In this chapter we explore the general concept of range imaging and its associated digital signal processing issues. The principle behind range imaging or echo location was developed early in the twentieth century [hal; edd; sko; woo]. The objective in range imaging is to measure the distance or range of targets in a scene and, perhaps, identify their structural type (e.g., a bird, a small airplane, or a large airplane). In some ways our sensory system provides us with a primitive tool for echo location. For instance, if a person shouts (transmits a burst of acoustic waves) in a mountain area, then multiple echoes can be heard (received) by that person. The timing of these echoes depends on the relative location (range) of the reflectors with respect to that person (transmitter and receiver). Moreover a bigger reflector is likely to produce a stronger echo. (This is not always true.) The latter measure is referred to as a target reflectivity.
A human-made echo location or range imaging system is an advanced version of the mountain experiment. The early echo location systems were based on some form of analog circuitry designed to accurately quantify the location of a target and its reflectivity. In the radar community a target distance from the radar transmitter/receiver is called the target range, and a target reflectivity is called the target radar cross section (RCS). The basic approach in the early radar (or sonar) range imaging systems was to ...