GPS Block IIIA

From Wikipedia, the free encyclopedia - View original article

GPS Block IIIA
GPS Block IIIA.jpg
Artist's impression of a GPS Block IIIA satellite in orbit
General information
ManufacturerLockheed Martin
Country of origin United States
BusA2100
ApplicationsNavigation satellite
Orbit regimesSemi-synchronous MEO
OperatorUS Air Force
Lifetime15 years
Production
StatusDevelopment
On order4
 
Jump to: navigation, search
GPS Block IIIA
GPS Block IIIA.jpg
Artist's impression of a GPS Block IIIA satellite in orbit
General information
ManufacturerLockheed Martin
Country of origin United States
BusA2100
ApplicationsNavigation satellite
Orbit regimesSemi-synchronous MEO
OperatorUS Air Force
Lifetime15 years
Production
StatusDevelopment
On order4

GPS Block IIIA, or GPS III is the next generation of GPS satellites which will be used to keep the Navstar Global Positioning System operational. Lockheed Martin is the contractor for the design, development and production of the GPS III Non-Flight Satellite Testbed (GNST) and the first eight GPS III satellites.[1] The United States Air Force plans to purchase up to 32 GPS III satellites. GPS IIIA-1, the first satellite in the series, is projected to launch in 2014.[2]

Overview[edit]

The United States' Global Positioning System (GPS) reached Fully Operational Capability on July 17, 1995,[3] completing its original design goals. However, additional advances in technology and new demands on the existing system led to the effort to modernize the GPS system. Announcements from the Vice President and the White House in 1998 initiated these changes. In 2000, the U.S. Congress authorized the effort, referred to as GPS III.

The project involves new ground stations and new satellites, with additional navigation signals for both civilian and military users, and aims to improve the accuracy and availability for all users.

Lockheed Martin was awarded the GPS III Space Segment contract on May 15, 2008. The first launch is projected for 2014.[4] Raytheon was awarded the Next Generation GPS Operational Control System (OCX) contract on Feb 25, 2010.[5]

Development[edit]

Block IIIA satellites use Lockheed Martin's A2100 bus structure. They are manufactured by Alliant Techsystems from lightweight, high-strength composite materials.[6]

The first and second GPS III satellites are on schedule for launch in 2014 and 2015.[2]

The U.S. Air Force awarded Lockheed Martin a $238 million contract for production of the third and fourth satellites in January 2012.[7]

Future Block III variants are planned to incorporate additional capabilities. They include Distress Alerting Satellite System (DASS) capabilities for search and rescue, as well as satellite crosslinks for rapid command and reduced age of data.[8]

New navigation signals[edit]

Civilian L2 (L2C)[edit]

One of the first announcements was the addition of a new civilian-use signal to be transmitted on a frequency other than the L1 frequency used for the existing GPS Coarse Acquisition (C/A) signal. Ultimately, this became known as the L2C signal because it is broadcast on the L2 frequency (1227.6 MHz). It is transmitted by all block IIR-M and later design satellites, but until the new OCX (Block 1) system is in place, the signal consists of a default message ("Type 0") that does not provide full navigational data.[9] OCX Block 1 with the L2C navigation data is scheduled to enter service in February 2016.[10][11]

The L2C signal is tasked with providing improved accuracy of navigation, providing an easy-to-track signal, and acting as a redundant signal in case of localized interference.

The immediate effect of having two civilian frequencies being transmitted from one satellite is the ability to directly measure, and therefore remove, the ionospheric delay error for that satellite. Without such a measurement, a GPS receiver must use a generic model or receive ionospheric corrections from another source (such as a Satellite Based Augmentation System). Advances in technology for both the GPS satellites and the GPS receivers have made ionospheric delay the largest source of error in the C/A signal. A receiver capable of performing this measurement is referred to as a dual frequency receiver. The technical characteristics of it are:

It is defined in IS-GPS-200.[12]

Military (M-code)[edit]

A major component of the modernization process, a new military signal called M-code was designed to further improve the anti-jamming and secure access of the military GPS signals. The M-code is transmitted in the same L1 and L2 frequencies already in use by the previous military code, the P(Y) code. The new signal is shaped to place most of its energy at the edges (away from the existing P(Y) and C/A carriers).

Unlike the P(Y) code, the M-code is designed to be autonomous, meaning that users can calculate their positions using only the M-code signal. P(Y) code receivers must typically first lock onto the C/A code and then transfer to lock onto the P(Y)-code.

In a major departure from previous GPS designs, the M-code is intended to be broadcast from a high-gain directional antenna, in addition to a wide angle (full Earth) antenna. The directional antenna's signal, termed a spot beam, is intended to be aimed at a specific region (i.e., several hundred kilometers in diameter) and increase the local signal strength by 20 dB (10× voltage field strength, 100× power). A side effect of having two antennas is that the GPS satellite will appear to be two GPS satellites occupying the same position to those inside the spot beam.

While the full-Earth M-code signal is available on the Block IIR-M satellites, the spot beam antennas will not be available until the Block III satellites are deployed, tentatively in 2014. Like the other new GPS signals, M-code is dependent OCX—specifically Block 2—which is schedule to enter service in October 2016.[13][11]

Other M-code characteristics are:

Safety of Life (L5)[edit]

Safety of Life is a civilian-use signal, broadcast on the L5 frequency (1176.45 MHz). In 2009, a WAAS satellite sent the initial L5 signal test transmissions. SVN-62, the first GPS block IIF satellite, continuously broadcast the L5 signal starting on June 28, 2010. However, like the L2C signal, the L5 broadcast will not include a data message until OCX comes online.[14] The L5 navigation data will not be transmitted until OCS Block 2 enters service,[9] though there is some speculation that it will be made available in Block 1.[13][11]

WRC-2000 added space signal component to this aeronautical band so aviation community can manage interference to L5 more effectively than L2

It is defined in IS-GPS-705.[15]

New Civilian L1 (L1C)[edit]

L1C is a civilian-use signal, to be broadcast on the same L1 frequency (1575.42 MHz) that contains the C/A signal used by all current GPS users. The L1C will be available with first Block III launch, scheduled for 2014,[16] however the full navigation data (like L5) is dependent on OCX Block 2 entering service.[9]

It is defined in IS-GPS-800.[17]

Block III satellite improvements[edit]

Increased signal power at the Earth's surface

Researchers from The Aerospace Corporation confirmed that the most efficient means to generate the high-power M-code signal would entail a departure from full-Earth coverage, characteristic of all the user downlink signals up until that point. Instead, a high-gain antenna would be used to produce a directional spot beam several hundred kilometers in diameter. Originally, this proposal was considered as a retrofit to the planned Block IIF satellites. Upon closer inspection, program managers realized that the addition of a large deployable antenna, combined with the changes that would be needed in the operational control segment, presented too great a challenge for the existing system design.[18]

Ground control segment improvements[edit]

The control segment determines the orbital position of satellites and transmits information to satellites in space to keep the GPS system operational and performing within specification. The Operation Control Segment (OCS) currently serves as the control segment of record. It provides the operational capability that supports global GPS users and keeps the GPS system operational and performing within specification. OCS successfully replaced the legacy 1970’s-era mainframe computer at Schriever Air Force Base in September 2007. After installation, the system helped enable upgrades and provide a foundation for a new security architecture.

In 2010, the United States Air Force announced plans to develop a modern control segment, which would act as a critical part of the GPS modernization initiative. OCS will continue to serve as the ground control system of record until the new system, Next Generation GPS Operation Control System[22] (OCX), is fully developed and functional.

The new capabilities provided by OCX will be the cornerstone for revolutionizing GPS’s mission capabilities, and enabling [23] Air Force Space Command to greatly enhance GPS operational services to US combat forces, civil partners and myriad domestic and international users.

The GPS OCX program also will reduce cost, schedule and technical risk. It is designed to provide 50% [24] sustainment cost savings through efficient software architecture and Performance-Based Logistics. In addition, GPS OCX expected to cost millions less than the cost to upgrade OCS while providing four times the capability.

The GPS OCX program represents a critical part of GPS modernization and provides significant information assurance improvements over the current GPS OCS program.

On September 14, 2011,[25] the U.S. Air Force announced the completion of GPS OCX preliminary design review and confirmed that the OCX program is ready for the next phase of development. The GPS OCX program has achieved major milestones and is on track to support the GPS IIIA launch in May 2014.

See also[edit]

References[edit]

  1. ^ "U.S. Air Force Awards Lockheed Martin Contracts to Begin Work on Next Set of GPS III Satellites" (Press release) Lockheed Martin 2013-02-25
  2. ^ a b "U.S. Air Force Awards Lockheed Martin GPS III Flight Operations Contract" (Press release) Lockheed Martin 2012-05-31
  3. ^ U.S. Coast Guard Navigation Center. "GPS FAQ". U.S. Department of Homeland Security. 
  4. ^ "U.S. Air Force Awards Lockheed Martin Team $1.4 Billion Contract To Build GPS III Space System" (Press release). Lockheed Martin. 2008-05-15. 
  5. ^ "Raytheon Wins Next-Gen GPS Award". Aviation Week. The McGraw-Hill Companies, Inc. 2010-05-01. 
  6. ^ "Lockheed Orders GPS 3A Satellite Buses from ATK"
  7. ^ http://www.lockheedmartin.com/us/news/press-releases/2012/january/0112_ss_gps.html
  8. ^ http://www.gps.gov/systems/gps/space/
  9. ^ a b c "New Civil Signals: Second Civil Signal". National Coordination Office for Space-Based Positioning, Navigation, and Timing. 2013-02-06. Retrieved 2013-11-21. 
  10. ^ "Control Segment: Next Generation Operational Control System". National Coordination Office for Space-Based Positioning, Navigation, and Timing. 2013-09-26. Retrieved 2013-11-21. 
  11. ^ a b c Kolibaba, Ray (2012-11-14). "GPS OCX Program Status". Stanford 2012 PNT Challenges and Opportunities Symposium. Retrieved 2013-11-21. 
  12. ^ "Interface Specification IS-GPS-200, Revision E" (PDF). Coast Guard Navigation Center. 2010-06-08. 
  13. ^ a b Divis, Dee Ann (January/February 2013). "More Than Money Worries: OCX and the New Civil Signals". Inside GNSS. Retrieved 2013-11-21. 
  14. ^ "New Civil Signals: Third Civil Signal". National Coordination Office for Space-Based Positioning, Navigation, and Timing. 2013-02-06. Retrieved 2013-11-21. 
  15. ^ "Interface Specification IS-GPS-705, Revision A" (PDF). Coast Guard Navigation Center. 2010-06-08. 
  16. ^ "Block III GPS Upgrade for Satellites to be Tested in Colorado by Lockheed Martin". TMCnet. 2011-12-13. 
  17. ^ "Interface Specification IS-GPS-800, Revision A" (PDF). Coast Guard Navigation Center. 2010-06-08. 
  18. ^ Lazar, Steven (Summer 2002). "Modernization and the Move to GPS III". Crosslink 3 (2): 42–46. 
  19. ^ "ILRS Meeting on Retroreflector Arrays". 
  20. ^ "Slides from ILRS Meeting on Retroreflector Arrays". April 2006. 
  21. ^ NASA Search and Rescue Mission Office : Distress Alerting Satellite System (DASS)
  22. ^ "GPS ADVANCED CONTROL SEGMENT(OCX)". 
  23. ^ "GPS III Operational Control Segment (OCX)". GlobalSecurity.org. 
  24. ^ "The USA’s GPS-III Satellites". Defense Industry Daily. 2011-10-13. 
  25. ^ "GPS Completes Next Generation Operational Control System PDR". Air Force Space Command News Service. 2011-09-14. 

External links[edit]