THE EMC IMPACT OF SPS OPERATIONS ON LOW EARTH ORBIT SATELLITES W. B. Grant and E. L. Morrison, Jr. Institute for Telecommunication Sciences K. C. Davis Battelle Memorial Institute - Northwest Laboratories Low orbiting (LEO) satellites have a significant probability of passing through an SPS main beam or principal sidelobes. This probability, and consequently the frequency of traversal, depends on the number of SPS stations in operation. Operational effects for LEO satellites depend on orbits, equipment complement and usage modes, and vehicle physical configuration. Existing and planned LEO systems include remote sensing, navigation and position fixing, and communications functions. Sensors include electro-optical devices, active and passive microwave systems, and particle detectors. The susceptibility of various operational and planned LEO satellites have been examined during the course of the SPS EMC evaluation program. Functional degradation for the electronic systems on LANDSAT, GPS, and the Space telescope is described in relation to the amplitude of the SPS illumination components. Analyses and tests include the modes of coupling to devices and subsystems, and performance effects in relation to satellite mission. The SPS energy coupling into LANDSAT subsystems is indicated in Figure 1. As diagrammed, the communications, sensor, power bus, and attitude control functions can be effected. Coupling would occur through the communications antennas, attitude sensor optical apertures and the optical apertures and thermal louvers of the multi spectral scanner (MSS) and thermatic mapper (TM). Energy coupling through the solar panels to the power units, which would transmit noise to the on-board computers and instrumentation, is not a problem because of circuit filtering and regulation presently designed into the systems. Exposed area and scanner locations on the satellite indicate that the optical aperture is the principal SPS energy coupling mode for these sensors. Figure 2 shows the SPS microwave beam geometry at LANDSAT orbit altitude of 704 km. From this field intensity estimation coupling energies can be calculated. From tests and analysis it was determined that at SPS main beam energy levels an increase in video channel noise of 8 percent was present in the TM/MSS instrumentation, and a decrease in modulation transfer function of 18 to 20 percent was induced which affects the spatial imaging capability by approximately 20% These data are still being analyzed to determine the complete effects. For the direct earth-station to satellite S-band link, a bit error rate (BER) increase of 70 to 85 percent would occur during a period of about 16 seconds while the satellite was exposed to the SPS main beam and principal sidelobes. For the wide band communication channel used to transfer information via the tracking and data relay satellite system (TDRSS) the BER would increase by 20 to 40 percent during a pass through the power beam. Mitigation techniques to be investigated include rejection filters and antenna modifications. For the TM and MSS, mitigation techniques to be confirmed include circuit filters, noise extraction in the data analysis process, and extended shielding for the detectors and colocated video amplifiers. Additional shielding for the video channel and scan control circuitry is recommended to eliminate jitter in the line scans, if the coupling is proven to be directly into these circuits and not through internal common connections.
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