Abstract
In this paper, the SOMOSTA (Soil Moisture Monitoring Station) experiment on the intercomparison of soil moisture monitoring from Global Navigation Satellite System Reflectometry (GNSS-R) signals and passive L-band microwave radiometer observations at the Valencia Anchor Station is introduced. The GNSS-R instrument has an up-looking antenna for receiving direct signals from satellites, and a dual-pol down-looking antenna for receiving LHCP (left-hand circular polarization) and RHCP (right-hand circular polarization) reflected signals from the soil surface. Data were collected from the three different antennas through the two channels of Oceanpal GNSS-R receiver and, in addition, calibration was performed to reduce the impact from the differing channels. Reflectivity was thus measured, and soil moisture could be retrieved. The ESA (European Space Agency)-funded ELBARA-II (ESA L Band Radiometer II) is an L-band radiometer with two channels with 11 MHz bandwidth and respective center frequencies of 1407.5 MHz and 1419.5 MHz. The ELBARAII antenna is a large dual-mode Picket horn that is 1.4 m wide, with a length of 2.7 m with −3 dB full beam width of 12° (±6° around the antenna main direction) and a gain of 23.5 dB. By comparing GNSS-R and ELBARA-II radiometer data, a high correlation was found between the LHCP reflectivity measured by GNSS-R and the horizontal/vertical reflectivity from the radiometer (with correlation coefficients ranging from 0.83 to 0.91). Neural net fitting was used for GNSS-R soil moisture inversion, and the RMSE (Root Mean Square Error) was 0.014 m3/m3. The determination coefficient between the retrieved soil moisture and in situ measurements was R2 = 0.90 for Oceanpal and R2 = 0.65 for Elbara II, and the ubRMSE (Unbiased RMSE) were 0.0128 and 0.0734 respectively. The soil moisture retrievals by both L-band remote sensing methods show good agreement with each other, and their mutual correspondence with in-situ measurements and with rainfall was also good.
Highlights
L-band microwaves have very good advantages in soil moisture remote sensing, for being unaffected by the atmosphere, and for the ability to penetrate vegetation, except in very dense forests
Extending Global Navigation Satellite System Reflectometry (GNSS-R) applications toward soil moisture remote sensing was first proposed by Zavorotny and Voronovich [2] in 2000
A brand-new method was proposed by Larson [5,6], who verified that multipath signals could be used to infer near-surface soil moisture
Summary
L-band microwaves have very good advantages in soil moisture remote sensing, for being unaffected by the atmosphere (clouds, aerosols, etc.), and for the ability to penetrate vegetation, except in very dense forests. Using GNSS (Global Navigation Satellite System)-reflected signals for remote sensing applications was put forward by ESA in 1993 [1]. Extending GNSS-R applications toward soil moisture remote sensing was first proposed by Zavorotny and Voronovich [2] in 2000. GNSS signals reflected from the land surface, the well-known flight campaign SME02 was launched by NASA and the University of Colorado in 2002 [3]. Signals) experimental field campaign was carried out by ESA and Starlab in 2009 to investigate the interaction between land surface parameters such as soil moisture, surface roughness, and vegetation biomass, and the scattered GNSS signal characteristics [4]. A brand-new method was proposed by Larson [5,6], who verified that multipath signals could be used to infer near-surface soil moisture
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