158.

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racy should be preserved. If either of the above conditions is not satisfied, the use of Equation (1.20) is recommended.

It should also be appreciated that surface topographic variations will also cause distortions in the geometric correction of images. The map to which the image is referenced represents the relationship between features reduced to some datum such as sea level, while the image shows the actual terrain surface. If the terrain surface is significantly above sea level, then the image pixel position will be displaced by an amount proportional to the pixel’s elevation above sea level (or whatever datum is used).

1.5 Remote sensing in the microwave region

The word ‘radar’ is an acronym derived from the phrase ‘RAdio Detection And Ranging’. Imaging microwave sensors are known as imaging radars These instruments transmit a signal in the wavelength range approximately 3 cm to 1 m, and receive reflection (backscatter) from the target. The level of backscatter for each pixel over the imaged area is recorded and the set of pixels forms the radar image. Remote sensing in the microwave region differs from optical remote sensing in a number of ways, the most important of which are:

1 Radar backscatter is related to the roughness and electrical conductivity of the target. This information is complementary to that which is acquired by optical and thermal sensors.

2 Energy in the microwave region can penetrate clouds.

3 Microwave imaging radars are active, not passive, instruments and thus can operate independently of solar illumination.

An increasing number of space-borne radar systems is now in orbit, and it is probable that radar imagery will play an increasingly important role in supporting our understanding and monitoring of our environment. The main disadvantage of active microwave systems vis-à-vis optical systems is their power requirements, for the sensor transmits, as well as receives, energy. Optical sensors passively detect reflected solar radiation. Passive microwave sensors, which are not considered in this chapter, detect microwave radiation that is generated by the target.

This section introduces radar remote sensing. In Section 1.6, basic ideas underlying the use of radar images, including geometrical effects and the main factors affecting surface reflection or backscatter at radar wavelengths, are introduced. Section 1.7 considers the extraction of surface information from this backscattered signal. One of the main problems associated with the interpretation of radar imagery is the presence of noise, or radar speckle. The use of filters to reduce the noise effect is described in Section 1.8.

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