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).
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 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.
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.