Evaluating the Performance of China’s Tianmu-1 GNSS-R Satellite Constellation Observations in Land and Ocean Remote Sensing Applications

China's Tianmu-1, equipped with the Global Navigation Satellite System Reflectometry (GNSS-R) payload, has formed a constellation of 22 small satellites since its batch launch in 2023. Since the TM-1 constellation has the largest number of GNSS-R satellites to date, TM-1 observations provide powerful samples from the Earth's surface. This study is the first to utilize ten months of TM-1 GNSS-R observations for land (soil moisture (SM), freeze-thaw (F/T), waterbody) and ocean (sea ice, sea surface winds, and significant wave height (SWH)) parameter retrievals. Compared with the reference satellite data, the root mean square error (RMSE) and correlation (R) of the TM-1 SM retrieval were 0.035 cm3/cm3and 0.61, respectively. Compared with in-situ data, the RMSE and R of the TM-1 SM retrieval were 0.050 cm3/cm3and 0.83, respectively. The accuracies of the TM-1 F/T retrievals were 96.5% and 92.0% compared with the reference satellite data and in-situ data, respectively. The accuracy of TM-1 water body detection exceeded 97% when compared to optical remote sensing products. TM-1 sea ice detection accuracy exceeds 98% compared to microwave radiometer products. The RMSE for TM-1 sea surface wind speed is 2.09 m/s with an R of 0.82, and the RMSE for SWH is 0.65 m with an R of 0.86. Additionally, the RMSE of TM-1 wind speed retrievals compared to NDBC buoy wind speeds is 1.84 m/s, with an R of 0.82. The RMSE of TM-1 SWH compared to the NDBC SWH is 0.46 m, with an R of 0.86. Both land and ocean remote sensing retrievals demonstrate the excellent performance of the TM-1 GNSS-R constellation. As a result, the TM-1 GNSS-R constellation holds promise as a valuable tool for Earth observation.