Radar has been around for more than 100 years and has progressed from being a scientific novelty to becoming the most versatile of marine electronics. You can use radar to find out where you are, to find your way into unfamiliar harbors, to dodge thunderstorms, or even to find fish by picking out flocks of feeding seabirds. By far its most important use it that for which it was first intended—collision avoidance.
Radar works by transmitting short, intense pulses of radio energy, then listening for the faint echoes that come back. Using radar effectively is a multi-stage process that involves making the most of the tiny scraps of energy that are reflected from distant objects or poor reflectors, then removing the clutter that is produced by reflections from things we don’t want to see, such as waves and sometimes clouds. After that, you can start on the third stage—interpreting the useful information you have received.
THE RIGHT SETTINGS
Brightness, contrast and color: These elements help you see contacts more clearly, so they need to be set up properly. Set the brightness control high enough for the picture to be visible, but not so high that it is dazzling. If you still have a unit with a monochrome display, adjust the contrast so it gives the clearest picture possible. Most color displays offer a choice of color palettes, and which one you choose is up to you. Most people prefer to have a dark background at night.
Gain: The gain control adjusts the receiver’s amplification. If gain is set too high, the screen will be filled with a mass of speckles. This is the visual equivalent of the noise you hear when the squelch control is wrongly adjusted on a radio. If it is set too low, real echoes will be missed.
First, increase the gain control until the screen is full of speckles, then reduce it until the speckles have disappeared from all but the center of the screen. At short ranges, it often pays to turn the gain down a little further, because this produces a less “blobby” picture. When looking for weak or distant targets, turn it back up until the speckles start to reappear.
|TUNING: Always make tuning adjustments in small steps; make sure the weak contacts are as strong and bright as possible.|
Tuning: Even though a radar listens for the echoes of its own transmissions, the receiver still needs to be tuned to the right frequency. Luckily, the “auto-tune” function on most modern radar sets is very good at doing this. If you want, or need to, tune your set by hand, set the gain first, then select a long or medium range and look for a weak contact on the screen. Next, adjust the tuning in small steps, either up or down, waiting about three seconds after each adjustment for the picture to be completely redrawn under the new setting. If the contact looks weaker, adjust the tuning back the other way. Keep going until the contact is as strong and bright as possible.
|CLUTTER CONTROL: Because it’s a crude tool, it must be used carefully in order to avoid removing boats, buoys, and even land.|
Clutter control: Some things produce echoes we don’t really want to see. Seawater, in particular, is such a good reflector that the center of a radar screen is often packed with big bright blobs generated by the radar energy being reflected from waves immediately around the boat. The sea-clutter control can remove these blobs, but it’s a crude tool that can also erase boats, buoys and even land.
To be sure it is working effectively, turn the sea-clutter control down to its minimum setting. That way you will know it’s doing no harm. Then, if sea clutter truly is a problem, you can turn the clutter control back up, as little as possible.
Finally, there is the interference-rejection control that erases visual clutter caused when your radar receives transmissions from another boat’s radar. This control can do nothing but good, so there’s no point in switching it off.
|SEA AND RAIN CLUTTER: Sea clutter is at the bottom of the photo at the center of the range rings; rain clutter is directly above on the vertical heading mark|
Rain-clutter controls can be equally dangerous. While they do remove smudgy contacts created by rain clouds, they also degrade echoes from genuine targets. The rule of thumb here is the same as for the sea-clutter control: turn it off when you don’t need it, and then use it as sparingly as you can.
Some radars have two rain-clutter controls, one usually affects only the area in the middle of the screen, while the other, often called “FTC,” affects the entire scanning area.
One foggy morning a 900ft container ship, heading west at 25 knots, collided with a 40ft sailboat that, until a few minutes before the collision, had been heading north at about 7 knots. The investigators criticized the ship’s conduct, but they also found that if both vessels had maintained their course and speed, the sailboat would probably have passed at least three quarters of a mile ahead of the ship. Unfortunately, the sailboat skipper misinterpreted his radar display and decided to stop—right in front of the ship. The lesson here is that the clearest radar contact in the world won’t do you any good if you can’t make sense of it.
True or relative: Until a few years ago one’s own vessel was always at the center of a radar screen, heading straight up. However, that’s no longer the case. True-motion displays, in which the center of the radar picture moves across the screen in step with the boat’s movement in the real world, are increasingly common.
In a collision avoidance situation, it’s best to switch off the true-motion function, and go to the relative-motion mode. Your vessel appears to be stationary at the center of the radar screen, while the rest of the world moves past it. There are a number of options to consider.
|NORTH UP DISPLAY: Data provided by an electronic compass enables the radar to be rotated so the display has North in the up position|
Head-up or North-up: All radars can display a head-up picture, in which a line pointing straight up from the center of the screen represents the subject vessel’s forward motion. This is useful, because things in front of the boat are at the top of the screen, things on the starboard side are on the right, and so forth. But if the boat yaws, the picture also yaws, and blobs representing other vessels turn into vague smears.
Data sent from an electronic compass to the radar allows it to rotate the head-up image to produce a cap north-up picture where north is at the top of the screen. In both formats, the boat’s heading is still represented by the straight line of the heading mark. Which display you prefer is up to you.
|COLLISION COURSE: If a contact is moving directly toward the center of the screen, and there is no change of course, you will share the same patch of water with the other vessel|
Will it hit me?: For over a century the Navigation Rules have advised that “risk shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change.” On a relative-motion display, where our own boat is at the center of a radar image, the logic is obvious. If a contact appears to be moving straight toward the center of the screen, there is a risk of collision.
One easy way to tell if this is happening is to use the feature known as “tracks, trails or wakes,” in which the past positions of each contact are displayed as a pale trail on the screen. If you can, set the trail length to six minutes or one tenth of an hour. This makes it easy to calculate a contact’s speed. For example if a trail is two miles long, the closing speed of the other vessel is two miles in a tenth of an hour, or 20 knots.
|PASS ASTEARN: A relative motion display shows the direction and distance at which a contact will pass closest to the center of the screen, your boat; it’s called the CPA, or Closest Point of Approach|
Close approach: A conventional compass can tell you whether there is a risk of collision, but it can’t tell you if you’re going to miss an approaching vessel by 500 or 2,000 yards. Radar can do this, as long as you keep a record of the movement of a contact across the screen, or if you look at the direction of the wake the contact leaves behind it.
|PASS AHEAD: If the contact is tracking so it will cross the heading marker and there is no course or speed change, the contact will cross in front of you|
If a contact is heading straight for the center of the screen for a few minutes, unless someone does something, the contact will continue to converge. Suppose, though, that the contact isn’t heading for the center, but is going to pass 1,000 yards from the center? The same principle applies here: unless someone does something to change the situation, the contact will keep moving in the same direction and at the same speed, until it does pass 1,000 yards from the screen’s center. The point at which the contact is closest to the center of the screen—the other ship is closest to your boat—is called the closest point of approach, or CPA.
Remember, when predicting a CPA, if it appears the contact will cross the heading marker, the ship will pass ahead of us and we will pass astern of it. But if the contact is going to pass behind us, that means we will pass in front of it—just like the boat that was hit by the container ship. It is important to understand this distinction.
Giving way in fog: Normal steering and sailing rules do not apply when visibility is restricted by fog or other conditions. This is when Rule 19 takes over. The rule says, among other things, that if you detect another vessel by radar, you should:
• Avoid altering course to port for a vessel forward of the beam, other than for a vessel being overtaken.
• Avoid altering course toward a vessel that is abeam or abaft the beam.
In other words, in restricted visibility, the Navigation Rules encourage you to alter course to port for a vessel on your starboard quarter, and to turn to starboard for everything else.