Thermal Sensors and Satellites
Thermal sensors or scanners detect emitted radiant energy. Due to atmospheric effects these sensors usually operate in the 3 to 5 μm or 8 to 14μm range. Most thermal remote sensing of Earth features is focused in the 8 to 14 μm range because peak emission (based on Wien's Law) for objects around 300K (27° C or 80° F) occurs at 9.7μm. Many thermal imaging sensors are on satellite platforms, although they can also be located on-board aircraft or on ground-base systems. Many thermal systems are multispectral, meaning they collect data on emitted radiation across a variety of wavelengths
Thermal Infrared Multispectral Scanner (TIMS)
NASA and the Jet Propulsion Laboratory developed the Thermal Infrared Multispectral Scanner (TIMS) for exploiting mineral signature information. TIMS is a multispectral scanning system with six different bands ranging from 8.2 to 12.2 μm and a spatial resolution of 18m. TIMS is mounted on an aircraft and was primarily designed as an airborne geologic remote sensing tool. TIMS acquires mineral signature data that permits the discrimination of silicate, carbonate and hydrothermally altered rocks. TIMS data have been used extensively in volcanology research in the western United States, Hawaiian islands and Europe. The multispectral data allows for the generate of three-band color composites similar other multispectral data. Many materials have varying emissivities and can be identified by the variation in emitted energy.
The thermal image to the right was captured the Thermal Infrared Multispectral Scanner (TIMS) and is a thermal image of Death Valley California. A color composite has been produced using three of the thermal bands collected by TIMS. There are a variety of different materials and minerals in Death Valley with varying emissivities. In this image Thermal Band 1 (8.2 - 8.6μm) is displayed in blue, Thermal Band 3 (9.0 - 9.4μm) is displayed in green and Thermal Band 5 (10.2 - 11.2 μm) is displayed in red. Alluvial fans appear in shades of reds, lavender, and blue-greens; saline soils in yellow; and different saline deposits in blues and greens.
Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)
Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is a sensor on-board the Terra satellite. In addition to collecting reflective data in the visible, bear and shortwave infrared, ASTER also collects thermal infrared data. ASTER has five thermal bands ranging from 8.1 to 11.6 μm with 90m spatial resolution. ASTER data are used to create detailed maps of surface temperature of land, emissivity, reflectance, and elevation. ASTER data is available for download through EarthExplorer.
Moderate-resolution Imaging Spectroradiometer (MODIS)
As previously discussed, MODIS has a high spectral resolution and collects data in a variety of wavelength. Similar to ASTER, MODIS collects collects reflective data and emitted, thermal data. MODIS has several bands that collects thermal data with 1000m spatial resolution. MODIS has high temporal resolution with a one to two day return time. This makes it an excellent resource for detecting and monitoring wildfires. One of the products generated from MODIS data is the Thermal Anomalies/Fire product which detects hotspots and fires.
A variety of the Landsat satellites have carried thermal sensors. The first Landsat satellite to collect thermal data was Landsat 3, however this part of the sensor failed shorty after the satellite was launched. Landsat 4 and 5 included a single thermal band (band 6) on the Thematic Mapper (TM) sensor with 120m spatial resolution that has been resampled to 30m. A similar band was included on the Enhanced Thematic Mapper Plus (ETM+) on Landsat 7. Landsat 8 includes a separate thermal sensor known a the Thermal Infrared Sensor (TIRS). TIRS has two thermal bands, Band 10 (10.60 - 11.19μm) and Band 11 (11.50 - 12.51μm). The TIRS bands are acquired at 100 m spatial resolution, but are resampled to 30m in the delivered data products.
Landsat TIRS and Applications
Irrigation accounts for 80% of fresh water use in the U.S and water usage has become an increasingly important issue, particularly in the West. Thermal infrared data from Landsat 8 is being used to estimate water use. Landsat 8 data, including visible, near infrared, mid-infrared, and thermal data are fed into a relatively sophisticated energy balance model that produces evapotranspiration maps. Evapotranspiration (ET) refers to the conversion of water into water vapor by the dual process of evaporation from the soil and transpiration (the escape of water though plant’s stomata). For vegetated land, ET is synonymous with water consumption. Landsat data enable water resources managers and administrators to determine how much water was consumed from individual fields.