The United States' Global Positioning System (GPS), having reached Fully Operational Capability on July 17, 1995,[1] has completed 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, 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.[2] Raytheon was awarded the Next Generation GPS Operational Control System (OCX) contract on Feb 25, 2010.[3]
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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.
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.[4]
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 (10x voltage field strength, 100x 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 2013.
Other M-code characteristics are:
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.
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.[5]
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.[6]
It is defined in IS-GPS-800.[7]
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[8]
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[12] (OCX), is fully developed and functional.
The new capabilities provided by OCX will be the cornerstone for revolutionizing GPS’s mission capabilities, and enabling [13] Air Force Space Command to greatly enhance GPS operational services to US combat forces, civil partners and myriad of domestic and international users.
The GPS OCX program also will reduce cost, schedule and technical risk. It is designed to provide 50% [14] 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,[15] 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.