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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 116 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. Possible Interim Operational Procedures The NRC committee believes that the most significant shortcoming of a GPS denial strategy is the current inability to operate in high levels of enemy jamming, while at the same time denying GPS to an adversary. The implementation of technical enhancements to military user equipment, such as direct Y-code acquisition capability, improved nulling antennas, better inertial aiding capabilities, enhanced signal processors, and improved L2 ionospheric corrections would assist in the optimal solution to this problem. Although the NRC committee believes that these technical capabilities are now available, unfortunately, such capabilities are not currently fielded by the military. GPS receivers are especially vulnerable during their signal acquisition phase. This weakness is magnified by the inability of most military GPS receivers to acquire the Y-code during periods when the C/A-code is being jammed. Future receivers capable of direct Y-code acquisition will go a long way toward solving this problem. In any event, tactics must be developed and put in place to facilitate acquisition during jamming. Some possible disciplines that can be implemented in the near-term are presented below. (1) Develop military procedures to remove jammers and DGPS stations. As with existing plans to destroy radars in a hostile area, plans and procedures should be developed to remove jammers and DGPS stations. (2) Acquire the Y-code outside the jamming area. Prior to entering the jamming area, the C/A-code can be used to acquire the Y-code. Once the Y-code is obtained, and while still within the active jamming area, PPS receivers should be operated continuously or be re-powered every few hours in order to maintain accurate time. Accurate time will aid in faster, direct reacquisition of the Y-code. This technique can be extended to aircraft-based GPS-guided munitions using low-powered C/A- code retransmissions aboard, or by hardwiring of time-transfer circuits. (3) Review training exercises, procedures, and policy manuals. The current training procedures and policy manuals should be examined to make sure U.S. troops are properly instructed to operate in both hostile jamming and denial jamming environments. For example, ground forces can make use of natural terrain and man-made obstructions to obtain some shielding from ground-based jammers.
