Characterization of GPS L1 EIRP: Transmit Power and Antenna Gain Pattern

Abstract

The GPS Equivalent Isotropically Radiated Power (EIRP), defined as the product of transmit power and antenna gain pattern, determines the directional radio frequency (RF) power incident on the Earth surface. EIRP is of great importance to the Level 1B calibration of the normalized bistatic radar cross section (NBRCS) of the Cyclone Global Navigation Satellite System (CYGNSS) mission. To address the uncertainties in EIRP, a ground-based GPS constellation power monitor (GCPM) system has been designed, built, calibrated, and operated to accurately and precisely measure the direct GPS signals. The calibration subsystem and low noise amplifier (LNA) are implemented on a PID (proportional–integral–derivative) controlled thermal plate with extremely stable temperature over the long term. The calibrated GCPM received power is highly repeatable and has been verified with DLR/GSOC’s independent measurements. The transmit power of the full GPS constellation is determined using an optimal search algorithm. Updated values for GPS transmit power have been successfully applied to CYGNSS L1B calibration and found to significantly reduce the PRN dependence of CYGNSS L1 NBRCS and L2 science data products, including the ocean surface wind speed and mean square slope (MSS). Additionally, a complementary technique of estimating the GPS EIRP has been designed using direct signal measurements from the CYGNSS zenith antenna to fully map all GPS transmitters. The CYGNSS zenith antenna measurements are demonstrated to be able to sample the transmit antenna gain pattern over the complete terrestrial service volume within a very short time. An absolute power calibration algorithm was designed using the CYGNSS GPS signal simulator (GSS) and CYGNSS Delay-Doppler Mapping Receiver (DMR) to convert the raw counts to received power in watts. By incorporating zenith antenna measurements, the uncertainty in GPS EIRP due to transmitter antenna pattern asymmetry can be reduced. The CYGNSS zenith antenna and receiver as a real-time GPS power monitor are also a powerful tool to capture any abrupt change in the GPS EIRP that may occur. The CYGNSS four-channel zenith signal, in combination with a yaw attitude correction algorithm, is used to retrieve the full transmit antenna pattern for the GPS constellation. The system design and absolute power calibration scheme of the GCPM are helpful to the future design and implementation of GNSS receiver systems. The calibrated GPS transmit power and full transmit antenna pattern will be useful to the system design, science investigation and engineering calibration of future GNSS reflectometry missions.