Safety At Sea Studies - 1995 Radar Reflector Tests Pg4
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Target Pattern Maps A Target Pattern Map (TPM) is a method of representing three-dimensional data on paper, where azimuth (horizontal) angles are shown on the horizontal X-axis, elevation data is shown on the vertical Y-axis, and the strength of the return is shown by color or gray-scale shading. Color is dramatic, but unfortunately expensive to reproduce. A study of GEC Marconi’s published TPMs reveals two interesting anomalies. First, the comparison to a 12" octahedral shows peaks of barely 2 m2, while a 12" diameter spherical octahedral reflector (such as the Davis) would be expected to have a peak RCS associated with the axis of the "pocket" of each trihedral reflector of 8.3 m2. The answer to this mystery may lie in the formula for the return for a triangular trihedral. An octahedral made from 8½" square plates would form triangular trihedrals, and would measure 12" (i.e. a 6" radius) across the largest dimension. The theoretical peak return for this device would be 2.2 m2, which corresponds to the Bell/Lark TPM. So the comparison must be to a 12" triangular octahedral, not a spherical octahedral such as the Davis Echomaster. The second problem with GEC Marconi’s data is with the characteristic of the peaks for the Firdell Blipper 210-7 shown in the TPM. A preponderance of peaks are shown greater than 3 m2, with a vertical interval from peak to peak of about 3°, the basis for GEC Marconi’s statements that the "gaps" are "only between 1 and 2 degrees wideiii". The vertical peaks and nulls are the result of interference effects between vertically spaced reflector elements, which will either add or cancel as the heel angle is changed. Both the size and the vertical spacing of the reported peaks, however, are consistent with a device much larger than the 210-7. A second series of tests were performed on 26 March, 1995 to investigate these assertions. The computer-controlled target pedestal elevation control was used to take data at elevation angles from -12 to +2 degrees and azimuth angles over 180 degrees without remounting the reflector. That range of angles was felt to be representative, and was within the time available in the chamber. Data was collected for the Davis Echomaster and the Firdell Blipper 210-5.
Interestingly, it was not possible to reproduce the results reported in the Bell/Lark paper. The Davis Echomaster showed the expected pattern, with a broad reflectance peak corresponding to the axis of each trihedral corner reflector, and a sharp peak matching the orientation of each planar surface. The returns associated with each trihedral reflector peaked at just over 6 m2, compared to a theoretical peak of 9 m2. The average (RMS) return was 2.6 m2 for the Davis over the area tested, consistent with the earlier tests. The Blipper results that we obtained were quite different from those that were reported in Bell/Lark. The areas of low reflectance extended much further in elevation than reported, with a vertical interval of 6 to 7° between peaks. The overall average RCS was 1.6 m2, only slightly higher than our previous more limited tests, and only a very few peaks exceeded 3 m2. The issue of the size of the gaps seems a bit moot given the small size of the peaks. Figures 8 and 9 show the TPM data that we obtained for the Davis Echomaster and the Firdell Blipper respectively. The 210-7 is approximately 10% larger than the 210-5 tested, and would be expected to show peaks about 40% higher but only 10% smaller vertically, not a significant difference. While TPM’s are certainly a more comprehensive way of looking at reflector performance, for these types of devices a simple horizontal scan at two or three heel angles is more than sufficient to characterize the reflector. The additional complexity of a Target Pattern map is not justified for any of the reflectors tested here. The first conclusion is that there is no substitute for size when it comes to radar reflectors. The devices that offer smaller size and lower windage simply don’t work as well. With regard to the Firdell Blipper, it is a well packaged and clever device, but the models tested were not large enough to have much real value aboard a vessel. Larger versions would accomplish what GEC Marconi claims, but are not practical on small vessels.The Davis Echomaster (in the "Double Catch Rain" position) and the Lensref performed the best of all of the devices tested. The Lensref has no nulls, which is a tremendous advantage in terms of being seen, but the overall reflectance is marginal. If a Lensref is fitted on a sailing vessel, it should be gimbaled or made adjustable. The Davis Echomaster had stronger peak reflectance, but also large holes, which means that a large target would not consistently be presented on a ship’s radar. None of the reflectors would be more than marginally useful in offshore situations where only S-band were being used, except perhaps in calm sea conditions. The marginal performance of radar reflectors in general does not mean that they should not be carried. On the contrary, anything that improves a vessels radar visibility is worthwhile, particularly short-handed vessels and those without radar themselves. Beyond that, it needs to be again pointed out that the best defense where shipping is concerned is a good offense. A ship’s radar may only see a sailboat three or four miles away, but that same sailboat can typically see the ship 12 miles away by radar, and visually at least 8 miles away in clear weather. The small boat is both better equipped and more highly motivated to avoid the potential collision. Footnotes: |
