It is not possible to measure precipitation, or indeed any variable, from a satellite directly; what is measured is electromagnetic radiation, nothing else. Everything has to be inferred indirectly from this through algorithms, some simple, some complex, some accurate, some not.

Usually the radiation is natural – either solar radiation in the visible and near infrared bands reflected or scattered without any change of wavelength by the earth's surface and by the atmosphere and its contents. Or the radiation can be in the thermal infrared and microwave bands, radiated by virtue of the surface and atmosphere having been heated by solar radiation, re-emitting the energy at longer wavelengths. These various wavelengths are measure by different types of instrument aboard different types of satellite. Another class of instrument aboard some satellites actively radiates microwaves and detects the scattered signal just as ground-based radar does. This chapter looks at the satellites, at their instruments and at how the measurements can be converted into estimates of precipitation.

Before starting, it is useful to say something about the satellites themselves.

Satellite orbits

There are three types of orbit into which satellites with remote sensing (RS) and communications capabilities are normally placed – geostationary, polar and skewed.

Geostationary orbits

Satellites with an orbital distance of 35,786 km from the earth circle the earth exactly once a day.