Single-Pass Soil Moisture Retrieval Using GNSS-R at L1 and L5 Bands: Results from Airborne Experiment

Joan Francesc Munoz-Martin,Daniel Pascual,Christoph Rüdiger,Raul Onrubia,Miriam Pablos,Hyuk Park,Adriano Camps,Alessandra Monerris,Jeffrey Walker

doi:10.3390/rs13040797

Abstract

Global Navigation Satellite System—Reflectometry (GNSS-R) has already proven its potential for retrieving a number of geophysical parameters, including soil moisture. However, single-pass GNSS-R soil moisture retrieval is still a challenge. This study presents a comparison of two different data sets acquired with the Microwave Interferometer Reflectometer (MIR), an airborne-based dual-band (L1/E1 and L5/E5a), multiconstellation (GPS and Galileo) GNSS-R instrument with two 19-element antenna arrays with four electronically steered beams each. The instrument was flown twice over the OzNet soil moisture monitoring network in southern New South Wales (Australia): the first flight was performed after a long period without rain, and the second one just after a rain event. In this work, the impact of surface roughness and vegetation attenuation in the reflectivity of the GNSS-R signal is assessed at both L1 and L5 bands. The work analyzes the reflectivity at different integration times, and finally, an artificial neural network is used to retrieve soil moisture from the reflectivity values. The algorithm is trained and compared to a 20-m resolution downscaled soil moisture estimate derived from SMOS soil moisture, Sentinel-2 normalized difference vegetation index (NDVI) data, and ECMWF Land Surface Temperature.

Highlights

Soil is a natural reservoir of water, making it the main supply store for plants to live.The surface storage is mainly depleted by the natural process of evaporation, percolation to lower layers in the soil, water uptake by plants, etc
Monitoring the soil moisture content of this near-surface layer of soil is crucial for sustainable irrigation of crop fields, forest fire risk prediction, assessment of vegetation senescence, and to have a better knowledge of the water cycle, which plays a key role in the climate feedback loops [3]
It is important to remark that the Dry flight was conducted after ∼1 month without rain and most of the area is not irrigated

Summary

Introduction

Soil is a natural reservoir of water, making it the main supply store for plants to live. The surface storage is mainly depleted by the natural process of evaporation, percolation to lower layers in the soil, water uptake by plants, etc. Moderate-to-high surface soil moisture (SM) values increase flood [1] risks and affect soil erosion [2] by wind and rain. Monitoring the soil moisture content of this near-surface layer of soil is crucial for sustainable irrigation of crop fields (smart irrigation), forest fire risk prediction, assessment of vegetation senescence, and to have a better knowledge of the water cycle, which plays a key role in the climate feedback loops [3]. Using L-band microwave radiometry, the European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS)

Results

Discussion

Conclusion