Launched on 31 January 2015, the NASA Soil Moisture Active Passive (SMAP) mission is designed to principally measure soil moisture and freeze/thaw state from space for all non-liquid water surfaces globally within the top layer of the Earth. The mission additionally provides a value-added Level 4 terrestrial carbon dataset derived from SMAP observations.
SMAP is now also building upon the legacy of Aquarius/SAC-D mission in delivering both soil moisture and derived sea surface salinity (SSS) observations for the world’s oceans. With the loss of the Aquarius mission on 7 June 2015 it became critical also to continue the time series of global salinity observations important to studies of the Earth’s water cycle. Because both Aquarius and SMAP shared a L-band feed-horn configuration, lessons learned from the algorithm development under the Aquarius mission could be applied to SMAP to retrieve SSS via SMAP. However, because of the larger swath coverage, spatial resolutions under SMAP are approximately 40 km instead of 100 km with Aquarius. The increased spatial coverage provides opportunities for applying SMAP data for higher resolution studies than Aquarius. With the initiation of SMAP science operations and data flows in April of 2015, the approximate 3-month overlap period between SMAP and Aquarius also allows for inter-calibration and comparative studies.
The primary SMAP salinity datasets include a Level 2 orbital dataset, in which data granules contain both the ascending and descending arcs of the orbit, and two Level 3 gridded datasets: an 8-day running average (linked to the day repeat cycle of SMAP) and monthly average. SMAP SSS data are archived and distributed via the PO.DAAC. SMAP soil moisture and L4 carbon datasets are available from the National Snow and Ice Data Center (NSIDC), with Level 1 SMAP radar data distributed by the Alaska Satellite Facility (ASF). These DAACs are the official NASA repositories for SMAP mission data.