Development of the Harry v3 GNSS-R Instrument

GNSS reflectometry is a method for measuring Earth surface properties by measuring the GNSS signals reflected from the Earth surface. By using GNSS reflectometry, surface properties, such as ocean wind speed and direction and soil humidity, can be known. Additionally, the absolute delay between the direct and reflected signal can be used for altimetry and target detection. This method have been proven by multiple satellite missions, such as TechDemoSat-1 from UK Space Agency and CYGNSS from NASA. The Australian Centre for Space Engineering Research (ACSER) are currently developing Harry v3 GPS-R instrument, with the aim of flight trial in CUAVA-2 cubesat. This instrument utilises two ACSER-developed Kea FPGA-based GPS receivers. One receiver provides a PNT solution and a shared clock for the entire system, while the other receiver is modified to generate delay-Doppler maps from the reflected signal. Additionally, the receivers can be configured to dump decimated IF samples through the USB interface. These receivers are controlled by Raspberry Pi Compute Module 4, which provides receiver control and reflection correlator steering, data storage in onboard SSD and interface to CUAVA-2 satellite bus over UART. Harry v3 are tested using replayed RF signals from airborne mission, and live signals from outdoor antenna. It generates DDMs with shape and signal strength that represents the reflection surface. However, the delay between direct and reflected signal has large variation, which reduces altimetry precision to 300m. This may be caused by imperfect synchronisation between the two receivers.