Thermal infrared imaging

Thermal infrared imaging is used to measure the radiance emitted by land, water, fires and cloud tops, recorded in a small number of discrete mid-wave infrared (MWIR) and long-wave infrared (LWIR) bands. It depends on a cooled or temperature-controlled detector and on onboard calibration. It works at night because the signal is thermal emission, but it does not see through cloud, and its mid-wave bands pick up reflected sunlight by day. Demonstrated implementations include the Landsat Thermal Infrared Sensor (TIRS) ^[1], the Moderate Resolution Imaging Spectroradiometer (MODIS) ^[2], the Visible Infrared Imaging Radiometer Suite (VIIRS) ^[3], the Sea and Land Surface Temperature Radiometer (SLSTR) ^[4], the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) ^[5] and the ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) ^[6].

The output is radiance or brightness temperature per band. Hydrosat and OroraTech each deliver that as a Level 1 product and surface temperature as a separate Level 2 product ^[7][8]. Land surface temperature and sea surface temperature retrievals, and thermal anomaly detection, build on this imagery.

Detectors are cooled or uncooled. MODIS uses mercury cadmium telluride focal planes cooled to 83 K by a passive radiative cooler ^[2], and VIIRS cools its mid-wave and long-wave focal planes to about 82.5 K in the same way ^[3]. TIRS uses quantum well infrared photodetector arrays held at 43 K by a two-stage mechanical cryocooler ^[1], and ECOSTRESS uses a mercury cadmium telluride array at 65 K with cryocoolers ^[6]. Cooled cameras give high calibration fidelity, but cryocoolers conflict with small-satellite size, weight and power limits ^[9]. NASA's Multiband Uncooled Radiometer Instrument flew a microbolometer in January 2023 with a constant-temperature control loop and a calibration paddle ^[10].

Agency instruments calibrate against a space view and onboard blackbodies. VIIRS uses a blackbody at about 292.5 K ^[3], SLSTR views blackbodies at 265 K and 302 K once per scan cycle ^[4], ECOSTRESS views two blackbodies every 1.29 seconds ^[6], and TIRS views its blackbody routinely ^[1].

Ground sampling varies widely. MODIS has 16 thermal emissive bands at 1000 m ^[11], VIIRS has seven at 750 m and 375 m ^[3], SLSTR samples at 1 km ^[4], ASTER has five at 90 m ^[5] and ECOSTRESS has five at 38 by 69 m at nadir ^[6]. TIRS has two LWIR bands with a 120 m requirement, a 100 m design and bands resampled to 30 m in delivered products ^[1]. Commercial imagers include SatVu HotSat, one MWIR band at 3.5 m ^[12], constellr SkyBee, four thermal bands at 28.9 m with accuracy under 2 K ^[13], and Hydrosat VanZyl, two LWIR bands at 70 m ^[7]. Thermal sensitivity is specified as noise-equivalent temperature difference (NEdT); the MODIS requirement is 0.05 K for bands 20, 29, 31 and 32 at 300 K ^[11].

Cloud, sunlight and calibration set the main limits. The method does not see through cloud; MODIS assigns bands 29 and 31 to 36 to cloud properties and cloud-top altitude ^[11]. By day, mid-wave radiance combines emission with reflected sunlight. He and Li (2011) found that this contamination depends on albedo, solar zenith angle and atmosphere, can produce an equivalent temperature error of up to 40 K over highly reflective surfaces and cooler targets, and weakens fire contrast, while at night the signal is emission alone ^[14]. SatVu lists its MWIR band as 3700 to 4950 nm at night and 4325 to 4950 nm by day ^[12]. Water vapour also affects the signal: TIRS flies two LWIR bands to separate surface temperature from atmospheric effects ^[1].

Calibration errors occur. Stray light from outside the TIRS field of view gave initial errors of minus 2.1 K in band 10 and minus 4.4 K in band 11 at 300 K ^[15]. A software correction in February 2017 reduced the variability ^[15], and Landsat 9 fixed the problem in hardware ^[1]. Uncooled microbolometers are sensitive to their own temperature: a 1 degree Celsius rise in chip temperature lowers the measured temperature by 7.2 degrees ^[9].