Measurement of GPS Antenna Gain Patterns for CYGNSS Using a Spaceborne Antenna Range

The NASA Cyclone Global Navigation Satellite System (CYGNSS) mission is a constellation of eight small satellites that receive global positioning system (GPS) signals reflected off the earth to support a variety of science applications. For remote sensing, the strength of the reflected GPS signal contains important information and must be known to a small fraction of a dB. This makes it very different from traditional GPS navigation applications, in which small amplitude variations have minor or negligible impact depending on the accuracy requirements of the specific application. Satisfying mission requirements necessitates precise knowledge of the GPS satellite transmit antenna pattern and the receive antenna pattern on each CYGNSS satellite. In this article, we present the antenna calibration approach that is used by the CYGNSS science operations center. GPS satellite and CYGNSS satellite antenna patterns are jointly estimated using several years of on-orbit CYGNSS measurements. The orbiting GPS transmit and CYGNSS receive antennas form a spaceborne antenna measurement range, which can be utilized to determine their patterns. The accuracy of the estimated antenna patterns can then be quantified using CYGNSS GPS science data as a metric. When compared to the limited publicly available GPS satellite antenna data or antenna chamber measurements of the CYGNSS satellites, the proposed on-orbit approach is able to introduce corrections that significantly improve the CYGNSS mission science performance.