What is SLAP and what is SLAP used for? The Scanning L-band Active/Passive (SLAP) instrument is an airborne simulator of NASA’s Soil Moisture Active/Passive (SMAP) satellite. Therefore, SLAP’s primary application is sensing soil moisture and soil freeze/thaw state, but it can also sense the salinity of coastal water. With a copy of SMAP’s digital back end, SLAP is also a great tool for observing manmade radio frequency interference (RFI). Like SMAP, SLAP was built with two main sensors: a passive microwave radiometer, the main soil moisture, freeze/thaw, and coastal salinity sensor for both SLAP and SMAP, and an active microwave radar—also sensitive to soil moisture and freeze/thaw, but more useful for mapping variations in soil or sea surface roughness and vegetation—factors that complicate the sensing of soil moisture, freeze/thaw, and sea salinity. SMAP’s radar stopped working shortly after launch, but SLAP’s radar works fine. Note SLAP’s radar is a scatterometer, not a SAR, so the footprint size is similar to the radiometer footprint size. SLAP also includes a thermal infrared (TIR) sensor to provide estimates of surface temperature needed by algorithms. This is an extra sensor that SLAP has; SMAP does not have a TIR sensor. Like other airborne instruments, SLAP can be used to verify satellite observations (calibration/validation or ‘cal/val’), to test new algorithms, and to improve existing ones. By comparing airborne and satellite observations of the same type at the same location at the same time, scientists can check and confirm that the satellite sensor is functioning properly. Algorithms use the raw radiometer or radar data to retrieve (compute) scientific quantities like soil moisture. SLAP data can be used to improve an existing soil moisture algorithm, to test a new soil moisture algorithm, or even to test an algorithm for a new quantity like ocean wind speed or sea ice type. Since SLAP observes at L-band, it can perform cal/val for other satellites with L-band sensors, too, including the Soil Moisture and Ocean Salinity (SMOS) and Copernicus Imaging Microwave Radiometer (CIMR) from the European Space Agency. Who built SLAP and when was it built? SLAP was designed and built at NASA’s Goddard Space Flight Center (GSFC) in 2013. When it’s not flying, SLAP lives at GSFC. Figure 1. Integration and testing with the outer cover removed inside the lab at GSFC How do you fly SLAP? When it’s time to fly, SLAP gets installed on an aircraft at NASA’s Langley Research Center (LaRC). SLAP has flown on Langley’s B200 and UC-12 King Air aircraft. SLAP could also be installed on other aircraft, if, for example, we needed to make observations from a higher altitude or at a faster airspeed than King Airs are capable of. However, the best spatial resolution and most accurate observations are obtained when we fly SLAP low and slow, so King Airs have been a great aircraft for SLAP. And with the King Air, we can fly SLAP nearly anywhere in the world, so there hasn’t been a huge need yet to install on other aircraft. With over 80 science hours over 30+ flights, SLAP has collected soil moisture measurements over the Delmarva peninsula, North Carolina, and Spain, freeze-thaw data in Canada, and supported an experimental signals-of-opportunity root zone soil moisture instrument in Oklahoma. Figure 2. SLAP on flying King Air aircraft Hydro
