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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 85 ionospheric corrections in the absence of L1. Modifications to military receivers to accomplish this are discussed later in this report. The current manual distribution of decryption keys is laborious and time consuming. The DOD is currently developing the means to distribute the keys electronically. Such a capability would greatly enhance the use of the encrypted L2 Y-code. The committee also believes that technology is available to upgrade the current P-code encryption method and suggests that the Air Force should explore the necessity of utilizing this technology. Modifications to the Block IIR satellites and the Block IIF request for proposal may be required if upgraded encryption methods are necessary. Changes to military receivers also will be required. Impact of A-S on Civil Users SPS receivers cannot directly track Y-code, which significantly limits civil access to measurements of the L2 signal for correction of ionospheric errors. Several Y-codeless approaches have been developed to overcome this problem.13 These techniques, however, have a lower signal-to-noise ratio than dual-frequency tracking. This creates difficulties in situations where the receiver is moving, is subject to multipath signals, or is operating in areas where signal attenuation exists, such as in an urban area or under foliage. Despite these limitations, less approaches are still being used for many surveying and scientific applications. SPS users also would benefit from access to an unencrypted L2 signal, because its bandwidth is approximately ten times as wide as the L1 signal. The wider bandwidth would improve resistance to interference and reduce vulnerability to multipath. Findings and Recommendations A-S is critically important to the military because it forces potential adversaries to use the C/A-code on L1, which can be jammed if necessary without inhibiting the U.S. military's use of the encrypted Y-code on L2. Further, encryption provides resistance to spoofing of the military . Although many civil users could benefit if A-S is turned off, as discussed in the previous chapter and above, their requirements can be met with other enhancements described in subsequent pages. A-S should remain on and the electronic distribution of keys should be implemented at the earliest possible date. In addition, the Air Force should explore the necessity of upgrading the current encryption method. Required receiver enhancements should be incorporated in future planned upgrades. 13 Some codeless approaches include (1) delay and multiply to recover the carrier and code phases, (2) squaring to recover the carrier phase, (3) cross-correlation of the L1 and L2 signals to measure the differential carrier phase and code pseudorange, and (4) P-code enhanced versions of these techniques.
