SLAP’s thermal infra-red (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 the 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.

Calibration

We calibrate the IR sensor when SLAP is sitting on the ground using a calibration blackbody target. Before start calibration, we first need to make sure that the temperature of the blackbody target is stabilized. We let the blackbody temperature equilibrate with the ambient temperature.  A high precision PRT (Platinum Resistance Thermometer) records the temperature of the target. There is small hole on the back of the blackbody surface where the PRT can be inserted to measure the temperature. To avoid an air gap in the hole, we use a thermal conductive grease before inserting the PRT.

To make the measurements, the blackbody should be placed as close to the IR sensor aperture as possible in a way that the center of the target is on the boresight line of the IR sensor. Ideally, the sensor should see nothing else but the blackbody (meaning the black surface at the bottom of the blackbody interior).

During calibration, the IR sensor is taking readings from the target, and the PRT sensor also records its temperature separately. Comparing the result between two measurements will give us the offset of the IR sensor which will be used to analyze the thermal IR data during science flights.