|
Radar Reflectors Radar Reflector Test provided by West Marine, for additional information dial (800)BOATING. INDEX Characteristics of Marine Radar The Reflectors Tested Octahedral Reflectors Davis Echomaster and Emergency Holland Yacht Equipment Other Trihedral-based Reflectors The Firdell Blipper Mobri High Gain Rotation Cyclops Non-Trihedral Reflectors Lensref Radar Flag The Tests The Results Target Pattern Maps Conclusions Footnotes Almost nothing provokes as much fear in the hearts of sailors as the thought of a collision with a ship. Rogue waves and killer storms, maybe, but there are a lot more ships than either of those. The good news is that a ship keeps a careful watch and they always use radar, so avoiding a collision is simply a matter of a good radar reflector. Or is it? Just how well do radar reflectors work, anyway? Thanks to the generous cooperation of SRI International, in Menlo Park, CA, we had an opportunity to find the answer to that question. West Marine provided samples of 10 commercially available radar reflectors, which were tested in SRI’s large radar test chamber, normally used for testing such things as satellite antennas and stealth bombers. Participating In the tests were Eldon Fernandes, the operator of the range and an employee of SRI; Dick Honey, a Sr. Principal Scientist at SRI; Stan Honey, Vice President of Technology for News Corporation and a former Research Engineer at SRI; Chuck Hawley, Technical Director for West Marine; and Jim Corenman, Sailor at Large.
Characteristics of Marine Radar A ship will typically use her X-band unit near shore, due to its higher resolution and ability to detect smaller targets. In conversations with ship’s officers, nearly all indicated that in offshore waters they depend entirely on the S-band unit set to a 24-mile scale. The advantage of S-band in this situation is longer range, less interference from rain, and reduced interference from sea clutter (a factor of about 2½, or -4 dB). This is not good news for radar reflectors, however, since performance falls off as the square of wavelength. This means that, at least in theory, a given reflector will have an S-band return of only one tenth (-10 dB) compared to its X-band performance. In a situation where the return from the sea state is the limiting factor, part of this loss is made up by reduced sea clutter, but the effective return will still be reduced to one fourth (-6 dB) compared to X-band. A digression on units of measurement … Radar reflector performance is normally characterized in terms of Radar Cross Section, or RCS, measured in square meters (m2). The measurement of RCS is referenced to a conductive (metal) sphere of the specified cross-sectional area, using the familiar p r2 formula for the area of its cross section. Performance of a reflector is expressed in decibels (dB) relative to some reference, typically a 1.0 m2 sphere, but occasionally some other reference. Decibels are a relative measurement, and are log-based, with 3 dB representing a factor of two, and 10 dB representing a factor of 10. So saying that a signal is 3 dB below a 1.0 m2 reference (or -3 dB) is the same as saying it is half as big, or 0.5 m2. One thing that helps put everything into perspective is to consider that the radar return from a typical duck is about 0.1 m2, so our 1 m2 reference sphere can be also described as 10 duck units, or 10 du’s. Not very big. So having properly introduced the technically hip decibel units, we will now go on and talk about things we can visualize, like square meters and duck units. Everyone understands that, when it comes to radar reflectors, more ducks is better. The central question, of course, is just how many ducks does it take to be seen by a ship? There are as many answers as there are ships and radar operators, but the typical response that keeps coming back is that the lower limit of detectability using X-band radar in a moderate sea is 1 to 3 m2 (10-30 du’s). GEC Marconi (the manufacturer of the popular Firdell Blipper), statesi that 2.5 m2 is "generally accepted in the radar business as the ‘threshold’ of radar detectability" at X-band, and proposes a minimum average RCS of 2.5 m2 (25 du’s), with no gaps below 2.5 m2 that are larger than 5° x 5°. In response to the question of how far away a sailboat can be picked up on radar, the most common answer is three to six miles without a radar reflector if it can be picked up at all, and a better chance of picking it up with a reflector. These numbers are purely anecdotal, based on conversations with ship’s officers, but are consistent with those reported in Radar Detectability and Collision Riskii. These detection ranges are certainly less than those of us who sail small vessels would hope for, and less than we’ve somehow been led to believe. What we will try to accomplish in the space that remains is to sort the truth from the claims, and bring some perspective to bear on the problem of visibility at sea. The one thing that is key to the performance of any reflector design is size. The reflective performance of any type of reflector is proportional to the fourth power of its linear size. In other words, doubling the size of a reflector results in an increase of effective area of 16 times, or a 12 dB increase. Stated differently, an increase in size of a reflector of 19% will double its performance. Further, as the smallest dimension of a reflector gets down to a few wavelengths of the radar signal, it quits acting as a reflector and starts to act as a lump of metal. Remember that a wavelength is 3.2 cm (1¼") for X-band, and 10 cm (4") for S-band. So small detectors must be looked at with a great deal of suspicion, as there really is no substitute for size. Another issue is the increasing reliance upon ARPA systems aboard ships. These systems automatically capture and track radar targets, and provide a warning to the watch when a close approach is predicted. An ARPA system will only work with targets that are visible on the radar, however, and typically a minimum of three consecutive "hits" is required on the ship’s radar before a blip is acquired as a target. This puts a premium not only on the strength of the return, but also its consistency.
The classic octahedral reflector is made of three planar circles or squares of metal intersecting at right angles, forming eight trihedral reflectors. In the usual "catch rain" position, one trihedral will face up and one down, and the remaining six are arrayed around a circle, three oriented 18° above the equator, and three 18° below. This optimizes the return from the "pockets", and avoids the nulls or gaps as best as is possible, but only at a 0° angle of heel. Considerations of heel angle has led to the "double catch rain" position (see fig. 2), with one planar surface oriented vertically along the vessel’s axis, and the other two planes ±45° from the vertical. This is not the ideal with no heel angle, but moves towards the "catch rain" position as the boat heels. Davis Echomaster and Emergency
We tested three octahedral-type reflectors. Two Davis products were included, the 6.25" radius spherical Echomaster, model 153, and the Emergency model 151. The Emergency model has circular plates of 5.5" in radius, and is constructed out of foil laminated over a foam core. It can be disassembled into three disks for storage if desired. The Echomaster Deluxe is constructed from anodized aluminum disks and has a radius of 6.25". It comes with a plastic and stainless bracket which attaches to the intersection of all of the plates, although it was removed for our tests. When installed, this bracket makes is easier to suspend or mount the device in the so-called "catch rain" position.
|
