A Multioctave 8 GHz--40 GHz Receiving System for Astrometry and Navigation

NASA's deep space spacecraft are navigated to their intended targets via astrometric measurements relative to the Celestial Radio Reference Frame. There is an increased emphasis on the use of K band (25.5 GHz--27 GHz) and Ka band (31.8 GHz--32.3 GHz) communications, both to increase the downlink data rates and because of spectral congestion and interference at lower frequencies. Onboard a spacecraft, the same equipment used for communications is used also for generating the radio signals for navigation, with an aspect of deep space navigation involving relative astrometry between the spacecraft and International Celestial Reference Frame (ICRF) sources. The IAU recently adopted the third ICRF, which now includes observations at K- and Ka bands as well as the legacy S-X--reference frame. We describe a project to develop a prototype wide-band receiver system for the Very Long Baseline Array (VLBA) that covers from 8 GHz to 40 GHz simultaneously, and that would provide the capability to observe reference frame sources at the deep space communication K- and Ka bands. The wide-band receiving system consists of a quadridge feed horn, a cryogenic package containing wide-band low-noise amplifiers (LNAs), and a downconverter to transform the received signals to the VLBA's signal transport system. The receiving system was tested at the Jet Propulsion Laboratory, California Institute of Technology, then deployed to the VLBA's Owens Valley antenna. We describe its performance, which includes detection of fringes in observations with other VLBA antennas.