Ground Based GNSS Reflectometry for Soil Moisture Determination

Efren Lopez Nava

Soil moisture is key for understanding the flow of water and energy between the surface of the earth and atmosphere, predict floods and droughts and also optimizing agricultural management.

Obtaining soil moisture data at the field scale with currently available in-situ techniques is a challenge since continuous observations are generally required and the sampling volumes are small.

Method

Among these techniques we have the gravimetric method and the TDR method. The gravimetric method consists of taking a sample of soil and drying it in an oven to compare the original weight with the dried one to estimate water content. In the TDR (Time-domain reflectometry) method a sensor is inserted in the ground and, using a pulse generator, a signal is sent. Finally analyzing changes in the waveform soil moisture is obtained.

The use of data from the Global Navigation Satellite Systems (GNSS) for remote sensing, based on utilizing Earth reflected signals, was first proposed in a pioneering work by Martin-Neira (Vey et al. 2015). Most scientists used GNSS exclusively to measure position, however, it was found out that using reflected GNSS signals (also known as multipath) can contribute to data acquisition about soil moisture.

High-precision GPS receivers can sense soil moisture fluctuations using the dual-frequency GNSS signals, which are in the L-Band with wavelengths of 19.05 and 24.45 cm. The reflected signals cover an area of approximately 1000 square meters and thus are a potential source of data acquisition.

It is possible to estimate soil moisture with measurements from a standard single ground-based dual-frequency geodetic GNSS receiver (Larson 2008). The power of the GNSS signal is recorded as signal-to-noise ratio (SNR). This SNR is the ratio of the GNSS signal power to the measurement noise given in a logarithmic decibel (dB) or decibel-Hertz (dB-Hz) scale. In operational applications, the SNR is used to check the signal quality and characteristics of electromagnetic noise in the close environment of the GNSS station. The SNR mainly depends on the power of the signal transmitted by the GNSS satellite, the antenna gain pattern, the elevation angle of the satellite from the horizon and the tracking algorithm in the receiver.