An imaging satellite is a telescope looking down. The bigger the mirror and the lower the orbit, the sharper the picture. But like your own eyes, it sees nothing through cloud, and nothing in visible light without sunlight.
In practiceCommercial satellites with a mirror of just over a metre at ± 600 km achieve ± 30 cm per pixel: you can see aircraft, vehicles and their shadows. Analysts use those shadows to estimate heights, and compare images from different days to find changes.
Sharpness is called GSD (ground sample distance): how many centimetres of ground one pixel covers. Physically it is limited by the mirror diameter, the wavelength and the altitude:
| Type of image | What you see | Use |
|---|---|---|
| Panchromatic | Black and white, the highest sharpness | Recognising and counting objects |
| Multispectral | A few colour bands, including near infrared | Vegetation, camouflage against natural backgrounds, terrain |
| Hyperspectral | Hundreds of narrow bands | Recognising materials, such as paint or nets against real foliage |
| Thermal infrared | Heat, also at night | Running engines, fires, recent activity |
Optical imaging only works in daylight and without cloud; in north-west Europe at least part of the sky is cloudy most of the time. A satellite in LEO also does not pass constantly (see revisit, 2.8). Whoever knows this plans their activities around it.