GPS Block IIIA
Artist's impression of a GPS Block IIIA satellite in orbit
|Country of origin||United States|
|Operator||US Air Force|
|Design life||15 years|
|Orbit regimes||Semi-synchronous MEO|
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. The United States Air Force plans to purchase up to 32 GPS III satellites. GPS IIIA-1, the first satellite in the series, was projected to launch in 2014, but is now significantly delayed.
- 1 Overview
- 2 Development
- 3 New navigation signals
- 4 Block III satellite improvements
- 5 Ground control segment improvements
- 6 Ground control segment: The GAO Perspective
- 7 See also
- 8 References
- 9 External links
The United States' Global Positioning System (GPS) reached Fully Operational Capability on July 17, 1995, 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 was projected for 2014. Raytheon was awarded the Next Generation GPS Operational Control System (OCX) contract on Feb 25, 2010.
Block IIIA satellites use Lockheed Martin's A2100 bus structure. They are manufactured by Alliant Techsystems from lightweight, high-strength composite materials. Each satellite will carry 8 deployable JIB antennas designed and manufactured by Northrop Grumman Astro Aerospace
The first GPS III satellite was originally scheduled for launch in 2014, but the March 2014 GAO report expects that the first satellite will launch no sooner than April 2016. The most significant issues causing delays appear to be with the navigation payload.
The U.S. Air Force awarded Lockheed Martin a $238 million contract for production of the third and fourth satellites in January 2012.
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.
||It has been suggested that portions of this article be moved into GPS signals#Modernization and additional GPS signals. (Discuss)|
||This article duplicates, in whole or part, the scope of other articles, specifically, GPS signals#Modernization and additional GPS signals. (December 2013)|
Civilian L2 (L2C)
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 can be transmitted by all block IIR-M and later design satellites. The original plan stated that until the new OCX (Block 1) system is in place, the signal would consist of a default message ("Type 0") that does not provide full navigational data. OCX Block 1 with the L2C navigation data is scheduled to enter service in February 2016, but that date does not reflect the two year first satellite launch delay projected by the GAO. Attempts are underway to try to decouple L2C availability from the OCX Block 1 schedule, starting with activation of the L2C signal in April 2014.
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:
- L2C contains two distinct PRN sequences:
- CM (for Civilian Moderate length code) is 10,230 bits in length, repeating every 20 milliseconds.
- CL (for Civilian Long length code) is 767,250 bits, repeating every 1,500 milliseconds (i.e., every 1.5 s).
- Each signal is transmitted at 511,500 bits per second (bit/s); however, they are multiplexed to form a 1,023,000 bit/s signal.
- CM is modulated with a 25 bit/s navigation message with forward error correction, whereas CL contains no additional modulated data.
- The long, non-data CL sequence provides for approximately 24 dB greater correlation protection (~250 times stronger) than L1 C/A.
- L2C signal characteristics provide 2.7 dB greater data recovery and 0.7 dB greater carrier tracking than L1 C/A
- The L2C signals' transmission power is 2.3 dB weaker than the L1 C/A signal.
- In a single frequency application, L2C has 65% more ionospheric error than L1.
It is defined in IS-GPS-200.
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. Like the other new GPS signals, M-code is dependent on OCX—specifically Block 2—which is scheduled to enter service in October 2016, but that date does not reflect the two year first satellite launch delay expected by the GAO. Other M-code characteristics are:
- Satellites will transmit two distinct signals from two antennas: one for whole Earth coverage, one in a spot beam.
- Modulation is binary offset carrier
- Occupies 24 MHz of bandwidth
- It uses a new MNAV navigational message, which is packetized instead of framed, allowing for flexible data payloads
- There are four effective data channels; different data can be sent on each frequency and on each antenna.
- It can include FEC and error detection
- The spot beam is ~20 dB more powerful than the whole Earth coverage beam
- M-code signal at Earth's surface: –158 dBW for whole Earth antenna, –138 dBW for spot beam antennas.
Safety of Life (L5)
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. The L5 navigation data will not be transmitted until OCS Block 2 enters service, though there is some speculation that it will be made available in Block 1. Efforts are underway to accelerate the testing and availability of the L5 signal, beginning with the transmission of the L5 signal from all satellites capable of doing so starting April 28, 2014, and further adding capabilities in December 2014.
- Improves signal structure for enhanced performance
- Higher transmission power than L1 or L2C signal (~3dB, or twice as powerful)
- Wider bandwidth, yielding a 10-times processing gain
- Longer spreading codes (10 times longer than used on the C/A code)
- Located in the Aeronautical Radionavigation Services band, a frequency band that is available world wide.
WRC-2000 added space signal component to this aeronautical band so the aviation community can manage interference to L5 more effectively than L2. It is defined in IS-GPS-705.
New Civilian L1 (L1C)
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. however the full navigation data (like L5) is dependent on OCX Block 2 entering service.
- Implementation will provide C/A code to ensure backward compatibility
- Assured of 1.5 dB increase in minimum C/A code power to mitigate any noise floor increase
- Non-data signal component contains a pilot carrier to improve tracking
- Enables greater civil interoperability with Galileo L1
It is defined in IS-GPS-800.
Block III satellite improvements
Increased signal power at the Earth's surface
- M-code: –158 dBW / –138 dBW.
- L1 and L2: –157 dBW for the C/A code signal and –160 dBW for the P(Y) code signal.
- L5 will be –154 dBW.
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.
- NASA has requested that Block III satellites carry laser retro-reflectors. This allows tracking the orbits of the satellites independent of the radio signals, which allows satellite clock errors to be disentangled from ephemeris errors. This, a standard feature of GLONASS, will be included in the Galileo positioning system, and was included as an experiment on two older GPS satellites (satellites 35 and 36).
- The USAF is working with NASA to add a DASS payload to the second increment of GPS III satellites as part of the MEOSAR system.
Ground control segment improvements
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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 (OCX), is fully developed and functional.
The new capabilities provided by OCX will be the cornerstone for revolutionizing GPS’s mission capabilities, and enabling  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% 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.
- OCX will have the ability to control and manage GPS legacy satellites as well as the next generation of GPS III satellites, while enabling the full array of military signals.
- Built on a flexible architecture that can rapidly adapt to the changing needs of today’s and future GPS users allowing immediate access to GPS data and constellations status through secure, accurate and reliable information.
- Empowers the warfighter with more secure, actionable and predictive information to enhance situational awareness.
- Enables new modernized signals (L1C, L2C, and L5) and has M-code capability, which the legacy system is unable to do.
- Provides significant information assurance improvements over the current program including detecting and preventing cyber attacks, while isolating, containing and operating during such attacks.
- Supports higher volume near real-time command and control capability.
On September 14, 2011, 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.
Ground control segment: The GAO Perspective
While the USAF has consistently claimed that the OCX segment has always been on time and on budget, the GAO has consistently disagreed. In March 2013 the GAO estimated that the OCX budget had increased 43% to a total cost estimate of $3.7 billion. As of March 2013, the USAF was still claiming an April 2014 “available for launch” date for the OCX segment. As of April 2014, OCX was far from complete and the GAO estimated OCX initial capability no sooner than October 2016.
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