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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 70 terminal guidance packages or access to differential corrections to meet accuracy requirements of a few meters (CEP). In order to take advantage of GPS accuracy, accurate knowledge of the target location is essential. Various target-determination techniques are being developed, but until they are available, munitions delivery, even with GPS, will still require highly accurate terminal guidance systems. Using an enhanced GPS with greater accuracy for guidance would provide two levels of benefit. First, the requirements on an inertial navigation device can be relaxed because more accurate GPS determination of position and velocity will be possible. Second, under some conditions where jamming is not likely, GPS could be used to provide a very economical and accurate stand-alone munitions guidance system. Integrity and Availability In general, improving the system ranging accuracy also improves integrity and availability. As noted in Appendix C, availability is the percentage of time that a user's positioning errors lie within a specified accuracy. If the ranging errors are decreased, then positioning errors will remain inside the accuracy bounds for higher DOP (Dilution of Precision) values. As a consequence, the amount of time that the system is available increases, especially in the presence of satellite outages. Improvements in availability for the SPS as ranging accuracy improves will be shown throughout this chapter by comparing availability values for Chicago, Illinois.1 Integrity checking algorithms also benefit from improved ranging accuracy. Most integrity algorithms, such as RAIM (Receiver Autonomous Integrity Monitoring), are based on consistency checks among redundant sets of measurements.2 Poor consistency indicates the possibility of a position solution error exceeding the protection limit. However, when the range errors are large, the consistency checks are not reliable except under very favorable satellites geometries. With improved range measurement accuracy, consistency can be reliably measured even under poor user-satellite geometries. SELECTIVE AVAILABILITY AND ANTI-SPOOFING The GPS was designed to provide our military forces with an advantage when engaged with other military forces, while still providing a reasonable positioning service to 1 Chicago, Illinois, was randomly chosen by the MITRE Corporation, which determined the availability values presented in this chapter using a GPS availability model developed for the FAA (Federal Aviation Administration). The analytical model accounts for individual satellite short-term and long-term failures and restorations for the 24-satellite constellation and assumes a conservative serial restoration strategy (that is, only one satellite can be replaced at a time). The GPS receiver was assumed to have an elevation mask angle of 5 degrees. 2 RAIM, which utilizes receiver software algorithms to detect unreliable satellites or position solutions, is defined in Appendix C.
