Uplink array concept demonstration with the EPOXI spacecraft

Uplink array technology is currently being developed for NASA's Deep Space Network (DSN), to provide greater range and data throughput for future NASA missions, including manned missions to Mars and exploratory missions to the outer planets, the Kuiper belt, and beyond. The DSN uplink arrays employ N microwave antennas transmitting at X-band to produce signals that add coherently at the spacecraft, thereby providing a power gain of N 2 over a single antenna. This gain can be traded off directly for N 2 higher data rate at a given distance such as Mars, providing for example HD quality video broadcast from earth to a future manned mission, or it can provide a given data-rate for commands and software uploads at a distance N times greater than possible with a single antenna. The uplink arraying concept has been recently demonstrated using the three operational 34-meter antennas of the Apollo complex at Goldstone, CA, which transmitted arrayed signals to the EPOXI spacecraft. Both two-element and three-element uplink arrays were configured, and the theoretical array gains of 6 dB and 9.5 dB, respectively, were demonstrated experimentally. This required initial phasing of the array elements, the generation of accurate frequency predicts to maintain phase from each antenna despite relative velocity components due to earth-rotation and spacecraft trajectory, and monitoring of the ground system phase for possible drifts caused by thermal effects over the 16 km fiber-optic signal distribution network. This paper provides a description of the equipment and techniques used to demonstrate the uplink arraying concept in a relevant operational environment. Data collected from the EPOXI spacecraft was analyzed to verify array calibration, array gain, and system stability over the entire 5 hour duration of this experiment.