Abstract
Soil moisture is an important geophysical parameter for studying terrestrial water and energy cycles. It has been proven that Global Navigation Satellite System Interferometry Reflectometry (GNSS-IR) can be applied to monitor soil moisture. Unlike the Global Positioning System (GPS) that has only medium earth orbit (MEO) satellites, the Beidou Navigation Satellite System (BDS) also has geosynchronous earth orbit (GEO) satellites and inclined geosynchronous satellite orbit (IGSO) satellites. Benefiting from the distribution of three different orbits, the BDS has better coverage in Asia than other satellite systems. Previous retrieval methods that have been confirmed on GPS cannot be directly applied to BDS MEO satellites due to different satellite orbits. The contribution of this study is a proposed multi-satellite soil moisture retrieval method for BDS MEO and IGSO satellites based on signal-to-noise ratio (SNR) observations. The method weakened the influence of environmental differences in different directions by considering satellite repeat period. A 30-day observation experiment was conducted in Fengqiu County, China and was used for verification. The satellite data collected were divided according to the satellite repeat period, and ensured the response data moved in the same direction. The experimental results showed that the BDS IGSO and MEO soil moisture estimation results had good correlations with the in situ soil moisture fluctuations. The BDS MEO B1I estimation results had the best performance; the estimation accuracy in terms of correlation coefficient was 0.9824, root mean square error (RMSE) was 0.0056 cm3cm−3, and mean absolute error (MAE) was 0.0040 cm3cm−3. The estimations of the BDS MEO B1I, MEO B2I, and IGSO B2I performed better than the GPS L1 and L2 estimations. For the BDS IGSO satellites, the B1I signal was more suitable for soil moisture retrieval than the B2I signal; the correlation coefficient was increased by 19.84%, RMSE was decreased by 42.64%, and MAE was decreased by 43.93%. In addition, the BDS MEO satellites could effectively capture sudden rainfall events.
Highlights
Soil moisture is a fundamental variable in the study of terrestrial water and energy cycles [1,2,3,4]
In the procedures of data processing, it should be noted that the orbital periods of Global Positioning System (GPS) (GPS satellites all belong to the medium earth orbit (MEO) satellites), Beidou Navigation Satellite System (BDS) MEO, and BDS inclined geosynchronous satellite orbit (IGSO) satellites are, respectively, about 12 h, 14 h, and 1 d
This means GPS and BDS IGSO satellites appear near the same locations in the sky every day, while BDS MEO satellites appear at different locations in the sky within seven days and repeat on the eighth day
Summary
Soil moisture is a fundamental variable in the study of terrestrial water and energy cycles [1,2,3,4]. Soil moisture retrieval based on SNR observations was first studied by Larson et al [17,18]; they demonstrated that soil moisture can be obtained by the Global Positioning System (GPS) multipath signals. The BDS consists of MEO, GEO, and IGSO satellites which provide better coverage of satellites in Asia than other satellite navigation systems [23] In this context, research on GNSS-IR based on the BDS is significant.
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