After separation from the launch vehicle, a spacecraft’s Guidance and Control system typically orients the spacecraft autonomously into a three-axis stabilized attitude for non-spinners. If an anomaly occurs, or if the spacecraft fails to orient itself appropriately, the Mission Operations team will want to observe spacecraft telemetry or may even be required to command the spacecraft before attitude stabilization. Antenna coverage in these cases is critical, since the command and control antenna may be rotating away from the ground station line-of-sight as the spacecraft tumbles. However, activating opposite-pointing hemispherical low-gain antennas on the spacecraft to obtain more complete angular coverage comes at the cost of presenting an interferometric signal to the ground station (or spacecraft receiver, depending on the radio-frequency subsystem configuration) which fluctuates as a function of the relative antenna positions and tumble rate of the spacecraft. Recent programs developed by The Johns Hopkins University Applied Physics Laboratory, specifically MESSENGER and STEREO, have investigated the capability of the Deep Space Network and Universal Space Network receivers to recover telemetry from a tumbling spacecraft at a low orbital altitude. Also investigated was the ability of a tumbling spacecraft utilizing a Small Deep-Space Transponder to register valid uplink commands, even in the presence of a second, closely-spaced uplink frequency, as expected for the two STEREO spacecraft.
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