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Boat Tech Guide: Radar Basics

Radar  - updated August 2009
by Chuck Husick

Put a radar on your boat and you will be able to "see" through fog, rain and darkness. In the hands of a competent operator radar is a remarkable aid to safe navigation. It can confirm your position relative to landmarks and navigation aids such as buoys and by detecting other waterborne traffic make a great contribution to collision avoidance. Once reserved for large yachts and ships, today's radar products are available at prices and sizes suitable for use on boats as small as 18 feet. Even the least costly radars provide quite complete operational capabilities, including on screen display of the range and bearing of targets. Electrical power consumption is modest and well within the capability of virtually any boat equipped with an engine. The ultimate value of a radar rests with the user's ability to understand and properly use the information on the screen. New users will find video tape and computer based radar training programs of great value.

Radar detects targets in a manner similar to the way we see things when using a flashlight in total darkness, when the only objects we can see are those illuminated by the flashlight's beam. Radar operates in a similar manner, except that it "sees" only the minute amount of the radio frequency energy it transmitted that has been reflected back to its antenna. A marine radar antenna constantly rotates, illuminating its surroundings with a continuous series of very short duration, rapidly repeated pulses of energy. A reflection is noted by the receiver and will be shown on the screen as coming from wherever the antenna was pointing at that moment. The distance at which the target is shown is calculated from the time that elapsed from when the pulse was sent until the reflection was received. The on screen intensity of the target depends on how much energy was reflected by target. The size of the target is generally related to the size of the reflecting surface, although a small, highly reflecting target may appear larger than a larger, less efficient reflector. Metal surfaces are generally the best reflectors while wood and fiberglass are stealthy targets and the reason a prudent mariner will mount a radar reflector as high as possible on a boat.

Radar is a real time image of what surrounds the boat and to be of real use should be installed at the helm station of the boat. The screen image must be easily viewable under all lighting conditions. Using today's technology TV like CRTs present the best quality images. The new monochrome and color LCD screen displays are constantly improving and offer the advantage of being waterproof.

Choosing a radar requires evaluation of a number of performance specifications. The maximum range specification and transmitter power rating are often quoted as meaningful measures of the suitability of a radar for a particular boat. Although of interest, they are not particularly appropriate measures on which to base a selection.

Radar energy, like the energy emitted from the VHF transceiver, travels in a generally straight line. Horizontally directed radar energy is soon well above the curved surface of the earth and can illuminate only those objects tall enough to protrude above the radar horizon. For example, for a radar mounted at a height of 22 feet above the water, an object must be more than 1200 feet high to be seen at a distance of 48 miles. (Mounting the radar at the top of a 100 foot mast won't help much, the shortest object visible at 48 miles will still have to be at least 880 feet high). Regardless of the size of the vessel or the maximum range or power of marine radar it is most often used to scan for targets not more than about 6 miles distant and will frequently be operated at ranges of less than 2 miles. Choosing a radar on the basis of maximum range is not a good idea. The typical magnetron equipped marine radar cannot detect and display targets that are less than about 200 feet from the antenna. (The receiver is necessarily "blind" to signal reflections from close-in targets because it must be desensitized in the immediate aftermath of the emission of the energy pulse from the magnetron transmitter. A CW radar can visualize targets within a few feet of the antenna).

Radar transmitter power for all but a few new continuous wave transmitter radars (where power is stated as average power) is defined as Peak Pulse Power. Although manufacturers typically provide increasing power levels in their longer range and most costly sets, a modest amount of power is usually sufficient for the relatively short ranges most often used. Two kW transmitters can provide excellent results. For most recreational boats, transmitter power becomes important only in very heavy rain conditions. Rain reflects and absorbs radar energy. The reflection is useful in showing us the location and shape of rain showers. The absorption of radar energy can prevent a radar from portraying targets it would normally detect. Higher transmitter power can be valuable in such cases, however there are some tropical downpour conditions that can absorb all of the energy of even the most powerful ship radar sets.

The size of the radar antenna plays a key role in determining the overall performance of a radar. Marine radars use the same antenna for both transmission and reception. The transmitter energy is carefully focused, much like the light from a well designed searchlight. However, unlike a searchlight, where the desired pattern of projected light is usually circular, the energy from the radar must illuminate a relatively wide vertical swath to ensure that the target area is well covered as the boat rolls and pitches in the sea. At the same time, a narrow horizontal beam is needed to allow objects close to one another in azimuth to be seen as separate targets and not as a single blob. Typical vertical beam angles are 12.5 degrees. Horizontal beam angles, which are largely determined by the length of the antenna range from about 2.4 degrees for the smallest antennas to 0.75 degree for antennas about 10 feet in length. Even the smallest antennas will provide useful target information at the relatively short ranges most often used by small craft.

The radar antenna, transmitter and receiver are usually packaged as a single unit, enclosed in either a radome or in a housing with a rotating, bar-like antenna mounted on its top surface. The smallest radome housed units are less than 12 inches in diameter and weigh less than 10 pounds, making them practical for installation on even quite small boats. The antenna should be mounted at a height that places it at least two feet above and four to five feet from the head of anyone on board and in a position where crew members will usually be at least five feet from the antenna. Mounting the antenna more than about 22 feet above the water will not make a worthwhile contribution to maximum range operation and can degrade the radar's ability to show important close-in targets. An radar mounting pole that places the radar about eight feet above the deck works well.

The power of pleasure boat radar transmitters ranges from about 2 kW to as much as 50 kW sets used on large yachts (with the exception of the new solid state continuous wave sets where average power may be as low as one-tenth of a watt). All of these power ratings are peak power. The transmitter delivers its power in extremely short pulses, usually on the order of 0.07 to 0.7 millionths of a second long. Pulses repeat at rates between 1000 and 3000 per second. The average power consumption of most radar sets is quite low, with small boat radar usually consuming less than 50 watts of 12 volt power. Providing this amount of power is rarely a problem, even on boats under sail with no engine or generator set operating. Solid-state, CW radar sets typically consume less than 20 watts, excluding the display screen unit.

Radar can be an invaluable aid to navigation. However, it is an adjunct to not a substitute for other means for knowing your position and that of other vessels. A radar equipped vessel is expected to have the set in operation whenever maneuvering in conditions of limited visibility.

Radar Detectors
Radar Detectors similar in function to those commonly used in cars to announce the presence of speed control radar are available for use on boats. These detectors are designed to detect energy at the frequencies used for marine radar and will not respond to typical speed detection radar sets. The intent in their use is to alert the mariner to the presence of a nearby marine radar. Such information can be of some use, with the value largely dependent on the nature of the marine traffic in the area. The devices are passive, they cannot provide any warning of your presence to the vessel's whose radar signals they detect.

Radar Reflectors
Most small vessels are difficult to detect with marine radar, in fact the typical fiberglass boat is a virtually stealthy target. A metal mast is a very poor radar reflector. To have a reasonable likelihood of being seen on another vessel's radar your vessel must provide a reflecting surface capable of returning a significant amount of incident radar energy back in the direction from which it arrived. Various forms of radar reflectors are available, ranging from seemingly simple corner reflectors made of three intersecting sheets of radar reflective material to complex reflector geometry's using varying forms of corner reflector or a Luneburg lens. A number of articles comparing the usefulness of the many types of reflectors have appeared in various publications. To many it appears that using two of the basic corner reflector types, each suspended at a reasonable height above the boat's structure is an effective way to enhance the likelihood of being seen on a ship's radar.





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