Technical Note on Quality Assessment of SPIRE L2 GNSS-R Soil Moisture

In this work we examined the SPIRE Soil Moisture Preview Product, obtained using GNSS-R technique; SPIRE collects soil moisture value from the first 5 cm of the surface. Oneadvantage of using this technique is penetration through vegetation, where optical sensorsand C-band radars have limited functionality.SPIRE data are generally well documented, with the description of all the variablespresents in the files and the presence of numerous metadata even if some informationshould be explained more clearly. L1 documentation deeply characterizes the sensor, thecalibration, and the geometrical aspects of the measurements; the delivered L2 ProductManual describes the retrieval algorithm and some examples of characterization of theproduct with respect to CYGNSS and SMAP, two datasets used in the calibration process.Soil moisture uncertainty is not present.Soil moisture Product has been intercompared with three different satellites mission:CYGNSS, SMAP and SMOS. The intercomparison region chosen is the south-eastAustralia, with about 30 days of data between June and July 2021. CYGNSS dataset hasbeen chosen for the use of their historical data in the calibration processes over the south-east Australia; CYGNSS performs GNSS-R measurements, and its retrieval also inspiredthe algorithm employed by SPIRE. SMAP and SMOS can be considered the state-of-the-art missions in the soil moisture measurements; they employ different measurementtechniques and can be considered independent datasets.