The Global Positioning System has a satellite NAVSTAR at an orbit altitude of 10,900 miles. The SPS beam geometry at NAVSTAR orbit altitude is shown in Figure 3. The power coupling modes are diagrammed in Figure 4. Coupling to sensors and communications is similar to the LANDSAT. There can be direct coupling through the thermal control louvers that control the temperature of the principal electronic functions; clock, computer, and command/control receiver and decoder components. Induced jitter in the internal clock and message decoder logic is estimated to be in the 10 percent to 65 percent range for SPS power coupling of 10 watts to 25 watts. The S-band communication receiver and associated processor would experience an increase in BER in the range of 50 to 1,000 times with the antennas exposed to SPS power densities to 10 mw/cm2 to 100 mw/cm2. The mitigation techniques would be very similar to LANDSAT. The space telescope is in circular orbit some 312 miles in altitude. Communications with earth are via TDRSS. The telescope is a Cassegrain configuration and other on-board instrumentation under study are: wide field/planetary camera, faint object spectograph, faint object camera, high resolution spectrograph, and high speed photometer/polarimeter. Figure 5 shows degradation in resolution for a 512 element array, charge coupled device (CCD) similar to one of the imaging devices on board space telescope. SPS effects and mitigation techniques for all system on board are under study. Effects through the sun sensors for satellites in general are insignificant; approximately 2 percent increase in noise, primarily because of SPS harmonics would be present. This noise would cause less than 2° to 5° orientation change of the solar panels over a period of 1 to 1.3 seconds of maximum SPS beam exposure, and be corrected within 2 to 5 seconds after the satellite departs the SPS beam. Figure 6 shows attitude error versus time for a star tracker sensor system used for satellite attitude stabilization. The lower curve shows the normal response as the system settles in with no outside influence. The upper curve shows the attitude error in arc-seconds where a satellite is in a stable position and the star tracker illuminated with a 15 mw/cm2, 2.6 GHz microwave signal. Since the satellite will be in the beam only short time, there may be between 3 to 8 arc- seconds of error introduced during a passage through the SPS beam, but as the lower curve shows this will settle out in 5 to 8 seconds after leaving the beam. The analysis of data from the above satellites will be extrapolated into guidelines for future satellites indicating the character of degradation expected for proposed electronic elements. This will include specifications regarding the physical configuration and testing procedures pointing toward satisfactory performance of future satellites operating in an SPS environment.
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