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A Semi-Empirical SNR Model for Soil Moisture Retrieval Using GNSS SNR Data

1
School of Electronic and Information Engineering, Beihang University, Beijing 100191, China
2
Beijing Vegetable Research Center, Beijing Academy of Agricultural and Forestry Sciences, Beijing 100097, China
*
Author to whom correspondence should be addressed.
Remote Sens. 2018, 10(2), 280; https://doi.org/10.3390/rs10020280
Received: 26 November 2017 / Revised: 8 February 2018 / Accepted: 9 February 2018 / Published: 11 February 2018
(This article belongs to the Special Issue Soil Moisture Remote Sensing Across Scales)
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Abstract

The Global Navigation Satellite System-Interferometry and Reflectometry (GNSS-IR) technique on soil moisture remote sensing was studied. A semi-empirical Signal-to-Noise Ratio (SNR) model was proposed as a curve-fitting model for SNR data routinely collected by a GNSS receiver. This model aims at reconstructing the direct and reflected signal from SNR data and at the same time extracting frequency and phase information that is affected by soil moisture as proposed by K. M. Larson et al. This is achieved empirically through approximating the direct and reflected signal by a second-order and fourth-order polynomial, respectively, based on the well-established SNR model. Compared with other models (K. M. Larson et al., T. Yang et al.), this model can improve the Quality of Fit (QoF) with little prior knowledge needed and can allow soil permittivity to be estimated from the reconstructed signals. In developing this model, we showed how noise affects the receiver SNR estimation and thus the model performance through simulations under the bare soil assumption. Results showed that the reconstructed signals with a grazing angle of 5°–15° were better for soil moisture retrieval. The QoF was improved by around 45%, which resulted in better estimation of the frequency and phase information. However, we found that the improvement on phase estimation could be neglected. Experimental data collected at Lamasquère, France, were also used to validate the proposed model. The results were compared with the simulation and previous works. It was found that the model could ensure good fitting quality even in the case of irregular SNR variation. Additionally, the soil moisture calculated from the reconstructed signals was about 15% closer in relation to the ground truth measurements. A deeper insight into the Larson model and the proposed model was given at this stage, which formed a possible explanation of this fact. Furthermore, frequency and phase information extracted using this model were also studied for their capability to monitor soil moisture variation. Finally, phenomena such as retrieval ambiguity and error sensitivity were stated and discussed. View Full-Text
Keywords: Global Navigation Satellite System Interferometry and Reflectometry (GNSS-IR); soil moisture; semi-empirical Signal-to-Noise Ratio (SNR) model; quality of fit; reconstruction Global Navigation Satellite System Interferometry and Reflectometry (GNSS-IR); soil moisture; semi-empirical Signal-to-Noise Ratio (SNR) model; quality of fit; reconstruction
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Han, M.; Zhu, Y.; Yang, D.; Hong, X.; Song, S. A Semi-Empirical SNR Model for Soil Moisture Retrieval Using GNSS SNR Data. Remote Sens. 2018, 10, 280.

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