北斗GEO卫星微动条件下的GNSS-R土壤湿度反演

全球定位系统GPS发射的L波段导航信号源已被用于估测地球表面的土壤湿度，但由于卫星具有一定的重返周期，所以对固定区域土壤湿度反演的性能受到不断变化的星下点轨迹的限制。北斗GEO（geostationary earth orbit）卫星是地球静止轨道卫星，同时又存在着周期性的微动，利用它的反射信号来反演土壤湿度受到其微动特性的影响。针对这个问题，本文研究了利用GEO卫星反射信号功率进行固定区域内土壤湿度反演的可行性。本文首先分析了GEO卫星的微动特征和反射区域的变化；然后采用包络提取和三次多项式插值的方法来恢复SNR的趋势变化规律，以消除因为GEO卫星轻微运动带来的周期性波动和噪声，并且根据现场收集的部分实验数据确定的校准参数值对反射系数进行校正；最后，我们使用测试数据进行土壤湿度反演，得到每半小时固定区域的连续土壤湿度反演值。结果表明，土壤湿度反演值与原位土壤湿度值的变化趋势基本一致，反演的平均绝对误差小于4.63％。可见，BDS GEO卫星信号可以作为GNSS反射计的重要数据来源。

Soil Moisture Inversion Based on GNSS-R under the Micro-motion Condition of Beidou GEO Satellites

The L-band navigation signal source emitted by the Global Positioning System (GPS) has been used to retrieve the soil moisture in the surface of the Earth, but their retrieval performance is limited by their continuously varying footprints due to the repeat cycle of satellites. The BeiDou GEO satellites are geostationary orbit satellites but with slight periodic fluctuations, so the soil moisture retrieval using its reflected signal is affected by its slight movement characteristics. In response to this problem, we studied the feasibility of using BeiDou system(BDS) geostationary earth orbit(GEO) satellites reflected signal power to perform soil moisture retrieval in a fixed area. In this paper, we first analyzed the micro-motion characteristics of the GEO satellites and the changes of reflection area; then we restored the trend of SNR by envelope extraction and cubic polynomial interpolation to eliminate the periodic fluctuations and noise caused by the slight movement of GEO, and we also determined the calibrated parameter values by employing part of the experimental dataset collected in the field; finally we retrieved the continuous soil moisture in the fixed area per half hour using the test data. The results show that the estimated soil moisture changes are largely consistent with the in situ soil moisture data; and the mean absolute error of the estimation is less than 4.63%. It is concluded that the BDS GEO satellites represent an important source of data for GNSS reflectometry.