Design, Development, and Prelaunch Calibration of a Low Cost 118.75 GHz Temperature Sounding Radiometer for CubeSat Missions

• Main Author: Periasamy, Lavanya
• Format: Text
• Language: unknown
• Published: CU Scholar 2019
• Subjects: 3D printed corrugated horn; CubeSat radiometer; Microwave filterbank; Offset reflector and feed; Prelaunch antenna calibration; Prelaunch radiometer calibration; Electrical and Computer Engineering; Other Electrical and Computer Engineering; Antarc*; Antarctica; National Snow and Ice Data Center
• Online Access: https://scholar.colorado.edu/eeng_gradetds/31; https://scholar.colorado.edu/cgi/viewcontent.cgi?article=1031&context=eeng_gradetds

The 118.75 GHz eight-channel, double-side-band scanning temperature sounding radiometer “MiniRad” for CubeSat missions is intended to serve as a demonstrator for a constellation of low cost, quick turn-around millimeter wave and higher frequency passive sounders and imagers for weather forecasting at high spatial and temporal resolution. This radiometer payload, built at the Center for Environmental Technology in partnership with the Colorado Space Grant Consortium and the National Snow and Ice Data Center at the University of Colorado at Boulder, can provide a 3D temperature map from the earth's surface to an altitude of 18~km. For precise prelaunch antenna calibration, an HE11 mode full wave electromagnetic field analysis was developed in Matlab for determination of an optimal feed horn and offset paraboloidal reflector geometry such that the main beam and spillover efficiencies of the system are maximized, and these and the antenna phase center location that maximizes phase efficiency are precisely known. Results from this analysis were also compared with HFSS and GRASP simulations of the antenna subsystem. The efficacy of employing a 3D-printed corrugated conical horn, operable between 110 and 127 GHz, as the feed for the reflector was addressed due to its very low cost and rapid manufacturability. Horn measurements indicated a reflection coefficient below -15 dB and an 89% average spillover efficiency at the main reflector subtending a 16° half-angle. The need for a compact intermediate frequency spectrometer for operation between 50 MHz and 7 GHz resulted in the design and development of an eight-channel lumped element filterbank with bandwidths between 0.25 and 2.2 GHz. Laboratory experiments implemented to characterize the MiniRad helped in achieving radiometer sensitivities close to theoretical limits. Initial performance obtained from airborne measurements over Antarctica during the NASA Operation IceBridge experiment in Oct-Nov 2016 suggested a well-focused scanning antenna subsystem and good separation between the radiometer channels. After final system integration, measurements obtained from prelaunch experiments indicated the antenna 3-dB beamwidth to be broader by ~0.1° compared to the idealized simulated pattern, and radiometer sensitivities that agreed to better than 0.5 K with theoretical estimates across all eight channels.