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 shall 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 (Baltsavias 1999, Flood 2001). 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. By measuring the time delay and knowing the speed of propagation of the pulse, the range (distance) from the instrument to the surface can be determined. By transmitting a continuous stream of pulses, a profile of the range can be built up, and if the position of the platform as a function of time is accurately known the surface profile may then be deduced.