Performance Assessment of the Radio Occultation Experiment for Commercial off-the-shelf Receivers on VELOX-CI Microsatellite

VELOX-CI is the second micro satellite of Satellite Research Center ( SaRC) in Nanyang Technological University [ 1, 2]. Since its launch on Dec. 16, 2015, VELOX-CI has performed a total of 136 radio occultation ( RO) missions, and collected 448 hours' GPS data with the lowest penetration altitude of 6 km. In this paper the radio occultation performance of the VELOX-CI's COTS receiver and the space-borne receivers are compared in the upper troposphere ( 10 similar to 60km) in tropical region ( latitude between 30 degrees N and 30 degrees S). VELOX-CI is designed to explore the potential of using the commercial-off-the-shelf ( COTS) GPS receivers for RO mission for the first time. The satellite has a near equatorial orbit with an altitude of 550km. Three GPS antennas are located at the zenith, forward-velocity, and after-velocity directions. The tracked GPS signal phase, carrier-to-noise ratio C/N-0 are stored and sent to ground for post processing. The ionospheric profiles and preliminary tropospheric profiles are presented previously in [ 3, 4]. In this paper, the VELOX-CI's GPS RO payload is described. The RO data inversion procedures are introduced and a refractivity profile comparison study is made for refractive index obtained with VELOX-CI RO, radiosonde measurements and COSMIC data.