Land monitoring using GNSS-R techniques: A review of recent advances

Abstract

Soil moisture is required to improve meteorological and climate predictions. Global soil moisture maps are nowadays produced daily from SMOS satellite data, with a basic spatial resolution of -50 km. Recently, using data fu sion techniques between SMOS and MODIS data, an operational service has been implemented at the SMOS Barcelona Expert Center to downscale SMOS data down to 1 km over the Iberian peninsula [11. However, despite SMOS operates in the passive microwave "protected" band from 1400 to 1427 MHz, radio fre quency interference may degrade the quality of the soil moisture (and sea salinity) retrievals or even prevent them [21. Signals of opportunity transmitted from Global Navigation Satellite Systems (GNSS) can be used for soil moisture, vegetation, snow, water level. .. monitoring after reflection (GNSS-R) on the Earth's surface. In principle, eventhough these signals can also be jammed, their structure and the way they are processed, makes them more robust in front of radio-frequency interference, while at the same time -in principle-can achieve also a better spatial resolution. In this work, the few different GNSS-R techniques are first revised, including their pros and cons. Then a few applications are revised, with special emphasis -but not exclusively-in those in which the UPC Remote Sensing Lab has been working.

GPS interferometric reflectometry: applications to surface soil moisture, snow depth, and vegetation water content in the western United States

“…The European Space Agency is planning a reflectometry mission from the International Space Station . Ground‐based GPS reflection instruments have also been built and successfully employed . In each of these cases, the instrument were designed to deliberately measure reflected signals.…”

“…36 Groundbased GPS reflection instruments have also been built and successfully employed. 37 In each of these cases, the instrument were designed to deliberately measure reflected signals. The development of GPS-IR has clearly benefited from these mission studies and ground-based reflectometry studies.…”

Software tools for GNSS interferometric reflectometry (GNSS-IR)

“…GNSS-IR is a method for estimating environmental parameters around a geodetic-quality GNSS site. Unlike other reflection techniques, where an antenna is designed to measure reflection signals (Löfgren et al 2011;Camps et al 2013) or a geodetic antenna is rotated to improve its ability to measure reflections (Anderson 2000), GNSS-IR uses data collected with (nominally) multipath-suppressing geodeticquality GNSS antennas in an upright orientation. GNSS-IR has been demonstrated and validated for measuring surface soil moisture (Larson et al 2008), snow depth (Larson et al 2009;Nievinski and Larson 2014c, d), permafrost melt (Liu and Larson 2018), tides (Larson et al , 2017Löfgren et al 2014;Roussel et al 2015), ice-up (Strandberg et al 2017), firn density (Larson et al 2015), and vegetation water content (Wei et al 2015).…”