Breathing Easy

Because sailboats are notoriously poor radar targets, many sailors purchase a passive radar target enhancer (RTE), or radar reflector, to improve the vessel’s signature. How effective are these devices? For my recent book Radar Reflectors for Cruising Sailboats, I developed a method for describing and comparing RTEs. We’ll use it here to compare a few of the common passive radar reflectors on the market.

The strength of the radar signal reflected by a target is related to the radar cross section, or RCS. A larger RCS means the target will be detected at greater range, by lower power radar sets, in poorer weather conditions and more consistently. Consistent detection is especially desirable for collision avoidance because your vessel may be missed by a human operator and it may be ignored by automatic radar plotting aid software.

Radar reflectors cannot be completely described by a single RCS value. Rather, the RCS depends on the orientation of the radar reflector relative to the radar that is painting it. The orientation, or aspect, is simply the relative bearing of the radar from your vessel and the elevation angle of the line of sight to the radar relative to your deck. For a radar dead abeam, the elevation angle is your vessel’s angle of heel; for a radar dead ahead or astern, the elevation angle is your vessel’s pitch angle. A complete characterization of a radar reflector includes the RCS at all bearing angles, or azimuths from 0° to 360° and all elevations angles from -90° to +90° – although a smaller range of elevation usually is adequate.

Polar diagram

Radar cross section is commonly presented in a polar diagram. To obtain polar diagram data, the target is mounted on a rotating platform in an indoor radar range (radar anechoic chamber). A carefully calibrated radar system records the strength of the reflected signal as the platform rotates through 360� and graphs RCS against azimuth.

A polar diagram describes the RTE as long as it is vertical (elevation is zero). It does not represent performance if the RTE is tilted away from the vertical, as would be the case if the RTE is mounted on a vessel that is rolling or pitching in a seaway or simply sailing at a constant angle of heel. Consequently, single polar diagrams do not provide enough information to compare radar reflectors that are to be used on sailboats. Manufacturers sometimes provide several polar diagrams for different tilt angles, but this is not common practice and, even when the data are available, it is not easy to visualize performance given multiple polar diagrams.

Analytic RCS diagram

The data visualization problem may be overcome by a single quantized, color-coded RCS diagram showing the data from many polar plots. Such a diagram is easier to interpret than multiple individual polar plots, but obtaining enough anechoic chamber data to produce a detailed diagram is costly and time consuming.

My solution was to develop analytic models of all common RTE, calculate RCS over the entire range of aspect, and present the results in a color-coded analytic RCS diagram. An analytic approach is possible because all RTE are made of a few basic elements that have been analyzed and described thoroughly in the technical literature. Basic elements can be combined analytically to represent any RTE if the physical structure is known. The color-coded diagram allows one to visualize RCS over a large range of aspect and compare RTE with a common display.

Included here is the analytic RCS diagram for a Davis EchoMaster 121/2-inch octahedral, mounted in the normal orientation. RCS magnitude is quantized so only six colors are needed. I have used green to indicate an RCS greater than 10 m2, yellow between 5 and 10, dark blue between 2.5 and 5, light blue between 1.25 and 2.5, purple between 0.625 and 1.25, and red less than 0.625 m2. The 2:1 spacing of contours allows six colors to cover a large range of RCS. Essentially, green is good. RCS decreases progressing from yellow to purple, and red means you probably won’t be detected.

Examining the diagram, the major feature is the eight circular areas of green, yellow and dark blue centered at about 35� elevation. These correspond to the eight corner cube pockets that are the main element of the octahedral. The narrow vertical and horizontal green areas come into play at certain aspects. This diagram is interpreted as follows. The main response consists of eight cones oriented about 35� above and below horizontal. There is good response at very small elevation angles, roughly �4�, but even at zero elevation, there are small ranges of azimuth for which the response is red. Consequently, this RTE would be marginally useful as long as the vessel on which it is mounted does not heel more than 4�, or heels about 35�. For heel angles between a few degrees and about 20� there is a lot of red. You would present a very small RCS to the radar, and you probably would not be detected at most azimuth angles. There is more red than green or blue over the range of aspect encountered by monohull sailboats, so the octahedral, in normal orientation, may not be the best choice. On the other hand, there is a lot of green and blue for elevation angles less than 4� so this would not be a bad choice for a vessel that does not tilt much.

RCS is not the only criterion for selecting an RTE. Cost, mounting, weight and windage aloft, power consumption, and reliability also are important, as is the application, i.e., collision avoidance, search and rescue, fixed navigation aids. Even the type of vessel and expected sailing conditions are important for collision avoidance because of different ranges of heel angle. However, the analytic RCS diagram succinctly summarizes the variation of RCS with aspect and enables one to compare RTE on a common basis.

Philip Gallman’s book is Radar Reflectors for Cruising Sailboats: Why They Work, How to Evaluate Them, and What the Limitations Are, published by U

By Ocean Navigator