B: Applications

The L2C Study: Benefits of the New GPS Civil Signal

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.

Scenarios
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.

Conclusions
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.

Acknowledgments
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, <http://www.navcen.uscg.gov/gps/modernization/L1/L1C-report-short.pdf>

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, <http://www.acq.osd.mil/dsb/reports/2005-10-GPS_Report_Final.pdf>

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Mobile RTK: Using Low-Cost GPS and Internet-Enabled Wireless Phones
May 1, 2006

Mobile RTK: Using Low-Cost GPS and Internet-Enabled Wireless Phones

Government regulation such as E911 and the promise of location-based services (LBS) are the biggest drivers for integrating positioning capability into mobile phones. The increasing sophistication of applications and refinement of map databases are continually tightening the accuracy requirements for GNSS positioning. In particular, location-based games and features such as “friend finder” sometimes require better accuracy than what is achievable with state-of-the-art network-assisted GPS (A-GPS) platforms.

Government regulation such as E911 and the promise of location-based services (LBS) are the biggest drivers for integrating positioning capability into mobile phones. The increasing sophistication of applications and refinement of map databases are continually tightening the accuracy requirements for GNSS positioning. In particular, location-based games and features such as “friend finder” sometimes require better accuracy than what is achievable with state-of-the-art network-assisted GPS (A-GPS) platforms.

Cellular standards for GPS assistance data exist for both control plane and user plane protocols. These protocols carry information that help the integrated GPS receiver to improve its sensitivity, speed up signal acquisition, and especially reduce the time to first fix. However, these approved standards do not contain sufficient information for the receiver to do carrier phase positioning.

Until now, no compelling reason existed for adding carrier phase positioning related features into cellular standards so that they could employ real-time kinematic (RTK) techniques. Generally, RTK-enabled devices on the market are expensive and intended primarily for geodetic and survey applications. Also, there has been no real need in the cellular world for the accuracy RTK provides. With evolving LBS applications, however, this situation is changing.

This article describes a solution called mobile RTK (mRTK), a system specifically designed and implemented for the cellular terminal use. Its design incorporates low-cost single-frequency A-GPS receivers, Bluetooth (BT) communications, and inertial sensors.

Basically, the technique involves exchanging measurements in real-time between two units — one designated as the reference and the other as the user terminal — and producing the best possible estimate of the baseline between the terminals using RTK techniques. We are developing the solution so that in the future it will be possible to add any other Global Navigation Satellite System (GNSS) measurements in addition to GPS measurements — or even instead of GPS measurements.

Using a simulator, we shall provide data that show it is possible to enable high-precision, carrier phase-based positioning in handsets with minimal additional hardware costs. Further, we shall describe some of the protocol aspects and especially the aspects of adding support for mRTK messaging to already existing cellular standards — GSM and UMTS. We believe that the mRTK solution will bring high performance to the mass market.

Moreover, additional GPS signals, such as L2C and L5, and other GNSSes such as Galileo will become operational in the near future. Consequently, it would be very beneficial to begin incorporating mRTK into the pertinent wireless standards now so that the infrastructure and the service providers will be ready when business opportunities present themselves.

. . .

mRTK Solution Overview
A plethora of RTK surveying solutions is available on the market today. Generally, they are characterized by the use of both GPS frequencies, L1 and L2, enabling ambiguity resolution in seconds over baselines of up to 20 kilometers, or even 100 kilometers with more time and under good conditions.  We must emphasize that this article does not claim to demonstrate similar performance and reliability as high-performance dual-frequency receivers.

We are designing the mRTK solution to work with low-cost, off-the-shelf GPS receivers with certain requirements (for example, the ability to report carrier phase measurements and data polarity). Therefore, performance degradations are expected in terms of time to ambiguity resolution, accuracy, and achievable baseline length.

. . .

Testing the System
The mRTK performance testing was accomplished using two identical hardware platforms containing 12-channel off-the-shelf high-sensitivity OEM GPS receiver modules and a 3-axis accelerometer. We constructed this test system to determine the physical limitations and requirements for the protocol and messaging aspects.

. . .

Performance
We conducted several experiments using the testing system and a GPS simulator. The simulator was configured to output data from the same eight satellites for both receivers with using several different baseline lengths varying from 0 meters to approximately 5 kilometers , and using scenarios for different GPS weeks.

. . .

Testing Protocol
The testing protocol used in the mRTK solution was designed specifically for use in research and development and as a reference design for proposed changes to the pertinent cellular standards. The protocol was designed to be as efficient as possible and especially to take advantage of the properties of TCP/IP. As TCP/IP already guarantees that transmitted data are error-free and also preserves the order of the data, our protocol did not need to include extensive error corrections and packet order counts.

. . .

Cellular Protocol Aspects
During the testing protocol design and implementation, several issues emerged concerning the addition of the mRTK feature into cellular protocols . . . User-to-user relative positioning is not recommended for control plane systems because it would require a lot of protocol and implementation work to get the binding of two terminals and relaying measurements between two terminals to actually work.

. . .

Future Work
This article has introduced a new concept called mobile Real-Time Kinematics and shows that RTK-like features are possible using low-cost components and existing cellular communication carriers. Even though a lot of development work remains on the mRTK algorithm side, the biggest challenge still involves cellular carriers and their standardization. Of course, even after standardization, the development of the infrastructure would require a huge effort.

Future work with the existing testing protocol includes more testing, especially field testing, and testing with different signal conditions and satellite constellations. The testing protocol itself should be modified with new features such as the VRS service. Using VRS, the baseline can always be kept very short, and accurate absolute positioning is available everywhere using mRTK.

One of the ideas that also need to be further developed is peer-to-peer protocols. In those protocols the mRTK measurements would be transmitted directly from one terminal to another without the use of a server in between.

As an example, this kind of protocol could be embedded into voice-over-IP (VoIP), in which the data channel for the voice encoding is already open and could easily accommodate other data transmissions that do not have strict real-time requirements, such as mRTK. Other peer-to-peer protocol means would exist, for instance, in WLAN, where the terminals are connected to the same subnet and would be able to open direct connections to each other.

The solution we have presented holds a lot of potential. Especially with the forthcoming satellite systems (e.g., Galileo and modernized GPS), the solution will significantly improve the accuracy of positioning in the mobile terminal. Nonetheless, the standardization of the mRTK features will require a lot of joint effort among terminal and network manufacturers and cellular operators.

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

Acknowledgments
This article is based in part on two papers, “Bringing RTK to Cellular Terminals Using a Low-Cost Single-Frequency AGPS Receiver and Inertial Sensors,” by L. Wirola, K. Alanen, J. Käppi, and J. Syrjärinne, and “Inertial Sensor Enhanced Mobile RTK Solution Using Low-Cost Assisted GPS Receivers and Internet-Enabled Cellular Phones,” by K. Alanen, L. Wirola, J. Käppi, J. Syrjärinne, presented at the IEEE/ION PLANS 2006 conference, © 2006 IEEE.

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photo-E160.jpg

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

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, http://ilrs.gsfc.nasa.gov/).

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, http://igscb.jpl.nasa.gov/). 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.

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By Alan Cameron
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GPS III, Block IIF Programs Hit New Delays

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

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
Using Maps as Automotive Sensors
April 1, 2006

Using Maps as Automotive Sensors

Accurate real-time positioning is a necessary condition for many land transportation applications. But positioning alone cannot ensure successful and safer navigation, let alone higher-order driver assistance and awareness applications. The most common accessory to positioning is a map database — of more or less sophisticated design with greater or lesser data content and granularity.

Accurate real-time positioning is a necessary condition for many land transportation applications. But positioning alone cannot ensure successful and safer navigation, let alone higher-order driver assistance and awareness applications. The most common accessory to positioning is a map database — of more or less sophisticated design with greater or lesser data content and granularity.

Another resource and tool for navigation, route guidance, and advanced assistive systems is the application of geometrical principles to the positioning and mapbase data to anticipate or project upcoming conditions and events along a route.

This article introduces the map database as a sensor in driver assistance and awareness applications, which begin with a map-matched position and then look ahead from that position to determine the most likely driving path (MLP). From this calculation, we can support assistance and awareness applications such as curve speed warning, predictive adaptive front lighting systems, adaptive cruise control, and forward collision warning.

We will explore the role of MLP in these applications as well as its use in modifying route guidance instructions and map-matched positions. Finally, this article will also take up the question of map requirements and the navigation system interface needed for such applications.

Path Prediction
Adaptive cruise control (ACC) and forward collision warning (FCW) require systems that can determine the primary target in the host vehicle lane and then accurately estimate the geometry of the road between the host and the target vehicle. Curve speed warning (CSW) also requires knowing the geometry of the intended driving path to warn the driver of going too fast for the upcoming curve. Predictive adaptive front lighting can use the predicted road geometry to swivel the headlamps in the road curvature direction.

. . .

Driver Assistance, Awareness
Visteon has used GPS and map databases as sensors. In a road departure crash warning (RDCW) field operational test funded by the U.S. Department of Transportation (DoT) and completed last year, Visteon developed a CSW functionality using a commercial navigation system and map database. The CSW system warns the driver when the vehicle is traveling too fast for an upcoming curve by processing the map database geometric and attribute information.

. . .

Land Vehicle Navigation
Route guidance is an essential feature in current land navigation systems. In this navigation feature, a driver feeds the navigation system with the desired trip destination. The route guidance algorithm calculates the route for the driver to follow. The driver may make mistakes in following the intended (calculated) route, and the route guidance system will have to adjust its instruction to correct this mistake.

. . .

Map Database as a Sensor
Current commercial map databases are designed for navigation purposes. The accuracy of these maps is sufficient for navigation in a large variety of road scenarios. However, they sometime fail in such situations as service drive/highway, highway/exit ramp, fork, complex overpasses, and mountain area/single road. All of these scenarios could lead to placing the vehicle on the wrong road or off the road.

. . .

Conclusion
Map database can provide detailed information of the road segment at the vehicle position and the road segments ahead of the vehicle. This information when processed can be used for advanced driver assistance and awareness applications Moreover, these systems should incorporate a map corrective/updating capability due to the changing nature of the roads and associated driving restrictions.

Map database errors can arise in such road scenarios as merging, road connections (overpasses), divided/undivided roads, and mountains areas In order to optimally use the map database, such error sources should be defined and modeled. Furthermore, inclusion of additional information such as height or elevation could extend the usage of the map for other automotive applications.  From a commercialization perspective, it is recommended to standardize the navigation system interface.

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

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Builders Notes: Russian GLONASS at the Stage of Active Implementation

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.

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Geodesy and Satellite Navigation

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.

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White House Defense Budget Proposes GPS Funds
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.

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By Inside GNSS
GNSS Solutions header web.jpg
March 1, 2006

Carrier Phase Ambiguity Resolution, GNSS Use In Cellular Telephone Systems, and New Antennas?

Q: Will I need a new antenna for the new GPS and Galileo signals? Will one antenna work for both systems?

A: To answer these questions, information will be presented on the GPS and Galileo signal formats, some antenna basic fundamentals with various user applications in mind, followed by some predicted performance assessment.

Q: Will I need a new antenna for the new GPS and Galileo signals? Will one antenna work for both systems?

A: To answer these questions, information will be presented on the GPS and Galileo signal formats, some antenna basic fundamentals with various user applications in mind, followed by some predicted performance assessment.

The well known “Basic GPS” signals are centered at L1 (1575.42 MHz) and L2 (1227.60 MHz), with the GPS Coarse/Acquisition (C/A) code, at a chipping rate of 1.023 Mcps (million chips per second) on L1. The Precise (P) code is transmitted with a chipping rate of 10.23 Mcps on L1 and L2; if encrypted, it is then called the P(Y) or Y-code when broadcast at the 10.23 Mcps rate.

For these binary phase shift key (BPSK) modulated signals we often use the null-to-null bandwidth (twice the chipping rate) to characterize the signal bandwidth, which is 2.046 MHz and 20.046 MHz for the C/A and P(Y) codes that are transmitted in phase quadrature, respectively. Note that for many high performance applications we often require additional signal bandwidth to include the power in the sidebands of the signal spectrum. This is a very important factor in considering antenna bandwidth for a particular application.

(For the rest of Dr. Chris Bartone’s answer to this question, please download the complete article using the PDF link above.)

Q: How will the new frequencies in GPS and Galileo affect carrier phase ambiguity resolution?

A: In the years to come, GNSS users will benefit from the availability of more satellites and signals with the coming of Galileo and the modernization of GPS. Galileo will consist of a brand new constellation of 30 satellites transmitting their signals on four frequencies. Four different navigation services will be offered, meaning that some of the signals and information is available for free to every user, but other services are either to be paid for or are only available to certain authorities.

The first milestone for GPS modernization is the availability of the L2C code for civil users. In the next phase, the L5 signal will also be available.

GNSS positioning will thus be possible with improved precision, reliability, availability and integrity. Still, for rapid and high precision positioning, carrier phase ambiguity resolution remains indispensable. Only with the ambiguities fixed to their correct integer values do the carrier phase observations start to act as very precise pseudorange observations. This implies that the probability of correct integer estimation, generally referred to as the success rate, should be very close to unity.

(For the rest of Sandra Verhagen’s answer to this question, please download the complete article using the PDF link above.)

Q: Aside from E-911 and E-112, how is GNSS used in cellular telephone systems?

A: While mobile positioning for E-911 and E-112 emergency services are becoming more pervasive, other important applications of GNSS exist that are less obvious. These fall into two main categories: namely, those associated with direct mobile user applications based on the mobile’s location and those associated with enhancing the performance of the overall cellular network.

A plethora of user applications based on mobile location are rapidly emerging including street map and direction finding, fleet position data logging and targeted advertising. No dominant “killer application” has emerged at this stage, but the steady accumulation of these minor location-sensitive services is rapidly making GNSS an indispensable component of cellular functionality and markets.

The other main application category of GNSS in cellular telephony is associated with the enhancement of the overall performance of the wireless network infrastructure from the perspective of network capacity and quality of service. First-generation cellular wireless systems were based on time division or frequency division multiplexing.

(For the rest of Dr. John Nielsen’s answer to this question, please download the complete article using the PDF link above.)

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Building Monitors

Building Monitors

Severe loading conditions such as strong winds and earthquakes acting on modern tall buildings and structures can cause significant loads and vibrations. Recent trends toward slender, flexible, and light-weight buildings have left a large number of buildings susceptible to wind-induced motion. Furthermore, human perception of building motion has become a critical consideration in modern building design.

Severe loading conditions such as strong winds and earthquakes acting on modern tall buildings and structures can cause significant loads and vibrations. Recent trends toward slender, flexible, and light-weight buildings have left a large number of buildings susceptible to wind-induced motion. Furthermore, human perception of building motion has become a critical consideration in modern building design.

More complex building shapes and structural systems further accentuate eccentricities between the mass center, the elastic center, and the instantaneous point of application of aerodynamic loads, and consequently will generate significant torsional effects.

Verifying dynamic structural analysis requires the development of direct dynamic measurement tools and techniques in order to determine the natural frequencies, damping characteristics, and mode shapes. Among these tools accelerometers have played the most important part in analyzing structural response due to severe loading conditions. However, they provide only a relative acceleration measurement. The displacement from acceleration measurement cannot be obtained directly by double integration.

In contrast to accelerometers, GPS can directly measure position coordinates, thereby providing an opportunity to monitor, in real-time and full scale, the dynamic characteristics of a structure. GPS used in the real-time kinematic mode (GPSRTK) offers direct displacement measurements for dynamic monitoring. Earlier studies by the authors and other researchers, referenced in the Additional Resources section at the end of this article, have shown the efficiency and feasibility of structural deformation monitoring by combining accelerometer and GPS-RTK.

However, GPS-RTK has its own limitations. For example, the measurement accuracy can be affected by multipath and depends strongly on satellite geometry. Moreover, the typical GPS-RTK 20Hz sampling rate will limit its capability in detecting certain high mode signals of some structures. The new 100Hz GPS-RTK systems need to be further tested in order to ensure the independence of the measurements.

In order to exploit the advantages of both GPS-RTK and accelerometers, two data processing strategies have typically been used, namely to convert GPS measured displacement to acceleration through double differentiation and compare it with the accelerometer measurements (what we refer to as forward transformation), or to convert the accelerometer measurements into displacement through double integration and compare it with GPS measured displacement (the reverse transformation).

The latter approach is much more challenging because we have to determine two integration constants in order to recover all the components of displacement (static, quasi-static and dynamic). If the structure to be monitored is subject to a quasi-static force, as in the case of a typhoon, this further complicates the analysis.

Although earlier research has proposed a lab-based threshold setting for accelerometers to deal with the quasi-static issue, we believe that avoiding this procedure and developing new ways to recover the false and missing measurements from GPS by acceleration transformation would provide a preferred approach.

This article discusses recent efforts to design such a system based on a new integration approach that employs the correlation signals directly detected from a GPS-RTK system and an accelerometer to transform one form of measurement to the other. The methodology consists of a Fast Fourier Transform (FFT) for correlated signal identification, a filtering technique, delay compensation, and velocity linear trend estimation from both GPS and accelerometer measurements. We also present results derived from its installation on structures in Japan that subsequently experienced the effects of an earthquake and typhoon.

(For the rest of this story, please download the complete article using the PDF link above.)

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Will Success Spoil GPS?
January 1, 2006

Will Success Spoil GPS?

Like some behemoth rocket ship launched in the 1970s, the Global Positioning System sails on through an expanding universe of users and applications, seemingly imperturbable, successful beyond the expectations of its creators, an enormous momentum carrying it into the third millennium.

Like some behemoth rocket ship launched in the 1970s, the Global Positioning System sails on through an expanding universe of users and applications, seemingly imperturbable, successful beyond the expectations of its creators, an enormous momentum carrying it into the third millennium.

To all appearances, GPS is prospering more than ever: a second full signal (L2C) is becoming available to civil and commercial users, a denser ground monitoring system being built out, improved accuracies squeezed out of the algorithms and operational practices at the Master Control Station in Schriever Air Force Base, prices dropping on products with more features and functions than ever, hundreds of millions of receivers in use around the world. A follow-on generation (Block IIF) of satellites with a third civil signal (at the so-called L5 frequency) is being built by Boeing for launch beginning in 2007.

Since its first satellite launch 28 years ago, GPS has blazed a trail for satellite-based positioning, navigation, and timing. Thanks to GPS, global navigation satellite systems have gone from being a technological unknown to becoming a widely recognized utility. GPS, a model and inspiration to its imitators across the oceans.

Or is it?

In fact, for some years now GPS has been a victim of its own success. Performing better than advertised, the system has suffered from budgetary pilfering for other defense programs and risks getting lost in the shifting maze of diffuse dual-use management responsibilities.

“History has shown that the Air Force has had chronic difficulty in adequately funding GPS, even in the absence of the more expensive GPS III satellites,” observes a high-level Defense Science Board (DSB) task force report on GPS issued late last year. “If the Air Force continues to use its GPS investments as a funding source to offset other space/aircraft programs, then GPS service continuity will remain in jeopardy even without the more costly GPS III.” (See article “Bold Advice” in this issue.)

Meanwhile, an Air Force Space Command projection puts the worst-case probability of the GPS constellation falling below its fully operational capability (FOC) of 24 space vehicles sometime between 2007 and 2012 as 20–40 percent. Indeed, the task force argues for a 30-satellite constellation to ensure robust coverage in “challenged environments.”

The timelines for the last three GPS satellite development and launch programs — Block IIR, IIR-M, and III — all slid to the right, as they describe schedule delays these days.

Intermittently starved for fuel, with sporadic guidance from the helm, will new resources reach the system before its speed inevitably begins to slow, threatening its being overtaken by other GNSS vehicles?

Okay, that’s the bad news.

The good news is that no one connected to the program wants to let one of the world’s leading U.S.-branded utilities slip into the shadow of the other GNSSes under development. And steps are under way to ensure that doesn’t happen.

New Game Plan

A long-awaited next-generation program, GPS III, spent well more than hundred million dollars on conceptual studies and several years jogging in place before receiving a renewed go-ahead from the Department of Defense (DoD). The Fiscal Year 2006 (FY06) federal budget allocated $87 million for GPS III. The FY07 budget will be finalized soon in Washington, and current indications are that GPS Block III will receive at least $237 million, according to the GPS Joint Program Office (JPO). Of course, GPS III funds have been zeroed out before.

Current plans call for GPS JPO decision this summer that chooses among proposals submitted for separate space vehicle (SV) and operational control (OCX) segment contracts. Once acquisition strategies are formally approved in Washington, release of the GPS Block III SV request for proposals (RFP) are expected to be released by mid-February and later in the spring for the OCX RFP, according to JPO.

“Minor adjustments are being implemented in the program planning to reflect an incremental development and delivery approach for both acquisitions that will provide increased GPS capability sooner and more frequently over the life of the program,” the JPO told Inside GNSS. Nonetheless, an upgrade in the control segment to accommodate the new generations of satellites is behind schedule, which means that the capability to operationally control those signals will not be available until 2009 at the earliest, according to the DSB task force.

Modernizing Technology

In terms of its fundamental design, the Global Positioning System is nearly 35 years old. More recent spacecraft designs using modern electronics, new rubidium clocks, better satellite management techniques, and navigation message enhancements have improved performance. But the design of the key resource for manufacturers and users, the GPS signals-in-space, is essentially the same as when the first satellite was launched in 1978: a C/A-code on L1 (centered at 1575.42 MHz) and P/Y-code military signals at L1 and L2 (1227.60 MHz).

Over the next five years, however, this situation will change dramatically.

Beginning with SVN53/PRN17, the first modernized Block IIR (IIR-M) satellite built by Lockheed Martin and launched last September 25, GPS has gained a new open civil signal at L2 (centered at 1227.6 MHz). A third civil signal, L5 (centered at 1176.45 MHz) will arrive with the Block IIF satellites now scheduled to begin launching in 2007.

Both IIR-M and IIF satellites will offer new military M-code signals at L1 and L2 with “flex power” capability of transmitting stronger signals as needed. The L5 civil signal will be broadcast both in phase and in quadrature, with the quadrature signal being broadcast without a data message. Air Force Space Command expects to have a full complement of satellites transmitting L2C and M-code signals by 2013; for L5, fully operational capability is expected by 2014.

Generally, the new signals will be characterized by longer code sequences broadcast at a higher data rate and with slightly more power. Beginning with the IIR-M satellites, the Air Force will be able to increase and decrease power levels on P-code and M-code signals to defeat low-level enemy jamming — a capability known as “flex power.”

These new signal features will support improved ranging accuracy, faster acquisition, lower code-noise floor, better isolation between codes, reduced multipath, and better cross-correlation properties. In short, the new signals will be more robust and more available.

Looking farther ahead, another civil signal at L1 is planned to arrive with the GPS III program. Under a GNSS agreement signed with the European Union in June 2004, this will be a binary offset carrier (BOC 1,1) signal similar or identical to that of the Galileo open signal. This is expected to simplify the combined use of GPS and Galileo signals. Nominal first launch date for a GPS III spacecraft is currently 2013.

Modernization will also take place in the ground control segment. Six GPS monitoring stations operated by the National Geospatial-Intelligence Agency (formerly the National Imagery and Mapping Agency) have been folded into the existing five Air Force GPS monitoring stations (which includes the Master Control Station at Schriever AFB, Colorado.) This will eliminate blank spots in coverage and support Air Force plans to monitor the integrity (or health) of civil signals as well as military signals.

New Political Structure

Under a presidential national security policy directive (NSPD) released in December 2004, a National Space-Based Positioning, Navigation, and Timing (PNT) Executive Committee and Coordination Office have taken over from the Interagency GPS Executive Board (IGEB). Mike Shaw, a long-time GPS hand on both sides of the civil/military interface, stepped in toward the end of 2005 as the first director of the PNT coordination office.

Establishment of the PNT committee — now cochaired by deputy secretaries of defense and transportation, Gordon England and Maria Cino, respectively — kicked GPS leadership up a notch from that of the IGEB. Other members include representatives at the equivalent level from the departments of state, commerce, and homeland security, the Joint Chiefs of Staff and the National Aeronautics and Space Administration.

The committee had met once shortly after its formation under President Bush’s NSPD, but a January 26 gathering marks its first with the current leadership. In addition to getting acquainted with one another and the PNT topic in general, the agenda covered such issues as the DSB task force report, modernization and funding of GPS, and the new UN International Committee on GNSS (see article "What in the World is the UN Doing About GNSS?" in this issue).

Without a director and coordination board in place, the executive committee was unable to get on with many of the tasks assigned it by the presidential directive, including writing a five-year plan for U.S. space-based PNT and appointing an advisory board of outside experts. With Shaw on board, the coordination board now has seven staff members detailed from agencies represented on the executive committee.

A charter for the advisory board has been drafted and awaits approval by the committee, as does a draft of an international PNT strategy prepared by the State Department under the direction of Ralph Braibanti, who heads that agency’s space and advanced technology staff.

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