Global Navigation Satellite System Reflectometry for Land Applications

Global Navigation Satellite System Reflectometry (GNSS-R) is based on the concept of receiving GPS signals reflected by the ground using a passive receiver. The receiver can be on the ground or installed on a small aircraft or UAV and collects the electromagnetic field backscattered from the surface of the Earth. The received signals are then analyzed to determine the characteristics of the surface. GNSS-R has gained increasing interest over the last two decades as an efficient tool for remote sensing, thanks to the development of positioning systems. GNSSR finds application in several fields as detection of land geophysical parameters, floods and hazard prevention, weather prediction models. In this thesis, the potentiality of GNSS-R technique as a remote sensing tool for soil moisture monitoring, vegetation biomass and altimetry are investigated. Unlike the radar system, where the transmitter and receiver are in the same place (monostatic radar), in the GNSS-R system the transmitter and receiver are separated by a significant distance (bistatic system). The distribution of the received power can be mapped as a function of delay and frequency forming the so-called Delay-Doppler Map (DDM). The peak power and the signal to noise ratio of the received signal of the DDM can be used to retrieve the desired parameters. The transmitted GPS signals are right-hand circularly polarized (RHCP). The surface scattered signal is predominantly left-hand circularly polarized (LHCP) for incidence angles typical of this application. Previous experimental works have already shown the capabilities of this technique to sense the soil moisture changes by using LH reflected signals. Recently, the use of both LH and RH reflected signals (polarimetric measurement) has been proposed to mitigate the effects of the soil surface roughness. In this dissertation, the GNSS-R signals were used to retrieve the soil moisture contents and the vegetation sensing by considering three retrieval methods: LH signals only; LH and RH direct signals; LH and RH reflected signals (polarimetric measurement). Several measurements campaign were performed in a static condition and on board. The signal to noise ratio of the in-sight satellites was obtained from the received power, and different algorithms to retrieve the permittivity applied with the signal to noise ratio. Two different GNSS-R systems were used during the experiments. The first system is based on commercial GPS receivers and antennas, and suited for ground-based measurements. The second system is a light, small and 4-channnel GNSS-R receiver prototype developed in the framework of the Italian project SMAT-F2 (System of Advanced Monitoring of the Territory- phase 2) by Istituto Superiore Mario Boella (ISMB) suited to be mounted on board of small aircraft. In the case of static measurements, the values of permittivity obtained from GNSS-R were compared with those obtained with in-situ soil moisture measurements based on TDR (Time Domain Reflectometry). In the case of flight measurements, three observables were defined: the reflectivity of LH, RH reflected signals and LH/RH (Polarimetric ratio). The polarimetric scattering properties in various environments, such as lakes, forests, trees, humid grass and normal grass were analyzed. The estimation of soil moisture and vegetation in the land field is attracting widespread interest in hydrology, climatology and carbon cycles. This work explores the possibility of sensing Earth surface by using GNSS-R technique with feasibility and portability. Several measurements were done on the ground and on board of small aircraft considering different land environments. During the polarimetric measurements, It was observed that the LH was rather more sensitive to SMC than to RH signal. The polarimetric ratio (PR) value was considerably independent of roughness and it proved to be an optimum parameter for soil moisture estimation. The results obtained show the potentiality of GNSS-R for land remote sensing applications (altimetry, soil moisture content, and vegetation biomass determination). Possible future work will cover snow depth, ice topography, sea wind and wave height.