Using radar targets of opportunity to build a monopulse calibration table

• US 20080042896A1
• Filed: 08/21/2006
• Published: 02/21/2008
• Est. Priority Date: 08/21/2006
• Status: Active Grant

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Northrop Grumman Corporation
• Inventors: Alon, Yair, Jacobs, Mark
• Granted Patent: US 7,548,189 B2
• Time in Patent Office: Days
• US Class Current: 342/174
• CPC Class Codes: G01S 13/4418 with means for eliminating radar-dependent errors in angle measurements, e.g. multipath effects
• CPC Class Codes: G01S 13/765 with exchange of information between interrogator and responder
• CPC Class Codes: G01S 7/4026 Antenna boresight
• CPC Class Codes: G01S 7/403 in azimuth, i.e. in the horizontal plane
• CPC Class Codes: G01S 7/4034 in elevation, i.e. in the vertical plane
• CPC Class Codes: G01S 7/4091 during normal radar operation

Abstract

In a radar system, a monopulse calibration table is constructed from live targets of opportunity. A center of gravity or weighted average of normalized signals ΔV received at SUM and DIFF channels from a live target are used to determine the target'"'"'s actual azimuth. Off bore sight angles (OBA) of the target are then determined from the target'"'"'s actual azimuth. Normalized received signal values of ΔV are converted to nearest-valued integers. The OBA s that correspond to each integer-valued normalized received signal are averaged and can then be plotted as a function of normalized received signal value ΔV. Different tables or plots can be constructed for elevation angles. An equation of a best-fit line the matches or at least closely approximates the plotted data is determined to smooth the actual data.

Claims

1. A method of building a monopulse calibration table from a live target of opportunity using signals received from the target at sum and difference antenna channels of a rotating radar antenna that receives signals within an angular-shaped beam, the center of which is the antenna'"'"'s bore-sight, the angle formed between a reference direction and the bore-sight being an azimuth angle, the method comprising the steps of:

a) at each of a plurality of N different azimuth angles, calculating a normalized received signal from a target of opportunity using a signal received from the target at a sum channel and using a signal received from the target at a difference channel;

b) identifying the azimuth angle of where the target of opportunity is actually located as the azimuth angle whereat the magnitude of the normalized received signal obtained in step a) is greatest;

c) for each of the other N azimuth angles of step a), calculating the difference between each of the N azimuth angles of step a) and the azimuth angle determined in step b) to obtain N, off-bore sight angles where each of the normalized received signals of step a) was determined;

d) storing each of the normalized received signal obtained at step a) with a corresponding off bore sight azimuth angles determined in step c).

2. The method of step 1 wherein the step of calculating a normalized received signal is comprised of the steps of:

dividing the magnitude of the signal from the sum channel by the magnitude of the signal from the difference channel to obtain a value Δ

V.

3. The method of claim 2 wherein the step of identifying the azimuth angle whereat the magnitude of the normalized received signal obtained in step a) is greatest is further comprised of

identifying the azimuth angle where the target center exists by calculating a weighted average of all azimuth angles for all corresponding Δ

V values.

4. The method of step 1 wherein the step of calculating a normalized received signal is comprised of the steps of:

determining a logarithmic value of the magnitude of the signal from the difference channel;

determining a logarithmic value of the magnitude of the signal from the sum channel;

subtracting the logarithmic value of the magnitude of the signal from the difference channel from the logarithmic value of the signal from the sum channel.

5. In a radar system, a method of determining off-bore sight angles of a target using a plurality of signals received from the target at sum and difference antenna channels of a rotating radar antenna at each azimuth angle of a plurality of azimuth angles, the method comprising the steps of:

a) at each azimuth angle θ

_i of a plurality of i azimuth angles, calculating a normalized received signal Δ

V_i for signals that are received at the antenna from a target from a V_sum signal and a V_diff signal;

b) calculating the target'"'"'s actual azimuth angle θ

_boresight as the sum of the product of all θ

_i·

Δ

V_i, divided by the sum of all Δ

V_i;

c) for each of the i azimuth angles θ

, of step a), calculating an off bore sight angle θ

_{off bore sight i} for each corresponding Δ

V_i by calculating the difference between each azimuth angle θ

_i, of step a) and the azimuth angle θ

_boresight determined in step b) to obtain i, off-bore sight angles, θ

_{off bore sight i}, whereat a corresponding Δ

V_i of step a) was determined;

d) correlating each of the normalized received signals Δ

V_i obtained at step a) to a corresponding off-bore sight angle θ

_{off bore sight angle} determined in step c).

6. The method of claim 5 further including the steps of:

converting each value of Δ

V_i to a nearest-valued integer, int(Δ

V_i).

7. The method of claim 6 further including the steps of:

storing each int(Δ

V_i) with a corresponding θ

_{off bore sight angle} to determine a target'"'"'s off bore sight angle.

8. The method of claim 5 wherein the step of calculating a normalized received signal is comprised of the steps of:

dividing the magnitude of the signal from the sum channel V_sum by the magnitude of the signal V_diff from the difference channel.

9. The method of claim 5 wherein each normalized received signal is determined by the steps of:

determining a logarithmic value of the magnitude of the signal V_diff from the difference channel;

determining a logarithmic value of the magnitude of the signal V_sum from the sum channel;

determining the difference between the logarithmic value of the magnitude of the signal from the difference channel and the logarithmic value of the signal from the sum channel.

10. The method of claim 5 wherein the step of identifying the azimuth angle whereat the magnitude of the normalized received signal obtained in step a) is greatest is further comprised of the steps of:

storing each of the N azimuth angles whereat a sum and difference signal ratio is determined;

storing each of the N sum and difference signal ratios;

sorting each of the N sum and difference signal ratios to determine the greatest ratio; and

identifying the corresponding azimuth angle where the greatest sum and difference signal ratio occurred.

11. The method of claim 5 further comprising the step of:

calculating a running average of the azimuth angles θ

for each corresponding Δ

V_i.

12. The method of claim 11, wherein the running average is calculated by the equation:

13. The method of claim 5 further comprising the step of:

assigning a plurality of signals received from the target at sum and difference antenna channels of a rotating radar antenna at a plurality of azimuth angles to a predetermined vertical angle segment.

14. The method of claim 5 further comprising the step of:

determining a best-fit line for a graph of all Δ

V and all θ

_{off bore sight} angles.

15. In a monopulse radar receiver, a method of building a monopulse calibration table from SUM and DIFF signals received from a plurality of live radar targets, the table having N entries, each of the N entries comprising a monopulse discriminant and a comprising a corresponding azimuth angle whereat signals from the live targets are obtained, the method comprising the steps of:

determining a first azimuth angle θ

that corresponds to a first monopulse discriminant, from a first live target;

determining a second azimuth angle θ

that corresponds to the first monopulse discriminant, from the first live target;

calculating a running average of the first and second azimuth angles θ

for the first monopulse discriminant for the first live target.

16. The method of claim 15 further comprising the step of:

identifying an azimuth angle of a second live target from the running average of the azimuth angles for the first monopulse discriminant obtained from azimuth angles obtained from the first live target.

17. The method of claim 15 wherein the running average of azimuth angles is determined by the equation:

18. The method of claim 15 further comprising the step of:

receiving signals from a live radar target that identifies the target'"'"'s elevation as being within an elevation range.

19. The method of claim 18 further comprising the steps of:

assigning the running average of the first and second azimuth angles θ

for the first monopulse discriminant for the first live target to a first range of elevation angles.

20. The method of claim 15 further comprising the step of:

calculating a best-fit line for azimuth angles and normalized received signals.

21. In a monopulse radar receiver, a method of building a monopulse calibration table from signals received from a plurality of live radar targets, the table having N entries, each of the N entries comprising a monopulse discriminant and a comprising a corresponding azimuth angle whereat signals from the live targets are obtained, the method comprising the steps of:

determining a first azimuth angle θ

that corresponds to a first monopulse discriminant, from a first live target;

determining a second azimuth angle θ

that corresponds to the first monopulse discriminant, from a second live target;

calculating a running average of the first and second azimuth angles θ

for the first monopulse discriminant obtained from the first and second live targets.

22. The method of claim 21 further comprising the step of:

identifying an azimuth angle of a third live target from the running average of the azimuth angles for the first monopulse discriminant obtained from azimuth angles obtained from the first and second live targets.

23. The method of claim 21 wherein the running average of azimuth angles is determined by the equation:

24. The method of claim 21 further comprising the step of:

receiving signals from a live radar target that identifies the target'"'"'s elevation as being within an elevation range.

25. The method of claim 21 further comprising the steps of:

assigning the running average of the first and second azimuth angles θ

for the first monopulse discriminant for the first live target to a first range of elevation angles.

26. The method of claim 21 further comprising the step of:

calculating a best-fit line for azimuth angles and normalized received signals.

27. A radar system comprised of:

a rotating antenna that receives signals at sum and difference antenna channels from targets at different elevations, within an angular-shaped beam, the center of which is the antenna'"'"'s bore-sight, the angle formed between a reference direction and the bore-sight being an azimuth angle;

a receiver coupled to the antenna and receiving signals from the sum and difference channels of the antenna in an elevation angle range;

a computer coupled to the receiver and receiving data that represents signals received at the sum and difference channels of the antenna; and

memory, coupled to the computer, said memory storing program instructions, which when executed cause the computer to;

a) calculate a normalized received signal from a target of opportunity in an elevation angle range at each of a plurality of i different azimuth angles θ

, using a signal received from the target at the antenna'"'"'s sum channel and using a signal received from the target at the antenna'"'"'s difference channel;

b) identify an azimuth angle θ

_boresight of where the target of opportunity is actually located, as the azimuth angle whereat the magnitude of the normalized received signal obtained in step a) is greatest;

c) for each of the other i azimuth angles of step a), calculate the difference between each of the i azimuth angles of step a) and the azimuth angle determined in step b) to obtain i, off-bore sight angles where each of the normalized received signals of step a) was determined;

d) correlate each of the normalized received signal obtained at step a) to a corresponding off-bore sight angle determined in step c).

28. The radar system of claim 27, wherein said program instructions cause the computer to:

e) determine a second target'"'"'s off-bore sight angle by correlating a normalized received signal obtained from the second target, to a normalized received signal value determined for the target of opportunity and an associated off-bore sight angle determined in step d).

29. The radar system of claim 27, wherein said program instructions cause the computer to:

divide the magnitude of the signal from the sum channel by the magnitude of the signal from the difference channel.

30. The radar system of claim 27, wherein said program instructions cause the computer to:

divide the magnitude of the signal from the difference channel by the magnitude of the signal from the sum channel.

31. The radar system of claim 27, wherein said program instructions cause the computer to:

determine a logarithmic value of the magnitude of the signal from the difference channel;

determine a logarithmic value of the magnitude of the signal from the sum channel;

subtract the logarithmic value of the magnitude of the signal from the difference channel from the logarithmic value of the signal from the sum channel.

32. A radar system comprised of:

a rotating antenna that receives signals at sum and difference antenna channels within an angular-shaped beam, the center of which is the antenna'"'"'s bore-sight, the angle formed between a reference direction and the bore-sight being an azimuth angle;

a receiver coupled to the antenna and receiving signals from the sum and difference channels of the antenna;

a computer coupled to the receiver and receiving data that represents signals received at the sum and difference channels of the antenna; and

memory, coupled to the computer, said memory storing program instructions, which when executed cause the computer to;

a) determine at each azimuth angle θ

_i of a plurality of i azimuth angles in an elevation angle range, a normalized received signal Δ

Vi for signals that are received at the antenna from a target in said elevation angle range;

b) calculate a target'"'"'s actual azimuth angle θ

_boresight in the elevation angle range as the sum of the product of all θ

_i·

Δ

V_i, divided by the sum of all Δ

V_i;

c) for each of the i azimuth angles θ

in the elevation angle range of step a), calculate the off bore sight angle θ

_{off bore sight} of each Δ

Vi by calculating the angle difference between each azimuth angle θ

_i, of step a) and the azimuth angle θ

_boresight determined in step b) to obtain i, off-bore sight angles, θ

_{off bore sight i}, whereat each of the normalized signals Δ

V_i of step a) was determined;

d) correlate each of the normalized received signals Δ

V_i obtained at step a) to a corresponding off-bore sight angle θ

_{off bore sight i} determined in step c).

33. The radar system of claim 32, wherein said program instructions cause the computer to:

determine when the phase angle difference between a signal received at the antenna'"'"'s sum channel and at the difference channel changes by 180 degrees.

34. The radar system of claim 32, wherein said program instructions cause the computer to calculate a normalized received signal Δ

Vi by;

dividing the magnitude of the signal V_sum from the antenna'"'"'s sum channel by the magnitude of the signal V_diff from the antenna'"'"'s difference channel.

35. The radar system of claim 32, wherein said program instructions cause the computer to:

divide the magnitude of the signal from the difference channel V_diff by the magnitude of the signal V_sum from the sum channel.

36. The radar system of claim 32, wherein said program instructions cause the computer to:

calculate a logarithmic value of the magnitude of the signal V_diff from the difference channel;

calculate a logarithmic value of the magnitude of the signal V_sum from the sum channel;

calculate the difference between the logarithmic value of the magnitude of the signal from the difference channel and the logarithmic value of the signal from the sum channel.

37. The radar system of claim 32, wherein said program instructions cause the computer to identify the azimuth angle whereat the magnitude of the normalized received signal obtained in step a) is greatest by:

storing each of the N azimuth angles whereat a sum and difference signal ratio is determined;

storing each of the N sum and difference signal ratios;

sorting each of the N sum and difference signal ratios to determine the greatest ratio; and

identifying the corresponding azimuth angle where the greatest sum and difference signal ratio occurred.

38. In a monopulse radar receiver, a method of building a plurality of monopulse calibration tables from signals received from a plurality of live radar targets at a plurality of different elevation ranges, each of the plurality of monopulse calibration tables determining the off bore sight angle location of targets within a corresponding elevation range, each of the tables having N entries, each of the N entries comprising a monopulse discriminant and a comprising a corresponding azimuth angle whereat signals from the live targets are obtained, the method comprising the steps of:

receiving a first signal that identifies a first live radar target as being within a first elevation range;

determining a first azimuth angle θ

that corresponds to a first monopulse discriminant, for the first live target;

receiving a second signal that identifies a second live radar target as being within said first elevation range;

determining a second azimuth angle θ

that corresponds to the first monopulse discriminant, for the second live target;

calculating a running average of the first and second azimuth angles θ

for the first monopulse discriminant obtained from the first and second live targets within said first elevation range;

assigning the running average of the first and second azimuth angles θ

for the first monopulse discriminant to a first monopulse calibration table for live targets within said first elevation range.

39. The method of claim 38 further comprising the steps of:

receiving a third signal that identifies a third live radar target as being within a second elevation range;

determining a third azimuth angle θ

that corresponds to a first monopulse discriminant, from the third live target;

receiving a fourth signal that identifies a fourth live radar target as being within said second elevation range;

determining a fourth azimuth angle θ

that corresponds to the first monopulse discriminant, from the fourth live target;

calculating a running average of the third and fourth azimuth angles θ

for the first monopulse discriminant obtained from the third and fourth live targets within said second elevation range;

assigning the running average of the third and fourth azimuth angles θ

for the first monopulse discriminant to a second monopulse calibration table for live targets within said second elevation range.

40. The method of claim 38 wherein said first signal is an IFF Mode C response.

41. The method of claim 38 wherein said radar is a 3-D radar and said first signal identifies a target'"'"'s range and horizontal distance and the target'"'"'s elevation is calculated by the equation:

42. The method of claim 38, wherein each of the calibration tables corresponds to a range of target elevation angles.

43. The method of claim 42 where the number of calibration tables is five.

44. The method of claim 38 further comprising the steps of:

determining an equation for a best fit line for each of the calibration tables.

45. The method of claim 44 further comprising the step of building a monopulse calibration table for live targets using the best fit line for the first monopulse calibration table and the monopulse discriminates and azimuth angles obtained from the first and second live targets.

46. The method of claim 45 further comprising the step of building a monopulse calibration table for live targets using both the best fit lines for the first and second monopulse calibration tables and the monopulse discriminates and azimuth angles obtained from the first, second, third and fourth live targets.

47. The method of claim 44 further comprising the steps of determining whether the best fit line is valid or invalid.

48. The method of claim 47 wherein the determination of a best fit line'"'"'s validity is determined by calculating a standard deviation of fit line points and measured data points.