Thermal Infrared Remote Sensing

Every object that is warm gives off invisible heat rays called infrared radiation. The warmer the object, the more it gives off. Thermal infrared remote sensing means using sensors on satellites or aircraft to measure this heat, so we can find the temperature of land, water, crops and cities from far away. It works day and night, because it does not need sunlight.

Why does it matter?

A normal camera shows what things look like. A thermal sensor shows how hot they are. This tells us things a normal picture cannot. A field of crops that is short of water becomes hotter, because the plants stop "sweating" water through their leaves. A city is hotter than nearby villages. A forest fire, a volcano or a leaking pipe shows up as a hot spot. So thermal data helps with farming, water management, disaster warning and climate study.

How does it work?

Think of how you feel heat from a hot iron without touching it. The sensor does the same thing from space.

1. The land surface gives off infrared energy according to its temperature.
2. This energy travels up through the air to the satellite.
3. The sensor measures how much energy arrives in certain wavelength bands.
4. Scientists correct for the effect of the air in between and for the type of surface, then calculate the land surface temperature (LST).

Why the 8 to 14 micrometre band?

A scientific rule called Wien's law says that the wavelength at which an object gives off the most energy depends on its temperature. For the Earth's surface, at about 288 K (around 15°C), the peak is close to 10 micrometres. Luckily, the air lets most radiation between about 8 and 14 micrometres pass through. This range is called an atmospheric window. So most thermal satellites look in this window. A second window at 3 to 5 micrometres is useful for very hot things like fires.

Emissivity: why surfaces differ

Two surfaces at the same temperature can give off different amounts of heat. The measure of how well a surface gives off heat compared with a perfect emitter is called emissivity. Water and thick vegetation have high emissivity. Bare rock and metal roofs often have lower emissivity. Sensors use several thermal bands so scientists can separate true temperature from emissivity.

Evapotranspiration: the key use

Evapotranspiration is the total water that goes into the air from soil (evaporation) and from plant leaves (transpiration). When a crop has enough water, it cools itself by transpiring, like we cool down by sweating. When it lacks water, it stops, and its leaves heat up. By measuring leaf and soil temperature, a thermal satellite can estimate how much water a field is using and whether it is under water stress, often before the crop looks dry to the eye. This helps plan irrigation and save water.

Main thermal satellites

• Landsat 8 and 9 (USA): Carry the Thermal Infrared Sensor (TIRS) with two bands near 10.9 and 12 micrometres, at 100 metre resolution.
• ECOSTRESS (NASA): A thermal instrument on the International Space Station since 2018, with about 70 metre resolution.
• TRISHNA (India-France): Planned for 2027. Its French thermal instrument (TIR) has four bands between 8 and 12 micrometres. Its Indian instrument (VSWIR) has seven bands in visible, near infrared and shortwave infrared light. It will see land and coasts at 57 metre resolution and oceans and polar areas at 1 km. It will revisit the same place about every three days, from a sun-synchronous orbit at about 761 km. Its planned life is 5 years. The name TRISHNA stands for Thermal infraRed Imaging Satellite for High-resolution Natural resource Assessment. Trishna also means "thirst" in Sanskrit.

Resolution and revisit: the trade-off

Resolution means the smallest area one pixel shows. Revisit means how often the satellite sees the same spot. Farmers need both: fine detail to see single fields and frequent visits to catch changes. Most older thermal satellites gave one or the other. TRISHNA aims to give both together.

India's position and uses

India is a farming country where agriculture uses the largest share of fresh water. Groundwater is falling fast in many states. Thermal data can support:

• Irrigation planning and crop water-stress alerts
• Drought monitoring
• Mapping urban heat islands in growing cities and planning heat action plans
• Watching coastal and inland water quality and temperature
• Tracking snow and glaciers in the Himalayas

Commonly confused concepts

• Thermal infrared vs near infrared: Near infrared (about 0.7 to 1.3 micrometres) is sunlight reflected by surfaces. Healthy plants reflect a lot of it. Thermal infrared (about 8 to 14 micrometres) is heat that surfaces give off by themselves.
• Land surface temperature vs air temperature: LST is the "skin" temperature of the ground or roof, measured by satellite. Air temperature is measured about 2 metres above the ground at a weather station. On a sunny day, a road can be much hotter than the air above it.
• Passive vs active sensing: Thermal sensors are passive: they only receive energy. Radar is active: it sends its own signal and reads the echo.

Issues, criticism and the way forward

• Clouds: Thermal sensors cannot see through clouds. During the monsoon, a large part of India is covered, so data gaps occur.
• Cost of fine detail: Getting fine resolution in thermal bands needs large, cooled detectors, which are costly.
• Using the data: Data helps only if farmers and state water departments can use it. Experts suggest linking satellite data to advisories in local languages and to schemes on water-use efficiency.
• Way forward: Along with TRISHNA, Europe and the US plan new thermal missions. Together, they could give near-daily high-resolution heat maps of the whole planet.

Concepts to Know

• Infrared radiation: Invisible light with longer wavelength than red light. We feel much of it as heat.
• Micrometre (µm): One-millionth of a metre. Used to measure wavelengths of light.
• Atmospheric window: A range of wavelengths that passes through the air without being absorbed much.
• Sun-synchronous orbit: A near-polar orbit in which the satellite passes over any place at about the same local time each day, so lighting conditions stay similar.
• Urban heat island: A city area that is hotter than the villages around it, because concrete, roads and buildings store heat and there are fewer trees.