This is the PI-produced JPL SMAP-SSS V4.0 CAP, level 3, monthly mapped sea surface salinity product from the NASA Soil Moisture Active Passive (SMAP) observatory. It is based on the Combined Active-Passive (CAP) retrieval algorithm developed at JPL originally in the context of Aquarius/SAC-D and now extended to SMAP. Associated file variables include: derived SSS and wind speed from SMAP and ancillary surface salinity from HYCOM. SMAP data begins on April 1, 2015 and is ongoing, with a 1 month latency in processing and availability. L3 products are global in extent and gridded at 0.25degree x 0.25degree with an approximate spatial resolution of 60km. Improvements with this V4.0 dataset include: addition of estimated SSS uncertainty, improvement of TB correction for high latitudes, modification of the land correction range to 1000 km from coast, change of the ancillary wind source from NCEP GDAS to NCEP GFS resulting in signicant improvements in SSS retrievals near storms, extension of the range of SSS retrievals to 45 PSU, and SSS retrievals also for large inland seas.

The SMAP satellite is in a near-polar orbit at an inclination of 98 degrees and an altitude of 685 km. It has an ascending node time of 6 pm and is sun-synchronous. With its 1000km swath, SMAP achieves global coverage in approximately 3 days, but has an exact orbit repeat cycle of 8 days. On board instruments include a highly sensitive L-band radiometer operating at 1.41GHz and an L-band 1.26GHz radar sensor providing complementary active and passive sensing capabilities. Malfunction of the SMAP scatterometer on 7 July for the surface roughness correction required for the surface salinity retrieval.