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Radar fundamentals (Part 5)

Posted: 19 Sep 2011     Print Version  Bookmark and Share

Keywords:Synthetic Aperture Radar  SAR  Doppler processing 

Each pulse has its return processed by azimuth and range, which allows separation over all locations in the radar beam, with resolution determined by the number of range bin and Doppler filter frequency banks. This is repeated at each PRF with the next phase correction value and the results are accumulated or integrated. Over the set of N PRFs equal to the number of N elements in the synthetic antenna, this is repeated.

After each set of N PRFs, the process repeats. The same point is measured N times and complex values representing both magnitude and phase are integrated over the measurements for each point. In this architecture, the number of virtual elements in the synthetic array is equal to the number of Doppler filters, which can also be set equal to the number of range bins. This is also the number of times each point is measured, and the results integrated. However, each of the different measurements for a given point is done at a different azimuth angle.

In reality, these two methods provide equivalent results, although the processing steps are different. The first method is conceptually easier for most people to understand. The second method has the advantage of lower computational rate. An intuitive diagram of the two different approaches is shown in figure 5.

 Doppler digital signal processing

Figure 5: An illustration of Line verses Doppler SAR processing.

Another way to look at this is that in line-by-line, a new narrow beam at right angles to the flight line is synthetically created each PRF. With Doppler processing many different azimuth beams are generated by each Doppler frequency bank during each PRF and the returns from each beam are summed over multiple PRFs.

SAR impairments
Several factors can degrade SAR performance. One of the most significant is non-linear flight path of an aircraft. We have seen how sensitive the phase alignments are to proper focusing, in fractions of the radar wavelength. Therefore, any deviations in flight path away from the parallel line of the radar scan path must be determined and accounted for. This motion compensation can be done using inertial navigation equipment and by using GPS location and elevation measurements. Another consideration is sidelobe return. When the sidelobe return from the ground beneath the plane is integrated over a wide azimuth and elevation angles, this can become significant despite the low antenna gain at the sidelobes. The design of the synthetic antenna, just like a real antenna, must take this factor into account. There are methods, similar to windowing in FIR filters, which can reduce sidelobes, but at the expense of widening the main lobe and degrading resolution.

Another issue is that the central assumption in SAR is that the scanned area is not in motion. If vehicles or other targets on the ground are in motion, they will not be resolved correctly and be distorted in the images. Shadowing is impairment. This occurs when a tall object shields another object from the radar's illumination, causing a black or blank spot in the range return. This becomes more prevalent when very shallow angles are used, which occurs when the aircraft is at low altitude and scanning at long ranges. At high altitudes, such as satellite mounted SAR, this is much less of an issue.

SAR radar implementation
SAR radar processing tends to be a bit less demanding than most other military detection and tracking radars. Many SAR radars are used in commercial applications, such as ground mapping. In some cases, SAR radars can be implemented in software, using floating point processors or DSPs. For higher performance SAR radars, FPGAs once again can provide a much higher level of signal processing and throughput.

It should be noted that FPGAs come in a variety of performance and density levels. Traditionally, the FPGA vendors offered high-end and low cost FPGA families. Any high performance DSP application tended to use the high end FPGAs. Several years ago, FPGA vendor Altera added a mid-range FPGA family named Arria. Mid-range FPGAs share common architectures with high-end FPGAs. The differences are primary in logic density and I/O speeds. For example, Altera Stratix V FPGAs have transceivers operating at 13 and 28Gbit/s, whereas Arria V FPGAs have transceivers operating at 6 and 10Gbit/s.

This concludes the 5-part article series on radar basics. I sincerely hope this information has been and will continue to be useful, while encouraging the reader to continue studying this fascinating and rapidly evolving field. Please feel free to comment on this article series and I will do my best to respond.

About the author
As senior DSP technical marketing manager, Michael Parker is responsible for Altera's DSP-related IP, and is also involved in optimizing FPGA architecture planning for DSP applications. Mr. Parker joined Altera in January 2007, and has over 20 years of DSP wireless engineering design experience with Alvarion, Soma Networks, TCSI, Stanford Telecom and several startup companies.

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