Earth observation platforms and sensors
Chapter 2 of Remote sensing.
Earth observation satellites
Earth observation satellites monitor land, sea, ocean currents, clouds and much else from space. Compared with other platforms they offer:
- coverage of large areas;
- frequent, repeated coverage of the same place;
- quantitative measurements, from radiometrically calibrated sensors;
- processing and analysis that can largely be automated;
- a low cost per area covered.
Each satellite and sensor is described by four kinds of resolution: spectral (which wavelength bands it measures), spatial (the ground size of a pixel), temporal (how often it images the same place) and radiometric (how finely it records brightness, see digital numbers).
| Satellite (sensor) | Pixel size | Revisit | Access |
|---|---|---|---|
| Sentinel-2 A/B/C (MSI) | 10, 20, 60 m; 13 bands | about 5 days with two satellites, more often at Danish latitudes | free, Copernicus Browser / Copernicus Data Space Ecosystem |
| Sentinel-1 (C-band radar) | about 10 m | about 6–12 days | free, as above |
| Landsat 8 and 9 (OLI, TIRS) | 30 m (15 m panchromatic, 100 m thermal) | 16 days each, 8 days together | free, USGS EarthExplorer |
| MODIS (Terra, Aqua) | 250–1000 m | daily | free, NASA |
| Commercial (e.g. Pléiades, WorldView, PlanetScope) | about 0.3–5 m | daily or on request | paid, some free for research |
Free imagery can be downloaded from the archives (the Copernicus Data Space Ecosystem for Sentinel, which replaced the Copernicus Open Access Hub in 2023; USGS EarthExplorer for Landsat) or processed in the cloud without downloading it (Google Earth Engine, the Copernicus Data Space Ecosystem’s own processing services).
Satellite orbits
A satellite follows an elliptical orbit around the Earth. The time one revolution takes is its orbital period. The path it traces on the ground is its ground track; as the Earth rotates beneath it, each revolution traces a different track. Earth observation satellites are usually placed in orbits that repeat their tracks after a fixed interval, the repeat cycle.
- Geostationary orbit. A satellite circling above the equator in the direction of the Earth’s rotation, with a period of 24 hours, stays above the same point. It must be about 36,000 km up. It always views the same, very large area, which suits weather satellites.
- Near-polar orbit. The orbit is tilted only slightly from the Earth’s axis, so the satellite passes close to the poles and covers nearly the whole Earth in one repeat cycle.
- Sun-synchronous orbit. A near-polar orbit at a height chosen so the satellite passes any given latitude at the same local solar time. Images of a place are then taken under the same light, apart from the seasons. Most Earth observation satellites, Sentinel-2 and Landsat among them, use one (Sentinel-2 passes Denmark late in the morning).
Airborne sensors
Sensors mounted on aircraft and drones, looking down or to the side, give images of very high spatial resolution (0.2 m or less). Their coverage is small and their cost per area high, so they are flown for one-time campaigns, whereas satellites monitor continuously. The Danish orthophotos are taken from aircraft.
Handheld spectrometers
Handheld spectrometers such as the FieldSpec measure the reflectance of a surface in the field, for instance from 325 to 1075 nm in 512 channels about 3 nm wide. The instrument measures radiation, which changes with the light conditions, so it is calibrated against a white reference, a surface that reflects (almost) 100 % at all wavelengths:
reflectance = radiation from the target / radiation from the white reference
A dark current measurement removes the signal the instrument produces by itself. Field measurements like these are used to understand satellite images and to develop new indices.
Radar (microwave) sensors
Radar satellites, such as Sentinel-1, work with microwaves, from about a centimetre to a metre. These long waves pass through clouds, whose droplets are far smaller than the wavelength, so radar sees the surface in any weather, day and night.
A radar is an active sensor: it sends out its own pulse and measures what returns, the backscatter. It usually works at a single wavelength, and it locates what it sees from the time the signal takes to return, which is why such satellites are called radar or SAR satellites. Synthetic-aperture radar (SAR) builds up, from the satellite’s movement, the effect of the very long antenna it would otherwise need.
The strength of the backscatter depends on the target:
- Surface roughness: a smooth surface (calm water, a road) reflects the pulse away from the sensor and looks dark; a rough surface scatters part of it back.
- Volume scattering: a tree canopy or dry soil scatters the pulse within its volume. Objects smaller than the wavelength are almost invisible to the radar.
- Water content: wet surfaces reflect more strongly.
Radar also sends and receives in different polarisations (horizontal or vertical). The difference between them shows the orientation of structures, such as upright stems in a field.