal to the best case improvements indicated by this article. MBOC will be less beneficial to very strong signals where the noise level is already less than the remaining correlated errors, like troposphere and unmitigated multipath.

Designing a receiver to use the MBOC code will be a significant effort. The resulting coder will likely have about double the complexity of the code generator that does not support MBOC. There will be a small recurring cost in silicon area, and power consumption will increase significantly. Overall, MBOC is desirable for our high performance applications. For many applications the costs are greater than the benefits.

Sheynblat/Rowitch – Yes, we do care about the decision of BOC versus MBOC. The proposed change to the GPS L1C and Galileo L1 OS signal to include BOC(6,1) modulation will perhaps improve the performance of a very tiny segment of the GPS market (high cost, high precision) and penalize all other users with lower effective received signal power due to their limited bandwidth. We prefer that GPS and Galileo implement the BOC(1,1) signal in support of OS location services.

Stratton – This decision does not appear to have much influence on our markets when viewed in isolation, but we would like to see GPS make the best use of scarce resources (such as spacecraft power) to provide benefits that are attainable under realistic conditions.

Studenny – Yes, we do care. GPS L5 needs to be complemented by a signal with similar properties at L1, the reason being that a momentary outage during precision approach on either L1 or L5 should not affect CAT-I/II/III precision approach continuity or integrity. We understand that there are constraints in selecting a new L1 signal; however the proposed MBOC waveform better supports this. This is keeping with supporting the FAA NAS plans and transitioning to GNSS for all phases of flight including precision approach.

Weill – Yes. Comm Sciences has established that the performance of current receiver-based multipath mitigation methods is still quite far from what is theoretically possible. It is also known that GNSS signals with a wider RMS bandwidth have a smaller theoretical bound on ranging error due to thermal noise and multipath. Since multipath remains as a major source of pseudorange error in GNSS receivers, I feel that the use of an MBOC signal for GPS and Galileo is an opportunity to provide the best possible multipath performance with evolving mitigation methods that take advantage of the larger RMS bandwidth of an MBOC signal as compared to plain BOC(1,1).

Q: Are the GNSS receivers of interest narrowband (under ±5 MHz) or wideband (over ±9 MHz)?

Fenton – Wideband. High precision GNSS receivers typically process all available bandwidth ~20 MHz (±10 MHz).

Garin – Our GNSS receivers are narrowband today, but we expect the widening of the IF bandwidth (or equivalently their effective bandwidth) to ±9 MHz, in the next 3-5 years, with the same or lower processing and power consumption.

Hatch /Knight – Our receivers are primarily wideband.

Sheynblat/Rowitch – The receivers of interest are narrowband. Low cost GPS consumer devices do not employ wideband receivers today and will most likely not employ wideband receivers in the near future. Any technology advances afforded by Moore’s law will likely be used to further reduce cost, not enable wideband receivers. In addition, further cost reductions are expected to expand the use of positioning technology in applications and markets which today do not take advantage of the technology because it is considered by the manufacturers and marketers to be too costly.

Stratton – All of our markets require wide-band receivers; however, the civil receiver/antenna RF characteristics are adapted to high-bandwidth C/A processing (where the bulk of RF energy is at band center). So the MBOC signal does raise some potential compatibility questions.

Studenny – Wideband.

Weill – I believe the trend will be toward wideband receivers for most applications. If one looks at the history of GPS receiver products, it is clear that there has always been competitive pressure to increase positioning accuracy, even at the consumer level. Not only is better accuracy a marketing advantage, but it has also opened up entirely new applications. The availability of wide bandwidth signals is a key factor in continuing to improve positioning accuracy. Although currently available receivers that can take advantage of wider bandwidth signals cost more and consume more power, the rapid rate of improving digital technology should make low-cost, low-power, wide bandwidth receivers available in the not-so-distant future. The availability of an MBOC signal would maximize the capability of such receivers.

Other Observers

Inside GNSS invited comments from a broad range of companies representative of most GNSS markets. In addition to those who fully responded to our questions, several offered abbreviated remarks:Garmin International, Inc. did not identify a spokesperson, but it submitted the following official statement: “It is Garmin’s policy not to disclose any information about future designs. However, we would like to indicate that we support the BOC(1,1) implementation over the MBOC.”Sanjai Kohli, Chief Technology Officer of SiRF Technology Inc., submitted the following official statement: “The existence of the BOC(6,1) chips in the MBOC signal won’t matter very much to SiRF. Still, to maximize the availability of weak signals, it would be preferable not to suffer any loss of signal power. Therefore, SiRF would prefer that all chips be BOC(1,1). Furthermore, it is doubtful that any advanced method of multipath reduction will be of much benefit for urban and indoor signal reception, since it is likely that the line-of-sight component of the weak signal is blocked.”

European Company – A large and well known European consumer products company could not obtain internal approval to answer the questions, but the following unofficial communication from a technical manager is of interest: “Our understanding about the pros and cons of MBOC as compared with BOC(1,1) is . . . that narrow-band receivers are not able to utilize the higher frequency components of the MBOC signal and they thus represent wasted power from their viewpoint. This is especially true for acquisition, because the acquisition bandwidth many times seems to be narrower than the tracking bandwidth, especially in those parallel acquisition receivers that are used in consumer products specified for weak signal operation. For such receivers the received signal power is critical in the acquisition phase, not so much in the tracking phase.”

L1C, BOC, and MBOC

Pertinent to the subject of this article is the remarkable way in which the L1C signal was designed. The original C/A- and P-code signals were designed by a small group of technologists under the direction of the GPS Joint Program Office (JPO). Although from the beginning GPS was understood to be a dual-use (civil and military) system, the signals were designed primarily from a military perspective.

Design of the L2C civil signal was led by a JPO deputy program manager representing the Department of Transportation (DoT) — but the process took place under extreme time pressure. The RTCA, Inc., with authorization from the Federal Aviation Administration (FAA), initially defined the L5 signal. The RTCA is a consensus-driven open forum, but its focus is almost exclusively on aviation.

In contrast, development of L1C was funded by the Interagency GPS Executive Board (IGEB), now superseded by the National Space-Based Positioning, Navigation, and Timing (PNT) Executive Committee. Representatives of the Department of Defense (DoD) and DoT co-chair the PNT Executive Committee: so, the central focus is on managing GPS as a dual-use utility. Reflecting this, the L1C project was co-chaired by a DoD representative and by a civil representative. (The civil co-chair was Dr. Ken Hudnut of the U.S. Geological Survey. A sequence of JPO officers represented the DoD: Captains Bryan Titus, Amanda Jones, and Sean Lenahan. Tom Stansell of Stansell Consulting served as project coordinator throughout.)

L1C development consisted of two key activities. The first was a study of the wide range of civil requirements and development of five signal structure options. A technical team conducted this part of the work, drawing on experts in all aspects of the signal, including spreading code, data modulation, forward error correction, and message format.

Several team members had deep experience developing civil user equipment, from consumer chipsets to high-precision survey receivers. Others were experts on aviation requirements. The second key activity is, to our knowledge, unique: a worldwide survey of GNSS experts to determine which of the five options to choose. The design process is complete, and a draft specification (IS-GPS-800) has been published.

The innovative MBOC proposal was developed quickly by a group of very competent U.S. and EU signal experts with both civil and military backgrounds. However, this team apparently had only one person with extensive experience in receiver manufacturing, and the timeline did not allow the opportunity for a broad survey to assess equipment manufacturers’ opinions about the design. Informal conversations with some industry representatives also revealed dissatisfaction with MBOC. Therefore, Inside GNSS decided to consult a number of experts from companies that build GNSS equipment to determine their thoughts about the MBOC concept.

Additional Resources

1. Agreement on the Promotion, Provision and Use of Galileo and GPS Satellite-Based Navigation Systems and Related Applications, June 26, 2004,

2. Hein, G. W., and J-A. Avila-Rodriguez, L. Ries, L. Lestarquit, Issler, J. Godet, and T. Pratt, “A candidate for the GALILEO L1 OS Optimized Signal”, Proceedings of ION GNSS 2005 – 13-16 September 2005, Long Beach, California

3. Hein, G. W., and J-A. Avila-Rodriguez, S. Wallner, J. W. Betz, C. J. Hegarty, J. J. Rushanan, A. L. Kraay, A. R. Pratt, S. Lenahan, J. Owen, JL. Issler, T.A. Stansell, “MBOC: The New Optimized Spreading Modulation Recommended for Galileo L1 OS and GPS L1C”, Inside GNSS, Volume 1, Number 4, pp 57–65, May/June 2006

4. Hein, G. W., and J-A. Avila-Rodriguez, S. Wallner, A. R. Pratt, J. Owen, J-L. Issler, J. W. Betz, C. J. Hegarty, S. Lenahan, J. J. Rushanan, A. L. Kraay, T.A. Stansell, “MBOC: The New Optimized Spreading Modulation Recommended for GALILEO L1 OS and GPS L1C”, IEEE/ION PLANS 2006, April 24-27, 2006, San Diego, California

5. IS-GPS-200: NAVSTAR GPS Space Segment / Navigation User Interfaces; IS-GPS-705: NAVSTAR GPS Space Segment / User Segment L5 Interfaces; Draft IS-GPS-800 for new L1C signal;

May 1, 2006

Galileo Light Show: First Laser Range Measurements to GIOVE-A

Space Geodesy Facility at Herstmonceux, East Sussex,UK

European researchers have obtained laser range measurements of 1.5-centimeter accuracy to GIOVE-A, the first element in the Galileo In-Orbit Validation phase. The observations were made through collaboration between the UK Space Geodesy Facility (SGF), Surrey Satellite Technology Limited (SSTL), and the European Space Agency (ESA). The SGF is operated by the UK Natural Environment Research Council, with support from the British National Space Centre and the Ministry of Defence.

European researchers have obtained laser range measurements of 1.5-centimeter accuracy to GIOVE-A, the first element in the Galileo In-Orbit Validation phase. The observations were made through collaboration between the UK Space Geodesy Facility (SGF), Surrey Satellite Technology Limited (SSTL), and the European Space Agency (ESA). The SGF is operated by the UK Natural Environment Research Council, with support from the British National Space Centre and the Ministry of Defence.

The SGF satellite laser ranging system at Herstmonceux, East Sussex, made the range measurements on April 8. The facility had been invited to attempt observations by SSTL, the prime contractor that built the GIOVE-A space vehicle launched from Kazakhstan’s Baikonur space center on December 25, 2005.

At the time of acquisition, GIOVE-A was more than 25,000 kilometers distant from the station, the furthest distance ranged by the facility. GIOVE-A orbits the Earth at a height of more than 23,000 kilometers, some 3,000 kilometers above the GPS and GLONASS satellites that SGF regularly observes.

In order to track the satellite from Herstmonceux, predictions of its path based upon its transmitted navigational signals were supplied to SGF by the GIOVE Processing Centre located at ESA ESTEC, The Netherlands. The predictions were sufficiently accurate for the sunlit image of the satellite to be detected at night by the system’s high intensity camera and allowed the observer to direct short laser pulses towards it.

Laser Ranging
The satellite laser ranging (SLR) technique uses small astronomical telescopes to emit short pulses of laser light towards specially equipped spacecraft and to detect those photons that are reflected back. The times of emission and reception are recorded to an accuracy of a few picoseconds such that the range to the satellite can be deduced from the measured time of flight to a precision of better than one centimeter. The satellites tracked by this technique are equipped with an array of quartz cube-corner reflectors that closely return to source the incoming laser pulses.

The technique is weather dependent as skies have to be fairly cloud-free, but measurements are carried out both during daytime and at night. The key to success in extracting signal from noise is narrow-band spectral filtering in the return optical path and spatial filtering through a range gate technique that arms the detector only a few tens of nanoseconds before the expected arrival time of returning photons.  During the satellite pass the time of flight inferred from every detected event is compared to the expected time of flight to the satellite and back.

A plot against time is built up of these observed-computed values, where true returns appear as a correlated track amongst the sky-noise events. Post processing extracts the real events, which are made available to the analysis community a few minutes after the end of the observations.

Unlike the microwave signals used by GNSS, the propagation time of laser light is not affected by the ionosphere; the tropospheric delay, amounting to a zenith range correction of two meters, can be estimated to millimeter accuracy using atmospheric mapping functions and local meteorological data. This lack of dependence of propagation delay on the variable effects of the ionosphere makes laser ranging a very strong technique for high-precision terrestrial reference frame (TRF) determination.

Laser range observations of a series of geodetic satellites, high-density small spheres encrusted with retro-reflectors and orbiting the Earth at heights of from 800 to 19,000 kilometers, are routinely used to monitor tiny changes in the location of the center of mass of the Earth, the origin of the TRF, that are driven by mass redistributions within the Earth system.

The technique is also used in precise orbit determination of Earth-observation altimeter and SAR satellites, complementing on-board tracking systems such as GPS and DORIS. This work and the operation of the worldwide network of SLR stations is coordinated by the International Laser Ranging Service (ILRS,

The GIOVE-A satellite is equipped with 76 quartz corner-cube retro-reflectors in an array to one side of its base, which is taken from the ESA document “Specification of GALILEO and GSTB-V2 Space Segment Properties Relevant for Satellite Laser Ranging, ESTEC, November 2005.”

During the design phase of the satellite, discussion took place among SSTL, ESTEC, and SGF in order to use SGF’s previous experience of ranging to the navigational satellites to help understand the difficulties and inform the choice of numbers of cubes to deploy, within available space and weight constraints, to enable a realistic link budget. For comparison, the two GPS satellites that are fitted with retro-reflectors have an array of only 32 cubes, and the newest GLONASS satellites have about 130 cubes.

The higher altitude of GIOVE-A reduces the return signal, which varies as the fourth power of the range, by some 55 percent relative to the signal from a GPS satellite; so, the greater number of cubes on GIOVE-A should ensure a return signal that is about 30 percent better than that from the GPS satellites, which from SGF’s Herstmonceux SLR facility are difficult targets.

SLR Results
Observations from Herstmonceux tend to support this analysis, with the Galileo laser return rate estimated at about 4 percent on average over the one-hour session; for the GPS satellites the rate is about 2 percent, and for GLONASS closer to 10 percent.

We can also use the laser range observations to estimate the accuracy of the predictions supplied by the GIOVE Processing Centre located at ESTEC. The differences between the observed and computed ranges imply that the predictions were accurate to about two or three kilometers, the major error being in the along-track direction.

Previous studies carried out by SGF and others have used laser range observations from the global network of ILRS stations to measure the quality of GPS and GLONASS orbits that are computed by the International GNSS Service (IGS, The laser ranging technique is a powerful, independent method of testing the accuracy of these orbits, which are determined from continuously transmitted navigational signals.

Given knowledge of the accurate location of the retro-reflector arrays on the satellites, SGF uses the laser range measurements to determine at a level of accuracy of better than one centimeter the radial distance to the satellite center of mass. The results suggest that the IGS orbits for GPS and GLONASS have an RMS radial accuracy of better than five centimeters and, interestingly, that there appears to be a radial bias in the IGS orbits of a few centimeters, in the sense that the IGS-determined orbits appear too big.

SGF and other researchers expect to use a similar technique to test the quality of any available precise orbital information derived from the GIOVE Processing Centre. In addition, once laser range data from the global network is available, it will be possible to determine SLR-only satellite orbits, which will further assist on-board clock characterisation in flight.

For figures, graphs, and images, please download the PDF of the article, above.


GPS III, Block IIF Programs Hit New Delays

The GPS program appears to be struggling on several fronts recently.

GPS III, the next-generation modernization project for the space and ground segments, is facing renewed uncertainty and possible schedule delays. At the same time, anticipated first launch of the follow-on block of satellites (Block IIF) with the new civil L5 signal has been postponed.

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By Inside GNSS

Civil L1 Signals: Galileo ICD, GPS L1C, New MBOC

Within weeks of a bilateral working group’s recommendation for a common civil GNSS signal design, the European Galileo and U.S. GPS programs have filed draft interface specifications (IS) or interface control documents (ICDs) for the new signals planned for the L1 frequency (around 1575 MHZ).

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By Inside GNSS
April 1, 2006

Builders Notes: Russian GLONASS at the Stage of Active Implementation

Modernized GLONASS-M satellite (left), A GLONASS-M satellite design (right)

On December 25, 2005, from the Baikonur Launch Site, there were three launched three navigation satellites belonging to the GLONASS system: one GLONASS satellite (the last of the first generation) and two new GLONASS-M satellites (see photo, above left).

This launch ensured the possibility of completing the flight tests for the modernized GLONASS system and performing the direct navigation determination using four GLONASS-M satellites simultaneously (two of these GLONASS-M satellites were launched earlier in 2003 and 2004).

On December 25, 2005, from the Baikonur Launch Site, there were three launched three navigation satellites belonging to the GLONASS system: one GLONASS satellite (the last of the first generation) and two new GLONASS-M satellites (see photo, above left).

This launch ensured the possibility of completing the flight tests for the modernized GLONASS system and performing the direct navigation determination using four GLONASS-M satellites simultaneously (two of these GLONASS-M satellites were launched earlier in 2003 and 2004).

Today, the orbital GLONASS constellation includes 16 satellites (12 GLONASS satellites and 4 GLONASS-M satellites). This article discusses the planned modernization of the GLONASS satellite navigation system with particular emphasis on the improved design of GLONASS-M satellites.

A New Commitment
The first satellite of the Russian navigation system GLONASS was launched on October 12, 1982, and the system was introduced into operation in 1993, being deployed to the complete constellation of 24 satellites in 1995. With 24 satellites in orbit, the GLONASS system can ensure the continuous global navigation for military and civil users by employing two types of signal: a signal of standard accuracy for civil users and a high-accuracy signal for military users.

When Russia faced new economical conditions in the 1990s, the financing for the space industry was reduced leading to the orbital GLONASS constellation reduction and decrease of its effectiveness.

Bearing in mind that the Space Navigation System GLONASS is a part of the national patrimony of Russia, in 2001 the president of Russian and the government of the Russian Federation ratified the policy directives setting out the intent to conclusively preserve and develop this navigation system. The Federal Target Program “Global navigation system” is one of these documents.

This program has been developed to be completed for the decade (from 2002 to 2011). During this period certain research and development activities shall be performed, including the ground experimental development for the prospective navigation spacecraft as well as flight and design tests; the ground control segment for the navigation system shall be modernized; the orbital constellation with the nominal number of satellites (24) shall be replenished.

GLONASS Modernization
The 10-year program of creation and operation of the modernized Russian Navigation System GLONASS space segment covers two stages: the current GLONASS-M satellites – at the first stage, and the proposed GLONASS-K satellites – at the second stage.

The GLONASS system is being modernized based on the following main conditions:

•     qualitative improvement of radio-navigation signal (introduction of the third frequency, increase in message rate, addition of new information into a navigation signal, etc), shift in the frequency bands keeping the possibility to work for the current existing users of the GLONASS system
•     improvement of the reliability and accuracy of the navigation support provided
•     increase of the satellite operation autonomy period and decrease of the level of the Ground Control Segment support needed to control the satellites
•     reduction of maintenance costs for satellite constellation as a result of increased satellite lifetime, reduced mass and resulted decrease of cost per satellite in-orbit
•     extension of the range of mission tasks to be performed.

With the preceding conditions implemented, there is kept the orbital configuration established earlier (three planes, with eight satellites in each plane), the orbital parameters (Н=19,400 km, i=64.8°, е=0) and the quantity of satellites in the nominal constellation (24 satellites). This enables the GLONASS operators to maintain the principles and methods of ballistic support of the satellite constellation and to provide the high-accuracy ephemeris.

New Generation Satellites
A GLONASS-М satellite, which is being developed at the first stage of the GLONASS Space navigation system modernization, has the following specific features as compared to a GLONASS satellite which is in use now:

1)     upgraded navigation radio signal
2)     implementation of intersatellite radiolinks to provide ranging measurements and data exchange between satellites located in the same plane and in different planes
3)     the stability of navigation signals increased up to 1·10-13 as a result of providing precision thermal stabilization of on board cesium frequency standards
4)     an improved dynamic model will decrease the level of unaccounted active forces impacting the satellite, mainly as result of increased accuracy of solar arrays pointing towards the Sun
5)     increased operational life of a satellite — up to seven years.

A GLONASS-М satellite can be injected into orbit by a cluster launch (three satellites by a single launch vehicle — see photo at the top of this article) from the Baikour Launch Site (using Proton LV and Breeze-M Booster) or by a single satellite launch from Plesetsk Launch Site using Soyuz-2 LV and Fregat Booster.

Spacecraft Design
A GLONASS-М satellite design (see Figure 1, above right) is based on a pressurized container inside which comfortable temperature conditions are maintained, ensuring the temperature range from 0 to 40°С, and local areas of temperature stabilization (near atom frequency standards) with ±1°С accuracy level. The temperature range is maintained by an active gas loop, shutter subsystem with electrical drivers and a set of controlled heaters. All dissipating mission equipment units are located outside the pressurized container on the antenna module in the areas not illuminated by the Sun.

Due to the fact that on board a GLONASS-M satellite there is a great amount of mission equipment units operating in the open space environment, the satellite design represents the intermediate stage between pressurized and non-pressurized design. In the nominal mode, the satellite longitudinal axis is continuously maintained pointed to the Earth, with accuracy of 0.5°, the satellite lateral axis is kept in the Sun-satellite-Earth plane with accuracy of ~0.5°, solar array axes are oriented towards the Sun with the accuracy of 2°. The orientation is provided by electrical wheels, periodically unloaded by electromagnets.

The propulsion subsystem being a single component thruster subsystem based on the catalytic thermal hydrazine separation method provides the possibility to form control torques within the initial orientation modes of the satellite, and to generate pulses for orbit correction. The orbit correction is performed after the satellite has been injected into orbit, while drifting to the designated orbital slot. High accuracy of the initial correction of orbital parameters allows keeping the satellite within the specified station limits (±5° latitude argument) without need for further corrections during the remaining lifetime.

An electric power subsystem based on nickel-hydrogen batteries and silicon solar arrays (30m2 area) provides electric power supply for onboard equipment of continuous stable voltage of 27+1-2 V and the power of up to 1400W, continuously in eclipse and illuminated orbit arcs. The onboard control subsystem based on an onboard digital computer provides data exchange between the equipment via MIL-STD-1553-B buses and performs the functions of control, diagnostic, intersatellite ranging data processing, calculation and generation of ephemeris time data.

For figures, graphs, and images, please download the PDF of the article, above.


Geodesy and Satellite Navigation

There has always been a love-hate relationship between geodesy and satellite navigation. Indeed, satellite positioning started life as an extension of terrestrial geodesy. When the first satellite, Sputnik 1, started orbiting the Earth in 1957, geodesists in several countries realised that satellites offered substantial potential as a geodetic positioning and navigation tool.

There has always been a love-hate relationship between geodesy and satellite navigation. Indeed, satellite positioning started life as an extension of terrestrial geodesy. When the first satellite, Sputnik 1, started orbiting the Earth in 1957, geodesists in several countries realised that satellites offered substantial potential as a geodetic positioning and navigation tool.

The basic technologies of terrestrial geodesy of the day, notably triangulation, traversing, and precise leveling, were slow and cumbersome, mainly because of the effect of the curvature of the surface of the Earth, which limited the range of measurements to theodolite observations between points situated on hilltops, observation towers, and triangulation masts.

The advent of EDM (electronic distance measurement) in the 1960s helped terrestrial geodesy, but it, too, was affected by the same limitation, namely the shortness of observable EDM ranges due to the Earth’s curvature.

Earth orbiting satellites did not suffer from this drawback. They could be viewed simultaneously from several points on Earth, and therefore direction and range measurements made, provided that the space vehicles were not obscured by high natural features or tall man-made structures. This led to several new satellite geodesy positioning methodologies.

The first of these was satellite triangulation, which was used initially to supplement and strengthen terrestrial triangulation networks. Satellite triangulation consisted of geodetic direction measurements derived from high power photographs of satellite orbits made against a stellar background of stars, with known right ascension and declination.

A few years later, this was followed by range measurements to satellites, made from Earth-bound EDM equipment to corner cube reflectors placed on the early satellites. The methodology used thus far was an extension of geodetic astronomy, with little reference to physical geodesy.

This situation changed significantly when geodesists realized that they could use the Doppler shift on the signal broadcast from a satellite to obtain differential range measurements that, together with the known Keplerian orbit of the satellite, could lead to a relatively fast positioning, or navigation, method. The Keplerian orbital motion of satellites is primarily based on the Earth’s gravity field, a subject of expertise by practitioners of physical geodesy.

This technical advance gave birth to Transit-Doppler, the first satellite navigation technology. Transit-Doppler was used in the late 1970s and early 1980s not only for the positioning of naval ships and of submarines surfacing in the polar regions, but also for the strengthening and scaling of national and continental terrestrial triangulation networks.

However, practitioners soon realized that positioning by Transit-Doppler to a reasonable degree of accuracy took several minutes, and, therefore, precluding its use as a full navigation methodology, which requires quasi-instantaneous positioning.

Enter GPS
These were the early days of a new global satellite positioning, navigation, and timing system, first called the NAVSTAR Global Positioning System, a name later shortened to just GPS. The rest is history. The early decision to base GPS on a constellation of 24 medium-Earth orbit satellites was taken on the advice, as you would expect, of geodesists at the U.S. Naval Surface Weapons Center in Dalgren, Virginia.

The close relationship between the early GPS and geodesy was further demonstrated by the adoption of WGS84, the World Geodetic System 1984, as the basis of the 3-D coordinate system of GPS. As GPS was born during the Cold War, it was declared a US military navigation system, with full access to NATO but only restricted access and down-graded positioning accuracies for civilian users.

This so-called Selective Availability (SA) gave the green light to the civilian geodetic community to come up with new methodologies that could counter the effects of SA. As always, human ingenuity did not disappoint, and two new differential techniques were developed. The first was the differential GPS (DGPS) technique, which improved relative positioning accuracies of GPS by at one order of magnitude, down to a few meters. As a result, DGPS soon became the standard methodology for the offshore positioning of oil platforms, pipelines, etc.

The next advance in improving the accuracy of satellite positioning was made on the advice of radio-astronomers, who proposed replacing the standard GPS pseudorange measurements, which are based on timing the modulated signal from satellite to receiver.

Instead, they suggested making measurements on the basic carrier frequencies of these signals, just as they did with extra-galactic signals arriving at, say, two widely spaced radio telescopes in so-called very long baseline interferometry (VLBI), leading as a by-product to the Cartesian coordinate differences between the two telescopes. This was the beginning of centimetric positioning by the carrier phase GPS method, which was later developed further by geodesists into kinematic GPS and centimetric navigation.

GPS had now become the universal high precision quasi-instantaneous positioning and navigation tool, creating the basis for hundreds of new applications. Again, geodesists led the way, concentrating on high precision scientific and engineering applications. These included surveying and mapping, positioning in offshore engineering, the monitoring of local crustal dynamics and plate tectonics, the relative vertical movements of tide gauges, and the continuous 3-D movements of critical engineering structures, such as tall buildings, dams, reservoirs, and long suspension bridges.

All of these applications required very high relative positioning accuracies, but not quasi-instantaneously as in the safety-critical navigation and landing of civilian aircraft. This came much later.

Geodesy and Navigation
Initially, GPS was considered as a standard navigation tool for military vehicles on land, sea, and air, but not for safety-critical civilian transportation. This was because, unlike military positioning and navigation, safety-critical civilian transportation not only requires quasi-instantaneous and accurate positioning, but also so-called “high integrity and good coverage.”

Geodesists will immediately realize that “integrity” stands for the geodetic concept of “reliability,” whereas “coverage” refers to the availability of a sufficient number of satellites that can be sighted by a receiver continuously and are not obscured by natural or man-made obstructions, such as high mountains, tall buildings, and the wings of an aircraft.

On its own, GPS cannot meet these requirements to the level required in safety-critical civilian transportation. Military transportation, on the other hand, has relatively modest requirements, which can be met by GPS. Indeed, you do not become a NATO Air Force pilot if you want a safe life. Flying as a passenger in a commercial airline is something else all together.

The penetration of satellite navigation, and primarily GPS, into civil aviation involved yet again, as you would expect, geodesists. They had to develop jointly with the civil aviation community the necessary theoretical and practical tools, which could be used to establish and quantify their requirements of accuracy, integrity, and coverage.

This involved the use of existing geodetic tools, such as the covariance matrix, the analysis of least squares residuals, and the well-established geodetic reliability measures. New tools were also introduced, such as the concept of RAIM or receiver autonomous integrity monitoring, based on the analysis of the least squares residuals.

Persuading Non-Geodesists
These geodetic tools, which were highly beneficial to the civil aviation community, initiated a fruitful, long-term collaboration between the two communities. However, this has not always been a straightforward and smooth relationship, and it involved — especially at the beginning — a deep suspicion of these “academic” geo-scientists. Here are a few notable examples of this love-hate relationship.

As a general rule, the existing civil aviation horizontal coordinates were based on latitudes and longitudes, with no particular reference to a reference datum. Heights in civil aviation were and still are based on barometric altimetry, on the assumption that all that matters is “the relative heighting between airplanes,” which is not affected significantly by a change in barometric pressure.

This assumption disregards, of course, the fact that the heights of natural features on the ground, such as mountains, do not change with changing barometric pressure. The first challenge was to convince the international civil aviation community that their horizontal coordinates, that is, latitudes and longitudes, required a proper geodetic datum and, as GPS was being contemplated as a future navigation tool, it made sense to adopt the same reference datum, namely WGS84. It took a while to convince the community to accept that.

The adoption of WGS84 led to the resurveying of most airports, runways, and various en route and landing navigation aids in order to bring them into WGS84, in preparation for the introduction of GPS. This led to the discovery of some large discrepancies, at airports and among navaids in many countries, between the existing horizontal coordinate values and their new WGS84 equivalents. Geodesists will be familiar with such occurrences, whenever they start dealing with a new community, whether they are civil or offshore engineers, oceanographers or meteorologists.

The first GPS receivers did not lend themselves to mass market adoption. Geodesists of a certain age will also remember some of the earliest commercial GPS receivers, such as the TI 4100 receivers, made by Texas Instruments. These early receivers operated by measuring sequentially four pseudoranges to four different satellites. Consequently, the receivers were programmed to first check the geometry of the satellites in view and decide on the best four in terms of geometrical configuration.

However, later on, with the emergence of new receivers that could measure all the available pseudoranges quasi-simultaneously, there was no need to carry on with measurements only to the “best four” satellites. One could track all available satellite signals and process these measurements by least squares, rejecting those with relatively large residuals, if any. This standard processing of observations is bread-and-butter stuff to surveyors and geodesists.

However, this was not the case with a number of navigation experts, who persisted on recommending the use of only the “best four” satellites for quite sometime, before they finally abandoned the practice.

A New Era of GNSS
Satellite navigation and positioning has changed substantially and significantly over the last 5 to 10 years. With Galileo in its development and in-orbit validation phase, the future developments in GPS IIF and GPS III, renewed interest in GLONASS, and satellite navigation initiatives in Japan, China, India, Australia, and several other countries, GNSS or the Global Navigation Satellite System is moving from being a concept, largely based on GPS alone, to a full global reality. A comprehensive program of GPS modernization currently under way aims to deliver significant improvements to both military and civil users.

The earliest mass-market applications of GPS involved road vehicles and mobile phones. In both cases, the twin aims are navigation (where am I, and how do I go to my destination?) and tracking (where is he, she, or it?). In the case of road vehicle tracking, successful applications include fleet monitoring (taxis or road transport companies), theft recovery of private cars, “black box” incident recorders, and the transport of hazardous or valuable cargoes.

Typically, most of these applications share three common features, namely prior knowledge of the proposed route, the continuous tracking of position and velocity by GPS, and the trigger of an alarm by a significant deviation.

Similarly, a number of GPS tracking applications use mobile phone technology (GSM or GPRS), but these are not as developed and widespread as vehicle tracking. Typically, these involve vulnerable people, such as young children, the elderly, key workers in some risky environments (for instance, railways), individuals with a chronic or contagious disease, and even VIPs.

Person tracking with GPS+telematics could also involve judicial cases (ordered by a court of law), of suspected criminals or anti-social elements. Other proposed applications include environmental information, location-based security, and location-sensitive marketing.

On its own, a GPS-enabled phone offers location and communication. This may answer the questions “Where is she or he?” and “Where am I?” but nothing more. However, when position and communication are combined with an appropriate geographic information system (GIS) database and a direction sensor, the combined system could answer two other very important questions, namely “What’s around me?” and “What’s that building over there?”

This could be achieved by a GPS+compass device, providing positional and directional data, which the mobile phone or the PDA transmits to a remote server. The server calculates the user’s position and identifies the building along the measured azimuth, gets the relevant information from the database, and sends it back to the client.

This is clearly valuable for the public utilities (water, gas, electricity, TV), shopping and leisure (restaurant menus, theater tickets), house hunting (details of the property advertised for sale), and of course, for visitors and tourists (museums, notable buildings, archaeological sites).

Leaving mobile phones aside, satellite navigation can also be used for location-based- security. For example, a briefcase or a portable PC can be programmed to unlock safely only in a specified location and nowhere else. This would minimize the risk of sensitive military or commercial material falling into the wrong hands.

Some working prototype systems already exist. Other location-and-context-based applications under consideration include the marketing and selling of goods, the reception of pay-TV, credit card security, spectator sports, road user charging and many others.

Indeed, the qualification of “critical application” is no longer restricted to safety-critical transportation, but it also applies now to financial-critical, legal-critical, security-critical, and business-critical applications as well. This creates a problem with standard off-the-shelf autonomous GPS receivers, which cannot operate indoors, because of signal attenuation and multipath.

Over the last few years, GPS chip and receiver manufacturers have tried, with some success, to develop high sensitivity GPS (or HS-GPS). The latest HS-GPS receivers, which incorporate up to 200,000 correlators operating in parallel, make it relatively easy to identify true pseudoranges from among the many signal and multipath reflections. Several manufacturers in the United States, Japan, Korea, and Europe, already advertise HS-GPS chips, and many other companies use such chipsets in their receivers.

GNSS Evolution
Like nearly all the technologies that preceded it, satellite navigation and positioning is going through the standard stages of development from birth to maturity. Older surveyors and geodesists may well remember the advent of EDM, using microwaves or lightwaves in the late 1960s and the 1970s. When the first EDM instruments were introduced, the distances measured were also measured with tapes, just in case.

Then came the second phase, when surveyors became fully confident about EDM and used it routinely for fast and precise range measurements. It took a few years and several critical mistakes in local mapping and national triangulation, to realize that EDM instruments could go wrong and that they had to be calibrated regularly in order to determine their accuracy and systematic biases.

The development of satellite navigation and positioning is following practically the same stages as EDM did 40 years ago. Only now we can formalize these successive stages of development of a technology and give them names by using Gartner’s famous “Hype Cycle Curve,” which was invented about 10 years ago in conjunction with new information technology products.

Using a simplified version, these successive stages of technology development are now formally called “Technology Trigger,” followed by “Peak of Inflated Expectation,” leading to “Trough of Disillusionment”, happily followed by the “Slope of Enlightenment,” and hopefully leading to the “Plateau of Productivity.”

As I write this, the first Galileo satellite, GIOVE-A, has been launched and tested successfully, opening a new era in satellite navigation. Hopefully, this will lead to the development of a large number of new critical applications — and involve close collaboration with geodesy and several other related disciplines — for the benefit of business, government and society.

Here is one last example about the strange relationship between geodesy and GPS. The U.S. delegation to the International Telecommunications Union (ITU) recently proposed to abolish leap seconds, and thus cut the link between Solar Time and Coordinated Universal Time (UTC) and ipso facto GPS Time.

At present, whenever the difference between UTC and Solar Time approaches 0.7 second, a leap second correction is made in order to keep the difference between them under 0.9 second. This is done every few years on the recommendation of the International Earth Rotation and Reference Systems Service, which monitors continuously the difference between Solar Time and UTC.

This leap second correction, which has to be applied every few years to GPS Time, apparently causes software problems because it has to programmed in manually. However, considering the difficulties that this change would cause to other scientific communities, such as astronomers, and even to users of GPS time itself for some critical applications, the U.S. proposal has now been postponed for the time being.

In conclusion, I must declare a conflict of interest. Although all the work I do at present involves GNSS, my academic background is clearly in geodesy. However, a change is in the air now, as safety-critical transportation is no longer the only critical application that has to be catered to. It has now been joined by several other emerging critical applications, notably financial-critical, legal-critical, security-critical and business-critical applications, which will also require nearly the same level of accuracy, integrity and coverage as safety-critical transportation.

This is where geodesy could step in again and create some new statistical tools, which will differentiate between the navigation and positioning systems on offer, and assess their suitability for the specific critical application.

For figures, graphs, and images, please download the PDF of the article, above.

March 2, 2006

White House Defense Budget Proposes GPS Funds

The Bush Administration’s Fiscal Year 2007 (FY07) budget proposal for the Department of Defense (DoD), announced in February, allocates $315,314,000 in advanced technology development for GPS, including work on the GPS III program. If approved by Congress, that would represent a sizable increase from the FY06 expenditures of more than $85 million and $33 million in FY05.

Read More >

By Inside GNSS
January 1, 2006

Bold Advice: Report of the Defense Science Board Task Force on GPS

Secretary of Defense Donald H. Rumsfeld

DSB Task Force Members

Robert Hermann
, Global Technology Partners, LLC
James Schlesinger, MITRE Corporation

DSB Task Force Members

Robert Hermann
, Global Technology Partners, LLC
James Schlesinger, MITRE Corporation

Task Force Members
John Darrah, Institute for Defense Analyses
William Delaney, MIT Lincoln Laboratory
Arnold Donahue, National Academy of Public Administration
Kirk Lewis, Institute for Defense Analyses
USAF Gen. James McCarthy (Ret), U.S. Air Force Academy
Steve Moran, Raytheon Corporation
Ruth Neilan, NASA Jet Propulsion Laboratory
Robert Nesbit, MITRE Corporation
Brad Parkinson, Stanford University
James Spilker, Stanford University
John Stenbit, Private Consultant, former Assistant Secretary of Defense for Networks and Information Integration (ASD NII)
USAF Gen. Larry Welch (Ret), Institute for Defense Analyses

Executive Secretary
Ray Swider, ASD NII

Recommendations from a high-level Defense Science Board (DSB) Task Force on the Global Positioning System, if implemented, would profoundly alter the way that GPS is managed and operated: a significantly redesigned and enlarged satellite constellation, a larger contractor role in running the system, more focused responsibility and authority for GPS, and permanent elimination of Selective Availability.

A memo from U.S. Secretary of Defense Donald Rumsfeld, drawing on the task force analysis and recommendations, has been drafted to provide guidance to departmental leaders and the Air Force officials responsible for overseeing and managing the program.

The DSB presented the group’s analysis and recommendations in a 109-page report, “The Future of the Global Positioning System,” signed by Under Secretary of Defense Kenneth Krieg and released publicly in early December. In a far-ranging critique, the report identifies potential gaps in sustainment of the GPS satellite constellation, delays in upgrading the operational control segment, and diffuse lines of authority within the Department of Defense (DoD). It calls for changes in how the United States funds GPS, how DoD manages the system and acquires military user equipment, and how the U.S. Air Force contracts for modernized GPS infrastructure and operates the satellites.

The ultimate significance of the report probably depends on the willingness and perseverance of its co-chairs, former defense and energy secretary Jim Schlesinger, and former National Reconnaissance Office director Robert Hermann, to advocate vigorously for the product of the task force’s labors.

Schlesinger briefed Rumsfeld and Deputy Secretary of Defense Gordon England, who also co-chairs the National Space-Based Positioning, Navigation, and Timing (PNT) Executive Committee (NPEC). The DSB report was on the committee agenda for its January 26 meeting.

Established by Krieg’s predecessor Michael Wynne, the task force’s objective initially had been framed to address competitive concerns in light of Europe’s move to implement its own GNSS, Galileo.

“Without significant DoD movement on GPS, the introduction of Galileo may marginalize GPS to an expensive military use only system,” Wynne wrote in an April 9, 2004, memo to DSB Chairman William Schneider, Jr.

Within a couple of months, however, the signing of a US/EU agreement on cooperation in GPS and Galileo matters broadened the focus of the task force — fortuitously, one might argue. President Bush’s policy directive on space-based positioning, navigation, and time further influenced the scope and emphasis of the group’s work.

An impressive mix of old GPS hands and former defense leaders comprised the task force — a gathering of what used to be known in less gender-sensitive days as “graybeards” or “wise men.”

“It was a unique confluence of expertise and leadership that we won’t have again for some time,” says Jules McNeff, vice president of strategy and programs for Overlook Systems Technologies, Inc., who staffed the task force and oversaw the drafting of its report. McNeff himself has more than 20 years invested in GPS, both inside and outside of DoD.

Rattling Cages
After 18 months of study, more than a dozen outside briefings, and deliberations, this “unique” task force produced a trenchant volume of solidly reasoned findings and recommendations (The full report can be download from the Internet at <>.)

Inevitably, such a collection of strong-willed, independent free-thinkers with a broad mandate produced some real zingers. Among those proposals:

•    Permanently eliminate Selective Availability (SA), the ability to degrade positioning accuracy in open civil signals “with the objective of deleting the hardware and software overhead for its implementation from throughout the future system.”

•    Change the constellation to a three orbital plane configuration with 30 satellites, rather than the current requirement of 24.

•    “Selectively” integrate technical personnel from private contractors into direct satellite monitoring and control operations at the Master Control Station at Schriever Air Force Base — a break from long-standing tradition of only uniformed Air Force personnel operating the satellites.

•    Prepare for discussions regarding possible use of Galileo services for military purposes by NATO member nations.

•    Require each U.S. military service to fund its own R&D program to best ensure position and timing information is integrated into equipment and operational capabilities. (The function is currently coordinated by the NAVSTAR GPS Joint Program Office.)

•    Designate a single focal point within the Office of the Secretary of Defense responsible for all GPS policy and oversight matters.

•    Limit GPS III satellite weight to permit launch of two satellites on a single mid-size launch vehicle, including the transfer, if necessary, of the Nuclear Detonation Detection System now on board GPS satellites to other host spacecraft.

Leadership and Capacity
Throughout the report’s analysis and recommendations, two concerns stand out: the task force’s strong desire to see a greater GPS system capability funded, built, and brought on-line in a timely fashion, and the perceived need to create a clear, unified line of authority and responsibility for GPS — what McNeff refers to as “a single belly button” that can be pushed to get GPS the attention it needs.

But just sustaining the GPS constellation at its current 24-satellite fully operational capability (FOC) level is at risk, according to the task force, as a result of budgetary uncertainty and delays in modernization programs. States of the task force findings:

“The current on-orbit inventory is 28 satellites; however, with expected failures, the AF Space Command December 2004 PNT Functional Availability Report reflects a nominal probability between 5–20 percent and a worst-case probability between 20–40 percent that the constellation will fall to fewer than 24 satellites in the 2007– 2012 period based on current satellite replacement schedules.”

Moreover, the capability to operationally control new GPS L2C and L5 signals will not be present in the GPS control segment until 2009 at the earliest, the report suggests.

Upgrading the Block IIR and IIF satellites to include M-code, L2C, and L5 signals, along with an annual rather than multi-year purchase strategy, nearly doubled the cost of those spacecraft. Looking ahead, the price tag for GPS III satellites will be nearly double that of the preceding generation. As a way to mitigate the expense of the GPS III program, satellites should be designed so as to allow two to be launched at the same time, even if this means eliminating unrelated functions such as NDS.

“The concern that the DSB has is that, if GPS III becomes another massive satellite, the department can’t afford it,” task force member Brad Parkinson told Inside GNSS. “GPS really needs 30 to 36 satellites, but the Air Force requirement is only 24.” Increasing the strength of GPS transmissions should not pre-empt the goal of populating the constellation with more spacecraft, adds Parkinson, who was the first director of the GPS Joint Program Office. “Geometry is more important than extra power.”

As for the leadership issue, one of the report’s recommendations proposes, “The Secretary of Defense should also clarify lines of authority and responsibility within the Department to eliminate ambiguity regarding GPS responsibilities that hinders decision making internally and that perpetuates the perception externally that the DoD has lost sight of its GPS stewardship responsibilities.”

Noting the President’s creation of the PNT Executive Committee, which occurred during the task force’s deliberations, the report says the new policy body “affords an opportunity for all stakeholders to correct deficiencies of the former Interagency GPS Executive Board [IGEB].” That assessment stems largely from the fact that the President’s national security directive creating the executive committee also elevated the level of its leadership to deputy secretaries of transportation and defense.

Nonetheless, reflecting the difficulty of the IGEB to gain sustained participation from its co-chairs, the task force recommends, “If Deputies do not routinely participate, then designated representatives to the . . . PNT Executive Committee . . . must be formally empowered to speak for and act on behalf of their respective Deputies for all matters coming before the [committee].”

Says Parkinson, “The [PNT] executive committee can do some good if it gets the attention of people who can make some changes.”

Mike Shaw, director of the National Space-Based PNT Coordination Office that will provide staff support for the executive committee, says he hopes the office will exercise “more insight responsibilities.” By this Shaw means looking into the agencies involved with GPS and identifying “disconnects” the prevent a common exercise of GPS policy, and then putting this information “in front of senior people” who can make the needed changes.
Glen Gibbons

NovAtel Inc.
The Defense Science Board (DSB) Task Force report focuses primarily on project strategies for correction of a number of known GPS deficiencies, with the impetus to fix things being driven by the potential future impact of Galileo. There are some good thoughts and several opportunities for improvement.

The President’s U.S. Space-Based Positioning, Navigation, and Timing (PNT) Policy announced in December 2004 already highlights areas where GPS and GPS assets are vulnerable to jamming. The DSB Task Force report once again highlights this area of vulnerability, especially for civilian users.

As the supplier of GPS reference receivers to the FAA Wide Area Augmentation System (WAAS) network and participant in the development and supply of the Galileo Reference Chain receivers for the Galileo ground control system, NovAtel has proposed an approach using a combination of antenna array and signal processing for protection of the NovAtel network reference receivers. These extensively tested and qualified national networks could substantially improve GPS signal monitoring – if only the GPS control segment could access data from the WAAS networks in US, Japan, Europe, India and even China.

NovAtel supports the initiative to permanently increase the GPS constellation to 30 satellites, and we are ready for the new L2C and L5 signals. More space vehicles means a greater probability of seeing good geometry signals, and more signals at different frequencies will improve system accuracy and signal reliability. The DSB report does not, however, appear to consider the combined use of Galileo and GPS, which together will provide up to 60 satellites. This will really improve signal reliability and usability!

Keeping pace with the coming of Galileo is a recurring theme and the threat of a competitive system runs throughout the report. However, it does not really support the need to actively monitor and use Galileo for national programs such as WAAS and Local Area Augmentation Systems (LAAS). NovAtel has already fielded a commercial dual-mode GPS/Galileo 16 channel receiver, which can provide users with the benefits of new signals and which works with both systems.

Such receivers could be readily added to the existing WAAS reference receivers. Moreover, NovAtel expects to be deeply involved in Galileo and GPS receiver development for many years to come.

As the world moves into a GPS/Galileo dual-constellation environment, where dual use will be pervasive, it seems strange that the U.S. Department of Defense may have to remain reliant on single-mode GPS while the rest of us benefit from the improved accuracy and reliability which GPS and Galileo together will provide.

Tony Murfin
Vice President, Business Development
NovAtel, Inc.

Lockheed Martin
The Defense Science Board report thoughtfully addresses many key issues as the government looks forward to and works to define future generations of the Global Positioning System. Many of the issues raised in the report have been examined by industry and the Air Force as part of the GPS III architecture and requirements studies.

The report will serve as an important resource as the Air Force finalizes its plans to acquire next-generation space and ground architectures. GPS III is a major focus area for Lockheed Martin, and we stand ready to help the Air Force create a next-generation system that will address the challenging military transformational and civil needs across the  globe, including advanced anti-jam capabilities, improved system security and  accuracy, and reliability.

Steve Tatum
Sr. Manager, Communications
Lockheed Martin Space Systems Co.

EADS Space Services
The DSB Task Force report provides a very interesting and fair overview of the Global Positioning System challenges from a performance, competitiveness and governance point of view in view of the upcoming European alternative “Galileo.” As a major actor of the future Galileo PNT system, EADS has a particular interest in the GPS evolutions and policy, especially in the field of cooperation with leading U.S. manufacturers.

The Task Force position is particularly appreciable for the navigation industry as it recommends promoting “opportunities for cooperation,” “true civil interoperability,” and considering “alternative means of funding and governance” for GPS to facilitate its international support. The underlying purpose of this collaborative approach is to improve the commercial and cost efficiency related to the PNT civil signals.

Through its recent site distribution agreement, Galileo has made a significant step forward and will provide in the near future increased satellite signal availability worldwide for navigation purposes. It is indeed a primary objective to promote the combination of the GPS and Galileo constellations for civil users in order to improve significantly the overall positioning accuracy and integrity.

Consequently, the report supports the definition of an international civil signal standardization allowing combined GPS-Galileo receivers. This common effort is necessary to facilitate a widespread usage and certification of the signals in the commercial sector. Therefore, all parties should sustain the systems interoperability with the “full disclosure of an open signal structure” and well defined geodetic and time reference transformations in receivers.

EADS also welcomes the task force proposal to “explore cooperative exchange of monitoring information” provided by the WAAS and EGNOS systems as well as a “collaborative approach” to manage and monitor both systems for better performances.

Finally, we consider that the adoption of a separate strategy and governance for the GPS military and civil activities would facilitate the system modernizations, international cooperation, and augmentations focused on the civil particular interests, while maintaining a superior military capability.

To conclude, we regret that tangible directives have still not been issued by the U.S. authorities in the direction initiated by the US-EU agreement of June 2004. It would add great benefits to the user community to initiate the creation of joint entities aimed at addressing the performance, the standardization, and the vulnerability of the GPS and Galileo signals across the Atlantic.

Martin U. Ripple
Director Galileo Program
EADS Space Services

The Boeing Company
We continue to execute on our commitment to GPS IIF production, with a goal to make the IIF the most capable and reliable navigation satellite to join the constellation. We also look forward to seeing the customer’s requirements for GPS IIIA when the request for proposal (RFP) is issued. We’re excited about the new IIIA program and await the competition.

I believe that the majority of the [DSB] comments relate to the future of GPS requirements and that is the purview of the GPS Joint Program Office (JPO).  Boeing is ready to respond to the requirements, with whatever DSB recommendations are included.  The job of the JPO is to take the opinions of all appropriate experts and meld them into a future roadmap and set of requirements which are sent to industry to propose and build. Again, Boeing stands ready to respond with a compelling proposal.

Mike Rizzo
Director, Navigation Systems
The Boeing Company

L-3/Interstate Electronics Corp.
In general, the DSB report is “right on.” Their assessments regarding current shortfalls and urgent needs bring to light the vulnerabilities that our current war fighter is faced with when depending on GPS. It is true that improved satellite coverage is needed for challenged (e.g. urban) access, modernized GPS availability to the war fighter is too far out in time, and enhanced anti-jamming (AJ) capability is not being adequately funded or fielded.

The report makes a good point about the need for sufficient, but not excessive AJ capability in user equipment. Industry has demonstrated scalable, cost-effective AJ solutions that include hardware and software-only augmentations that satisfy the DSB’s recommended minimum acceptable level of 90 dB jamming resistance. These capabilities are easily and readily incorporated into user equipment, yet there are few programs in place to incorporate and deploy it.

Agencies like the Office of Naval Research (ONR) and Air Force Research Lab (AFRL) are financing technology programs that include AJ improvements for GPS; however, these are not pointed at fielding new equipment for the war fighter. Case in point — the Modernized Receiver Card Development Program, which is in place to help establish “proof of design” for modernized GPS does not require this type of AJ enhancement.

Hopefully, with the promulgation of the DSB report more military agencies will recognize the emerging jamming threat and programs will begin requiring the deployment of more AJ capability for GPS user equipment.

L-3/IEC agrees with the report’s assessment that the new PRONAV security architecture is essential to providing the needed Information Assurance improvements to military GPS (although one might argue with the details in the DSB’s comparison of performance benefits that PRONAV provides).

However, one must be cautious with considering the permanent removal of SA or, even more importantly, with opening up DoD acquisition policies to allow non-military GPS equipment. The gamble is the price our war fighter pays by having the wrong positioning, navigation, and timing (PNT) information because he’s using vulnerable commercial GPS signals. That price can be the difference between life and death.

Carlton Richmond
Chief GPS Technologist
L3 communications, Interstate Electronics Corp