Thermal Imaging

Thermal imaging is a remote sensing technique that measures the thermal infrared (TIR) radiation naturally emitted by the Earth's surface. All objects emit various levels of electromagnetic (EM) radiation, depending on their temperature and material properties. Satellite sensors can detect this EM radiation, and use it to infer the temperature of Earth’s surface.

Thermal imaging operates in the TIR band, specifically within the 8 - 14 µm longwave infrared (LWIR) and 3 - 5 µm midwave infrared (MWIR) ranges. These bands align with the natural emission materials commonly found on Earth’s surface, and also fall within the atmospheric window - the spectral interval where radiation passes through the atmosphere with minimal absorption by molecules. This makes these bands ideal for accurate, uninterrupted observations of temperature. Thermal data is vital for monitoring wildfires, droughts, urban heat islands, agriculture, and broader climate and weather patterns. Ultimately, thermal imaging is essential for understanding environmental dynamics, informing resource management, and improving responses to natural hazards. 1) 2)

The history of thermal imaging goes back to 1800, when astronomer Sir William Herschel, best known for discovering Uranus, identified infrared radiation. Although not the earliest attempt by humans to detect heat (rudimentary methods date as far back as 400 BC), Herschel's discovery laid the foundation for modern thermal imaging. By the late 19th century, devices like the bolometer, invented by Samuel Pierpont Langley, could detect subtle variations in thermal radiation, including the heat emitted by a cow from 400 m away. In the 1920s, photography capable of capturing infrared light emerged, but it was during the mid-20th century, spurred by World War II, the Korean War, and the space race, when thermal imaging technology rapidly advanced, primarily for military and medical use. The transition to satellite-based thermal imaging began in the 1960s with NASA’s Nimbus program, whose early weather satellites carried basic infrared radiometers to measure cloud-top temperatures. These pioneering efforts marked the beginning of spaceborne thermal remote sensing, which would later expand into fields such as Earth observation and astrophysics. 3) 4)

Different types of TIR detectors are used on satellites to measure the infrared radiation emitted from the Earth’s surface, such as Quantum Well Infrared Photodetectors (QWIPs) and microbolometer arrays. QWIPs, used by Landsat-8’s Thermal Infrared Sensor (TIRS), operate by trapping electrons in quantum wells until they are excited to a higher energy state by absorbing infrared photons. The resulting movement of electrons generates an electrical signal that can be processed into an image. Microbolometers, on the other hand, detect changes in electrical resistance caused by infrared-induced heating of the sensor material. There are various detection methods and materials available, each with its own advantages and limitations: some require cooling for improved sensitivity, while others can operate uncooled but may offer lower sensitivity or resolution. 5) 6) 7)

The satellite scans areas of interest on Earth, and regions with higher temperatures emit stronger infrared signals, producing more intense detector responses. These signals can be converted into thermograms, where each pixel corresponds to a temperature measurement. Thermograms are then used for applications such as measuring land surface temperature and detecting wildfires.

Example Products

Land Surface Temperature (LST)

Land Surface Temperature (LST) maps represent the temperature of the Earth’s uppermost surface layers, providing critical insights into the thermal state of soils, vegetation, urban structures, and water bodies. These maps are widely used across a range of scientific fields, including meteorology, climatology, hydrology, ecology, and biophysical and biochemical research. LST is derived from ground-based instruments, airborne sensors, and spaceborne platforms. LST products also serve as an independent source of temperature data, complementing in situ observations and atmospheric reanalysis products. This makes them crucial for climate monitoring and policy frameworks such as the UNFCCC Paris Agreement. The Global Climate Observing System (GCOS) has hence defined LST as an Essential Climate Variable (ECV). 8) 9)

LST maps are derived by feeding TIR measurements from satellite sensors through retrieval algorithms that account for various atmospheric and surface effects. TIR radiation emitted from the Earth’s surface may be absorbed or scattered by atmospheric molecules, interfering with the measurements received at the sensor. Additional uncertainties arise from the satellite’s viewing angle; when the satellite’s viewing angle deviates significantly from nadir, the apparent land surface geometry changes. This affects the observed radiation due to factors like surface roughness, slope, and mixed land cover types within the sensor’s field of view. These geometric effects must be accounted for in the retrieval algorithm to ensure accurate temperature estimation. Clouds pose a major challenge, as they are largely opaque in the thermal infrared spectrum, obscuring surface temperatures entirely. 10) 11)

Additionally, accurate LST retrieval depends heavily on knowledge of emissivity, which is the efficiency with which a surface emits thermal radiation relative to a perfect blackbody. Emissivity varies with surface material, texture, and moisture content. To account for this, advanced algorithms, such as split-window techniques or single-channel methods, combine spectral data with atmospheric profiles and land cover models. These methods work best within the TIR atmospheric window (typically 8–14 µm), where absorption by gases is minimal, allowing more accurate surface temperature readings.

Land Surface Temperature Climate Change Initiative Project

The LST-CCI (Land Surface Temperature Climate Change Initiative) project, funded by ESA, aims to improve current satellite LST data records to meet the Global Climate Observing System (GCOS) requirements for climate applications. LST-CCI’s goal is to provide an accurate long-term record of LST maps spanning the past 25 years. To achieve this, the project merges data from several satellites, beginning with the Along Track Scanning Radiometer (ATSR) aboard the ERS-1 and ERS-2 missions (1995 - 2002), and finishing with data from the Sea and Land Surface Temperature Radiometer (SLSTR) onboard Sentinel-3 (2016 - present). The dataset offers global coverage at 0.05° spatial resolution and three-hourly temporal intervals, supporting detailed climate analysis and modelling. It includes rigorous in situ validation across all major land cover types and collaborates closely with the International LST and Emissivity Working Group (ILSTE) to ensure product quality and relevance for climate users. 12) 13) 14)

Urban Heat Mapping

Urban heat maps are specialised thermograms that focus on cities and built-up areas, and are closely related to LST maps. Their primary purpose is to monitor urban heat islands, which are areas within cities that become significantly hotter than surrounding rural regions. This effect is largely due to the concentration of heat-retaining surfaces like roads and buildings, reduced vegetation, and limited shade. By using LST data, these maps help identify urban hotspots where temperatures are elevated. Monitoring urban heat is important, as excessive heat can increase energy consumption, worsen air pollution, and lead to more cases of heat-related illnesses and fatalities. Accurate urban heat mapping supports better urban planning, such as installing green or cool roofs and increasing green spaces to reduce local temperatures and improve quality of life. 15) 16)

Thermal Anomaly and Sulfur Dioxide (SO2) Products

Thermal anomaly maps are derived from LST data and are used to detect areas where surface temperatures deviate from the average conditions expected for a specific location and time of year. Volcanic activity such as lava flows, domes, fumaroles, and eruptive vents often give rise to these anomalies, particularly within the TIR band 10.4 - 12.5 µm. Using satellite TIR data, researchers can map and monitor these anomalies, providing valuable information that complements in situ observations. Changes in the size, intensity, or location of these thermal anomalies can serve as indicators of evolving volcanic hazards and potential eruption precursors. Satellite-based monitoring is especially useful for observing remote or hazardous regions that may be inaccessible or unsafe for ground-based measurements. 17) 18) 19)

Beyond thermal anomaly maps, TIR data can also be used to estimate volcanic gas emissions, offering insights into the magma characteristics beneath a volcano. During eruptions, volcanoes release large volumes of gases such as H2O, CO2, SO2, and aerosols, which can pose hazards to aviation, impact the climate, and harm the local environment. Some of these gases, particularly SO2, have distinct absorption features in the TIR spectrum. By observing the radiance of the Earth's surface through volcanic plumes and applying retrieval algorithms, scientists can estimate gas concentrations. SO2, for instance, absorbs strongly in the 7 - 9 µm wavelength range, appearing as a dip in the TIR spectrum. By comparing these measurements to unaffected areas, the concentration of SO2 can be estimated. This method has been applied using data from instruments like ASTER onboard NASA’s Terra satellite to monitor Miyakejima volcano in Japan, and MODIS onboard Terra and Aqua for eruptions at Hekla in Iceland and Mount Cleveland in Alaska. 20) 21) 22)

Active Fire and Radiative Power Products

Wildfires, whether naturally occurring or human induced, have devastating impacts, resulting in loss of life, destruction of property, and long-term damage to surrounding ecosystems and the climate. Satellite-based thermal imaging is a key tool in detecting, tracking, and understanding wildfires globally, especially in remote or inaccessible areas. These sensors detect temperature anomalies in the mid- and TIR spectrum, identifying active fire fronts and calculating Fire Radiative Power (FRP), which is a measure of the energy released by a fire and is used to estimate intensity and emissions. Several major satellite instruments produce thermal data products specifically designed for active fire detection and FRP analysis. 23)

Related Missions

Sentinel-3

Sentinel-3, part of Copernicus, the European Union’s Earth observation program, is a constellation of two radar imaging satellites operated by ESA and supported by EUMETSAT, launched in February 2016 and April 2018 respectively. The Sea and Land Surface Temperature Radiometer (SLSTR) onboard Sentinel 3A and 3B includes channels in the TIR, and aims to provide global and regional SST and LST data for climatological and meteorological applications. SLSTR data contributes to the LST-CCI project, aiming to provide an accurate long-term record of LST maps spanning the past 25 years. 24)

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Landsat-8/9

Launched in February 2013 and September 2021, respectively, Landsat-8 and -9 are operated by NASA and the United States Geological Survey (USGS). Landsat-8 features a Thermal Infrared Sensor instrument (TIRS), a QWIP based instrument, which provides data used to measure evapotranspiration, map urban heat fluxes, monitor lake thermal plumes, identify mosquito breeding areas and provide cloud measurements. Landsat-9’s TIRS-2 is nearly identical to TIRS with minor improvements in redundancy in electronics.

Landsat-8: Read more 

Landsat-9: Read more

Terra/Aqua

Launched in December 1999 and May 2002, respectively Terra and Aqua are joint missions within NASA’s ESE (Earth Science Enterprise) program between the US, Japan and Canada. Terra and Aqua both carry the Moderate-Resolution Imaging Spectroradiometer (MODIS), while only Terra carries the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). Both instruments have been used in monitoring urban heat, LST, and volcanoes, with MODIS data also contributing to the LST-CCI dataset. Additionally, ASTER provides high resolution (90m) in the TIR bands, making it especially applicable to urban heat mapping.

Terra: Read more

Aqua: Read more

ECOSTRESS

Launched in June 2018, ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) is an International Space Station (ISS) based mission operated by NASA, and assisted by USGS. ECOSTRESS carries the Prototype Hyperspectral infrared imager Thermal Infrared Radiometer (PHyTIR), with five of the six spectral bands operating in the TIR spectral band. PHyTIR monitors plant and soil temperatures,  producing high resolution LST for the monitoring of water stress and evapotranspiration.

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HotSat-1

Launched in June 2023, HotSat-1 operated by SatVu is the first satellite to offer commercially available high-resolution thermal infrared imaging of Earth. The sensor offers a spatial resolution between 3.5 and 6.8 meters, depending on the imaging angle, and can detect temperature differences as small as 2 °C, marking a significant step forward in the accuracy of thermal imaging. The satellite is designed to monitor heat variations from industry and infrastructure, supporting applications in activity monitoring, security intelligence, and climate impact analysis.

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References  

1) UP42, “An Introduction to Thermal Infrared,” 21 December 2023, URL: https://up42.com/blog/introduction-to-thermal-infrared 

2) Medium, “Thermal Imaging with Satellites,” 1 August 2021, URL: https://chrieke.medium.com/thermal-imaging-with-satellites-34f381856dd1 

3) USGS, “The History and Importance of Airborne Thermal Infrared Imaging in Yellowstone National Park,” 30 September 2024, URL: https://www.usgs.gov/observatories/yvo/news/history-and-importance-airborne-thermal-infrared-imaging-yellowstone 

4) Thermography Medical Clinic, “The History of Thermography,” URL: https://thermographymedicalclinic.com/the-history-of-thermography/ 

5) Natural Resources Canada, “Thermal Imaging,” URL: https://natural-resources.canada.ca/maps-tools-publications/satellite-elevation-air-photos/thermal-imaging 

6) Axioms optics, “Microbolometers vs Photodetectors for IR Thermography,” URL:https://www.axiomoptics.com/blog/microbolometers-vs-photodetectors-for-ir-thermography/#:~:text=The 

7) NASA,”Thermal Infrared Sensor,” URL: https://landsat.gsfc.nasa.gov/satellites/landsat-8/spacecraft-instruments/thermal-infrared-sensor/ 

8) MDPI, “Intercomparison of In Situ Sensors for Ground-Based Land Surface Temperature Measurements,” 15 September 2020, URL: https://www.mdpi.com/1424-8220/20/18/5268#:~:text=In%20the%20thermal%20infrared%20spectral%20atmospheric%20window,both%20the%20actual%20surface%20temperature%20and%20emissivity.&text=Estimation%20of%20LST%20from%20Brightness%20Temperatures”

9) ESA, “Land Surface Temperature,” URL: https://climate.esa.int/en/projects/land-surface-temperature/ 

10) University of Reading, “Understanding the Land Surface Temperature Retrievals from Satellites,” 7 March 2025, URL: https://research.reading.ac.uk/met-darc/2025/03/07/understanding-the-land-surface-temperature-retrievals-from-satellites/ 

11) Science Direct, “Enhancing the spatial and temporal resolution of satellite-derived land surface temperature in urban environments: A systematic literature review,” March 2025, URL: https://www.sciencedirect.com/science/article/pii/S2212095525000616#s0005 

12) RUB, “ESAClimate Change Initiative Land Surface Temperature Project,” URL: https://www.geographie.ruhr-uni-bochum.de/projekte/00105-cci.html.en 

13) CEDA Archive, “ESA Land Surface Temperature Climate Change Initiative (LST_cci): Collection 1,” URL: https://catalogue.ceda.ac.uk/uuid/57cfc8b38d914abc8de02b647e879e66/ 

14) CEDA Archive, “ESA Land Surface Temperature Climate Change Initiative (LST_cci): Monthly Multisensor Infra-Red (IR) Low Earth Orbit (LEO) land surface temperature (LST) time series level 3 supercollated (L3S) global product (1995-2020), version 2.00,” URL: https://catalogue.ceda.ac.uk/uuid/785ef9d3965442669bff899540747e28/ 

15) HEAT, “Urban Heat Islands,” URL: https://www.heat.gov/pages/urban-heat-islands 

16) London, “Major Summer Heat Spots Using Landsat-8 Thermal Satellite Data,” URL: https://data.london.gov.uk/dataset/major-summer-heatspots-using-landsat-8-thermal-satellite-data 

17) Earth Observatory, “Land Surface Temperature Anomaly,” URL:https://earthobservatory.nasa.gov/global-maps/MOD_LSTAD_M

18) ASPRS, “Description of Thermal Anomalies on two active Guatemalan Volcanoes Using Landsat Thematic Landmap Imagery,” URL: https://www.asprs.org/wp-content/uploads/pers/1995journal/jun/1995_jun_775-782.pdf

19) USGS, “Thermal Anomaly Map of Yellowstone National Park Based on a Landsat-8 Nighttime Thermal Infrared Image From 9 January 2021,”  URL: https://www.usgs.gov/media/images/thermal-anomaly-map-yellowstone-national-park-based-a-landsat-8-nighttime-thermal 

20) NASA, “The use of multispectral thermal infrared image data to estimate the sulfur dioxide flux from volcanoes: A case study from Mount Etna, Sicily, July 29, 1986,” URL: https://ntrs.nasa.gov/citations/19950036479 

21) Atmospheric Measurement Techniques, “Retrieval of Sulphur Dioxide From the Infrared Atmospheric Sounding Interferometer (IASI),” URL: https://amt.copernicus.org/articles/5/581/2012/amt-5-581-2012.pdf

22) Science Direct, “Sulfur dioxide flux estimation from volcanoes using Advanced Spaceborne Thermal Emission and Reflection Radiometer - a case study of Miyakejima volcano, Japan,” 1 June 2024, URL: https://www.sciencedirect.com/science/article/abs/pii/S0377027303004153 

23) Copernicus, “Wildfire Impact: How is it Monitored & Measured,” 13 August 2024, URL: https://climate.copernicus.eu/wildfire-impact-how-it-monitored-measured#:~:text=Satellite%20imagery%20is%20a%20key%20tool%20in,identify%20the%20areas%20that%20have%20been%20burned 

24) Copernicus SentiWiki, “SLSTR Instrument – Sentinel-3,” URL: https://sentiwiki.copernicus.eu/web/s3-slstr-instrument