Soil moisture retrieval based on LHCP and RHCP GNSS-R signals

Soil moisture can be retrieved using radar techniques or L-band microwave radiometry. Recently, Global Navigation Satellite System Reflectometry (GNSS-R) techniques have been demonstrated to be useful for many applications as sea-state retrieval and soil moisture retrieval. In this paper, GNSS-R signals reflected from the ground are acquired by means of a commercial receiver. These signals are raw sampled and digitalized, before being processed into Delay Doppler Maps (DDMs) and Delay Waveforms (DWs) with an open loop scheme. In this open loop approach, data are continuously acquired, independently on the tracking status of the receiver. An antenna with two channels, left and right circularly polarized and a low level of cross-polarization is used for the acquisition. The value of the permittivity is computed from the SNR with a standard algorithm based on LH polarized reflected signals, and an improved algorithm based on LH and RH reflected signals in order to keep into account the roughness of the surface. Various in situ measurement campaigns on a terrain with known characteristics are described. The values of the permittivity evaluated from the GNSS-R signals are compared with the results obtained by local measurements based on Time Domain Reflectometry technique (TDR). The measurements were performed with a three-rod sensor and a Tektronix Metallic Cable Tester 1502 instrument.