Coherent Combination of GPS III L1 C/A and L1C Signals for GNSS Reflectometry

With the evolution of global navigation satellite systems (GNSSs), more GNSS satellites and civilian signals are available for GNSS reflectometry (GNSS-R). Developments of new onboard processing strategies can improve the observation performance of spaceborne GNSS-R. To this end, this article proposes a new processing method by coherently combining reflected global positioning system (GPS) III level 1 (L1) C/A and L1C signals. By exploiting the additional signal component, the signal-to-noise ratio (SNR) of the reflected signal can be significantly improved. Moreover, by taking advantage of the narrower autocorrelation function of the combined signal, the spatial resolution and the performance of geophysical applications can be significantly improved. The proposed method has been validated by processing cyclone GNSS (CYGNSS) raw intermediate frequency data, including the direct and reflected signals from GPS III satellites. The results indicate that the SNR of the combined reflected waveform can be improved by ~2 dB compared to the L1 C/A waveform. Moreover, the SNR of the combined signal can be improved more efficiently using a longer coherent integration interval compared to the L1 C/A signal. Preliminary altimetric results demonstrate a 35.3%-61.6% improvement in the ranging standard deviation and a 22.4%-64.4% improvement in the median absolute deviation compared to L1 C/A measurements. In addition, the correlation coefficient between combined measurements and wind speed improves by 26.3% on average compared to L1 C/A measurements and 45.7% for high winds. This article presents a novel GNSS-R onboard signal processing method with improved performance, which can provide a reference for the design of future GNSS-R instruments.