Optical: systems and elements – Mirror – With mirror surface of varied radius
Patent
1995-07-24
1998-09-22
Dzierzynski, Paul M.
Optical: systems and elements
Mirror
With mirror surface of varied radius
359867, 356 3, 356255, 343914, 343912, 343895, 342 47, G02B 510, H01Q 1500
Patent
active
058123318
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to reflectors for polarimetric radars, intended in particular for use in calibrating a radar or for use as a beacon.
2. Description of the Related Art
It is known to use a right dihedral made of metal with a rectilinear line of intersection or "ridge" for calibrating a polarimetric radar, but that technique requires the relative positions of the reflector and of the radar to be very precisely controlled so that the incident beam lies within one of the planes of symmetry of the dihedral, because of the extreme sensitivity of the response of the dihedral as a function of angle, and in particular as regards its equivalent radar area (E.R.A.) of cross-polarization.
The use of such a technique is therefore limited in practice to laboratory calibration.
Recent studies have been made of reflectors constituted by two conductive surfaces arranged as if they had been generated by displacing a right-angled V-shape along a curved path, such reflectors being called "right pseudo-dihedrals". The published studies have dealt more precisely with the cases of a circular ridge and of an elliptical ridge, which can improve the reflective characteristics and facilitate the use of the reflector, as explained in the study "Theoretical and Experimental Study of a Crosspolarization S.A.R. Calibrator" by J. C. Souyris, P. Borderies, P. F. Combes and H. J. Mametsa, published in the proceedings of the SECOND INTERNATIONAL WORKSHOP ON RADAR POLARIMETRY (Nantes, September 1992), but those solutions do not provide improvement of all desired performance measures simultaneously, and compromise is therefore required.
SUMMARY OF THE INVENTION
The present invention provides a right pseudo-dihedral radar reflector which gives improvement of all desired performance measures simultaneously.
One object of the invention is to provide a pseudo-right dihedral radar reflector which makes on-site calibration possible, in the presence of on-board antennae, the orientation of which is known with approximate accuracy.
A further object of the invention is to provide a right pseudo-dihedral radar reflector which backscatters a sufficient level of cross-polarization energy in as wide a solid angle as possible, this being of particular benefit for beacon and for identification.
A still further object of the invention is to provide a right pseudo-dihedral radar reflector suitable for constituting a cross-polarization standard having as constant a response as possible over a given angular domain.
An object of the invention is also to provide a right pseudo-dihedral radar reflector having a cross-polarization pattern which is angularly enlarged without its pattern being modulated by undesirable ripple.
All these objects are attained in the invention when the ridge of the right pseudo-dihedral is in the form of a portion of a helix.
Preferably, the ridge extends over no more than one spiral turn of the helix.
Preferably, the tangent at a point of the helix subtends an angle of 45.degree. with the axis of the helix.
The orientation of the V during its theoretical displacement along the ridge varies in accordance with a relationship chosen as a function of the given trajectory of the polarimetric emission-reception system.
This reflector, placed in front of a polarimetric radar, i.e. a radar whose emission and reception take place along two orthogonal linear polarizations H and V., allows the radar to be calibrated in cross-polarization; H emission--V reception or V emission--H reception, and this takes place over a large angular range without requiring a change in the orientation of the dihedral.
This reflector, which may be made entirely of metal, allows a radar to be calibrated at an airport or a heliport, or situated on any type of vehicle (for example, a satellite), or on a tower, providing the radar lies in the angle of the calibration cone coming from the reflector.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained below by means of comparative examples, with ref
REFERENCES:
patent: 4823131 (1989-04-01), Bell
Borderies Pierre
Christophe Florent
Dumon Patrick
Lopez Jean-Marc
Mametsa Henri-Jose
Centre National d'Etudes Spatiales
Dzierzynski Paul M.
Sikd-er Mohammad Y.
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