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SLAP Instrument

SLAP consists of three separate instruments colocated on a scanning airborne platform, a passive radiometer, an active scatterometer (radar), and a thermal infrared sensor. The three instruments make coincident measurements and have similar footprints on the ground. The instrument platform uses a conical scanning geometry similar to that of the Soil Moisture Active Passive (SMAP) satellite mission.

Radiometer

The SLAP radiometer operates at 1.4 GHz and features the same fully polarimetric observations with similar radio frequency interference (RFI) detection and mitigation capabilities as the SMAP radiometer. The beam width of the radiometer is 19° in the range direction and 12° in the azimuth direction yielding an oval footprint on the ground. Footprint size is also a function of aircraft altitude; during SLAPEX F/T, radiometer footprint size was approximately 250×500m for the linear transect flights and 400×800m to obtain complete spatial coverage of the SMAP grid cell.

Scatterometer

The active sensor in SLAP utilizes simple scatterometer design with a single string used as transmitter and simultaneous V and H chains used as receivers.  It provides VV, HH, VH and HV polarization channels. It operated at a frequency between 1.2-1.3 GHz with flexibility for adjusting to avoid interfering with other spectrum users.

Thermal Infrared

SLAP’s thermal infrared (TIR) sensor is used to estimate the physical temperature of each footprint on the ground during a science flight. This information is needed by many algorithms—for example, the L2 passive soil moisture algorithm. SLAP uses an off-the-shelf CTF-SF15-C1 made by Micro-Epsilon for this measurement.

The accuracy of such a TIR estimate of physical temperature largely depends on knowledge of the TIR emissivity of the contents of the footprint, which is not always well known. Even when it is known, the TIR sensor measures the skin temperature—the temperature of the top—most thin surface of soil, vegetation, etc. To obtain the physical temperature needed by the soil moisture algorithm, an adjustment is often required.

In spite of these limitations, having a coincident TIR observation may provide a more accurate estimate of physical temperature than model-based alternatives. And, in some cases, the estimate can be quite useful.