Many serious sailors, cruising and racing sailors alike, are equipping their boats with radars or radars integrated with GPS receivers and chart-plotters. The radar technology matured, the instruments are reliable, and reasonably priced. Let's face it; radar in the hands of a competent operator is fun to use adding a new dimension to the boating experience

Radar's primary function: Collision avoidance

The primary function of radar (RAdio Detection And Ranging) is to help one "see" in fog, rain, and other conditions (including darkness) that restrict visibility. Radar is a remarkable aid to collision avoidance. For this purpose, most experienced mariners operate their radars in the "Head Up" or "Course Up" mode. This means that the top of the radar display is oriented with the bow of the boat, and targets on the display are relative to the boat's heading. "Head Up" or "Course Up" mode facilitates collision avoidance because it is easy to glance at the radar display and visualize other vessels in the vicinity, as well as their relative positions and movement.

On a radar display, a target echo moving toward you on a steady bearing represents a serious threat of collision and deserves your immediate attention: Maneuver to avoid. A temporary, but significant change in course (60o or more) will signal to the other vessel your intention to avoid. This will be more readily apparent than a reduction in your speed. Resume course when the threat has passed.

A target echo that is moving away from your vessel represents no threat of collision. A target echo that is moving steadily "down scope" in a straight line that is reciprocal to your course, moving aft on the display at the same speed that you're moving forward, is likely stationary. It may be an anchored vessel, or a navigation aid.

It is useful, many say legally required, to maintain a serial plot of radar targets that represent a potential close encounter. Most modern radars can do this internally using a feature described as Automatic Radar Plotting Aid (ARPA) or Mini-ARPA (MARPA). Absent such capability, the Captain or radar operator should maintain manual plots of approaching targets using a maneuvering diagram, or at least a sheet of clear acetate draped over the radar display. Marking the radar echoes with a grease pencil or felt-tip pen helps to distinguish between a stationary target and a moving target that represents a potential collision. We recommend practicing this plotting exercise in conditions of good visibility to develop skill and confidence in ability to do it when visibility is restricted.

Radar is also a valuable navigational tool. This is not widely appreciated, as nearly every boat with radar also has GPS capability, and as we all know, GPS navigation today is extremely reliable and accurate. With WAAS-enabled receivers, GPS is dependably accurate to within about 10 feet… 95% of the time. Even older GPS receivers (those marketed prior to the advent of WAAS in 2003) yield amazing accuracy, usually less than one boat-length. GPS has become, therefore, the default navigational instrument.

Radar, nonetheless, can supplement GPS navigation, or even substitute, should the GPS receiver fail. Furthermore, radar is a great navigational back up when visibility is so occluded that a buoy or other GPS waypoint cannot be observed during an approach. Keep this in mind:

For navigation, most mariners set the radar in the "North Up" mode so that the radar display correlates with integrated chart-plotters and printed charts. When visibility is poor, if you and your radar can positively identify even a single prominent aid to navigation, the range and bearing of that aid, as determined by radar, fixes your position.

What's a "prominent" aid to navigation?

• A large steel or aluminum buoy, or even a smaller buoy equipped with a radar reflector.

• The end of a stone or steel breakwater, especially one marked by a tall latticework structure creating a strong echo on the radar display.

• An offshore lighthouse or "Texas tower."

• A peninsula, especially one with tall buildings and other structures providing good radar reflections.

• A tangent of land, coastlines, or even small islands with prominent cliffs or fabricated structures along the shore (rather than low-lying beaches, marshes, or flat land, which do not provide good radar echoes).

• Bridges.

Most day-marks and small harbor buoys (especially the plastic ones) are not reliably useful for navigating with radar because they usually do not provide good radar echoes unless equipped with metal reflectors.

Bear in mind that, as shown on the display, radar ranges are somewhat more accurate than radar bearings to a navigational aid (or another vessel). Should your radar picture be superimposed over a chart-plotter, or show as a dual "split-screen" display with the chart-plotter, that's even better.

Racons (radar beacons)

Racons are particularly useful for radar navigation. Racons - - also called radar beacons, radar responders, or radar transponder beacons - - are receiver/transmitter devices installed on navigational aids such as prominent buoys and lighthouses near the entrance to busy harbors and along heavily traveled commercial routes. This accompanying illustration shows a racon being prepared for installation on a lighthouse.

A racon, upon receiving a radar pulse, responds by transmitting a unique Morse code character (series of dots and dashes) back to the originating radar set. This identifying code is displayed on your radar monitor as part of the echo, or in a small window or highlighted area near the echo. You can match this code to that on your chart or chart-plotter, thus making a positive ID on the nav aid. A radar range and bearing to such a mark will fix your position with great accuracy.

In order to conserve battery power, racons installed on buoys are programmed to operate 50% of the time. These racons are normally active for 20 seconds, and then off for the next 20 seconds. Racons installed on shore, where battery life is not a factor, are normally programmed to operate 75% of the time. Racons do not operate 100% of the time in order to ensure that the racon code never completely masks the echo of the radar target itself.

Racons are becoming more and more common as important aids to navigation. For example, to provide navigational assistance to vessels transiting the Straits of Florida, and to protect the fragile coral reefs of the Florida Keys National Marine Sanctuary, a string of racons has been installed between Miami and Loggerhead Key in the Dry Tortugas, 70 nautical miles west of Key West. NOAA and USCG personnel installed the racons, but the owners of the 613-foot container ship Houston covered their costs. In 1997, the Houston grounded in the Keys and tore through approximately 400 meters of reef, crushing and breaking corals along the way.

Some precautions

This illustration describes how the racon is indicated on a navigational chart. The magenta circle around it indicates that green buoy "11" is equipped with racon; its code is -… :

One must be cautious when interpreting radar echoes. A small highly reflective target may appear bigger than a large poorly reflective target (on-screen intensity and size are not reliable indicators). In addition, water in the air absorbs transmitted radar energy, so rain or heavy fog may obscure the radar echo of a navigational aid or even another vessel as displayed on your scope.

Also, as reliable as modern radar has become, it is not fail-safe, and you should not become wholly dependent upon it. Have back-up systems in place: Your GPS receiver should be on the line and your loran receiver if you have one. As important, you or your navigator should be maintaining a back-up plot on the paper chart and in the log.

Finally, of course, do not be seduced by radar. You MUST post a lookout. In today's modern era, there is no excuse for not knowing your position... a collision at sea can ruin your day.

Captain Bernie Weiss is a delivery skipper based in Stamford, CT. As Atlantic Yacht Delivery (www.AtlanticYachtDelivery.com), Cap'n Bernie re-positions sailing and motor yachts along the Atlantic coast, Maine to Florida. When not at sea, he trains, lectures, and organizes workshops on electronic navigation, seamanship, boat-handling, engine maintenance, and related subjects.