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Radar fundamentals (Part 5)

Posted: 19 Sep 2011     Print Version  Bookmark and Share

Keywords:Synthetic Aperture Radar  SAR  Doppler processing 

Synthetic Aperture Radar ("SAR") is typically employed to map ground features and terrain. It is also known in literature as Synthetic Array Radar. Both names make sense, though "Synthetic Aperture Radar" will be used here. It is used for a wide variety of military and commercial applications. It can be made to map almost arbitrarily fine resolution ground features or used to more coarsely map larger areas in with comparative effort.

SAR resolution
The key parameter in ground mapping is the resolution. SAR systems can be designed with abilities to differentiate using dimensions from few centimeters to hundreds of meters, depending on if the purpose is to map a military installation, an urban area, or the contours of a mountain range. The range is basically limited by the transmit power of the radar and can operate at resolutions at much greater than visual, at long ranges, and is unaffected by darkness, haze, or other factors impacting visual detection.

As with video, the quality of images depend upon the pixel density (pixel stands for "picture element"). The equivalent of pixel density in radar is a voxel, or "volume element". The voxel is defined by the azimuth, elevation, and range. The minimum voxel size is dependent upon the radar resolution capabilities. The voxel spacing is basically the distance that two points on the ground can be distinguished from each other. Radar resolution capabilities, in turn, are dependent upon range resolution and main lobe beam width capabilities.

The voxel spacing or density should in general be at least 10 times the dimensions of the objects being mapped to achieve useful images. A 1m resolution is feasible for detecting buildings that are at least 10m long and wide.

As precision range detection is a fundamental requirement, high PRF (Pulse Repetition Frequency) operation is unsuitable for SAR due to the range ambiguities. Low PRFs are used instead, to eliminate range ambiguities over the distances from the aircraft to the ground being mapped. Maximum Doppler rates tend to be low, as the only motion is due to the radar-bearing aircraft flight path. Due to the nature of SAR, the relative motion is often substantially less than the aircraft flight speed. Use of a low PRF, while restricting the usable Doppler range, enhances the precision of Doppler frequency detection within that restricted range. This is an advantage in high resolution SAR mapping.

Pulse compression
Range resolution is dependent upon the precision of the receive pulse detection arrival delay. This can be achieved by a very short transmit pulse width, which has the disadvantage of low transmit power level due to the short duration. Or very high levels of pulse compression can be used, which allow relatively long transmit pulses and therefore long integration times at the receiver, with the receive operating on higher power returns. This increases the SNR and allows for longer range mapping. A high level of pulse compression can be achieved by using long matched filters (correlation to the complex conjugate of transmit sequence) and transmit sequences with strong autocorrelation properties. The only consequence is a higher level of computations associated with the long matched filter. The speed of light, and therefore radar waves, is about 1m per 3ns (3x10-9). Since the path is roundtrip, the range appears to become half this. So for about 1 m range resolution requires a 6ns timing detection precision. To achieve this level of correlation would require a transmit sequence with phase changes with at least 160MHz rate. This requires radar transmit frequency width of at least the same bandwidth.

 elevation processing

Figure 1: An illustration of elevation processing using range binning.

The elevation of the antenna main lobe does not need to be narrowly focused. In a SAR radar system, the antenna is directed to the ground at an angle off to the side (figure 1). As the elevation angle decreases, the radar beam will be directed at a steeper angle a ground location closer to the flightpath of the aircraft, with a shorter range. The different portions of the beam elevation will therefore map to different ranges, and the return sequence can be directed into different range bins. The precision of the range detection capability translates into the degree of elevation resolution attainable.

Azimuth resolution
The other requirement for precise ground mapping is for a very narrow angular resolution of the main beam in the azimuth. As discussed in a previous installment, the narrowness of the radar beam depends upon the ratio of the antenna size to the wavelength. To achieve a "pencil" like radar beam requires either a very large antenna or very high frequency (and short wavelength) radar. Both are impractical for airborne radar. The antenna size is necessarily limited by the aircraft size. Extremely high frequency radars tend to be useful only at very short range, due to both atmospheric absorption and scattering.

Instead, a virtual large antenna or a virtual antenna of large "aperture" is used. The forward motion of the aircraft is used to transmit and receive from many different points along the flight path of the aircraft. By focusing the radar main beam the same area of ground during the aircraft motion, the returns from different angles at different times created by the aircraft motion can be synthesised into a very narrow equivalent azimuth main lobe using signal processing techniques. The end result is as if an antenna of great length (up to a kilometer) was used. Because this is done using radar returns over several hundred milliseconds, this technique works for stationary targets, so is ideal for ground mapping.

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