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摘要:
对利用北斗导航系统地球静止轨道（GEO）卫星干涉信号功率反演土壤湿度的方法进行研究。针对现有研究主要建立纯经验模型反演土壤湿度、反演方法单一的问题，提出一种半经验的反演方法。所提方法针对北斗GEO卫星星地之间几何构型较为稳定的特点，利用相邻两天的干涉幅度信息抵消发射信号功率的影响，实现了土壤反射系数的反演。基于交叉极化影响下的反射系数理论模型构建了一种半经验的土壤湿度反演模型，通过仿真与实验对所提方法和模型进行验证。仿真结果表明：所提方法和模型能更好地适应反演过程中出现的非线性情况。实验结果表明：土壤湿度反演的均方根误差小于0.02 cm3/cm3，相关系数超过0.8。
Abstract:Soil moisture retrieval using Beidou geostationary Earth orbit (GEO) interference signal power was studied. Current researches mainly establish empirical models for retrieving soil moisture. Therefore, a semi-empirical retrieval method was proposed. Taking the advantage of Beidou GEO satellite’s stable geometric configuration relative to the earth, this method utilized the interference amplitude metric obtained in two consecutive days to cancel the influence of the power of the transmitted signal, therefore, achieving the retrieval of the reflection coefficient. Then, based on the reflection coefficient theoretical model under the influence of cross-polarization , a semi-empirical soil moisture retrieval model was constructed. Finally, the proposed method and model are validated through simulations and experiments. The simulation results showed that the proposed method and model can better adapt to the nonlinear situation encountered in the retrieval process. Finally, the experiment results show that the root mean square error of soil moisture retrieval is less than 0.02 cm3 ·cm−3, and the correlation coefficient exceeds 0.8.
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图 2 RHCP与LHCP分量反射系数模值
Figure 2. Modulus of RHCP and LHCP component reflection coefficient.
图 4 交叉极化影响下的GEO卫星载噪比仿真
Figure 4. Simulation of GEO satellite C/N0 under effect of cross-polarization
图 5 无交叉极化影响下的GEO卫星载噪比仿真
Figure 5. Simulation of GEO satellite C/N0 without effect of cross-polarization
图 11 4号星的反射系数反演结果
Figure 11. Retrieval results of reflection coefficient for satellite No.4
图 14 不同训练数据量下的反演性能
Figure 14. Retrieval performance under different number of training data
表 1 土壤湿度反演仿真结果统计
Table 1. Statistical results of simulation of soil moisture retrieval
极化情况 反演模型 均方根误差/
（cm3·cm−3）相关系数 有交叉极化 半经验模型 0.012 3 0.9962 一阶模型 0.083 2 0.8090 二阶模型 0.0793 0.8297 无交叉极化 半经验模型 0.0145 0.9959 一阶模型 0.0346 0.9741 二阶模型 0.0324 0.9775 
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表 2 实验期间GEO卫星高度角变化情况
Table 2. Variation of GEO satellite elevation angle during experiment
(°) 卫星号 最低高度角 最高高度角 1 36.7389 39.5104 2 31.8213 34.3298 3 42.4344 44.8786 4 25.6247 27.0136 5 14.7317 16.9901
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