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Radar visibility

Feb 28, 2012

To the editor: The recent article by Philip G. Gallman on the visibility of yachts as seen by professional ship radars (Professional radar, poor visibility, Issue #199, January/February 2012) was thought provoking. I’d like to know a bit more about the calculation of the red curves in the two graphs, the “actual returns” of the signal a ship receives from a point source radar reflector as on a small yacht. What I think I’m seeing here is interference between the signal returned from the direct beam from the ship, and that returned from a beam reaching the yacht after reflecting from the ocean. There’ll be an assumption of a flat sea, most likely, and of either 3-cm or 10-cm radar. The path difference would vary with separation of the two vessels, hence the phase difference between the two reflections, and thus the strength of the reflected signal. Is this correct?

If so, might the yachtsman improve the situation by mounting a second radar reflector, perhaps five or 10 meters above the first? And, if this would help, what separation of the reflectors would be optimal, or are there too many variables to permit a meaningful recommendation?

—Emanuel Laufer sails a C&C 34 Ceol Mor, is based in Halifax, N.S., and teaches celestial navigation with the Canadian Power & Sail Squadron.

Philip Gallman responds: Emanuel Laufer is quite right in his understanding of the “actual returns” chart. Echo strength depends on several parameters: wavelength, radar power, height of the radar antenna above sea level, sea state, radar cross section of the reflecting object, height of the reflector above sea level, and, of course, the distance between the radar and target. I estimated all parameters used in calculating the charts from documentation for the ship-sailboat collision I refer to in the article. In particular, I assumed X-band and sea state 3. I am confident of my assumptions because my calculated blind zones match the radar screen dumps from the collision very well. Different parameters would give different results.

One might be able to optimize performance for one specific radar antenna height by mounting a second radar reflector. However, any other antenna height or a different ship could produce worse rather than better performance. In short, the second radar reflector would help in some encounters and make matters worse in others. I do not recommend multiple radar reflectors.