Coastal GNSS-R Code Delay Altimetry Using GPS L5 Signals

In recent years, with the gradual completion of different GNSS constellations, the available L-band navigation signals have become more abundant. Not only can they be used for PNT (Positioning, Navigation, and Timing), but the reflected signals can also be used for retrieving a variety of geophysical parameters such as sea surface height. Coastal GNSS-R (Global Navigation Satellite System Reflectometry) code delay altimetry obtains the path delay measurements between reflected signal and corresponding direct signal based on ranging code. And the height from sea surface to antenna can be calculated according to the geometric relationship of sea surface altimetry. In order to achieve path delay with higher accuracy, we performed coastal GNSS-R altimetry experiment on a trestle using GPS L5 signals with high ranging code rate. The digital IF (intermediate frequency) data at 25 MHz sample rate were collected during the experiment. Then, self-developed software-defined receiver was used to process the IF data. As the code modulation methods of in-phase (I component) and quadrature-phase (Q component) on GPS L5 carrier are different, we tested the accuracy of I and Q component for sea surface altimetry, separately. In addition, considering different coherent integration time have a certain impact on the accuracy, 1 ms and 10 ms coherent integration time are adopted to compute the path delay based on I and Q component. The results show: for 1 ms coherent integration, the RMSE of I and Q component are 0.70 m and 0.61 m, respectively; while the coherent integration time is 10 ms, the RMSE of I and Q component are 0.54 m and 0.43 m, respectively. To sum up, increasing coherent integration time can improve the accuracy; the altimetry accuracy of the Q component signal is better than that of I component signal.