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MITRE Helps Make GPS More Accurate and Powerful September 2003
The first Global Positioning System (GPS) satellite system was launched 25 years ago by the U.S. military as a guidance and navigational tool. As the technology has matured, its availability and accuracy have improved dramatically. GPS enables the military to guide missiles and pinpoint the exact location of aircraft, soldiers, and tactical equipment—a critical capability in Operation Iraqi Freedom. The GPS satellite launched in March, and the ones to be launched in the future, will extend the capabilities even further—military signals will be more powerful and civil services better protected from interference. "MITRE has played an integral role in the design of new signals and in the government's decision to add more GPS frequencies," says Alan Moore, MITRE's GPS project leader. "This made it possible to remove the global accuracy limitations on civil uses of GPS. It also provided civil users with a tenfold improvement in accuracy." This improvement helped spur many civil GPS applications. These include aviation, commercial fleet management, precision agriculture, surveying and mapping—and recreational uses (e.g., hikers and boaters). MITRE continues to work with both military and civil agencies to balance their needs for navigation services and to solve common problems. MITRE's work involves operational applications, augmentation systems, signal processing, antenna engineering, radio frequency design, ground control system software design, and overall system architectures. "The GPS satellites being launched next year will expand the system and increase GPS capabilities considerably," says Moore. "A second and then third civil frequency will be broadcast later to avoid interference and provide improved accuracy and redundancy. At the same time, two new military signals will be transmitted. Their separation from civil signals provides added protection and avoids interference with civil operations during conflicts." Moore notes that the constellation of satellites (there are 28 today) will eventually be fully populated with more powerful satellites. "The worldwide civil community and the U.S. military and its allies will have robust and extremely accurate navigation anywhere, all the time," he says. GPS's Three Components Today's small handheld GPS receivers belie the complex system behind them. Three major components make up a GPS system: a space segment, a user segment, and a control segment. The space segment consists of a minimum of 24 satellites that orbit 11,000 nautical miles above the earth's surface. Their orbits guarantee that at least four satellites will be visible all the time. The satellites send precisely timed radio signals to the user segment, which includes receivers used by people and vehicles in the air, on water, on the ground, and in space. The control segment is a network of ground stations located around the world that monitors the GPS signals. The ground stations precisely determine the locations and time measurements of the satellites, and then they send back to the satellites information to correct position errors in their existing broadcast for transmission to the users. MITRE's work involves all three segments. Better Jam-Proofing for the Military One of the problems of GPS from the beginning has been its susceptibility to hostile interference and jamming. In the late 1990s, the GPS Joint Program Office asked MITRE to help make GPS-equipped platforms more robust. MITRE used its experience in electronics systems engineering and architecture planning to develop a multipronged approach. "Our success exceeded initial expectations, and we've continued to work on this problem, gradually improving GPS jam-proofing capabilities," says Moore. For example, MITRE played an important role in developing and deploying a new military signal (M-code), which enables a more powerful signal that is more difficult to jam, but does not interfere with reception of civil signals. MITRE staff developed essential parts of the novel design, along with theory to predict its performance. We also developed the first-of-its-kind transmitter and receiver hardware for M-code, as well as advanced integrated circuits for receiver processing. This equipment is being used as reference hardware for initial qualification testing for M-code-capable satellites, the first of which will be launched in 2004. The M-code satellite will use a technique called flex power to raise the power of selected military signals by a factor of five. Even higher power improvements will be seen in the future on the new GPS III satellites. This block of satellites will also offer improved accuracy and integrity. MITRE is also improving the robustness of GPS receivers. Our engineers designed the frequency domain interference suppressor, which suppresses narrow-band interference without affecting navigation accuracy. It can be used with new equipment or on existing platforms with legacy receivers. In addition, we are designing smaller and higher performance antennas for nulling/beamforming. These will allow the military to install advanced anti-jam capabilities on a wide range of platforms. "We also developed an adaptive space-time processor that cancels broadband and narrowband interference while preserving the integrity of GPS signals," says Moore. "The processor also cancels multipath interference where the jamming energy appears to be coming from multiple directions and with multiple differential delays. This technology has already been applied to missile applications and is in development for aircraft." GPS Critical to Civil Aviation
MITRE is also working on major augmentations to the basic GPS constellation. Most important of these is the Federal Aviation Administration's Wide Area Augmentation System (WAAS). It's the cornerstone of the next-generation civil aviation navigation service. "WAAS combines ground- and space-based equipment to augment the standard positioning service of the GPS," says Kelly Markin, program manager in the communications, navigation and surveillance division of MITRE's Center for Advanced Aviation System Development (CAASD). "The WAAS system will provide navigation for en route, terminal, and approach phases of flight in the National Airspace System," says Markin. "It'll provide vertically guided instrument approaches to thousands of runways where such service would otherwise be uneconomical or physically impractical." CAASD also supports a Local Area Augmentation System (LAAS) that goes further to provide highly accurate navigation signals close to major airports. In addition to LAAS specification activities, our staff is also providing technical expertise to evolve LAAS into an advanced capability that could support landings with runway visibilities as low as 600 feet. Another safety method initially developed by MITRE is receiver autonomous integrity monitoring, or RAIM. "This method also ensures signal integrity," says Walter Scales, a principal communications engineer in CAASD's communications, navigation and surveillance division. "It takes advantage of the large number of GPS satellites usually in view," says Scales. "Signals from redundant satellites check the consistency of a position's determination. This integrity checking is available to aviation users now and will provide an additional integrity check to the monitoring MITRE designed into WAAS service." Looking Further Abroad Because of MITRE's expertise in both the DOD's and civil use of GPS, we work closely with the GPS Joint Program Office and the Interagency GPS Executive Board—a consortium of representatives from the DOD and many other government agencies—to solve common issues with GPS across government and commercial organizations. MITRE has also been playing an integral role in a broader area of satellite navigation—coordinating with the GPS Joint Program Office on the design and development of a new European satellite navigation system called Galileo. "Galileo offers the potential for dramatically improved civil navigation services, if it is compatible and interoperable with GPS," says MITRE's John Betz, director of special projects. "Yet we need to work together to avoid incompatibility between Galileo, which is a civil system, and GPS—especially the military aspects of GPS." Betz and Chris Hegarty, a senior principal engineer in CAASD, have been part of a joint United States/European Commission Technical Working Group that is identifying solutions for the GPS JPO to ensure compatibility between GPS and Galileo. "We have excellent working relationships with our European counterparts, have made good progress and have established a much better understanding of the opportunities and obstacles," says Betz. "Yet there are significant sticking points that still need to be resolved." To aid understanding, MITRE has built hardware prototypes to demonstrate feasibility and performance of some signal designs. Hegarty points out that when Galileo is operational sometime after 2008, "civil users will be able to use signals from a 60-satellite joint constellation, providing great benefits to aviation users, as well as many other users." As GPS continues to evolve and other satellite navigation systems are fielded, there clearly will be continuing challenges, providing more opportunities for MITRE to provide creative solutions that make a difference to civil and military users worldwide. "There will always be users who want higher accuracies and more robust navigation services. We're helping provide those enhancements," says Alan Moore. —by David Van Cleave Related Information Articles and News
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