The Moon as a Calibration Load for the Breadboard Array

The calibration of radio antenna performance is a well-understood process. For system noise measurements in the current Deep Space Network (DSN), a noise diode is used along with an ambient absorber that is placed over the feed to make a direct measurement of the system noise as well as to characterize the diode. For future arrays, consisting of a large number of antennas, it will be necessary to minimize the amount of hardware, electronics, and time spent on each antenna. This work describes an effort to use the moon as the standard calibrator for the system noise injection diode. This work was inspired by the recent demonstration of consistency of very accurate pre-existing lunar brightness temperature maps, physical optics characterization of the 34-m beam-waveguide antenna patterns, and Mooncentered measurements at the DSN’s research and development antenna, DSS 13. The brightness temperature maps and antenna patterns were convolved to yield expected antenna temperatures for comparison with the Moon-centered observations. Due to limitations caused by atmospheric effects, the absolute calibration of the system diode will have larger error bounds as compared with the use of an ambient aperture load. However, results are very adequate for rapid, ongoing checks of the health and noise level of the receiving system. We describe the proposed method as used on the DSN Array breadboard antennas, with results and error analyses.