Retrieving Of Soil Moisture Using GPS Signal To Noise Ratio Observations

Soil moisture plays a key role in natural environment.Soil moisture provides an important observation window for monitoring regional hydrological fluctuations.Soil moisture variations are one of the key factors that have an impact on the response of ecosystems to climatic change.The stable and precise soil moisture monitoring provides an important monitoring tool for hydrological,climatic,agricultural and disaster prevention and control in a region.Common soil moisture monitoring methods such as on-site sampling are accurate but they are time and labor consuming and unfriendly to environmental protection.Electromagnetic probes are difficult to adapt due to complex equipment,and also these two methods can estimate soil moisture at small scale.A wide range of moisture estimating remote sensing satellite data may have limited temporal resolution and lack of ground data validation.This thesis goal is reducing extra labor and data costs and providing long-lasting stable soil moisture monitoring.This thesis research on the method of soil moisture using ground-based GPS receiver SNR observations and the factors which influence them.The algorithm can be used to measure the soil moisture within a radius of 25 meters from the station.The main research contents and results of this article are as follows:(1)This paper introduces the principle of retrieving soil moisture by using GPS signal-to-noise ratio.The multipath and SNR characteristics are summarized,and the relationship between multipath and SNR is discussed.SNR data with low elevation angle is greatly influenced by multipath effect.Through the method of polynomial fitting using a low order polynomial,direct signals are separated from reflected signals,and the conversion of the unit is carried out.The phase parameters are obtained by L-S spectrum analysis and nonlinear least square fitting,and then the soil moisture is obtained by model inversion.(2)This thesis studied the comparison between phase and reflector height.To demonstrate the comparison between phase and reflector depth two GPS stations(p041 and p038)are used as an example.For the purpose of comparison R~2 was done between the values of phase and reflector height with soil moisture.Results obtained from the correlation between phase and soil moisture were good compared to correlation between reflector height and soil moisture.The R~2 between phase and soil moisture were 0.808 and 0.702 for p041 and p038 respectively.In case of reflector height the R~2 was 0.606 for p041 and 0.689 for p038.In comparison to p041 station reflector height showed good results at p038 due to its flatness and having no or less vegetation.(3)This thesis studied the influence of various factors on signal to noise ratio data.1)Changes in soil moisture due to precipitation will cause changes in the phase of the signal-to-noise ratio data,and the amplitude of the signal-to-noise ratio increases with increasing moisture content.2)when the signals is reflected back from a vegetation canopy it losses energy,causing a decrease in amplitude.To find out effect of vegetation effect on SNR two types of data were used,one was for vegetated surface and the other was for bare surface.For vegetated soil the amplitude peak was 2.8v/v while for bare soil the amplitude peak was 2 v/v.3)Snow cause changes in the frequency of the reflected signal which can be used for inversion of snow depth.Two days data with different snow depths were used,when the snow depth was 0.53m,the reflected signal intensity or amplitude was 2.5volts/volts,when the snow depth was 0.15m,the reflected signal intensity was close to 0.15volts/volts.