SMAP¶
The Soil Moisture Active Passive (SMAP) mission, launched by NASA in January 2015, measures global soil moisture and freeze/thaw state every 2–3 days using an L-band radiometer with a 40 km footprint. An enhanced product on a 9 km grid exploits oversampling through Backus-Gilbert optimal interpolation. In Spire’s products, SMAP data is used both for calibration and as a primary input to the fusion algorithm.
GNSS-R (CYGNSS and Spire)¶
GNSS Reflectometry (GNSS-R) measures reflections of navigation signals from the Earth’s surface. This bistatic radar technique is sensitive to surface dielectric properties (soil moisture) and less affected by surface roughness and vegetation compared to conventional radar.
CYGNSS: NASA’s 8-satellite L-band GNSS-R constellation, launched in December 2016, covering 38°N–38°S with 2–4 Hz sampling.
Spire: Multi-GNSS (GPS, Galileo, BeiDou, QZSS, GLONASS) constellation providing global coverage, including polar regions.
Spire uses a change detection algorithm to derive soil moisture from GNSS-R reflectivity, calibrated against SMAP observations.
SMOS¶
The Soil Moisture and Ocean Salinity (SMOS) mission, launched by ESA in 2009, provides L-band radiometer observations of surface soil moisture at approximately 35–50 km resolution. SMOS data is integrated into the MSSM fusion as an additional input source (data_source values 6 and 7).
Sentinel-1¶
Sentinel-1, part of the EU Copernicus programme, provides C-band SAR imagery globally. The Level-1 Ground Range Detected (GRD) backscatter data from Interferometric Wide swath mode is used in Spire’s downscaling methodology to produce D-ESSM and D-HSSM products.