The vast majority of all satellites are in low Earth orbit (LEO). Being close to the Earth is good for sensors and links, but a satellite flies over an area in a few minutes.
| Advantage | Disadvantage |
|---|---|
| Sharp images: the camera is close | A satellite is above a point for only a few minutes per pass |
| Little delay (a few milliseconds) | Continuous coverage needs dozens to thousands of satellites |
| Cheaper to launch, smaller transmitters suffice | The residual atmosphere causes drag: without an engine a satellite descends and burns up within months to years |
Uses: Earth observation (optical and radar), SIGINT, the ISS, and the new proliferated constellations of hundreds to thousands of satellites, such as Starlink (± 550 km) and the military layers of the US Space Development Agency (see 2.8).
Space debris
The higher in LEO, the longer debris lingers. At ± 400 km it is gone within a few years; at ± 800 km it stays for decades to centuries. That is why the Chinese ASAT test of 2007 at ± 865 km was so damaging (see 6.2).