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Aerospace and Defense

GPS III Passes Preliminary Design Review

GPS IIIA. Lockheed Martin graphic

The Lockheed Martin team developing the next-generation GPS III satellites has successfully completed a major program milestone, the preliminary design review (PDR) conducted by the U.S. Air Force’s GPS Wing.

Underlining the importance placed on meeting a 2014 first-launch schedule, nearly 150 representatives from the GPS Wing and user communities, including representatives from the Department of Defense, the Joint Chiefs of Staff, Air Force Space Command, the Department of Transportation, and the Federal Aviation Agency participated in the four-day Space Vehicle PDR at Lockheed Martin Space Systems facilities in Newtown, Pennsylvania.

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By Inside GNSS
April 17, 2009

GPS Modernization Snapshot: WAAS, L2C, Launch Delays

Although satellite launch delays continue to plague the GPS program, planning and paperwork continue to make solid gains.

The U.S. Air Force announced that it will begin broadcasting a transitional civil navigation (CNAV) message on the GPS L2 civil signal (L2C). The signal is now being transmitted on the six IIR-M satellites in orbit as a dataless sequence without modulation. And, on October 31, 2008 the GPS Wing completed an integrated baseline review (IBR) of the GPS IIIA program.

Although satellite launch delays continue to plague the GPS program, planning and paperwork continue to make solid gains.

The U.S. Air Force announced that it will begin broadcasting a transitional civil navigation (CNAV) message on the GPS L2 civil signal (L2C). The signal is now being transmitted on the six IIR-M satellites in orbit as a dataless sequence without modulation. And, on October 31, 2008 the GPS Wing completed an integrated baseline review (IBR) of the GPS IIIA program.

The Federal Aviation Administration has also issued a performance standard for the Wide Area Augmentation System (WAAS) and reported that it has now published 1,333 localizer performance with vertical guidance (LPV) approach procedures based on WAAS. The LPVs cover runways at 833 airports.

In the near term, however, the launch schedule for GPS IIR-M7 and IIR-8 remains up in the air pending replacement of faulty components, a 40-second timer that triggers separation of the third stage booster from the GPS satellite. The problem has prevented any further expansion of the GPS constellation since IIR-M6 was launched March 15, 2008.

The GPS Wing hopes to launch IIR-M7 with its experimental L5 signal payload in March 2009. (UPDATE: It was launched successfully on March 24, 2009). Air Force GPS program managers had aimed to launch IIR-M7 this year to conduct an early test of the new L5 signal — standard on Blocks IIF and III. Now they just hope to meet an August 2009 deadline to secure the International Telecommunications Union frequency allocation in the aeronautical radionavigation service band.(UPDATE: Signal transmitted successfully on April 10, 2009)

That requirement has become more urgent as launch of the first IIF satellite has slipped to the August-October 2009 timeframe, according to the GPS Wing. The Boeing-built spacecraft was expected to complete its final environmental testing in November and be ready to ship to the Cape Canaveral launch site in February 2009.

Back to the Good News. According to a press release issued November 3 by the GPS Wing at the Space and Missile Systems Center, Los Angeles Air Force Base, operators at the 50th Space Wing and the 2nd Space Operations Squadron at Schriever Air Force Base, Colorado, will upload new software to the IIR-M satellites next year, enabling the first broadcast of the transitional message on the L2 frequency (1227.60 MHz).

The modernized L2C signal was designed with several significant advantages over L1 C/A-code signal, including a lower tracking threshold and better cross-correlation protection. The data portion of the L2C signal is also different: instead of the current “legacy” navigation (LNAV) structure with subframes of data repeating in a fixed pattern, the CNAV structure has individual messages that can be broadcast in a flexible order with variable repeat cycles.

The CNAV structure, as defined in Interface Specification (IS)-GPS-200D, allows up to 63 different message types, of which 15 types have already been defined, according to the GPS Wing. The 15 CNAV message types will be incrementally phased in over time, with the first CNAV message to broadcast being the “default message,” also known as Message Type 0.

Type 0 consists of a 12-second, 300 bit–long message that includes a preamble, satellite pseudorandom noise (PRN) number, message type ID (=0), GPS time of week, a sequence of alternating 1s and 0s, and a cyclic redundancy check (CRC) parity block. The GPS Time of Week will change every 12 seconds, as will the CRC bits.

Meanwhile, the successful GPS IIIA IBR paves the way for the establishment of an integrated cost, schedule, and technical baseline for the program, according to the GPS Wing. The contract, awarded earlier this year to a team led by Lockheed Martin, provides for development and production of the first two GPS IIIA satellites with an initial launch set for 2014.

WAAS Performance Standards. Publication of the WAAS Performance Standard (PS) on October 31 follows on the heels of an updated version of the GPS Standard Positioning Service Performance Standard in September.

WAAS is the multi-billion-dollar U.S. satellite-based augmentation system (SBAS) developed under an FAA contract by Raytheon Corporation and designed to provide real-time differential corrections, integrity messages (satellite signal “health”), and ranging signals that WAAS-capable equipment can use to improve navigation. It is used primarily by private general-aviation pilots, business and regional aircraft, and some cargo aircraft.

Leo Eldredge, FAA’s GNSS program manager, characterizes the WAAS PS as “a composite of the current specifications and standards that WAAS already complies with.”

Like the GPS SPS standard, the WAAS PS defines the WAAS signal-in-space (SIS) characteristics, navigation message, and performance requirements. For instance, LPV is designed to provide 16-meter horizontal accuracy and 20-meter vertical accuracy 95 percent of the time. LPV status also calls for a 6.2-second time-to-alert when the system is not meeting specified requirements.

Actual performance has exceeded these levels for WAAS, however. For example, vertical error has not been observed to exceed 12 meters in the history of WAAS operational service.

More Than ILS. The November 6 announcement marks a milestone for WAAS-supported LPV approach procedures, which now surpass the number of approach procedures based on its ground-based predecessor, the Category-I instrument landing system (ILS).LPV enables pilots to use instrument flight rules for approach and landing operations down to a decision height of 200 feet. FAA is scheduled to declare the full LPV performance (FLP) phase of WAAS operational early next year.

As the GPS system continues to modernize, FAA will update the WAAS PS to include, for instance, the L5 civil signal that will enable dual-frequency positioning in protected aeronautical radionavigation bands.

“For civil aviation purposes, we depend on the commitments contained in the SPS PS as the basis for our commitment to provide service through augmentation and the approvals for aviation use of standalone GPS,” Eldredge told Inside GNSS. “Before we could approve use of L5, for either standalone use or as part of an augmented service, the commitment to provide that service would first need to be provided by DoD in a PS.”

Historically, he added, the DoD includes the new signals in a performance standard after the full operational capability (FOC) has been achieved. The Air Force is adding the second civil signal (L2C) and L5 on the IIR-M and IIF space vehicles (SVs), respectively, and later L1C on the GPS III satellites.

Eldredge added that the FAA plans to upgrade WAAS to use the L5 signal at the monitoring stations and in aircraft avionics, which will eventually require an update to the WAAS PS. The GPS master schedule currently shows L5 achieving FOC in 2018 and a recent Federal Register announcement regarding use of L2 semi-codeless GPS requires the agency to complete the transition of L2 to L5 by December 31, 2020. WAAS currently uses L2-semicodeless at the monitor stations only.

By
April 16, 2009

About GPS

The fully operational GPS constellation has 24 satellites. In 2007, 30 are actually in orbit.

GPS 21st Century Milestones (2001-2009)

The Global Positioning System (GPS) is the first and only fully functional Global Navigation Satellite System. Developed and operated by the U.S. Air Force for the Department of Defense (DoD), GPS is designated by executive and congressional action as a dual-use system available without user fees for civil and military use.

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By Inside GNSS
April 15, 2009

About GLONASS

GLONASS is the Russian Federation’s GNSS—literally. The Russian acronym stands for GLObal’naya NAvigatsionnaya Sputnikovaya Sistema, or Global Navigation Satellite System.

Chronologically the world’s second GNSS system, both the program (established in 1976) and the first launch of a GLONASS satellite (October 12, 1982) followed the corresponding United States GPS milestones by a few years.

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By Inside GNSS
March 25, 2009

What Race? What Competition? The Four GNSS Systems

Munich’s high-level satnav summit the first week in March opened with a plenary titled, “The Worldwide Race in GNSS” and closed with a panel, “The Competition among the Big Four.”

Despite the provocative session titles, few speakers were willing to admit that either a race or a competition was under way in the GNSS world.

Munich’s high-level satnav summit the first week in March opened with a plenary titled, “The Worldwide Race in GNSS” and closed with a panel, “The Competition among the Big Four.”

Despite the provocative session titles, few speakers were willing to admit that either a race or a competition was under way in the GNSS world.

Visa problems reportedly kept a Chinese spokesperson from joining the GNSS “race” session, and the “competition” panel was opened by GPS pioneer Brad Parkinson invoking the motto of GNSS interchangeability: “Any four [satellites from any system] will do.”

Indeed, one way of looking at the Summit’s premise is that the United States already won both the race and the competition in late 1993 with a declaration of initial operational capability (IOC) and 24 operational GPS satellites on orbit. The Russian Federation came in second in 1995.

End of story.

But within the conference’s dozen panel discussions and inevitable hallway conversations lurked many indications that the race continues and the competition is fierce.

China squeezed out a few additional details on its implementation plans, announcing that three more Compass satellites would be launched this year, including one in the first half of 2009, and seven in 2010. Russia announced its decision to put CDMA signals on the new GLONASS civil L3 band centered at 1208 MHz.

Galileo representatives put a brave face on a program that continues to encounter adversity at home and abroad. As did U.S. officials for a GPS program that has had nearly a yearlong halt in its launch schedule due to a questionable component in the Delta II rocket, and now may have encountered new problems in the next-generation Block IIF satellites.

The general downplaying of a GNSS competitive race might best have been reflected in the observation of Mike Shaw, director of the U.S. National Coordination Office for Space-Based Positioning, Navigation, and Timing (PNT): “The race should not be among the provider nations and the services they offer. They [GNSS providers] should focus on the issues of compatibility and interoperability. The race is really in the [GNSS equipment and services] industry sector.”

Despite the denials of competition, a race of sorts is being run in the GNSS world. What kind of race? Arguably, it’s a marathon. All of the programs have planning processes under way that reach to 2020 or beyond.

Other aspects of the situation, however, give the impression of a sack race, with two or more GNSS providers running in tandem under bilateral and multilateral accords, each with one leg in the same sack. Or within a few years it could even be likened to leapfrog as each round of system modernization propels a GNSS provider temporarily to the front of the pack.

GLONASS. In some ways, the GLONASS program — after an allocation of more than 100 billion rubles (nearly $3 billion) in funding for its 2002–2011 modernization effort — has progressed most steadily in recent years.

With the three newest satellites from a launch last December now in operation, GLONASS has a 20-bird constellation — including 19 modernized space vehicles (SVs), the most in more than a decade. Some 17 of the spacecraft are broadcasting a second full civil signal on the GLONASS L2 frequency, the only such GNSS system doing so.

Its signal-in-space user range error (URE) is down to 1.8 meters — still high compared to GPS’s 1-meter URE, but within the 3.7 meters called for in the GLONASS Interface Control Document (ICD) and several times better than the UREs of just year ago.

By the end of last year, GLONASS was typically providing a standalone receiver with five-meter positioning accuracy using pseudoranges.

Launches have taken place regularly as scheduled over the past few years, and another six satellites are set to go up in triple launches in October and December this year. If successful, that should bring the GLONASS constellation to full operational capability (FOC) with 24 satellites early in 2010.

But that’s not all. The next-generation GLONASS-K will begin launching next year and include a CDMA (code division multiple access) signal on L3, which will more closely align with other GNSS systems that the system’s legacy frequency division multiple access (FDMA) design.

A decision about new GLONASS signals at the L1C and L5 frequencies depends on negotiations by a U.S./Russia working group, but could lead to additional CDMA signals, said Sergey Revnivykh, deputy director of the Russian space agency’s Mission Control Center.

The stable progress in rebuilding and modernizing GLONASS has even drawn interest from players in the mobile phone industry. Nokia has been investigating the use of GLONASS for its handsets.

And, at the Munich summit, Frank van Diggelen, technical director and chief navigation officer for Broadcom Corporation, a semiconductor company that targets mobile handset manufacturers, appeared to compare GLONASS’s prospects favorably to Galileo.

“If GLONASS, which almost has a complete constellation, finds its way onto consumer devices, then consumers will have access to 65 satellites (GPS 31 + SBAS 7 + QZSS 3 + GLONASS 24 = 65),” van Diggelen said. “This may be enough.”

In a worrisome aside for Europe’s system, which is counting on mobile phones playing a prominent role in downstream markets, he added, “Galileo may simply be too late to matter.”

COMPASS. As for Compass, if China executes its currently announced schedule for satellite launches, it would mark a substantial acceleration in that program. Given the caution with which Chinese officials have announced their plans, the 10 satellites in two years commitment could well be met.

Indeed, a Chinese representative indicated that the Compass program is under pressure from Beijing to show progress in bringing the planned five civil and five restricted services online. The schedule also suggests that China has a lot of satellites already built and ready to fly soon.

Autonomous positioning accuracy for the open service is expected to be at least 10 meters, according to Jing Guifei, chief of the international cooperation division in the National Remote Sensing Center of China (NRSCC).

A wide area differential service providing one-meter real-time positioning and a short message service (SMS) is also part of the Compass program, Jing said.

As the “newcomer” to the GNSS field, in the words of Yin Jun, director of the European Affairs Division of China’s Ministry of Science and Technology (MOST), Compass “is not in the same place at the start of the race.”

Yin stressed that GNSS should not be a “competitive” exercise. “We need coordination among system providers,” he said. Although a “regional” capability is expected once the first 10 Compass satellites are in place, Yin said a global Compass service would not arrive until between 2015 and 2020.

GPS. As the leading GNSS provider, the United States might be thought to have the luxury of improving on a real and existing system with 31 operational SVs on orbit. In fact, the GPS program is in the midst of a full-blown modernization phase.

Launch of a modernized GPS Block IIR satellite — SV IIR-20(M) — is scheduled for March 24, the first since discovery of a faulty component in the Delta 2 booster last June led to a suspension of launches.

A demonstration payload for the new L5 civil signal is on the IIR-20(M), and faces an August 2009 deadline to meet an International Telecommunications Union requirement for securing primary GPS access to the frequency.

The last IIR-M should go up in August, according to Col. Dave Buckman, PNT command lead for Air Force Space Command at Peterson Air Force Base, Colorado.

Launch of the first Block IIF spacecraft is scheduled for October 2009, although anomalies discovered in the signal generator of the second IIF now under construction has introduced some uncertainty into the plan.

GPS produced a one-meter URE in 2008, Buckman said. The GPS III satellites, which will carry the new civil L1C signal, are designed to have a URE that is four times better.

Galileo. Turning at last to Europe’s Galileo, the laborious process of contracting out the fully operational capability (FOC) system development continues. In Munich, Fotis Karamitsos, European Commission director-general for transport and energy, and Paul Verhoef, head of the Galileo unit, indicated that agreements with companies winning the lead contracts for six work packages should be signed between September and the end of this year.

Discussions at the Summit revealed tensions around negotiations with China about a frequency overlay of Compass signals on the security-oriented Public Regulated Service as well as the question of whether the costs to build Galileo can be kept within the €3.4-billion limit agreed by the European Council and the European Parliament.

In answer to a question at the March 3 opening plenary, Karamitsos insisted that “we have no reason to believe that FOC won’t be delivered on time and on budget.”

Responding to a comment that “several member states” and private companies have already suggested creating a “light” version of Galileo — fewer services, signals, and/or satellites, Karamitsos said he that the European Union (EU) member states have a “legal obligation to deliver the full system. Galileo satellites will be acquired in blocks of 10, 8, and 8.

Karamitsos complained of “people negotiating through the press,” adding, “In this time of economic constraints it doesn’t make sense for our industry to try to make money over” the amount allocated for the program.

According to one European source, the reference was to Surrey Satellite Technology Ltd. (SSTL), a UK firm whose acquisition by EADS Astrium closed in January as well as EU members uninterested in using the PRS. SSTL, which specializes in smaller, economical satellite designs, built Galileo’s GIOVE-A satellite now in orbit.

SSTL, along with its bidding partner OHB System AG (OHB), has been short-listed as a candidate for the Galileo FOC space segment (with EADS as the other contender) and are preparing for the submission of a “refined proposal” to the European Space Agency.

Versus Compass. Meanwhile, the issue of the Compass/Galileo signal overlay — which recalls an earlier attempt to overlay the PRS on the GPS M-code — continues unresolved after two meetings between Chinese and EC representatives. Some years ago, China attempted unsuccessfully to gain access to the encrypted PRS, which requires unanimous agreement of EU member states before a non-EU nation can do that.

“PRS needs spectral separation,” insisted Paul Verhoef, head of the EC’s unit for Galileo and intelligent transport, who acknowledged that negotiations with China are “going slower than we hoped.”

China’s ambitious launch schedule, which requires final decisions on Compass’s frequency plan, increases the urgency of the dialog. “We hope to get agreement [with Galileo] before we launch, but we cannot wait to do the validation and development of the system,” Jing said in response to a question from the Munich audience.

The situation reflects the ill will that has arisen since the two sides signed agreements in 2003 and 2004 to cooperate on Galileo, including a €200-million Chinese contribution to program development.

In the session on competition among GNSS systems, Yin said that China’s industry had found it hard to compete for contracts in the Galileo FOC procurement, even though the nation had allocated €70 million for the in-orbit validation (IOV) phase. “Several IOV cooperation projects could not be implemented smoothly, due to obstacles and barriers,” he added.

By
January 16, 2009

Future Waypoints

2009
1st Quarter. Publication of 2008 Federal Radionavigation Plan (FRP).

March 24. Tentative launch date for a modernized GPS Block IIR-M satellite — IIR-20(M) —with an experimental L5 signal payload, from Cape Canaveral, Florida

Summer. Launch of IIR-21(M), last of the GPS Block IIR satellites built by Lockheed Martin Company

2009
1st Quarter. Publication of 2008 Federal Radionavigation Plan (FRP).

March 24. Tentative launch date for a modernized GPS Block IIR-M satellite — IIR-20(M) —with an experimental L5 signal payload, from Cape Canaveral, Florida

Summer. Launch of IIR-21(M), last of the GPS Block IIR satellites built by Lockheed Martin Company

Summer. Publication of an initial National PNT Architecture Transition Plan

3rd Quarter. Next-Generation GPS Control Segment (OCX) Award Phase B contract

October. GPS IIIA satellite program, Key Decision Point KDP-C, Defense Space Acquisition Board (DSAB), review results of GPS III Capabilities Insertion Program and possible acceleration of capabilities for subsequent phases

Fall. Launch of first GPS Block IIF satellite

Fall. Deployment of L2C civil navigation (CNAV) message type 0 on Block IIR satellites

September 14–18. Fourth meeting of International Committee on GNSS (ICG-4) in St. Petersburg, Russia
Throughout the year: launch of three to four Compass satellites

2010

  • Critical design review (CDR) and authorization to build OCX
  • Military GPS User Equipment (MGUE) contract award: Ground-Based — GPS Receiver Application Module (GB-GRAM) [GPS Wing]
  • First Galileo In-Orbit Validation satellite launch
  • 24 GLONASS-M satellites on orbit. Launch of experimental GLONASS-K spacecraft
  • Throughout the year: launch of seven to eight more Compass satellites

2011

  • GRAM Standard Electronics Module Type E (GRAM-S/M) contract award for air and maritime platforms [GPS Wing]
  • Flight test of first CDMA signals on GLONASS; full constellation with on-orbit spares

2012

  • Civil Aviation. Design approval for GPS local area augmentation system (LAAS, known generically as ground-based augmentation system) Cat-III landing system
  • OCX Block 1 available

2013

  • L2C signal initial operational capability (IOC)
  • OCX Block 2 Available
  • Fully operational capability (FOC) Galileo constellation completed

2014

  • M-code (with flex power capability) on 24 satellites
  • Launch of first GPS IIIA satellite
  • Full-rate production, GB-GRAM

2015

  • Full-rate production, handheld MGUE and GRAM-S/M [GPS Wing]

2016

  • L2C signals transmitting all data types on 24 satellites

2017

  • Fielding of DoD modernized military GPS user equipment

2018

  • L5 signal transmitting on 24 satellites
  • WAAS approach procedures published to all instrument runways in the National Air Space

2020

  • L1C IOC

2021

  • L1C signal on 24 satellites (FOC)
By
January 14, 2009

Autonomous Integrity

In trying to ensure integrity of GNSS navigation systems for civil aviation, various approaches have produced a range of different concepts, most of which assume the computation of a protection level. This computation is usually accomplished either autonomously (that is, entirely based on information gathered by the user receiver) or with some degree of external assistance.

In trying to ensure integrity of GNSS navigation systems for civil aviation, various approaches have produced a range of different concepts, most of which assume the computation of a protection level. This computation is usually accomplished either autonomously (that is, entirely based on information gathered by the user receiver) or with some degree of external assistance.

Such information may be provided by integrity augmentation systems (for example, space-based or ground-based augmentation systems — SBAS and GBAS). It may also be provided directly by the GNSS constellation, as it is foreseen with the future GPS III — remarkably enough, the GPS SPS Performance Standard already includes integrity performance specifications — and Galileo.

Autonomous protection-level computation techniques, however, have never been seriously considered as reliable sole means for ensuring integrity in safety-of-life (SoL) applications, not only because of the poor performances achieved, but also due to the somewhat delicate assumptions all of them rely upon. As a result, such techniques have mostly been considered as complementary to external integrity systems. One example: GPS+receiver autonomous integrity monitoring (RAIM) is not allowed as a primary navigation means for precision approach operations.

Recently, in regard of the improvements on accuracy and reliability expected from the future constellations GPS III and Galileo, new approaches have been proposed for the apportionment of integrity requirements. This is reflected, for instance, in the conclusions presented in the Phase I report of the USA GNSS Evolutionary Architecture Study (GEAS). The report suggests that the allocation of the burden for providing integrity should be balanced towards the user receiver, thus conferring user-based integrity (that is, receiver autonomous integrity) a higher responsibility.

User-based integrity is also gaining importance due to the emergence of a new field of GNSS applications, the so-called liability-critical applications (i.e., those where undetected GNSS large position errors can generate significant legal or economic negative consequences). Some leading examples of such applications are road tolling/congestion charging (both for highways and city areas), law enforcement (e.g., speed fining or surveillance of parolees) or “pay as you drive” insurance schemes.

Unlike air navigation, liability-critical applications often take place in harsh operating environments dominated by local effects such as multipath. Under such conditions these applications cannot always be monitored or aided by external (global, regional, or even local) augmentation systems.

Even in civil aviation, some landing operations could also be subject to large multipath that could put the navigation integrity at risk. For those scenarios the proposed technology would mitigate the associated risk.

One key assumption of conventional RAIM schemes is that simultaneous faulty measurements are extremely unlikely. This single-fault assumption, however, fails to hold in a typical liability-critical application scenario, where multipath is the primary source for large measurement errors and will quite frequently affect more than one measurement at a time. The single-fault assumption also fails to hold in the future air navigation scenario, where the large number of satellites made available by the joint use of several constellations (GPS/Galileo/GLONASS) will significantly increase the probability of multiple simultaneous faults.

Other assumptions common to all existing RAIM schemes include one or another statistical model of the individual measurement errors, trying in particular to bound the tails of their distributions. This sort of assumption is somewhat risky and difficult to verify, especially when the target confidence level is very high, as in the case of SoL applications such as civil aviation.

Moreover, under heavy multipath conditions most statistical assumptions of this nature just do not hold as errors caused by multipath are strongly dependent on the geometric characteristics of the local environment. (An especially acute example of this is non-line-of-sight (NLoS) multipath — that is, when a signal is tracked by GNSS equipment as it reflects from some surface despite the fact that a direct view of the satellite is occluded by some obstacle.) Hence, it is almost impossible to come up with a statistical characterization of such errors that can be used for integrity monitoring.

In this article we present a novel technique for autonomous computation of protection levels, the isotropy-based protection level concept, or IBPL for short. This technique makes no particular assumption on the statistics of individual measurement errors and provides coverage against multiple fault conditions. It takes advantage of a possible future multi-constellation scheme as its performance improves rapidly with the amount of satellites used for positioning.

Discussion in this article will show that asymptotic performance of the IBPL with respect to the number of satellites is comparable to that obtained with SBAS protection levels. This fact makes the IBPL a very promising technique, not only for liability-critical applications (the framework where it was born) but also, and very particularly, for SoL applications. We believe that IBPL fits remarkably well in the scheme proposed by the GEAS panel mentioned earlier, which recommends a shift of the integrity responsibility towards the on-board equipment.

Furthermore, as a fully autonomous method, the IBPL-based approach does not require integrity information to be transmitted on the GNSS or SBAS signal in space. This dramatically simplifies the interoperation of multiple GNSS constellations for integrity purposes, avoiding the problem of combining different integrity concepts from the various constellations or augmentation systems.

Autonomous Integrity: Two Approaches
For liability-critical applications, particularly in urban areas, local effects such as multipath — especially NLoS multipath — are by far the main source of errors and, consequently, the main threat to accuracy and integrity. In this framework, the conventional notion of faulty measurement as a large measurement error caused by a satellite malfunction is no longer useful.

. . .

IBPL: the Concept
The IBPL algorithm does not implement measurement rejection techniques but rather computes a protection level based on the all-in-view least squares solution. Of course, other IBPL solutions are possible, for instance, when different subsets of measurements are used and the one with smallest IBPL is selected. However, in its simplest form (as described in this article), this algorithm is a strict ECA concept implementation. On the other hand, this does not exclude the possibility that some refinements can be made for open-sky applications by including some kind of fault detection/exclusion mechanism.

. . .

Validating IBPL Integrity
Of course, we need to validate this new protection level concept and its underlying isotropy assumption in terms of the achieved integrity, and that must be done by experimentation with real data. We have to show that the theoretical confidence level of the isotropy-based protection level is satisfied in real life. Our discussion here cannot be considered as a full validation of the IBPL concept, but it provides significant information about the validity of the proposed algorithms.

. . .

IBPL Performance Results
From the same open-sky test run for IBPL integrity validation we derive performance figures in the form of accumulated histograms of protection level sizes.

. . .

Asymptotic Convergence of IBPL to SBAS PL
Another remarkable property of the IBPL concept is its convergence to the definition of PL currently used in SBASs (see Annex J of RTCA/DO-229D cited in the Additional Resources section at the end of this article).

. . .

About the Isotropy Assumption
Once the isotropy assumption has been accepted, the level of integrity achieved with the IBPL concept can be proven mathematically, and is therefore incontrovertible. The only controvertible point of the method is the isotropy assumption itself, or, more precisely, the extent to which this assumption represents the real world.

. . .

Conclusions
The isotropy-based protection level concept arose as the result of investigations concerning GNSS liability critical applications, in particular in urban environments. The authors found, however, that this notion also shows a great availability performance in open-sky environments and could therefore become a major breakthrough in open-sky SoL applications such as civil aviation. Isotropy-based protection levels are completely autonomous, are easily computable in real time, and rely on a single, quite verisimilar and verifiable hypothesis.

Unlike other approaches for integrity being defined as part of the GEAS initiative, the IBPL method does not require, in principle, any ground monitoring, though detection and exclusion of faulty satellites by the ground segment would help guarantee isotropy, leaving the protection level computation to the user — through the IBPL — and thus simplifying ground segment design.

IBPL’s sensitivity to the number of satellites becomes a clear advantage in open sky. With currently no more than 10 satellites in view on average (GPS only) and 20 or even more when considering either GLONASS or the future European Galileo system, this PL concept will predictably yield great performances, with smaller protection levels than those achieved nowadays by existing SBASs such as the U.S. Federal Aviation Wide Area Augmentation System or the European Geostationary Navigation Overlay Service.

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

Additional Resources
[1] Cosmen-Schortmann, J., and M. Martínez-Olagüe, M. Toledo-López, and M. Azaola-Saenz, “Integrity in Urban and Road Environments and Its Use in Liability Critical Applications,” Proceedings of the Position Location and Navigation Symposium (PLANS) 2008, Monterey, California, May 6–8, 2008
[2] GNSS Evolutionary Architecture Study, Phase I – Panel Report, February, 2008, available on-line at <http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/library/documents/media/GEAS_PhaseI_report_FINAL_15Feb08.pdf>
[3] RTCA Inc., RTCA/DO-229D, “Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System Airborne Equipment,” 2006

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