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EXECUTIVE SUMMARY 8 Recommendations that Enhance GPS Performance for Military Users As stated above, GPS was originally designed to provide our forces with a military advantage. In the past, DOD has depended on a strategy of global signal degradation, through SA, to reduce the GPS signal accuracy to civilian and unauthorized users, while providing a more accurate, encrypted signal to authorized users. However, as stated above, the committee believes that the military usefulness of SA is severely diminished and that it is urgent that the DOD focus its attention on denial of all useful signals to an opponent, for example, through jamming and spoofing techniques, including jamming of the unencrypted C/A-code, rather than relying on SA The NRC committee therefore recommends several military receiver enhancements that would support such a strategy. The development of receivers that can rapidly lock onto the Y-coded signals in the absence of the C/A- code should be completed. The deployment of direct Y-code receivers should be given high priority by the DOD. Nulling antennas and antenna electronics should be employed whenever feasible and cost effective. Research and development focused on reducing the size and cost of this hardware should actively be supported. The development of low-cost, solid-state, tightly-coupled integrated inertial navigation system/GPS receivers to improve immunity to jamming and spoofing should be accelerated. The development and operational use of GPS receivers with improved integration of signal processing and navigation functions for enhanced performance in jamming and spoofing should be accelerated. Military receivers should be developed that compensate for ionospheric errors when L1 is jammed, by improved software modeling and use of local-area ionospheric corrections. In the interim time before such enhancements can be fielded by the military, various operating disciplines, which are discussed in Chapter 3, can be used to minimize the impact of C/A-code jamming on the ability to acquire the Y-code directly. Recommendations that Enhance GPS Performance for All Users (Civil, Commercial, and Military) In view of the rapidly expanding use of GPS, the NRC committee believes that GPS must be capable of continuous operation in all foreseeable contingencies. This capability is critical. The one area where the NRC committee found limited redundancy was in the operational control segment (OCS). Although the NRC committee determined that the Air
EXECUTIVE SUMMARY 9 Force has several experiments planned to improve the system, it believes there are some additional improvements that can be made to the OCS that would increase stand-alone accuracy, availability, and integrity; improve the overall reliability of the system; or simplify day-to-day operations. Recommendations that would result in greater stand-alone GPS accuracy and integrity include uploading more current clock and orbit information to all satellites, increasing the number of monitor sites, reducing the clock and ephemeris errors, and improving Block IIR and Block IIF integrity monitoring capability. In addition, the NRC committee found a need for (1) a simulator to test software and train personnel, (2) modern receivers at the monitor stations, and (3) a permanent, backup master control station. Specifically, the NRC committee recommends: Additional GPS monitoring stations should be added to the existing operational control segment. Comparison studies between cost and location should be completed to determine if Defense Mapping Agency or Air Force sites should be used. The operational control segment Kalman Filter should be improved to solve for all GPS satellites' clock and ephemeris errors simultaneously through the elimination of partitioning, and the inclusion of more accurate dynamic models. These changes should be implemented in the 1995 OCS upgrade request for proposal. Procurements for the replacement of the monitor station receivers, computers, and software should be carefully coordinated. The new receivers should be capable of tracking all satellites in view and providing C/ A-code, Y-code, and L1, and L2 carrier observables to the OCS. Upgradability to track a new L4 signal also should be considered. OCS software also should be made capable of processing this additional data. Firm plans should be made to ensure the continuous availability of a backup master control station. A simulator for the space and ground segment should be provided as soon as possible to test software and train personnel. The operational control segment software should be updated using modern software engineering methods in order to permit easy and cost-effective updating of the system and to enhance system integrity. This should be specified in the 1995 OCS upgrade request for proposal. The planned Block IIR operation should be reexamined and compared to the accuracy advantages gained by incorporating inter-satellite ranging data in the ground-based Kalman Filter and uploading data at some optimal time interval, such as every hour, to all GPS satellites.
