Introduction

Chapters 5 to 7 considered passive sensors, detecting naturally occurring radiation. In this chapter and the next we shall discuss active sensors, which emit radiation and analyse the signal that is returned by the Earth's surface or atmosphere. We have already identified three possible classifications of remote sensing systems, distinguishing between passive and active and between imaging and non-imaging, as well as classifying them according to the wavelength of radiation employed. We can also classify active systems according to the use that is made of the returned signal. If we are principally concerned with the time delay between transmission and reception of the signal we shall call the method a ranging technique, whereas if we are also (or mainly) interested in the strength of the returned signal we shall call it a scattering technique. The distinction between the two cannot be made entirely rigorous, but it provides a useful way of thinking about active remote sensing systems. It is clear that ranging systems are simpler both to visualise and, because of their less stringent technical demands, to construct, and we shall therefore consider them first. In chapter 9 we discuss the scattering techniques.

Laser profiling

Laser profiling (or laser altimetry) is the simplest application of the lidar (LIght Detection And Ranging) technique. Conceptually, it is extremely straightforward. A short pulse of ‘light’ (visible or near-infrared radiation) is emitted towards the Earth's surface by the instrument, and its ‘echo’ is detected some time later.