This is but a brief review of a complex subject and is intended to give an overall impression rather than a detailed account. The aim is to clarify what can be sensed remotely, and to put remote sensing into context with ground-based measurements.

What is remote sensing?

Remote sensing (RS) is not the transmission of data from in situ, ground-based sensors at a remote site to a distant base. That is telemetry (Chapter 12).

Whereas in situ measurements are made by sensors in direct contact with the variables they measure, RS measurements are made entirely by sensing the electromagnetic radiation reflected from, or emitted by, the surface of the earth and its atmosphere. Astronomy provides a good example of RS (apart from the spacecraft that have soft-landed on other planets), and photography and, more recently, electronic imaging have become the sensors and recorders of astronomy.

A platform from which RS measurements are made can be simply a mast on the ground, an aircraft, balloon, rocket or spacecraft in orbit around the earth. It is the last that has had the biggest impact on RS and so satellite RS is the main concern of this chapter. However, many of the instruments and techniques used on spacecraft are also used on the other platforms, particularly aircraft. But aircraft are expensive to fly and so are used mainly where an occasional or single measurement is needed and where a local, detailed, low altitude view is necessary.