Uplink array calibration via lunar Doppler-delay imaging

Coherent arraying of transmitting antenna signals at a distant spacecraft provides an effective gain proportional to N², where N is the number of antennas in the array. This 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. However, successful uplink arraying of X-band (~7.2 GHz) carriers transmitted by large 34 meter antennas separated by several hundred meters on the ground requires differential Doppler compensation and proper initial phasing to ensure coherent addition in the far-field of the array. This article describes a spacecraft-independent technique to phase-calibrate the array, where the calibration phases for two and three-antenna arrays are determined via lunar Doppler-delay imaging. This approach is required whenever the spacecraft range is large enough to render spacecraft-assisted calibration impractical due to long round-trip light times. The accuracy of the resulting calibration technique is evaluated, and long-term stability of the calibration phase-vector discussed in this article.