Radar Reflectors for Submarines

Randy L. Haupt, Daniel Aten, Sue Ellen HauptThe Pennsylvania State University

Applied Research LaboratoryP. O. Box 30

State College, PA 16804-0030

Introduction

Boat collisions are of great concern, especially in very crowded areas, like theChesapeake Bay, where large commercial ships intermingle with a myriad ofsmall pleasure boats. To prevent collisions, the boats use a maritime radar that ishorizontally polarized at 9.41 GHz. These radars provide plenty of warning whenthe boat targets are large and metal. Many pleasure boats, however, are small andhave limited metal parts resulting in a very small radar cross section (RCS),especially in the fore and aft directions [1].

This paper presents our investigation into the use of small passive reflectors toincrease the RCS of a submarine while it is near other ships. In particular we lookat computer models of the Echomax 305 [2] that is currently used by the US Navyand compare to other alternative possibilities.

Figure 1. Elliptical cylinder model of sail.

Figure 2. Alternative sail model.

Computer Model of the Submarine

Since the sail is the dominant scattering surface protruding above the water, weestimated the RCS of the sail using two two-dimensional method of moments

978-1-4244-3647-7/09/$25.00 ©2009 IEEE

modes. The first model is an elliptic cylinder (Figure 1). The second is a shapethat is a half cylinder fore (r=0.75m), flat sides (length=2.1m), and a wedgeshaped aft (side=2.1m) a shown in Figure 2. The two-dimensional results fromthis model (G'2D) can be accurately converted to three dimensional RCS (G'3D)

using the simple formula [3]

(1)

The dimensions of the models do not correspond to any particular submarine butseem similar to the dimensions of some submarine sail pictures found on the web.

Reflector Models

The Echomax 305 is made from thin sheet aluminum and consists of a stack ofthree quad trihedral comer reflectors separated by circular plates (Figure 3). Thereflector is placed inside a cylindrical plastic shell that has a very small effect onthe RCS, so is ignored in the computer model. The peak RCS of this reflector isadvertised as 17.1 dBsm. This was confirmed with a numerical model of thereflector. It is 71 cm tall and 32.2 cm in diameter. The second reflector is a .3 m2

flat plate placed perpendicular to the sides of the two-dimensional sail models asshown in Figure 4 and Figure 5. The peak RCS of the flat plate is 30.5 dBsm.

Figure 3. Computer model of the Echomax 305.

Figure 4. Elliptical cylinder with reflector.

Figure 5. Alternative sail model with reflector

Results

The RCS of the two-dimensional sail models with the reflectors are shown inFigure 6 and Figure 7. In both cases, the addition of the flat plate reflectors raisedthe fore and aft RCS but not the port and starboard RCS. The Echomax 305 maybe able to raise the RCS of the aft RCS of the alternative sail model but not theelliptical cylinder model.

Conclusions

The Echomax 305 was designed to give a more azimuthally symmetric RCS.Since the submarine has a large RCS at port and starboard already, a better designis possible. A flat plate placed against the side of the sail appears to have a muchbetter chance of increasing the submarine RCS compared to the Echomax. Thistype of reflector would also be smaller and easier to store and handle.

References

[1] P.G. Gallman, Radar Reflectors for Cruising Sailboats, Canada: UlyssianPublications, 2005.

[2] http://www.echomax.co.uk/EchomaxProducts.htm.[3] C.A. Balanis, Advanced Engineering Electromagnetics, New York: Wiley,1989.

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Figure 6. RCS of the elliptic cylinder with (solid line) and without reflector(dashed line).

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Figure 7. RCS of the alternative sail model with (solid line) and withoutreflector (dashed line).