Monopulse radar system and method for improved low elevation tracking

• US 4,005,421 A
• Filed: 05/30/1973
• Issued: 01/25/1977
• Est. Priority Date: 05/30/1973
• Status: Expired due to Term

• Current Assignee: Westinghouse Electric Company LLC (Brookfield Asset Management Incorporated)
• Original Assignee: Westinghouse Electric Company LLC (Brookfield Asset Management Incorporated)
• Inventors: Dax, Peter R.
• Primary Examiner(s): Tubbesing, T.H.
• Application Number:
  US05/365,245
• Time in Patent Office: 1,336 Days
• Field of Search: 343/16 M, 343/7.4, 343/7 A, 343/7.3
• US Class Current: 342/148
• CPC Class Codes: G01S 13/4418 with means for eliminating radar-dependent errors in angle measurements, e.g. multipath effects

Abstract

A method and monopulse radar system for improved tracking of low elevation targets in the presence of multipath signals through the use of an off-boresight tracking technique preferably in conjunction with a novel monopulse radar antenna sum and elevation difference signal channel patterns.

Claims

1. In a monopulse tracking radar system, apparatus for tracking low elevation targets comprising:

antenna means for receiving wave energy reflected from a target, said antenna means having a predetermined boresight;

means for generating an elevation tracking error signal related to an angular difference in elevation between a line-of-sight to the target and the antenna means boresight in response to the received wave energy;

means for driving said antenna means in a direction tending to minimize said elevation tracking error signal; and

,

means for disabling said driving means in response to approximate coincidence between actual antenna means boresight elevation angle and a predetermined lower limit of antenna means boresight elevation angle, wherein said antenna means includes;

a sum receiving channel for providing a sum signal in response to received wave energy;

an elevation difference receiving channel for providing an elevation difference signal in response to received wave energy;

means for modifying said sum signal and said elevation difference signal so that, with the antenna means boresight positioned in approximate coincidence with said predetermined lower limit, the ratio of said elevation difference signal to said sum signal for targets below said antenna means boresight is approximately the same for wave energy received along a direct path and for wave energy received along a reflected path.

2. The apparatus of claim 1 wherein said predetermined lower limit varies as a function of target range.

3. In a monopulse tracking radar system, apparatus for tracking low elevation targets comprising:

antenna means for receiving wave energy reflected from a target, said antenna means having a predetermined boresight;

means for generating an elevation tracking error signal related to an angular difference in elevation between a line-of-sight to the target and the antenna means boresight in response to the received wave energy;

means for driving said antenna means in a direction tending to minimize said elevation tracking error signal;

means for disabling said driving means in response to approximate coincidence between actual antenna means boresight elevation angle and a predetermined lower limit of antenna means boresight elevation angle;

said antenna means includes;

a sum receiving channel for providing a sum signal in response to received wave energy;

an elevation difference receiving channel for providing an elevation difference signal in response to received wave energy;

means for modifying said sum signal and said elevation difference signal so that, with the antenna means boresight positioned in approximate coincidence with said predetermined lower limit, the ratio of said elevation difference signal to said sum signal is approximately the same for wave energy received along a direct path and for wave energy received along a reflected path;

said predetermined lower limit varies as a function of target range; and

said antenna means includes a cluster of four feed horns approximately symmetrically disposed about said antenna means boresight, said modifying means including two additional feed horns disposed respectively above and below said cluster of four feedhorns and operatively coupled thereto.

4. In a monopulse tracking radar system, apparatus for tracking low elevation targets comprising:

antenna means for receiving wave energy reflected from a target, said antenna means having a predetermined boresight;

means for generating an elevation tracking error signal related to an angular difference in elevation between a line-of-sight to the target and the antenna means boresight in response to the received wave energy;

means for driving said antenna means in a direction tending to minimize said elevation tracking error signal; and

,

means for disabling said driving means in response to approximate coincidence between actual antenna means boresight elevation angle and a predetermined lower limit of antenna means boresight elevation angle, wherein said disabling means includes;

means for determining said lower limit of antenna means boresight in response to target range;

means responsive to said lower limit and said actual antenna means elevation boresight angle for supplying said elevation tracking error signal to said driving means with said antenna means elevation boresight angle above said lower limit and for supplying said lower limit to said driving means with said actual elevation boresight angle in approximate coincidence with said lower limit.

5. In a monopulse tracking radar system, apparatus for tracking low elevation angle targets comprising:

antenna means for receiving wave energy, said antenna means including means defining a selectively positionable antenna elevation boresight, a sum signal channel and an elevation difference signal channel, said sum and elevation difference signal channels producing respective sum and difference signal patterns referenced to said antenna boresight in response to received wave energy,

said sum and elevation difference signal patterns each having a zero amplitude value for wave energy received at an elevation boresight angle below the antenna boresight, the zero amplitude value of the elevation difference signal pattern occurring at a smaller off-boresight angle than the zero amplitude value of the sum signal pattern;

means operatively connected to said sum and elevation difference signal channels for generating an elevation tracking error signal, said elevation tracking error signal having an absolute peak amplitude value at a predetermined off-boresight angle and lower absolute amplitude values generally symmetrically disposed on each side of said peak amplitude value; and

,

means for limiting the elevation of the antenna boresight to positions above a predetermined angular elevation.

6. The apparatus of claim 5 including means responsive to said limiting means for driving said antenna means in a direction tending to null said elevation tracking error signal, said driving means being disabled by said limiting means at said predetermined angle elevation.

7. The apparatus of claim 5, including means for determining target range and for supplying an indication thereof to said limiting means, said predetermined angular elevation being a function of target range.

8. In a method for monopulse radar tracking, a method for tracking low elevation targets comprising:

transmitting wave energy toward a target;

receiving, with respect to a predetermined antenna boresight, wave energy reflected from the target;

generating an elevation tracking error signal related to an angular difference in elevation between a line-of-sight to the target and the antenna boresight in response to the received wave energy;

driving said antenna boresight in a direction tending to minimize said elevation tracking error signal; and

,

discontinuing the driving of the antenna boresight approximate coincidence between actual antenna boresight elevation angle and a predetermined lower limit of antenna boresight elevation angle, including the steps of;

summing the received wave energy to provide a sum signal;

subtracting wave energy received through two legs of an elevation difference receiving channel to provide an elevation difference signal;

modifying the sum signal and the elevation difference signal so that, with the antenna boresight positioned in approximate coincidence with said predetermined lower limit, the ratio of said elevation difference signal to said sum signal is approximately the same for wave energy received along a direct path and for wave energy received along a reflected path.

9. The method of claim 8 including the steps of varying said predetermined lower limit as a function of target range.

10. In a method for monopulse radar tracking, a method for tracking low elevation targets comprising:

transmitting wave energy toward a target;

receiving, with respect to a predetermined antenna boresight, wave energy reflected from the target;

generating an elevation tracking error signal related to an angular difference in elevation between a line-of-sight to the target and the antenna boresight in response to the received wave energy;

driving said antenna boresight in a direction tending to minimize said elevation tracking error signal; and

,

discontinuing the driving of the antenna boresight approximate coincidence between actual antenna boresight elevation angle and a predetermined lower limit of antenna boresight elevation angle, including the steps of;

determining said lower limit of antenna boresight in response to target range; and

,

driving antenna boresight in elevation in response to said elevation tracking error signal with said antenna boresight elevation angle above said lower limit and fixing said antenna boresight elevation at said lower limit with said actual elevation boresight angle in approximate coincidence with said lower limit.

11. In a monopulse tracking radar system, apparatus for tracking low elevation targets in the presence of multi-path signals comprising:

means for transmitting wave energy toward a target;

means for receiving, with respect to a predetermined antenna boresight, wave energy reflected from an actual target and from its image target to form sum (Σ

) and difference (Δ

) vector signals representative of said actual target and said image target;

means for shaping the elevation error function (E), the curve E equaling Δ

/Σ

, to form a peak amplitude value at a predetermined off boresight angle with a decreasing amplitude symmetrically on each side of said peak amplitude value; and

means for setting the boresight elevation angle such that the value of the error signal E due to said wave energy received in the direction of the target and in the direction of the image target are equal.

12. In a method for monopulse radar tracking, a method for tracking low elevation targets in the presence of multi-path signals comprising:

transmitting wave energy toward a target;

receiving, with respect to a predetermined antenna boresight, wave energy reflected from an actual target and from its image target to form sum (Σ

) and difference (Δ

) vector signals representative of said actual target and said image target;

shaping the elevation error function (E), the curve E equaling Δ

/Σ

, to form a peak amplitude value at a predetermined off boresight angle with a decreasing amplitude symmetrically on each side of said peak amplitude value; and

setting the boresight elevation angle such that the value of the error signal E due to said wave energy received in the direction of the target and in the direction of the image target are equal.