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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 87 Guidelines and Technical Considerations In studying possible options for the addition of another civilian frequency, a set of guidelines and technical considerations was developed as follows: (1) The signal must not interfere with the military's jamming techniques for denial of GPS signals. Any signal enhancement should preserve and maximize the ability of the military to deny the GPS signal to adversaries through local jamming of any unencrypted s without adversely impacting the L2 Y-code signal. The use of encryption on the Y-code effectively denies its use to unauthorized parties. (2) The signal must be backward compatible. A significant investment has been made in receiver purchases and existing receiver performance must not be degraded; although existing receivers may not be able to take advantage of the new signal. (3) The frequency allocation for the signal must be considered. The signal should be assigned a frequency in the L-band spectrum that has a reasonable chance of receiving an official allocation from the Federal Communications Commission and, in some cases, the International Telecommunications Union as well. By using an L-band frequency, the cost of receiver modifications should not increase substantially.17 Unfortunately however, because many of the proposed mobile satellite communication services (Iridium, Globalstar, and others) plan to use L- band frequencies, L-band frequency allocation is difficult to attain. In light of this potential problem, a preliminary assessment was undertaken to identify possible L-band frequencies that could be used for transmission of an additional GPS signal.18 Based on this preliminary assessment, it appears that several sub-bands have promise for the proposed signal, and several frequencies were selected as potential candidates. Although these frequencies are included in Table 3-5, in-depth investigation and coordination will be required before a specific frequency band, wide or narrow, can be selected. 17 The addition of an L signal would not affect the operation of existing receivers, but manufacturers would have to 4 modify future receivers (add another channel, and change the correlator and processor) to take advantage of a new L-band signal. If a frequency much greater than L-band is used, additional antennas would have to be added to the receivers, and the satellite transmitted power would have to increase. 18 A preliminary analysis of the L-band spectrum allocation that was conducted by Mr. Melvin Barmat, Jansky/Barmat Telecommunications Inc., Washington D.C., January 1994, is shown in Appendix I.
