The Tests
We were very fortunate in being able to secure the use of the radar testing facilities at SRI International in Menlo Park, California. The tests were conducted in SRI’s large anechoic chamber, which measures 20’ x 20’ x 40’. The target is centered near the far end of the chamber, on a radar-transparent pedestal that can be rotated through 360°, and the axis of rotation can be tilted up to 12°. A calibrated broad-band microwave transmitter/receiver is located in the wall at the opposite end of the chamber to accurately measure the reflected signal.

The walls, ceiling and floor are completely covered with semi-conductive, radar absorbing foam pyramids which absorb any stray radar signals and prevent any reflections back to the receiver, except those from the device under test. A retracting gangway extends through a door in one wall (also covered with foam pyramids) to provide access to the test pedestal.

The chamber is calibrated in terms of absolute RCS, and optimized to measure the very small radar returns from certain types of military aircraft. The background return is on the order of -60 dB (a millionth of a square meter). Calibration was checked before and after each days testing.

An HP minicomputer provided data logging and azimuth-elevation control of the test pedestal. Data was taken simultaneously at 3.05 GHz (S-band) and 9.41 GHz (X-band), recorded directly to disc and plotted to a laser printer. The data was subsequently converted to text files and transferred to floppy disc files for further analysis.

The first series of tests were performed during a two-day period, October 20-21, 1994. For each test, the gangway was extended, and the reflector being tested was secured to the test pedestal with non-reflecting tape and foam. The chamber was then closed, and the reflector was rotated at 1° increments through 360° while data was recorded. Each rotation required about 20 minutes.

Fig. 3. Test orientation for a "heeled" reflector (catch rain position shown).

Most of the reflectors were tested twice. A first test was done with the major axis perpendicular to the radar beam, corresponding to a reflector mounted upright on a vessel with no angle of heel, and viewed from every angle around the vessel. A second test was done with the reflector secured at an angle to the pedestal, corresponding to a reflector mounted upright on a heeled sailboat while a ship steams a circle around it. In this latter case the radar beam strikes the reflector at angles along an inclined plane, both above and below its equator (see fig. 3).

The Results
The results of selected individual tests are shown in graphical form as figures 4 to 8. Radar Cross Section in m² is shown as polar plots for both X-band and S-band, indicating the strength of the reflected signal in that would be seen by a ship steaming in a circle around a reflector located in the center. For each plot the outer ring represents the minimum RCS threshold (2.5 m² for X-band and 1.0 m² for S-band), and the inner ring represents 1.0 du, one duck unit.

Using the criteria for minimum reflectance described above, Table 1 was prepared as a summary for all of the devices tested. For each radar band, the first column indicates the average Radar Cross Section (RCS) in square meters for the reflector, using the recommended RMS (Root Mean Square) average. The second column indicates the percentage of angles that were greater than the minimum threshold (2.5 m² for X-band), an indicator of the "visibility" of the reflector, or the probability of being seen by a ship at an unknown horizontal angle. The third column indicates the largest angle that was less than this threshold, i.e. the angular width of the largest "blind spot". For S-band, a minimum threshold of 1.0 m² was used, reflecting the 4 dB reduction in sea state return experienced at S-band, allowing a smaller return to be detected.

The table data are sorted first by X-band "visibility", the percentage of return greater than the threshold, then by average RCS for the reflectors with no return above the threshold.

Using this criteria, the Davis Echomaster was the clear winner, but showed the deep nulls associated with an octahedral reflector. The peaks were as high as 25 m², but these peaks were too narrow to have any real significance. S-band performance was lower than X-band, but by less than the expected factor of 10.

Fig. 4. RCS plot of the Davis Echomaster in the double catch rain position with 0° heel.

The best overall performance for the Davis was not in the often-recommended "catch rain" position, but in the "double catch-rain" position, which has the advantage of very little degradation of performance with heel. The average RCS was 5.0 m² upright, and 4.5 m² heeled 20°. Visibility was not great, however, at less than 50% (see fig. 4).

The vertex-up position provided the best performance with a 0° heel angle, but quickly deteriorated as the reflector was heeled, and is not recommended.

The Lensref was a close second in this tabulation, with an average RCS of 2.4 m² (see fig. 5). Interestingly, this is just a fraction below the somewhat arbitrary threshold of 2.5 m². If we arbitrarily assign a threshold of 2.0 m² instead of 2.5 m², then the Lensref goes straight to the top of our chart, with virtually 100% of the return greater than the threshold (i.e. no gaps), an outstanding performance amongst this lot. Only three of the Lensref data samples came in at less than 2.0 m², and then only by one or two hundredths, truly splitting ducks. Performance of the Lensref on S-band is pretty marginal at 0.4 m² average RCS, due to its small size compared to the wavelength.

Fig. 5. RCS plot of the 8" diameter Lensref with 0° heel.

The limited angle of heel is a serious limitation for the Lensref for a sailing application, as angle of heel is often greater than the 18° limit of the device. Fitting a gimbal would solve this limitation neatly, but would need some engineering to avoid uncontrolled swinging, as the Lensref is not a lightweight device.

The poor performance of the Firdell Blipper was surprising, given its popularity and reputation. When measured at X-band with no heel, the 210-5 model fitted to most boats was only visible over 7% of the horizontal angles, and only a few peaks exceeded the 2.5 m² threshold (see fig. 6). The average return was 1.4 m², or 14 ducks, and the largest gap was over 90°. When heeled to 20°, the performance of the 210-5 deteriorated about 20%. The larger 210-7 model had a 20% higher average return of 1.7 m². Measured at S-band, the 210-7 performed about 50% better than the smaller unit, and was "visible" for 26% of the angles compared to 8% (with a 0° angle of heel).

Fig. 6. RCS plot of the Firdell Blipper 210-5 with 0° heel.

The Radar Flag reflector gained a high ranking in its flat configuration, spread out in a vertical plane. In this orientation it exhibited a very strong return perpendicular to the plane of the flag, but almost no return at other angles. It was "visible", with a return above the threshold, for only 9% of the angles. In a more typical "drooped" orientation, the Radar Flag was essentially invisible, with an average return of only 0.4 m² (4 ducks), and not above the threshold at any angle (see fig. 7).

Fig. 7. RCS plot of the Radar Flag (draped) with 0° heel.

The Davis Emergency reflector is the last of the lot to provide a return above the threshold at any angle, but is far from its bigger brother. The other reflectors were generally limited by their small size.

The two Mobri reflectors performed as might be expected, and were essentially invisible. Only the larger 4" diameter (2" radius) device came anywhere near detectability, with an average return at a 0° angle of heel of just over 1 m², with no deep nulls. On S-band, the average return was almost 0.5 m², not enough to be detected, but better than most. When heeled, however, things fall apart and the return drops to a few duck units (see fig. 8). The smaller Mobri is invisible under all conditions, and, with its minimal windage, might make a nice addition for the Stealth Bomber.

Fig. 8. RCS plot of the 2" radius Mobri with 20° heel.

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