Avoiding stealth

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Equipping your boat with radar and learning how to operate it properly is only part of the collision-avoidance story. The other part involves making sure that other vessels can pick you up on their radar. Anyone who owns a wood or fiberglass sailboat has achieved, at no extra cost, what the government has spent millions of tax dollars trying to achieve: virtual invisibility on radar.

The Cyclops 3 reflector is composed of two trihedral corner reflectors, one facing forward, the other aft, along with two dielectric lens reflectors facing port and starboard. The Cyclops is in a plastic enclosure designed for masthead mounting.
   Image Credit: Courtesy North Seas Navigator, Inc.

The solution is some sort of radar reflector. These run the gamut from simple and inexpensive to sophisticated and expensive. Buying one of these devices is a logical step toward increasing your radar visibility.

Radar is an echo-imaging device. A brief-duration radio frequency energy pulse is transmitted from a highly directional antenna, after which the set's receiver, using the same antenna, attempts to detect the very minute return signal that may reflect from various targets. No reflection, no information.

The power of the typical marine radar, often thousands of watts for even a small set, may sound impressive, especially when compared with the 25 watts produced by a fixed-mount VHF radio, or the one to five watts typical for a handheld VHF radio. However, this apparently impressive number is peak, not average, power, which is typically no more than a few watts. High peak power is necessary to provide reasonable assurance of target detection. The strength of the transmitted energy pulse is rapidly diminished by the distance over which it must travel. The transmitted radar energy does not behave like a highly focused, non-dispersing laser beam. In order to ensure target detection when the vessel rolls or pitches, it must be spread over a rather wide vertical angle, usually ±12°. Horizontal beam angles must be narrow in order to permit separation of closely spaced targets.

Typical horizontal dispersion angles for slotted waveguide antennas are 5.7° for a 16-inch antenna, 4° for a 21-inch antenna and 2.4° for a 42-inch antenna. The longer the antenna, the narrower the horizontal beam angle. (Radar units with long antennas, often more than 10 feet long, are commonly used on tugboats on inland rivers. Although antennas of this length are associated with sets having maximum range capability of more than 70 miles, their use on the rivers is dictated by their ability to provide excellent target discrimination. They can clearly show closely located targets as separate objects, while a shorter antenna, with its wider beam angle, would show the targets as one return.)

Diminished returns

By the time the radar's signal reaches a distant target the energy is spread over a wide area. Under the best of circumstances, only a small fraction of the radiated energy hits a target's surface. Of this, an even smaller portion will be reflected back toward the radar that originated the signal. The strength of this reflected energy will be further diminished by the distance it must travel back to the receiver. In addition, except under ideal conditions, a significant amount of both the incident and reflected energy may be absorbed by Earth's atmosphere. Heavy rain can totally blind most marine X-band radars, even at quite short distances.

Today's radar transmitters, antennas, receivers, and information processing circuits are truly excellent; however, without a decent return signal they are worthless. In order to gain the safety that comes from being visible on a vessel's radar we have to lend a helping hand. Our vessel needs to be a good reflector of radar energy. Marine radar sets operate on either X-band, centered at 9.41 GHz (9,400 MHz) with a wavelength of 3.2 cm (1.25 inches), or S-band, centered at 3.05 GHz, with a wavelength of 10 cm (3.9 inches).

Small-craft radar is primarily X-band, while large ships are fitted with both X- and S-band sets. The lower-frequency S-band offers advantages both in its ability to penetrate rain and in the reduction of confusing echo effects from the ocean's surface. The advantages of the lower-frequency radar are in part offset by its relatively poorer ability to detect and display small targets and to separate closely spaced targets. Ships typically rely primarily upon their S-band equipment when in the open sea, using X-band radar when in coastal areas. This fact can be important for those who voyage on open waters and wish to be as visible as possible on both X- and S-band radars. Regardless of the type of radar reflector used, it will be significantly less effective for S-band radar.

A radar reflector must be made of a material that is opaque at the frequency of the radar. Metal, even in the form of thin foil, is ideal for this use. In order to achieve satisfactory performance, the dimensions of a reflector must be a substantial multiple of the wavelength of the incoming radar energy.

Although pleasure craft are not governed by the requirements of the International Maritime Organization (IMO) or its referenced standards, IMO guidelines can be useful for determining desirable minimum performance for large vessels. The ISO standard for ship's radar reflectors requires a 10-square-meter effective equivalent reflecting area (radar cross-section, RCS) for an X-band radar. (A 10-square-meter RCS is the equivalent of a theoretical sphere with a diameter of 1.78 meters or 5.8 feet.) The specification requires that, when the reflector is within ±3° of horizontal, this reflecting capability must exist over at least 240° in azimuth, with no degradation greater than 6 db over an azimuth angle in excess of 10°. When the reflector is tested between ±15° from horizontal, the maximum degradation may not exceed 12 db. (Each decrease of 3 db is equivalent to a 50% decrease in reflecting area.) A radar reflector of a size practical for use on a small boat cannot provide a 10-square-meter RCS. From a practical standpoint, it is generally agreed that the minimum effective X-band reflector for small craft should be equivalent to a sphere whose projected area is 2.5 square meters (26.9 square feet). This radar cross-section will perform very poorly with S-band radar.

Reflectors dependent on shape

A sphere or a flat plate is a poor choice as a practical radar reflector. The sphere will reflect radar energy from any direction equally well or, perhaps more to the point, equally poorly. Depending on the relative angle of the arriving radar energy, a flat plate will work either very well or not at all. In this regard, the reflecting characteristic is like that of a mirror illuminated by visible light. Since there is no way of knowing at what angle the radar energy will arrive, using a flat plate reflector is pointless. An effective radar reflector must be able to present a significant RCS regardless of the direction from which it is illuminated by a searching radar signal.

The familiar, highly reflective highway lane markers, often called cat's eyes, are a type of corner reflector. The highly reflective coatings used on road signs are often made of miniature glass beads that act as corner reflectors. If three mirrors are placed at right angles to one another, a beam of light arriving from anywhere within a reasonably wide range of angles of incidence will be reflected back to the source of the light. Three metallic surfaces, placed at 90° to one another, can reflect radar energy, just as mirrors reflect light. The efficiency of the reflector will vary with the shape of each of the three plates. While square plates will be the most efficient, they will interfere with energy arriving at highly oblique angles and limit the effective angular range of the reflector. Plates in the form of quarter circles offer improved angular acceptance at the expense of somewhat less reflection efficiency. Plates in the form of triangles allow the widest possible angular performance, although with the least reflection efficiency of the three possible choices. Trade-offs are a part of technology.

Corner reflectors have some inherent problems. The performance of any corner reflector is close to zero when the incident energy arrives in-plane with any of the reflecting surfaces. An effective reflector must minimize the chance that incident energy from a searching radar falls on the reflector at a poor angle. If a vessel always sailed on a precisely even keel, with no angular motion about the roll, pitch, or yaw axis, designing and installing an effective radar reflector would be simple. Three corner reflectors, stacked one above the other, with each rotated 30° about the vertical axis, would provide an effective reflector for a radar signal arriving from any azimuth angle. Unfortunately, for both radar reflectors and those susceptible to seasickness, boats are rarely still.

One answer to the need for satisfactory reflector operation over a wide and ever-changing range of incident angles is the use of multiple corner reflectors, typified by the familiar radar reflectors. When tested in a laboratory, these devices will exhibit, as a function of incident angles, variations of as much as 20:1 in their radar cross-section. At angles at which the reflector is good, it can be very, very good and, to continue along the nursery rhyme theme, when they are bad, it is very, very bad. In addition to the variations inherent in the basic structure of the corner reflector, small angular variations from the desired 90° interplane relationship of the reflecting surfaces can reduce effectiveness. Corrosion may also have a negative effect on reflection capability. On the basis of lab tests in which the reflector is fixed in relation to the radar beam, tetrahedron-shaped reflectors tend to perform poorly overall.

Reflector never still

In the real world, however, the radar reflector is virtually never still. It is constantly moving about, presenting ever-changing attitudes to the incoming radar energy. The effect of vessel motion is usually amplified by the fact that these reflectors are usually hung in the rigging. Although the reflector's motion may at first seem a disadvantage, it may work to the benefit of radar visibility. As the reflector moves about, a poor incident angle may become more advantageous. This equation works both ways, changing what was a good reflection situation into an ineffective one. Since the advantageous incident angles of the multiple-corner reflector type outweigh the disadvantageous incident angles, the end result is positive.

We may be able to capitalize on the highly variable performance of simple corner reflectors by using more than one on a boat. Without undertaking a rigorous mathematical analysis, it seems reasonable to assume that, when two similar reflectors are non-rigidly mounted in a boat's rigging, the chance of both being in either the best or worst angular orientation to radar illumination is slight. Perhaps, while one is at its best angle, the other is at its worst, or possibly both are at some intermediate position. Given the relatively low cost of such reflectors, this may be a reasonable approach to solving the stealth avoidance problem.

There are radar reflectors that use technology other than the corner reflector. Some use dielectric lenses to reflect incident radar energy. Others use specialized dielectric lens configurations, such as a Luneburg Lens. When tested in a laboratory such devices perform very well, often far exceeding the reflection efficiency of a corner reflector of similar dimensions.

The Lensref uses a Luneburg Lens construction in which multiple layers of dielectric material act in a manner similar to a reflecting optical lens. The layers of dielectric material concentrate incident radar energy at a single reflective surface. The concentrated energy is then redirected back to the source via the same dielectric layers. Since the construction of this type of reflector is constant throughout 360° of azimuth, the performance of this type of reflector can be much more consistent than a corner reflector. Performance beyond a definite range of heeling angles is limited by the compromise necessary in the construction of the lens. Increasing the working vertical angle of the lens reduces the overall effectiveness of the device. Published data indicates that this reflector operates reasonably well up to about 18° of heel. Properly mounted in the rigging, this device should be at an acceptable angle much of the time. Its small size, eight inches in diameter, can be an advantage on many boats.

Corner reflectors and dielectric lenses

The Cyclops 3 reflector is composed of two trihedral corner reflectors, one facing forward, the other aft, combined with two dielectric lens reflectors facing port and starboard. The reflectors are housed in a plastic enclosure specified for masthead mounting. The corner reflector placement, facing fore and aft, recognizes that motion about the pitch axis is generally less than motion about the roll axis.

Test data collected at the British Defense Research Agency laboratory shows outstanding performance within a range of ±3° from the horizontal. As long as the angle from the horizontal is within this ±3° range, and with the Cyclops 3 mounted on a masthead or on top of some other structure, the resulting radar return should be very satisfactory. (As with all reflectors, shadowing of the reflector, caused by placing it close to a metal mast, will create a range of azimuth angles over which the reflector's performance will be seriously degraded.) Performance falls off as the angle of heel increases, but this happens with most reflectors. However, up to ±15° of heel, the azimuth angles over which performance was degraded were reasonably small. These areas of poorest performance were concentrated at ±40° of heel from directly ahead and dead aft.

The Cyclops 3, shaped like a squashed football, measures almost 18 inches in length, 17 inches in width and nearly 10 inches high. It weighs 18.5 pounds. Installation on the masthead may create some challenges for the installation of VHF antennas and wind systems. This device is also available in two smaller sizes, the Cyclops 1 and 2. The reflective value of these smaller units is not covered by the test data supplied by the manufacturer.

As with all problem solutions, there is a cost-benefit relationship associated with radar reflectors. Basic corner reflectors are available at prices from about $20 to $60. The Lensref is typically priced at about $475. The Cyclops 3 is $495.

There is an alternative to being a passive radar target. An active device can be fitted to a boat that will, when illuminated by radar energy, amplify the incoming radar signal and re-transmit it, enhancing the signal returned to the searching radar. The Ocean Sentry, Radar Target Enhancer, made in the U.K. by McMurdo Ltd. and marketed by Pains-Wessex, can substantially improve the radar signature of any vessel. Although somewhat similar to the operation of a radar beacon (racon) or the radar transponders used in aircraft, this device radiates very little power, typically 0.8 watts. However, even this small amount of energy can significantly improve the radar visibility of a target when compared with the few microwatts of a reflected radar signal. The Enhancer is composed of an X-band receiver (it does not operate on S-band) and a microwave amplifier housed in a 2.5-inch-diameter, 20.5-inch-long, 2.4-pound cylinder. In the absence of an incoming radar signal only the receiver is powered up, drawing 0.035 amps from the 12-volt bus. When a signal is detected the amplifier operates, enhancing the radar return. Power consumption with the amplifier on totals 0.250 amps. Average power consumption is minimal. Coverage is constant through 360° in azimuth.

McMurdo Ltd. claims normal operation for Ocean Sentry for angles up to 12° from vertical. The manufacturer specifies that the device is useful at ranges from 0.5 to 12 nautical miles. The specified minimum radio frequency gain is 55 db; typical gain is 58 db. Referenced to the radar cross-section of the theoretical sphere mentioned above, these gain specifications yield 25- and 50-square-meter reflecting areas (RCS). The Ocean Sentry is best installed at the masthead or on a separate antenna mast where it will not be shadowed by metallic structures. A small control box is mounted near the helm and provides the power switch, a self-test switch and a radar-nearby audible alarm. List price for the device is $1,995, with a street price on the order of $1,300.

Being seen by another vessel's radar is surely worthwhile. This assumes, of course, that someone on the other vessel is looking at his or her radar. In addition to being a cooperative target, it is clearly worthwhile to be an active participant in collision avoidance. Small but highly effective radar sets, many with waterproof displays, are available for less than $2,000.

Combined with one or more radar reflectors, a radar set can take much of the anxiety out of navigation in areas with limited visibility.


Contributing editor Chuck Husick is a sailor, pilot, and Ocean Navigator seminar instructor.

By Ocean Navigator