The Soil Moisture Active Passive (SMAP) platform and mission were launched on January 31, 2015. The onboard instruments began observations soon after in April 2015. Although SMAP was designed to measure soil moisture from space, its L-band radiometer can also be used to measure sea surface salinity (SSS) and sea surface wind speed (SSWS).

Instrument Description

SMAP stands for Soil Moisture Active Passive. It is the name of the mission, a satellite observatory consisting of both an active radar and a passive radiometer sharing a common L-band feed horn. Unfortunately, the active radar instrument stopped transmitting on July 7th, 2015 due to an anomaly involving the radar's high-power amplifier. The L-band passive radiometer continues to function normally. The primary mission of SMAP is to measure soil moisture. However, we use the L-band radiometer to retrieve sea surface salinity SSS.

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. In approximately 3 days it completes global coverage with an exact repeat cycle of 8 days. More detailed information about SMAP is available in the SMAP Handbook. Also refer to the NASA SMAP website.  We summarize some details of the SMAP instrument here:

Satellite Platform SMAP
Altitude 685 km
Equator Crossing Time
(Local time zone)
6pm ascending /6am descending
Antenna Size 6.0 m
Swath Width 1000 km
Orbit Inclination 98 deg
Off-Nadir View Angle 40.0 deg
Resolution 39 x 47 km
Repeat Cycle 8 days

Data Products

We obtain two products from SMAP:  Sea Surface Salinity (SSS) and Sea Surface Wind Speeds (SSWS)

Sea Surface Salinity (SSS)

Sea Surface Wind Speed (SSWS)