191.

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perpendicular to the illuminated surface. The look angle is complementary to the depression angle. If the illuminated surface is assumed to be flat, then one can also regard the incidence angle as the complement of the depression angle. The antenna beam width β is related to antenna length L and wavelength λ by the expression

 

The combination of azimuth and range resolution determines the ground resolution of each pixel on a radar image.

It can be inferred from Equation (1.23) that the shorter the pulse duration τ, or the smaller the value of θ, the finer the range resolution. The depression angle θ varies across an image. The value of θ in the near range is relatively larger than that in the far range (Figure 1.13). Thus, the ground range resolution will also vary with respect to θ.

Equation (1.24) shows that the smaller the values of β and d, the finer the azimuth resolution will be. Thus, near ground range has a higher resolution than that in far ground range because both are smaller in near range than that in far range (Figure 1.13). According to Equation (1.24), one can use a long antenna length and short wavelength to obtain finer azimuth resolution. However, shorter wavelengths are more likely to be affected by the atmosphere and, furthermore, antenna length is constrained by physical limitations. For instance, to obtain an antenna beam width of 1×10³m at a wavelength of 20cm, then the required antenna length will be 200m (Equation (1.24)). Clearly, if finer azimuth resolution is sought in terms of increasing antenna length, then serious practical difficulties will be encountered. An alternative strategy is to use a synthetic aperture radar (SAR), where the term aperture means the opening used to collect the reflected energy that is used to generate an image. In the case of radar, this opening is the antenna, while in the case of a camera, the opening is the shutter opening.

SAR increases the antenna length not in physical terms but by synthesising a long antenna using the forward motion of a short antenna, a process that requires more complicated and expensive technology. SAR uses the Doppler principle in order to synthesise a longer antenna. The Doppler effect is the change in wave frequency as a function of the relative velocities of transmitter and reflector. A radar sensor can image a given target repeatedly from successive locations, as illustrated in Figure 1.14. Here, the frequency of the waveform reflected by the target will increase from location a to b, because the distance between the sensor and the object is reducing. As the platform moves away from the target, from b to c, the frequency of the returned signal decreases. SAR uses the Doppler information to

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