An Adaptive Integration Algorithm for Improved Coherent Reflection Measurement in GNSS-R Instruments

Current GNSS-R instruments, such as those used aboard the CYGNSS and TDS-1 satellites, form delay-Doppler maps (DDMs) using fixed integration schemes. The reflected GNSS signal is coherently integrated with a local replica signal and then non-coherently integrated to form a DDM measurement. For scenes dominated by diffuse scattering (i.e. most land and ocean surfaces at typical incidence angles), this approach works well given the short reflected signal coherence time that does not vary significantly over the typical range of incidence angles. However, the coherence properties of the reflected signal change significantly in some circumstances, such as over inland water bodies, coastal areas, wetlands, and at grazing incidence angles over the ocean and land. In this study, we investigate the possible benefits of a receiver adapting its coherent integration time to the instantaneous properties of the reflected signal. The algorithm proposed uses the complex DDM samples to estimate the coherence time efficiently on-the-fly. These estimations are then used to adapt the coherent integration time in the receiver to form DDM measurements. Example results using raw signal data collected from CYGNSS will be presented.