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RADAR FOR MARINERS
By DAVID F. BURCH
McGraw-Hill EducationCopyright © 2013 David F. Burch
All rights reserved.
How Radar Works
Radar (radio detection and ranging) is an onboard electronic navigation instrument that measures the range and bearing of landmasses and vessels in its vicinity. It works by sending out a rotating beam of microwave pulses and detecting the pulses that are reflected back from objects around it. It works like a depth sounder, pointed toward the horizon rather than the bottom. As with a depth sounder, what we see on the radar screen are only electronic blips or echoes of the targets, not realistic representations. Also like a depth sounder—or a flashlight scanning a dark room—the radar beam only "sees" what is in view of the beam at the moment. The radar's internal display electronics must paint a picture on the radar screen as the beam scans the horizon, refreshing it every 3 seconds. It takes some practice to read a radar screen and interpret what is really out there. We cover this in Chapter 3. As we shall see, some objects are better radar reflectors than others, which gives rise to the term radar target. A good radar target is one that sends back a strong, well-defined image to the radar screen. Figure 1-1 illustrates schematically how a radar operates.
Isolated targets such as other vessels, large buoys, islets, or drilling platforms are easier to interpret than large, irregular landmasses. At longer distances, isolated targets appear as simple dots or small line segments. As they get closer their target sizes increase, but unless an object is big and fairly close, the size of the echo on the radar screen (of a ship or buoy, for example) is not a measure of the actual size of the target. This concept is explained further in Chapter 3. The image or echo of a target seen on a radar screen is sometimes called a blip.
The basic components of a radar system are an antenna, the radar display unit, and a power source—a typical radar consumes 30 to 40 watts when transmitting. The radar display unit includes a radar screen and a set of controls (knobs, buttons, and sometimes a track ball). There are sophisticated electronics in both the display unit and the antenna, and it is generally most efficient to have a professional electronics technician install and calibrate the system before use (installation options are discussed in Chapter 7). After proper installation it runs dependably and requires little attention as a rule, although basic performance monitoring as discussed in Chapter 13 is always prudent. Radar is a powerful broadcasting device, so an FCC (Federal Communications Commission) Ship Station Radio License (or international equivalent outside the United States) may be required in some cases (see Chapter 7). If you already have a license for a marine radio, the radar can be added to it without a new license.
What you see on the radar screen are your surroundings to a maximum distance equal to the selected range setting. The word range is used many ways in navigation, but for radar, it simply means the distance from your vessel to the radar target. Typical small-craft radars have maximum ranges of 16 to 36 miles—ship radars extend out to 72 miles or more—but we have to cover more background before we can appreciate the significance of these numbers. On the plan view used on radar display screens, your vessel is in the center of the screen. Dead ahead is usually straight up (at the top of the radar screen), your surroundings to starboard are in the right half of the radar screen, your port side is on the left, and aft at the bottom. Modern radars offer options to this display mode, but this head-up mode is the most fundamental and still the most common in small-craft radar. Head-up display is illustrated in Figure 1-2, which shows a radar image overlaid on the chart region it is viewing. It is similar to the schematic view of Figure 1-1, but with real data.
Radar units are often loosely referred to by their maximum range. A "32-mile radar" has a maximum range of 32 miles but can also be set for 24 miles, 16 miles, 8 miles, and so on. In the past (as well as in many units today), the available range options were fixed values. In some modern units, the user can choose any range desired up to the maximum range of the unit. Also in the past, radar screens were circular, so if you selected a display range of 6 miles, the circumference of the display was 6 miles away in all directions. Most modern radars screens are rectangular, with the display shifted slightly toward the bottom to show an extra ring forward. Today, a range setting of 6 miles will more likely mean that you can see 6 miles to the right, left, and aft, and approximately 7 miles dead ahead, as can be seen in Figure 1-3. In all modern units, however, there is always an Offset function that lets the user arbitrarily shift the center location for longer looks in any direction. This is covered in Chapter 8.
RANGES, BEARINGS, AND BUOYS
To navigate from what we see on the radar, we need numerical values of the ranges and bearings to the various targets shown on the radar screen. The very convenient, electronic way of measuring these values with radar is one of the primary virtues of the instrument.
Suppose we are looking for a buoy about 2 miles ahead on the starboard bow according to the chart and our GPS position. The first step in locating this buoy on the radar screen is to select the appropriate range scale. If the range scale were set to only 1 mile, we would not see the target because it is more than 1 mile away. When we increase the Range (by just pushing a button or turning a knob) to the 3-mile scale, however, we should see a target about twothirds of the way out from the center in the top right quadrant of the screen. If we increase the Range again, to 6 miles, the target should remain at the same relative bearing on the screen, but will now be nearer the center, just one-third of the way out, and will register as a smaller blip. If our range options were just 3 or 6 miles, the 3-mile scale would be the better choice. Generally the smallest range that shows the target of interest will give the best results for range and bearing measurements. This lowest range, however, might not offer the best overall perspective for orientation, so one of the basic things we learn in all aspects of radar usage is frequent switching of ranges to keep an eye on things up close and at a distance.
A large buoy, or one with a specially designed radar reflector on it, would show up much like a small vessel—just a little blip on the screen. The radar cannot generally tell you which of these you might be seeing—buoy or vessel—but if you were expecting a buoy, that would be your first guess of what the target is. A small buoy might not show up at all at 2 miles off because it does not have enough reflecting surface to send back a detectable signal, but larger ones will show up nicely. We cover this subject more in Chapter 3.
The first estimate of a numerical value for the actual range to the buoy comes from the range rings. Each range scale on the radar comes with a set of predefined concentric rings at specific ranges. On the 3-mile or 6-mile ranges, these rings are typically drawn 1.0 mile apart. At 12 miles they might be at 2.0 miles apart, and so on. Modern units often let the user select both the sequence of ranges as well as the ring spacing within them, so you might see a 6-mile range with either 1- or 2-mile ring spacing. A typical radar shows the active Range and Range Ring settings prominently in one corner of the display, such as R 3.0, RR 1.0. If we notice that a target is just inside the first range ring, then we know it is just less than 1 mile off. But we can do much better than that.
Every modern radar has a function called variable range marker (VRM). It is a range ring for which you can control the radius. The VRM is operated differently on different units, but all do the same thing. Press a button to turn it on, and then press another button to vary the range of the ring. We cover the finer points of using the VRM in Chapter 4; for now, just set the VRM ring to coincide with the closest edge of the buoy target, and then read the value numerically from the VRM readout—in this case it might read 1.89 nm. You can measure very accurate distances this way if you have good radar targets—a term that will become more clear as we proceed.
If you are moving toward the buoy, you can watch it move down the screen, getting closer to you. Its motion is easy to detect, because it will move off the VRM that you had set on its previous position. Readjust the VRM to see how much closer it is now. The value of the VRM cannot be overstated. It is not just for watching isolated buoys or vessels moving on the screen, but also for more general radar navigation. More specific examples are given in Chapters 5 and 12. We will also very shortly cover other ways to measure the distance to a radar target.
A bearing to the buoy is just as easy to obtain. Here the tool is called the electronic bearing line (EBL). Press a button to turn it on and a prominent radial line—along with its digital bearing—will appear in one corner of the radar display. Press another button (or turn a knob, depending on your model) to rotate the line to the right or left. Adjust the EBL until the line goes through the center of the buoy target and read off its bearing, such as 048 or 048 R. In head-up display mode, these bearing measurements are usually in relative units, meaning relative to the bow of your boat, also called ship's heading or head. In this example, the buoy was located 48° to the right of the bow at the time of measurement. Using relative units, dead ahead is 000, starboard beam is 090 R, aft is 180 R and port beam is 270 R. If the EBL had read 300 R, then the buoy would have been 60° to the left of the bow. Figure 1-4 illustrates relative bearings.
If we want a compass bearing to the buoy that we can use to look for the buoy on deck, we need to correct the relative output of the EBL for our actual heading. Do this by adding the relative bearing to your course heading. If our compass course is 200 C at the time of the reading, for example, then the compass bearing to the buoy (located 48° to the right of the bow) is 248 C. When targets are on the port side of the boat, with relative bearings greater than 180, it is best to add the relative bearing to your heading and then subtract 360 as needed. In the second example above, 200 (ship's heading) + 300 (relative bearing to buoy) = 500, and 500 - 360 = 140 degrees, which is the compass bearing to the buoy. For this and numerous other reasons, every nav station should have a simple calculator on hand at all times (large keys and a large display are assets). It will prove useful often, as in compass conversions, speed-time-distance computations, and for some special quick computations needed for radar, which we discuss later on.
(Later we discuss options in modern radars that let us read actual compass bearings or even true bearings directly from the radar without manually correcting for vessel heading, but this requires an optional heading sensor input to the radar.)
It is simple to get a bearing for any target we see on the radar using the EBL, but we must be mindful of several factors when we interpret the result. First, even though electronic bearings are typically specified to within a tenth of a degree, it is rare that the accuracy of the intended measurement has this level of precision. In the head-up example given above, we had to correct for our course heading of 200 degrees to get the buoy's compass bearing. But we may not have been precisely on course at the time we noted the EBL reading. The ship's heading will typically swing around a bit in a seaway, so the EBL will move on and off the target once it is set. In short, the accuracy of the bearing will depend on the accuracy of your heading knowledge at the time of the measurement. With head-up display, careful coordination with the helmsman is needed for precise bearing data; another solution is a digital heading output right on the radar screen so that you can record the EBL and heading at the same time. In Chapter 7 we cover even more convenient interface options.
When taking bearings to tangents of large radar targets (as opposed to centers of small, well-defined targets like buoys), other considerations affect accuracy as well. We cover these in Chapter 10.
Another way to measure ranges and bearings with modern radar is to use a cursor mode. In this mode a track ball or track pad is used to move a prominent crosshair cursor around on the radar screen while at the same time displaying the range and bearing to the cursor location somewhere on the radar screen. Want the range and bearing to a headland? Just roll the cursor over to that location on the screen and read it off digitally in a bottom corner of the screen. This is a very convenient option, which sometimes includes a further option to draw an EBL and VRM circle through the cursor location by pressing buttons.
The cursor is generally the preferred method of finding target range and bearing. It has the advantage of being quick but the disadvantage of not leaving permanent marks on the screen, as the VRM and EBL do. Underway you will have call for both types of measurements, which opens the opportunity for errors. A potentially serious error would be to set the cursor on a headland and then read the output from the VRM, which is inadvertently set on some other location. Often the outputs on the screen are very similar and just marked by different icons. It is often true with modern electronics that the more convenient our tools, the more careful we have to be in using them. Sample measurements are shown in Figure 1-5.
Bearing and range measurements are fundamental to radar usage. To a radar operator, VRM and EBL are acronyms as common as GPS or radar itself. We will use abbreviations throughout the rest of this book. VRM and EBL have several crucial applications that cannot be replaced with the cursor mode feature, but when just a value of range or bearing is needed, the cursor mode will generally be the first choice for the job. In Chapter 2 we cover a related tool called the electronic range and bearing line (ERBL). In Chapter 4 we cover ways to optimize the accuracy of the measurements.
HEAD-UP DISPLAY MODE
Until recently, small-craft radar had only one display mode available—namely, the head-up mode using relative bearings for the EBL as discussed above. This is still an option on all radar systems, and still the only option on many units in use today. It is also in many ways the most fundamental mode and the easiest to interpret. You think of your boat in the center of the screen, pointed straight up, and things you see around you on the radar screen are located relative to your vessel. Look to the right of your boat for things on the right of the screen; look back for things on the bottom of the screen, etc. The translation from radar image to the world around you is particularly direct and intuitive if your nav station faces forward. It is perhaps a bit less intuitive if your nav station faces athwart-ships or even aft, but in all cases this is a direct, easily interpreted radar display mode. Your heading line remains vertical, from the center to the top of the radar screen, regardless of your vessel heading or the orientation of the display unit within your vessel. No special inputs to the radar are needed.
In head-up mode, when your course changes to the left, or counterclockwise, all radar targets rotate a corresponding amount to the right, or clockwise, but no matter which way you head, the heading line remains straight up, and the top of the radar screen is the direction the bow is pointed. In head-up mode, the EBL reads out bearings to radar targets in relative units. Suppose the EBL is set on a target at 048 R (48° on the starboard bow), and then we turn away from it, 10° to the left. The heading line and the EBL will not move on the radar screen as we turn, but the target will rotate right 10°, and is now 58° on the starboard bow. This illustrates the fundamental advantage of the head-up mode—things are where they appear to be on the radar—but it also illustrates an inherent disadvantage when it comes to identifying what we see on the radar when we are moving. If our heading is swinging about as we proceed along our course, as it would in choppy water or a seaway, target positions will "smear" as they rotate back and forth in response to our heading swings. They get painted onto the screen wherever they happen to be when the radar beam passes across them.
Excerpted from RADAR FOR MARINERS by DAVID F. BURCH. Copyright © 2013 David F. Burch. Excerpted by permission of McGraw-Hill Education.
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