Uplink Arraying Experiment with the Mars Global Surveyor Spacecraft

For the first time ever, a pair of 34-m antennas was employed to simultaneously transmit and combine 7.15-GHz (X-band) carriers to a spacecraft in deep space, yielding up to four times greater signal power than either antenna separately. Antenna arrays have the potential to synthesize the equivalent of a very large antenna aperture in a robust and cost-effective manner, by appropriately combining the signals from a large number of smaller antennas. Prior to this experiment, only single 34-m or 70-m antennas had been used to uplink signals and commands to spacecraft. During this experiment, the Mars Global Surveyor spacecraft received the sum of the two carriers, measured the combined power level, and verified the predicted array gain resulting from the combining operation. Although great care had to be exercised to minimize differential Doppler between the two signals due to Earth rotation, it was demonstrated that the combined signal remained stable on timescales of 10 to 20 minutes, enabling the application of novel uplink array calibration algorithms, which may enable routine calibration of uplink arrays consisting of a large number of antennas in the future.