The Global Positioning System: A Shared National Asset (1995)
Chapter: Appendix L Enhanced Signal Structures for the Military
Appendix L
Enhanced Signal Structures for the Military
A significant increase (approximately 10 dB) in anti-jam capability could possibly be achieved on the Block IIF satellites by employing another wide-band signal, occupying perhaps 100 MHz to 200 MHz. Such a broad signal would require that the carrier be at Sband (approximately 3 GHz) or higher frequency. The move to a higher frequency also would reduce nulling antenna size and increase its performance. Such a high frequency would also provide increased immunity to the effects of ionospheric scintillation, which can degrade receiver performance when it is present.1
To demonstrate the anti-jam effectiveness of a wide-band, fine ranging signal, calculations for seven possible signal scenarios (with various bandwidths, antennas, and inertial aiding) have been performed for jammers operating at power levels of 100 watts and 10 kilowatts. In each case, the jammers were assumed to be co-located with the target. At these two power levels, code- and carrier-tracking thresholds were estimated as a function of range from the jammer. For many applications, the key parameter is not the minimum range for signal lock, but the minimum range for acceptable range error. Therefore, the minimum range-to-jammer for a 1-meter range error was also determined. It is important to distinguish two quite different operating scenarios: direct attack and loiter. In direct attack, the range-to-target is closed as rapidly as possible. Once GPS is lost, guidance to the target is by inertial guidance alone. Mission success then depends upon the remaining distance to target as well as the inertial drift rate. By contrast, in loitering scenarios such as remotely piloted vehicle reconnaissance and other scenarios involving sustained area-wide high accuracy, loss of GPS means loss of high accuracy positioning, as inertial drifts can quickly exceed mission error bounds.
Table L-1 summaries the seven signal scenarios. Scenario 1, 2, and 3 with Y-code signaling (20-MHz bandwidth) were considered as baseline for comparison with the other scenarios, each with a 100-MHz chipping rate (200-MHz bandwidth). A high chipping rate direct-sequence modulation was chosen to improve both the jamming margin and pseudorange accuracy. Under the assumption that a wide region of the L-band would be hard to come by and that beam-forming antenna structures are large at L-band, a fourfold
