FPGA-based GNSS receiver design for reflectometry applications

Research has shown that Global Navigation Satellite System (GNSS) signals reflecting off the Earth’s surface can be detected by receivers in low Earth orbit (LEO). The weak reflected signal properties are analyzed to characterize geophysical properties such as soil moisture, sea surface height, ocean surface wind speed and sea ice detection. This method of remote sensing is known as GNSS reflectometry (GNSS-R).

Commercial GNSS receivers have historically been designed to only detect and process direct GNSS signals and cannot be repurposed to collect relevant data for reflectometry. Data collected by some orbital receivers have been made public; however, due to the high volume of data, data are truncated and are insufficient for science applications. Therefore, to be able to develop and test new algorithms, a custom GNSS-R receiver is designed and implemented.

The developed GNSS-R receiver is implemented using field programmable gate array (FPGA) technology. The GNSS-R receiver prototype uses 1-bit signal resolution resulting in a compact design requiring minimal FPGA resources. Several field results show that the receiver prototype can successfully track direct and reflected GNSS signals in real-time. The observations indicate that the carrier-to-noise density ratio of signals reflecting from the surface of water was on average approximately 7 dB higher than the C/N_0 recorded when tracking land reflections. The difference in C/N_0 between water and land reflections is significant enough to conclude that a GNSS-R receiver using a one-bit quantization GNSS signal can be used for reflectometry applications.

To increase the sensitivity of the receiver to weak reflected signals (-140 dBm), the FPGA-based signal processing module is enhanced using the alternate half-bit method. The receiver sensitivity improved from -35 dB to -46 dB (signal-to-noise ratio).

Another challenge for GNSS-R receivers is generation of high-resolution delay Doppler maps (DDMs). DDMs provide insight into the reflecting surface characteristics based on signal scattering. The developed GNSS-R receiver demonstrates that high resolution DDMs can indeed be generated in real-time using 1-bit GNSS signal resolution. Data collected by the CYGNSS spacecraft are used to test and validate the receiver implementation. Results show that distinct DDMs are generated for land and water reflections and correlates (in terms of SNR) with research conducted using 2-bit signal resolution.