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5. Mapping: Hearing the Picture
- Johns Hopkins University Press
- Chapter
- Additional Information
Most modern radar screens present the data on a display laid out in the form of a two-dimensional map. Perhaps the display shows range against azimuth angle, as in figure 5.1—this is typical of surveillance radars that are searching the sea surface, or looking for incoming low-level missiles or airplanes . Some displays show plots of range versus speed, or azimuth versus elevation. The purpose of these types of displays is to convey the target information to the radar operators in the most efficient manner. Thus, in a military context, the radar operator will want to know the location of a hostile target relative to other hostiles and relative to friendly forces. So the target is projected onto a two-dimensional map that allows the operator to quickly assess the situation. For air traffic control (ATC) radars the targets are not hostile, they are cooperative and they want to be seen by the airport radar. The same type of display is used, however: the targets are plotted on a map of the airport and surroundings, so that the ATC operators can guide aircraft safely down onto the correct runway. Both ATC and military radars these days convey further information, by annotating the maps and by colorcoding the displayed targets, so that the operators know about target altitude and speed, and possibly about target type. In all these cases, the background map is just that—background. It places the targets in context, and is not of itself very interesting to the radar operators . So, such background maps do not have to be very detailed. Sometimes 5 Mapping: Hearing the Picture the background is simply a contour or street map—this works if the radar is in a fixed location, as for ATC radars. In other cases the map is simply the set of clutter echoes for each resolution cell. That is, the echoes from each cell (deemed to be clutter, and not targets) are displayed in their proper location, but dimmed or plotted in a different color that allows the targets to stand out clearly. Mapping radars stand apart from the radars described so far in this book, and the displays of these radars consist of far more than simply the background clutter maps of conventional radar displays. One man’s clutter is another man’s target. For imaging radars, the topographical features are the targets, and these are resolved and displayed with much, much higher resolution than is conventionally possible. I will explain how these radars work. Enter SAR SAR stands for Synthetic Aperture Radar, for reasons that will soon be abundantly clear. SAR systems, and their acoustic counterparts sidescan sonars, 122 Blip, Ping & Buzz Figure 5.1 Typical surveillance radar display. Here we see targets on the screen, displayed on a plot of range versus azimuth angle. U.S. Navy photo by Photographer’s Mate 3rd Class Andrew S. Geraci. [3.81.30.41] Project MUSE (2024-03-28 09:20 GMT) represent the way that humans do remote sensing.1 When it is time for me to stack up our human remote sensing achievements against those of the world champions—the bats—it will be SAR systems that I send forward to represent humanity. Human remote sensing began rather later than most radar development, in that most people consider it to be a post–World War II innovation. It is unusual also in that its origin is, by general acknowledgment, the unique achievement of one country. As we saw in chapter 1, most of the radar techniques in use today had their origins in World War II and most were invented several times over in different countries. SAR came into being in the 1950s, in the United States. Carl Wiley of the Goodyear Aircraft Corporation first thought of it in 1951. He saw a way to use the Doppler effect to improve radar cross-range resolution.2 At about the same time, researchers at the University of Illinois hit upon a different way of processing Doppler information to improve resolution. In the next few paragraphs I will show you how SAR works, without mentioning Doppler very much, since the explanation in terms of Doppler is rather mathematical. You should bear in mind, though, that the ultimate reason why SAR achieves high angular/cross-range resolution is because it exploits the Doppler information contained in the radar echoes. A great deal of processing is necessary to turn the echoes into a SAR map, and it was not...