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TECHNICAL ENHANCEMENTS FOR FUTURE CONSIDERATION 133 conflict, the desired signal power at the receiver could be increased. For example, if a 3-meter, steerable-reflector L-band antenna (or phased-array antenna) could be added to the spacecraft, then a gain of approximately 20 dB would be obtained, which would increase range-to-jammer penetration by a factor of 10.12 While adding a 3- meter steerable antenna to the GPS satellites is a very significant change with attendant complexity, weight, and cost penalties, this is clearly a preferable approach to simply boosting the overall L2 transmitter power. In summary, in addition to increasing the L2 transmitted power, military anti-jam capabilities can be further improved by using a new, very wide-band signal (approximately 200 MHz), a spot beam, or some combination of both. ENHANCEMENTS FOR HIGH-PRECISION USERS GPS Transmit Antenna Calibration High-accuracy users of GPS rely on differential carrier phase measurements to obtain millimeter- to centimeter-level results. High accuracies are obtained because for the differential measurements, most satellite- based errors are common mode errors and cancel in the differencing process. One error, however, that does not cancel is the error due to variations of the effective location (phase center) of the transmitting GPS antenna. These variations are a function of the angle to the user, primarily the angle off the GPS antenna array boresight. Satellites that require precise orbit determination, such as Topex/Poseidon, are vulnerable to this error because the satellites view the GPS antenna from large angles off boresight. The maximum boresight angle to receivers on the ground is about 13 degrees, while the angle to a satellite in an orbit at 1,300 kilometers altitude is about 17 degrees. Variations of a few centimeters in the GPS transmit antenna phase center would induce variations of about 10 centimeters in the altitude of the Topex orbit. Even for ground-based measurements, these effects may contribute a small (~10-9 x baseline length) error. Phase variations are expected to be much greater at larger boresight angles. For such applications, knowledge of the transmit antenna phase variations is needed to reliably obtain centimeter, or subcentimeter, accuracy. By measuring the transmit antenna phase center, the error currently limiting accuracy of very-high precision users can easily be eliminated. Since elaborate antenna measurements are already being made prior to launch, it should be relatively simple to make the measurements required to determine the actual phase center.13 12 Since the nominal GPS antenna has a gain of + 11 dBiC L and + 13 dBiC L , at 14.3 degrees off axis, the benefit of the 2 1 postulated spot beam is about 20 dB. 13 B. R. Schulper, B. L. Allshouse, and T. A. Clark, ''Signal Characteristics of GPS User Antennas," Navigation: The Journal of the Institute of Navigation 41, no. 3 (1994).
