This article addresses the performance of active radar target enhancers (RTEs) as a companion piece to “Radar Reflector Performance,” (Issue No. 152, March/April 2006), analyzing the performance of passive radar reflectors. As in the previous article, the multi-color analytic radar cross-section (RCS) diagram is used to describe and compare performance. The tradeoff between active RTE and passive reflectors and unique features of active RTE are discussed.
Active vs. passive
Passive reflectors are made of flat metal plates or lenses assembled in such a way to provide strong reflections of incoming radar pulses over as large a range of azimuth and elevation angles as possible. The size of the device as a radar target, the radar cross section (RCS), may be calculated analytically from the reflector’s physical configuration. Active units are made of a receiving antenna, electronic amplifier and transmitting antenna. These units capture an incoming radar pulse, amplify it and transmit it back toward the radar. RCS is also used to describe the size of an active device as a radar target, and RCS may be calculated analytically from the antenna pattern and the gain of the amplifier, as I did to generate the analytic RCS diagrams shown here.
Analytic RCS diagrams for two active RTEs, the Sea-Me and the Ocean Sentry, are shown. A third active unit, the Tideland Signal TE-70X, is similar in performance specifications to the Ocean Sentry but is beefed up for the commercial aids to navigation market. It was omitted because it costs more than $4,500. A fourth unit, the Kannad Activ’Echo, is similar to the Sea-Me in price and general performance except for smaller output power. It was omitted because detailed information was not available. Links to manufacturers may be found on the “Radar Reflector” page of www.TheRadarReflectorSite.org.
The recommended “stated performance level” for radar reflectors is 7.5 square meters. Any of the green shades in the analytic RCS diagrams indicate at least 10 square meters, and red indicates less than 0.625 square meters. Green is very good and red means that the unit probably won’t be detected. The two darker shades of green indicate larger RCS than found in most passive reflectors.
The primary tradeoff between passive and active units is performance vs. cost. Active RTEs generally provide much larger RCS over a large range of azimuth and elevation than possible with practical passive reflectors. Variation of RCS with changes in aspect is much smoother for active than with passive units, and isolated small regions of poor response are virtually eliminated. This improved performance comes with a price penalty. Purchase cost is in the $950-to-$1,400 range versus $50 to $530 for passive reflectors, and this does not include the higher installation cost because of the required electrical work.
Amplifier saturation
Unique to active RTEs, limitations in the electronics produce an effect at short range that must be considered. As an active RTE approaches a radar, the incoming pulse gets more powerful and the retransmitted pulse increases proportionally, keeping the effective RCS constant, but only as long as the amplifier can provide the required output power. At some point, as the range decreases the desired output power exceeds the capability of the amplifier and the amplifier saturates. As the RTE continues to approach the radar, the incoming pulse power keeps increasing but the outgoing pulse power stays fixed at whatever the amplifier’s limit is, and the effective RCS steadily decreases. The nominal RCS will only be achieved if the range is great enough that the amplifier does not saturate. At shorter range, the effective RCS is reduced.
Since high-power radars overload the amplifier at greater range, the effect is more noticeable with more powerful radars. Given the power limits of the active units discussed here, one might expect a highpower commercial radar to overload them at about seven miles and a lowpower recreational radar to overload them at about one mile. At shorter ranges, the benefit of active RTEs over passive reflectors decreases.
Amplifier saturation should not affect detection in good weather. In clear weather, the target must stand out against the system’s electronic noise. Even when saturation occurs, the returned pulse power steadily increases as the RTE approaches the radar, even though the rate of increase is slower than at ranges for which saturation does not occur. Once the detection threshold is exceeded, the returned pulse power will remain above threshold, and the RTE will continue to be detected. Detection in bad weather is another matter. In bad weather, the RTE must also stand out against the background environmental clutter from rain or waves. Since the effective RCS decreases as the RTE approaches the radar, at some point it becomes smaller than the clutter-limited detection threshold, the RTE no longer stands out against the clutter and won’t be detected.
As an example, I estimate that the effective RCS of a nominally 34-square-meter active RTE (typical value for the Sea-Me) begins to decrease at seven miles from a 50-kW, big-ship radar and is reduced to about 3 square meters at about three and a half miles. In this case a common passive reflector might provide better detection in wave clutter at ranges of importance to collision avoidance than the more expensive active RTE.
Other considerations
There are several other issues with an active RTE of which the potential purchaser should be aware. Like all electronic devices, these are subject to failure. They also consume power. Standby current consumption is generally small, 60 mA for the Sea-Me, but even that adds up and could be important for long-distance voyagers.
There are unique mounting issues. First, the units are so small that mounting in front of the mast, as is usually done with passive reflectors, would produce a very large radar shadow aft. You are almost forced to find some other mounting location such as on a pole, on the spreaders or at the masthead. The unit must also not be mounted where it is exposed directly to the beam of your own radar for fear of burning out the front-end electronics.
The available active units do not operate at S-band, only X-band. From a practical point of view this should not be a serious concern. Under the SOLAS Convention, vessels more than 300 gross tonnage must carry an X-band radar, and vessels more than 3,000 gross tonnage must, in addition, carry an S-band radar (or a second X-band radar). The implication is that X-band radar would always be active.
Availability of active RTE increases the number of options for improving a vessel’s radar signature and makes the selection process more complicated. Each design, whether it is a passive radar reflector or an active radar target enhancer, provides a different balance of performance, complexity and cost. Enough designs are available to suit your specific needs.
Philip Gallman is an avid sailor who lives in northern Virginia with his photographer wife Minnie.
