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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 111 The Block IIR inter-satellite communications crosslinks should be used to relay integrity information determined through ground-based monitoring. PERFORMANCE IMPROVEMENTS TO ENHANCE THE MILITARY USE OF GPS From its very inception, the force enhancement capabilities of GPS for both U.S. and allied armed forces has been one of the system's most important capabilities. This remains true today, despite the fact that civilian and commercial use of the system has grown rapidly. The current DOD policy is to secure for both the United States and its allies the full accuracy of GPS by using the encrypted Y-code on both L1 and L2, while denying that accuracy via SA to a potential enemy who, like most civilians, will have the C/A-code available only on L1. However, with the widespread use and proliferation of DGPS, the accuracy degradation produced by SA is routinely eliminated and, in many cases, civilians have access to more accurate signals than the military. As the cost of DGPS equipment decreases, differential technology and capability will proliferate. Differential systems will therefore become difficult to identify and render inoperative in a conflict situation. Furthermore, because adversarial forces are far less likely to be concerned with collateral damage, the 100-meter (2 drms) stand-alone accuracy of the SPS already poses a risk for our forces operating in a theater of war. Earlier in this chapter the NRC committee recommended that the DOD concentrate future efforts towards the denial of GPS capability to an enemy by jamming the L1 signal, the L4 signal (if added), and other frequencies that may be employed by enemy forces to broadcast differential corrections. This strategy implies that U.S. forces must be properly equipped to operate in a high jamming environment generated by both U.S. military and enemy jammers. Based on this objective, the remainder of this section recommends several near- term technical enhancements to improve the overall performance of military GPS user equipment operating in the presence of spoofing, jamming, and interference. The greatest improvement in user equipment performance will result from the combined implementation of all five recommended enhancements in a single integrated system. Possible operational procedures that could be used prior to the availability of each recommended enhancement also are discussed. Recommended Technical Improvements to Military User Equipment Rapid, Direct Y-Code Acquisition Current military receivers are designed to first acquire the more powerful C/A-code before handing over to the encrypted Y-code. Upon receiver restart, or following a loss of signal lock, a PPS receiver must go through acquisition in which a two-dimensional time-frequency search is carried out by trial correlations. With current receivers, this search conventionally is done serially, resulting in seconds to minutes of acquisition time for the C/ A-code prior to Y-code hand-over, depending upon the amount of signal blockage
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 112 experienced and the movement of the vehicle carrying the receiver. If the L1 signal is jammed, the current receivers cannot acquire the C/A-code and as a result are denied access to the encrypted Y-code as well as dual- frequency ionospheric corrections. One receiver improvement that would enhance military access to the encrypted Y-code in a jamming environment would be the ability to acquire the Y-code rapidly without first acquiring the C/A-code. A method for improving L2 ionospheric corrections in an L1 jamming environment is addressed later in this chapter. In order to obtain direct Y-code access, the signal acquisition processing capability of current PPS receivers must be improved through the use of massively parallel correlators, built using application-specific integrated circuit (ASIC) technology.51 The technology is now available that would allow the incorporation of at least 1,000 parallel correlators per receiver at a reasonable cost. This would allow direct Y-code acquisition within 2 seconds in a non-jamming environment, without prior acquisition of the C/A-code. This would allow faster receiver time-to-first-fix after power-down and would enhance signal availability after a blackout interval.52 The ability to directly acquire the Y-code on L2 would ensure that the selective denial of the L1 signal and the C/A- code through spoofing and jamming would eliminate or seriously degrade an enemy's use of GPS without impacting U.S. capabilities. According to experts in the field of military receiver technology, the technology for direct Y-code acquisition is in hand and in fact, the current military ''Plugger" receivers do try to directly reacquire the Y-code after signal loss.53 A military receiver with the capability to initially acquire the Y-code directly could be developed in 9-15 months depending on: (1) the amount of input received from the military regarding specifications; (2) the level of trade-off accepted between jamming-to-signal ratio versus the amount of time for direct Y-code acquisition; and (3) the ASIC development.54 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. 51 Massively parallel correlators using ASIC technology, permit the receiver to compare, at very fast speeds, the internally generated pseudorandom noise codes to the received codes, which contain data about the satellite's position and time the code was transmitted. 52 See Appendix K for calculations showing a direct Y-code acquisition time of 2 seconds with current ASIC technology. 53 Personal conversation with Mr. Tyler Trickey, Rockwell-Collins, February 1995. 54 Information provided to the NRC committee by Mr. Charles Trimble of Trimble Navigation Ltd., 31 March 1995.
