Remote Sensing - a tool for environmental observation

# 1 summarizes the most important letter codes with

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3.1 summarizes the most important letter codes with their frequency and wavelength. ──────────────────────────────── Band Wavelength (cm) Frequency (GHz) ──────────────────────────────── Ka 0.8 - 1.1 40.0 - 26.5 K 1.1 - 1.7 26.5 - 18.0 Ku 1.7 - 2.4 18.0 - 12.5 X 2.4 - 3.8 12.5 - 8.0 C 3.8 - 7.5 8.0 - 4.0 S 7.5 - 15.0 4.0 - 2.0 L 15.0 - 30.0 2.0 - 1.0 P 30.0 - 100.0 1.0 - 0.3 ──────────────────────────────── Table 3.1 Radar wavelengths and frequencies used in remote sensing (Sabins, 1987). 3.4 Spatial Resolution The spatial resolution in the look direction (range) and azimuth direction (aircraft heading) of a radar system is determined by the engineering properties of the antenna. An important property is the depression angel ( γ ), the angle between the horizontal plane and a beam from the antenna to the target (figure 3.2). The depression angle is steep near the aircraft and shallower at far range. The second important factor is the pulse length ( τ ). Pulse length is the duration of the emitted pulse and is measured in μsec. Hence, the spatial resolution in the range (look) direction varies with the distance from the aircraft. Figure 3.3 illustrates the principle of spatial resolution in the range (look) direction. The spatial resolution in the azimuth direction (aircraft heading) is determined by the width of the terrain strip illuminated by the radar beam. As the distance between radar pulse and aircraft increases, the radar beam gets wider. Consequently, at near range the resolution is smaller than in the far range. Figure 3.4 illustrates the principle of azimuth resolution. The angular beam width is a function of the distance from the aircraft but also of the wavelength of electromag- netic energy used. The angular beam width of a radar system is inversely proportional to antenna length. Therefore, spatial resolution improves with longer antennas, but there are practical limitations to the maximum antenna length (depends on the size of the aircraft or satellite). Formulae to compute the spatial resolutions of radar systems can be found in Sabins (1987).

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41 Figure 3.2 Depression angle and incidence angle (Sabins, 1987). Figure 3.3 Radar resolution in the range (look) direction (Sabins, 1987).
42 Figure 3.4 Radar beam width and resolution in the azimuth direction (Sabins, 1987). 3.5 SAR: Synthetic Aperture Radar In the previous section it was discussed that apart from wavelength, the antenna length was an important factor to determine the spatial resolution of a radar system: spatial resolution improves with longer antennae. Traditional radar systems used an antenna of the maximum practical length or real aperture. The Synthetic Aperture Radar or SAR employs a relatively small antenna that transmits a broad beam. The Doppler principle (and special software) are employed to synthesize the azimuth resolution of a very narrow beam. Hence, a technical trick is used to increase the antenna length. Figure 3.5 shows the SAR principle.

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43 Figure 3.5 Synthetic Aperture Radar (SAR) system (Sabins, 1987).
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