Study on the impact of hardware delay on spaceborne GNSS-R interferometric altimetry

A semi-physical simulation test system was designed to realistically simulate the spaceborne GNSS-R interferometric altimetry process. The system comprises an actual GNSS-R interferometric altimetry receiver, a GNSS simulator hardware system, and pre-simulated control command files for GNSS simulator signal generation.The GNSS simulator, manufactured by Spirent Communications in the UK, can simulate direct GNSS satellite signals in spaceborne observation scenarios. By reading fixed-format command files, it generates reflected signals with specified relative delays, Doppler shifts, and power levels. This experiment primarily focuses on the BeiDou Navigation Satellite System. Due to limited BeiDou code types available in the GNSS simulator, the B1 frequency uses B1C code (center frequency 1.57542 GHz), while the B2 frequency employs B2A code (center frequency 1.191795 GHz).The GNSS-R interferometric altimetry receiver was developed by the National Space Science Center of the Chinese Academy of Sciences and is compatible with the BeiDou Navigation Satellite System (BDS), Global Positioning System (GPS), and Galileo Navigation Satellite System (GAL). During the semi-physical simulation test, the receiver simultaneously processes three signals generated by the GNSS simulator: 1) GNSS satellite positioning signals for receiver self-positioning and reflection signal satellite selection; 2) GNSS satellite direct signals for interferometric processing; and 3) GNSS satellite reflected signals for interferometric processing. The receiver's primary output observation data is dual-frequency interferometric delay waveform maps (Delay Mapping, DM). Each waveform contains 448 delay sampling points with a sampling rate of 112 MHz, resulting in a sampling delay interval of approximately 2.6767 meters. The interferometric delay waveform maps are generated at a frequency of 5 Hz.