Approach radar with array antenna having rows and columns skewed relative to the horizontal
A monopulse precision landing or other land-based radar system uses an array antenna and a beamformer to produce sum, azimuth difference and elevation difference beams relative to the array. The rows and columns of the array are skewed relative to the horizontal for tending to reduce the effects of multipath propagation on altitude estimates. A coordinate converter converts target information from the skewed antenna coordinates to ordinary vertical and horizontal coordinates.
1. A land-based approach radar arrangement comprising:
an array antenna for at least receiving target signals, said array antenna comprising a plurality of antenna elements arranged in an array of rows and columns;
beamforming means coupled to said plurality of antenna elements, for controlling at least one electrical parameter of signals traversing said antenna elements, for forming at least a sum beam, and for forming a difference beam;
array antenna mounting means, for mounting said array of antenna elements on land with both said rows and said columns skewed relative to a horizontal plane in an earth-fixed coordinate system; and
angle determining means coupled to said beamforming means, for converting a target position in the antenna coordinates to a target position in horizontal and vertical coordinates.
2. An arrangement according to claim 1 , wherein said array of antenna elements is divided about one of said rows and columns into first and second portions, each of said first and second portions having substantially the same number of said one of said rows and columns, and said beamformer forms said elevation difference beam by taking the difference between signals received by said first and second portions of said array of antenna elements.
3. An arrangement according to claim 1 , wherein said array plane is tilted back relative to the horizontal plane.
4. An arrangement according to claim 1 , wherein said tiltback of said array plane relative to said horizontal plane is in the range of 1° to 45°.
5. An arrangement according to claim 1 , wherein said skew of each of said rows and columns relative to the horizontal plane is 45°.
6. An arrangement according to claim 1 , wherein said array of antenna elements has a rectangular shape.
7. An arrangement according to claim 6 , wherein said rectangular shape is a square shape.
8. An arrangement according to claim 1 , wherein said difference beam is an elevation difference beam in antenna coordinates.
9. An arrangement according to claim 1 , wherein said sum beam and said difference beam are generally orthogonal to an array plane defined by said rows and columns.
10. A method for providing land-based precision approach radar services, said method comprising the steps of:
providing an array antenna for at least receiving target signals, said array antenna comprising a plurality of antenna elements arranged in an array of rows and columns to thereby define a planar array plane;
beamforming said target signals for forming at least a sum beam generally orthogonal to said array plane, and for forming a difference beam, also generally orthogonal to said array plane;
mounting said array of antenna elements on land with both said rows and said columns skewed relative to a horizontal plane in an earth-fixed coordinate system; and
converting a target position in the antenna coordinates to a target position in horizontal and vertical coordinates.
11. A method according to claim 10 , further comprising the step of tilting said array back in order to direct at least one of said sum and difference beams generally upward.