Extrasensory Perceptions

The night sky over the Chesapeake Bay Bridge and Tunnel flashed white with lightning, like a silent artillery barrage. The storms were so far up the bay the sound of the thunder never reached Sonata, the 1981 Pearson 36 cutter...
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The night sky over the Chesapeake Bay Bridge and Tunnel flashed white with lightning, like a silent artillery barrage. The storms were so far up the bay the sound of the thunder never reached Sonata, the 1981 Pearson 36 cutter my wife, Liz, and I were living aboard. We’d left Little Creek at Cape Henry, Virginia, late that afternoon and were heading offshore bound for points north.

Every 10 minutes I checked our heading on the compass and GPS, then studied the radar display mounted at the helm. The bridge showed up on the green LCD screen as a giant black swath at a range of about 5 nautical miles. A black blob shaped like a beetle crept along the lower edge of the screen—an inbound containership. It was just another night on watch for Mr. Radar and me.

I’ve been a fan of radar since the 1990s, when I did a lot of sailing in Maine fog. After too many days spent stressing out in pea-soup visibility, I finally installed radar on my old Bristol 24. By the time I traded up to the Pearson, I considered radar to be must-have equipment.

How Radar Works

A traditional magnetron radar set “sees” the world around it by rapidly transmitting a series of microwave pulses (1,000 pulses per second or more) from a rotating antenna. The pulses travel at the speed of light and (hopefully) reflect off distant objects back to the antenna. The radar’s processor times how long it takes a pulse to hit a target and come back as an echo and calculates the distance to the target.

The radar’s processor can also establish a bearing to a target in relation to the scanner. According to Chapman Piloting & Seamanship, “typically a radar bearing is accurate to within 5 degrees of the true bearing...Radar ranges are inherently more accurate than radar bearings.” This is worth keeping in mind, but in most cases a radar bearing on an object is just fine for navigation purposes. However, if you have a choice between radar and visual bearings, the visual bearing is the one to use.

More recently, we’ve seen the development of frequency modulated continuous-wave radar, more commonly called “broadband” radar. These transmit a continuous wave on the X-band radio spectrum, as opposed to pulses, and measure echo returns based on frequency timing. The unit is constantly transmitting and receiving, requiring two antennas to operate simultaneously within the radome, whereas pulse radar has one antenna that alternates between transmitting and receiving.

The advantage of broadband radar is its ability to detect objects at very close range, typically within about three yards of the boat, and its superior target definition and target separation in tight quarters. These units transmit at just 0.1 watt, or a tenth the power of a cell phone, so you can mount the radome just about anywhere without worrying about microwave radiation. They also consume much less electrical power. With broadband radar’s solid-state technology, there’s no magnetron to warm up, and tuning is less of an issue, too.

Broadband radar has been around for several years now and is being hailed for its excellent performance, especially at close range. It is more useful than pulse radar out to about 5 nautical miles. At longer ranges, however, pulse radar is more effective, precisely because it is more powerful.

What Radar Sees

What you see on a radar screen largely depends on a target’s ability to reflect emitted microwaves. This is a function of surface shape and material. Rocks and metal reflect better than wood and fiberglass. Flat objects reflect better than round ones. Fiberglass and wooden sailboats generally make terrible radar targets, particularly if they are not carrying any sort of radar reflector.

In general, a more powerful transmitter is better at bouncing signals off targets in rain or fog, which can absorb microwaves. Pulse radar sets with a 2- or 4-kW transmitter and maximum ranges between 16 and 24 nautical miles are sufficient for the needs of most sailors. The antenna is also important. Longer antennas deliver narrower horizontal beam angles, which in turn increase bearing resolution, making it easier to differentiate between targets that are close together.

Using radar to determine distance off a shoreline can be dangerous. Radar is unlikely to see a low sandy beach at a range of 2 miles or more, because such a beach presents hardly any reflective surface. This means you could be on the beach before you know it. Microwaves basically travel in straight lines, so they can’t see beyond the immediate horizon unless an object is visible above it. The superstructure of a large ship barely visible on the horizon will, for example, appear as a small target. You won’t see how big it really is until it gets closer.

Interpreting what you see on the radar screen takes practice, something that is best done on clear days. Usually the easiest objects to identify are large vessels, which normally appear as distinct lozenge-shaped echoes, and squalls and thunderstorms, which appear as angry irregularly shaped blobs.

Improving Your Vision

All radars now have automatic tuning, but pulse radars sometimes need manual tweaking. If it’s raining or choppy out, the radar will detect the rain and waves and clutter up the screen with echoes. In rainy conditions, you can reduce rain clutter (and the sensitivity of the unit) with the FTC (Fast Time Constant) control. Suppressing rain clutter makes it so just the leading edge of a long-duration echo (i.e., a hard target) shows up on the screen. The sea clutter, or Sensitivity Time Control, function reduces the short-range amplification of wave echoes. These show up as black dots when the STC is adjusted properly. If sea-clutter suppression is set too high, both the sea clutter and targets will disappear. If set too low, targets disappear in the clutter.

As for the gain control, which adjusts the receiver’s sensitivity, a happy medium is best. Set the gain too high and you’ll miss the QE II. Too little gain, however, can blind your radar to weak targets.

Radar has been a helpful addition to my navigator’s toolbox for well over a decade, but I always keep in mind that it is just that—a helpful device, not a panacea. If you have radar on your boat, you should make liberal use of it, but always be sure to rely on other tools as well.

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