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pdfSpacecraft Reconnaissance of Asteroid and Comet Interiors ( 2015 )

Introduction: The National Radio Astronomy Observatory's Green Bank facility has several antennas available for spacecraft communications, Doppler tracking and radar, and provides unparalleled support infrastructure for data processing and recording equipment. The 100 meter Robert C. Byrd Green Bank Telescope (GBT) is currently capable of receiving signals at frequencies from 300 MHz to 86 GHz and has an aperture efficiency of over 30% at 100 GHz [1]. It is also the largest, steerable dish antenna in the world with sky coverage as far as -45 degrees declination. State of the art receivers on the GBT contribute to excellent system noise figures in the 0.3 to 0.5 dB range at frequencies below 40 GHz. This, in addition to the tremendous collecting area of the GBT, results in unprecedented sensitivity across the entire frequency range. The GBT has been used for spacecraft tracking in notable ESA and NASA missions. During its decent, the Doppler shift of the Huygens probe uplink to the Cassini spacecraft was tracked by the GBT after the probe’s launch from the Cassini spacecraft until its landing on the surface of the Saturnian moon Titan. Difficulties with the onboard Doppler wind experiment made the data collected by the GBT the primary source of information regarding wind velocities encountered during the decent of the Huygens probe [2]. The Doppler shift of the UHF links from the Phoenix Mars lander was tracked in a similar manner to aid in investigating Martian winds and to help determine the point of landing. After landing, the UHF links continued to be tracked, and showed the Doppler shift due to the relative motion of Mars with respect to the Earth. Numerous projects have used the GBT in bistatic radar experiments and include imaging of nearby asteroids [3] and hidden geological features below the Lunar surface [4]. A particularly exciting GBT radar result was the discovery of Mercury’s molten core [5]. In addition to the GBT is the NRAO 140-ft Telescope, a fully operational, 43 meter antenna useable through 27 GHz. This antenna is currently an Earth station for the RadioAstron project [6], performing wideband data and X-band Doppler tracking downlink tasks, and is capable of uplinking reference tones should the spacecraft’s onboard references become unreliable. Other fully operational antennas at Green Bank include the 20 meter antenna, which is capable of high slew rates and good efficiency to K-band and a 13.7 meter antenna with good efficiency to Ku-band. Three 26 meter antennas, which are currently not in operation, have good efficiency through Ku-band. Green Bank has extensive infrastructure to support signal processing and recording of received signals. Fiber optic links from all antennas to the Jansky Lab building are in place. A shielded control room suite provides climate controlled and secure locations for signal processing and recording equipment, a utility power feed of 12,000 kVA with high reliability, and protection of the site antennas from RFI through its location in and coordination of the National Radio Quiet Zone, provide users of Green Bank facilities with an unparalleled advantage in performing their missions and experiments.