Antenna Digital Beamforming on Spire's GNSS-Reflectometry CubeSat Constellation

Spire operates a constellation of satellites that perform remote sensing of the Earth using the Global Navigation Satellite System (GNSS) signals. Spire's STRATOS payload has been modified for sensing the Earth's surface using GNSS-Reflectometry (GNSS-R). This has found successful application in sensing the ocean wind / wave state and for retrieving volumetric soil moisture amongst others physical properties. In this paper the motivation for using antenna digital beamforming is described. The design elements that have been added, include the internal calibration, the digital signal processing and phase control. The calibration method uses the injection of a spread-spectrum signal into each of the radio inputs. This allows for absolute and relative calibration of signal amplitude and phase. A novel RF coupling device is devised to make a realisable RF coupler multitude of input channels. The scheme is tested on a two antenna system and the expected 3 dB improvement in receiver sensitivity is demonstrated.