Aerospace and Defense

July 4, 2009

Making Sense of GPS Inter-Signal Corrections

Figure 1

The downloadable PDF (above) contains bonus material not available in the print edition. Appendices and other information on this subject is available at the bottom of the page.

The current version of the master GPS Interface Specification document (IS-GPS-200 Rev D March 2006) contains a new dual-frequency ionosphere correction algorithm that is to be used with the modernized GPS space vehicles (SVs) and their next-generation modernized GPS signals.

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June 16, 2009

ESA Signs Galileo IOV Launch, FOC Satellite Contracts

René Oosterlinck, ESA director of the Galileo Program and Navigation-related Activities (center), and Jean-Yves Le Gall, chairman and CEO Arianespace (left), sign the Galileo In-Orbit Validation Launch Services Contract in the ESA Pavilion at the Paris Air Show, Le Bourget. Paul Verhoef, programme manager of EU Satellite Navigation Programs at the European Commission. ESA photo — S. Corvaja, 2009

On June 15, the European Space Agency (ESA) signed contracts for launch services on the Galileo in-orbit validation (IOV) satellites as well as two additional contracts for “long lead items” needed to build the full operational capability (FOC) Galileo constellation of satellites.

The first contract, with Arianespace, will provide launch services for the four IOV Galileo satellites that will be placed in orbit by the end of 2010. Two Soyuz rockets, each carrying two Galileo spacecraft, will launch from Europe’s Spaceport in French Guiana.

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By Inside GNSS
June 4, 2009

Changes in Store for U.S. Military GPS Equipment Development and Acquisition

DAGR 2008. Rockwell Collins image

Development and acquisition of military GPS user equipment (MGUE) are taking on new dimensions across the board — institutionally, procedurally, and technologically.

Along the way, the changes could redefine relationships within the Department of Defense (DoD) and between the agency and industry.

At the agency level, a proposal is forthcoming to “stand up” a positioning, navigation, and timing (PNT) user equipment joint program office (JPO) that would incorporate UE responsibilities (and budgets) now exercised by the GPS Wing (GPSW).

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By Inside GNSS
May 25, 2009

GAO Report on GPS Satellite Constellation Status: The Pushback

Two weeks after a Government Accountability Office (GAO) report warned of potential gaps in the GPS satellite constellation, reassurances from Department of Defense (DoD), U.S. Air Force, GPS program officials, and industry are slowly restoring calm to an anxious public.

Coupled with a May 7 congressional hearing (and subsequent media coverage) that fanned the flames, the report described scenarios — for instance a two-year setback in launching the first GPS III spacecraft — that could lead to deterioration in the quality of GPS service due to delays in building new generations of satellites and past program management problems. The news coverage drew primarily on the GAO report and prepared statements presented at the hearing and not on the much more interesting dialog with subcommittee members captured on video or on the assumptions and methodology underlying the constellation analysis.

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By Inside GNSS
May 21, 2009

New Leaders to Gather for First Obama-Era PNT ExCom Meeting

Mike Shaw, NCO Executive Director

A June 18 meeting of the National Space-Based Positioning, Navigation, and Timing (PNT) Executive Committee (ExCom) will bring together leaders of the Obama administration in the key interagency setting for GPS issues.

Established under a 2004 National Presidential Security Directive, the ExCom advises and coordinates federal departments and agencies on matters concerning the Global Positioning System and related systems. The deputy secretaries of defense and transportation and co-chair the committee, which includes equivalent-level members from seven other federal agencies.

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By Inside GNSS

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.

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