Satellites use a variety of sensors to observe and measure different phenomena on Earth and in space. These sensors operate across the electromagnetic spectrum, detecting visible light, infrared radiation, microwaves, and more to gather valuable data. There are two main categories of satellite sensors – passive and active.
Passive Sensors
Passive sensors detect natural electromagnetic radiation emitted or reflected by the Earth and its atmosphere. They measure the amount of incoming energy to identify objects and temperatures.
Radiometers
Radiometers are passive sensors that measure electromagnetic radiation in specific wavelength bands across the spectrum. They can detect visible light, infrared, and microwaves:
- Visible light radiometers measure sunlight reflected off the Earth’s surface in the visible spectrum. They produce images of the Earth in true color. The Landsat satellites and MODIS instrument on Aqua and Terra have visible light sensors.
- Infrared radiometers detect infrared energy emitted as heat. They can measure land and sea surface temperatures, cloud properties, and greenhouse gases. Examples are the Advanced Very High Resolution Radiometer (AVHRR) on NOAA satellites and the Atmospheric Infrared Sounder (AIRS) on Aqua.
- Microwave radiometers operate at microwave frequencies to penetrate cloud cover and measure energy emitted from the Earth’s surface and atmosphere. The Special Sensor Microwave Imager (SSMI) on Defense Meteorological Satellites is a microwave radiometer used for weather forecasting.
Spectrometers
Spectrometers measure the spectral signature of sunlight reflected off the Earth across many wavelength bands. They can identify the chemical composition and physical properties of materials.
- Imaging spectrometers produce images with high spectral resolution. NASA’s Earth Observing-1 (EO-1) has the Hyperion imaging spectrometer with 220 spectral bands.
- Non-imaging spectrometers don’t produce images but provide detailed spectral data. The Ozone Monitoring Instrument (OMI) on Aura measures ozone and other gases.
Photometers
Photometers measure the intensity of visible or ultraviolet light from the sun reflected by the Earth. They are used to monitor ozone, aerosols, and cloud properties. The Ozone Mapping and Profiler Suite (OMPS) on the Suomi NPP satellite includes photometers.
Active Sensors
Active sensors emit their own electromagnetic radiation and measure the signals reflected or backscattered from the Earth. They provide their own source of illumination to collect data day and night.
Radar Altimeters
Radar altimeters emit radio wave pulses towards the Earth and measure the return time to determine altitude above the surface. They are used for topographic mapping and measuring sea level, ice sheet thickness, and wave height. The Jason series and Sentinel-3 satellites carry radar altimeters.
Scatterometers
Scatterometers emit microwave pulses and measure the backscattered signals off the ocean surface. From this they can estimate wind speed and direction over the oceans. The Advanced Scatterometer (ASCAT) on the Metop satellites is an example.
Synthetic Aperture Radar (SAR)
SAR uses the motion of the satellite to simulate a large antenna aperture and achieve high spatial resolution radar images. The waves penetrate clouds and darkness. SAR is used for topographic mapping, monitoring deforestation, and measuring soil moisture. Examples are the SAR instruments on the European Space Agency’s Sentinel-1 satellites.
LiDAR
LiDAR (Light Detection and Ranging) uses laser pulses to measure distance by timing the return signal. From aircraft, LiDAR provides detailed topographic maps and vegetation structure data. NASA’s ICESat satellites had a LiDAR instrument to measure changes in ice sheet elevations.
Radiometers
Some radiometers are active sensors that include their own small microwave transmitters to measure temperatures, humidity, precipitation, and other weather variables. The Global Precipitation Measurement (GPM) Core Observatory has a dual-frequency precipitation radar.
Other Satellite Sensors
In addition to remote sensing instruments, satellites carry other sensors to monitor the space environment and the satellite’s health and position.
Magnetometers
Magnetometers measure the strength and direction of magnetic fields. They are used to study Earth’s magnetic field and space weather. The Magnetospheric Multiscale (MMS) mission has magnetometers to study magnetic reconnection.
Charged Particle Detectors
These measure the properties of ions and electrons in space. They characterize the plasma environments around Earth and other planets. The Van Allen Probes include the Relativistic Electron‐Proton Telescope to study radiation belt particles.
GPS Receivers
Global Positioning System (GPS) receivers on satellites precisely determine the satellite’s location in space and time. This allows accurate geo-referencing of data from imaging instruments.
Star Trackers
Star trackers orient the satellite by recognizing star patterns. They provide precise measurement of the orientation of instruments like cameras and antennas.
Accelerometers
Accelerometers measure non-gravitational forces like drag and solar pressure acting on the satellite. This helps maintain the satellite’s position and orientation.
Resolution
The resolution of a satellite sensor determines its ability to distinguish targets and features. There are four types of resolution:
- Spatial resolution is the size of a pixel in an image, which depends on the instrument’s instantaneous field of view. High resolution systems like GeoEye-1 offer less than 1 meter pixels.
- Spectral resolution is the number and width of wavelength bands measured. Hyperspectral sensors like EO-1 Hyperion acquire hundreds of narrow bands.
- Radiometric resolution is the ability of a sensor to discriminate differences in signal strength. More bits means the sensor can detect smaller variations in energy.
- Temporal resolution is how often the sensor revisits the same area, from multiple times per day for geostationary satellites to every 16 days for Landsat.
Data Processing
Once received by ground stations, the raw data from satellite sensors undergoes processing to generate usable products:
- Calibration – Converting signals to meaningful units like reflectance or temperature using pre-launch calibration data.
- Georeferencing – Assigning geographic coordinates to each pixel based on the satellite’s position and orientation.
- Orthorectification – Correcting for terrain distortion in images using digital elevation models.
- Cloud removal – Identifying and removing cloudy pixels from images using temperature thresholds or reflectance.
- Atmospheric correction – Removing the effects of the atmosphere on reflectance data using radiative transfer models.
Applications
Satellite sensor data has countless applications across many fields:
- Weather forecasting – Tracking storms, rainfall, winds, fog, and other weather with visible, infrared, and microwave imagery.
- Climate monitoring – Building long-term records of temperature, precipitation, vegetation, sea ice, and more to identify climate trends.
- Agriculture – Assessing crop health and yields, soil moisture, and drought conditions from space.
- Disaster response – Identifying areas damaged by floods, fires, and other hazards through changes detected with satellite sensors.
- Transportation – Navigating ships through sea ice and monitoring aviation hazards like volcanic ash plumes.
- Urban planning – Mapping cities and analyzing patterns of urban growth over time.
- Ecology – Studying ecosystem changes like deforestation and wetland loss.
- Geology – Identifying mineral deposits, mapping faults and geologic features, and monitoring volcanoes.
Future Outlook
With the advancement of sensor and satellite technologies, we can expect continued improvements in the quality, coverage, and applications of remote sensing data from space. The next generation of satellite missions promises to enhance our understanding of climate change and natural hazards while benefiting humanity.
