Three-Antenna Doppler-Delay Imaging of the Crater Tycho for Uplink Array Calibration Applications

Uplink arrays have the potential to greatly increase NASA’s deep-space command and communications capabilities and to provide greater reach during spacecraft emergencies. However, due to long round-trip light times, it is not practical to phase up and track the carrier phase of each antenna element using real-time measurements at the spacecraft. This article describes a method for utilizing the Moon as a natural far-field reflector for calibrating uplink arrays by extending well-known Doppler-delay radar imaging techniques to an array of transmitting antennas. A small, highly reflective feature on the Moon is selected as the calibration target, and the phase vector corresponding to maximum reflected power from the target is selected. The theoretical basis of “Moon-bounce” calibration is presented, and experimental results obtained with arrays of up to three 34-m antennas of the Deep Space Network are described.