Posted: August 1, 2013
Passive radar reflectors provide peace of mind.Electronics manufacturers have done a wonderful job bringing reliable, high-quality marine electronics of all sizes and applications to market. But there are times when old-school technology may be the best application for your situation, such as being seen during times of limited visibility.
Automatic Identification Systems (AIS) have revolutionized how boats are seen and identified electronically in any conditions. In restricted visibility, you want to make sure you’re seen, but what do you do if your boat’s helm doesn’t have the real estate for another piece of equipment, there’s no room for another VHF antenna, or you don’t have funds for additional equipment. Here’s where old-school radar-reflector technology may be the solution. But not all reflectors are created equal.
In order to understand how a radar reflector works, it is important to understand how radar works. A radar system basically sends electromagnetic pulses in an intended direction and detects targets or objects by measuring the difference in the time it takes for the pulses to reflect off an object and return to the emitting unit. It doesn’t matter whether the objects are moving or not.
How They Work
A radar reflector is a device that is attached to a boat to make the boat more visible on radar. Passive radar reflectors consist of several pieces of metal that intersect with each other to create a geometric shape that reflects the radar pulse so the boat appears on the radar monitor. There are a number of different styles and designs available, and several boating safety organizations have tested a range of radar reflectors to see which models and designs seem to work best, obtaining varying results — some of which seem to indicate that even with a radar reflector, a boat can sometimes be hard to spot on radar.
The best analogy would be to imagine trying to get a reflection from your spotlight by shining it at a mirror fastened to another boat in choppy seas. The rougher the seas, the more erratic the reflection.
A radar reflector needs to be positioned as high as possible on the boat to ensure the device achieves its maximum reflective range. Sometimes, if the reflector is not positioned properly, it reflects pulses inefficiently, leading to unwanted “nulls” or blind spots. The larger the reflector, the greater the radar cross section (RCS).
There are several radar reflectors on the market, but there are two main types of passive reflector technology in use: the octahedral corner reflector and the Luneburg lens. Octahedral reflectors produce characteristically strong peak reflections separated by areas of very little reflection. In other words, slightly different orientations of the radar reflector cause dramatically different RCS. This causes the size of the radar target to vary as the radar reflector yaws and pitches. The target appears and disappears due to interruption of the reflected pulse to a vessel using radar.
Multi-element reflectors attempt to smooth out the peaks and nulls by using a number of corner reflectors, an approach that results in a somewhat more uniform reflection over azimuth and altitude. Luneburg lenses focus the radar energy to a reflective band inside the element, which is reflected back on a reciprocal bearing to the source, producing a reflective pattern with a very uniform range of operation with limited nulls.
An effective radar reflector is one that will consistently return echoes through 360 degrees, up to 20 degrees of heel, with few gaps in its reflectivity pattern. Marine radar reflectors are not foolproof, however. Sometimes, even if you follow all the necessary and standard procedures, the reflectors can fail. But the margin of error is quite low.
Wooden and fiberglass sailboat skippers know they have a radar problem, and most equip their vessel with one of the various passive reflectors. Powerboaters tend to think their vessel is substantial enough to reflect back a reasonable amount of radar pings, and many of them are correct. A lot of vessels support enough metal hardware and internal machinery to do a fair job, but in rough water the effectiveness diminishes.
Today, pleasureboats and ships move fast. Ships’ bridges are lightly manned, meaning the use of radar is critical for collision avoidance. The bottom line: I’m not satisfied to look merely my size in traffic, especially with ships that are elephants compared to my 40-foot mouse. I’m not one to draw attention to myself normally, but on the rare occasion of cruising in low-visibility situations, I want to have a swagger and be a large, strong target on the radar screen of other boats and ships in the area.