GB2227368A - Radar reflector - Google Patents

Abstract

A radar reflective balloon 1 contains deformable sheets 3A, 3B of plastics material 7, which form the corners of a radar reflector when the balloon is inflated. The inflated balloon, when tethered to a raft, provides an indication of its location by Doppler shifting incident radar, due to its spinning. Wind vanes 6A, 6B are provided on the outside of a housing 5 connected to the balloon. The vanes may be feathered so as to act as a governor. <IMAGE>

Description

Radar Reflector This invention relates to radar reflectors. In particular, but not exclusively, the invention relates to emergency radar reflectors which are used at sea by mariners, to either locate life rafts adrift or possibly mariners who have been lost overboard.

Similar radar reflectors have been suggested in the past, reference should be made to UK patent No. GB 139572, in which the radar reflector comprises substantially a balloon tethered to the raft so that the balloon acts as a radar reflector.

According to a first aspect of the present invention an improved radar reflector, comprises: a radar reflective sheet attached to an inner surface of an inflatable balloon.

Preferably a small pressurized gas canister is connected to the balloon. The balloon is then released to rise above the surface of the waves and is tethered by way of a length of cord or string either to a life raft or the individual.

The balloon is preferably fabricated from a material which is substantially transparent to radar and contains material which is reflective to radar. Preferably the radar reflective material comprises a thin plastics material sheet which has been coated with a radar reflective substance, and which radar reflective sheet is attached to the inner surfaces of the balloon at certain points. Due to the foldings in the radar reflective sheet within the balloon, and because of the generally deformable nature of the sheet within the balloon, when deflated, the balloon may be folded substantially flat and is relatively compact. On inflation the balloon expands so that it opens out the folded radar reflective sheet.

Preferably the radar reflective sheets are attached to portions of the balloon so as to substantially form a retro-reflecting corner radar reflector.

Advantageously, vanes are attached to the balloon.

These vanes rotate the balloon thereby aiding location, by enabling a Doppler shift, in any incident radar signals, to be produced. By filtering the resulting reflected signals in a search vessel, clutter (reflection from the waves themselves) is reduced to a minimum and and the position of the balloon becomes apparent.

The invention will now be described by way of an example only and with reference to the accompanying Figure 1, which illustrates a schematic cross-section through a balloon.

An emergency radar reflector comprises a rubber or synthetic rubber balloon 1, inflated by gas such as Helium 2b. Connected to the inside of the envelope of the balloon at specific points 13b, 13c, etc there are a plurality of deformable radar reflective sheets 3a, 3b, constructed so as to form retro-reflective corners within the balloon when it is inflated, said sheets having specific edges in common with another. These sheets may be fabricated from a plastics material and preferably have a metalised or radar reflective coating.

The balloon when inflated is tethered to a raft or mariner by way of a cord 4. In a preferred embodiment the housing 5 contains a Helium gas canister (not shown) which is directly connected to the mouth of the balloon so as to inflate it. The Helium gas canister inflates the balloon when operated. On the outside of the housing 5 are wind vanes 6. These wind vanes act to spin the housing and hence the balloon relative to the axis of the cord.

Incident radar waves from a search vessel or station pass through the transparent membrane and impinge on the reflective material where they are retro-reflected the radar returns are then detected at the search vessel or station. Because the balloon is spinning, the radar returns are Doppler shifted. This enables the shifted radar returns to be filtered from randomly reflected radar off the sea surface and hence an unambiguous radar location of the balloon is possible.

Preferably the vanes are feathered so as to act as a governor. Thus as the angular velocity of the balloon increases, the feathering mechanism opens out the vanes which then act to stabilize the rotational speed, and hence give a substantially constant Doppler shift to impinging radar waves. This makes filtering easy, as there is no need to scan the frequency spectrum at a receiving station because the Doppler shift occurs at a stable frequency band.

Another possible use for such radar reflectors could be as decoys to enemy radar. Balloons could be strategically launched or "laid" at sea and assembled with timing devices so they could launch themselves automatically in strategic zones when desired.

Claims (7)

1. to. A radar reflector comprising: a radar reflective sheet attached to an inner surface of an inflatable membrane.
2. 2. A radar reflector as claimed in claim 1 wherein means are provided to rotate the inflated membrane.
3. 3. A radar reflector as claimed in claim 1 or claim 2, wherein the rotating means comprise a wind vane.
4. 4. A radar reflector as claimed in claim 3 wherein the wind vane has a feathering mechanism.
5. 5. A radar reflector according to any previous claim wherein the radar reflective material comprises metallised plastic sheets.
6. 6. A radar reflector according to any previous claim wherein the sheets define a retro-reflecting corner.
7. 7. A radar reflector substantially as described and with reference to Figure 1.