Digital polarimetric system

• US 6,911,937 B1
• Filed: 11/13/2000
• Issued: 06/28/2005
• Est. Priority Date: 11/12/1999
• Status: Expired due to Fees

• Current Assignee: Exelis Incorporated (L3Harris Technologies, Inc.)
• Original Assignee: ITT Manufacturing Enterprises, Inc. (ITT, Inc.)
• Inventors: Sparrow, Mitchell J., Cikalo, Joseph
• Primary Examiner(s): Gregory, Bernarr E.
• Application Number:
  US09/711,168
• Time in Patent Office: 1,688 Days
• Field of Search: 356364-370, 342 13- 20, 342 82-103, 342/177, 342/149, 342150-154, 342/175, 342/188, 342189-197, 342361-367, 333/1, 333/21.R, 333/21.A, 333/24.R, 333/24.1, 333/24.2, 333/24.3
• US Class Current: 342/188
• CPC Class Codes: G01S 7/024 using polarisation effects in waveguides H01P1/165; for aerials H01Q, e.g. H01Q15/22, H01Q15/24, H01Q19/195
• CPC Class Codes: G01S 7/2921 based on data belonging to one radar period

Abstract

A digital polarimeteric system employs a signal time stretching technique and digital signal processing of the time-stretched signal to accurately measure the polarization of a received RF signal with commercially available digital hardware. A superheterodyne receiver down converts received RF signal components to IF, and analog-to-digital converters sample the signal components at much lower sampling rates than would normally be required to accurately measure the signal polarization. Each signal sample is “time stretched” by storing each sample in M locations in a memory, such that N samples occupy M×N memory locations. A digital signal processor applies incremental phase shifts to the digital samples until a phase-shifted combination of the digital samples yields a minimum null output. The phase shifts producing the minimum null identify the polarization of the received signal. The stretching and digital processing yield the required number of samples per cycle of the received signal for accurate polarization measurement, thus effectively increasing the digital sampling frequency. Because each sample is stored in M memory locations, each incremental phase shift corresponds to 1/M^th of the actual sampling interval, thereby providing the high phase resolution required to accurately measure polarization.

Claims

1. A method for measuring the polarization of a received signal, comprising:

(a) time stretching polarization components of the received signal by representing each of a number of digital samples of the polarization components of the received signal as a plurality of time-stretched samples to produce time-stretched digital polarization components;

(b) computing a measurement value from phase-shifted combinations of the time-stretched digital polarization components, such that phase shifts producing a minimum measurement value correspond to the polarization of the received signal.

2. The method of claim 1, wherein the measurement value is a difference/sum ratio of phase-shifted combinations of the time-stretched digital polarization components, and the minimum measurement value is a null value of the difference/sum ratio.

3. The method of claim 2, wherein the null value of the difference/sum ratio corresponds to phase values φ

and γ

, wherein a phase and amplitude difference between the polarization components of the received signal are a function of phase values φ and

γ

.

4. The method of claim 3, wherein the time-stretched samples of the time-stretched digital polarization components are stored in memory, and wherein the phase values φ

and γ

are respectively represented by integer numbers of memory location shifts.

5. The method of claim 1, further comprising:

(c) sampling the polarization components of the received signal to generate the digital samples.

6. The method of claim 1, wherein (a) includes storing each of the digital samples of the polarization components of the received signal in a plurality of memory locations to form time-stretched sequences of digital samples representing the time-stretched digital polarization components.

7. The method of claim 1, wherein an effective phase resolution of the time-stretched digital polarization components is greater than that of the digital samples of the polarization components.

8. The method of claim 1, wherein a phase resolution of the time-stretched digital polarization components is a function of a sampling rate of the polarization components of the received signal and a number of memory locations in which each of said number of samples is stored.

9. The method of claim 1, wherein the digital samples of the polarization components comprise N samples per signal cycle, where N is a positive number, and wherein (a) includes storing each of the N samples in M memory locations, such that M×

N samples are stored for each signal cycle, where M is a positive integer.

10. The method of claim 1, wherein the polarization components of the received signal are horizontal and vertical polarization components.

11. The method of claim 1, wherein the time-stretched digital polarization components include first and second time-stretched polarization components, and (b) includes:

(b1) shifting samples of the first time-stretched polarization component by k sample positions to produce a time-shifted first polarization component, where k is an integer;

(b2) adding samples of the time-shifted first polarization component to samples of the second time-stretched polarization component to produce a first sum signal;

(b3) shifting samples of the first sum signal by j sample positions to produce a time-shifted first sum signal, where j is an integer;

(b4) subtracting samples of the time-shifted first polarization component from samples of the second time-stretched polarization component to produce a first difference signal;

(b5) adding samples of the first difference signal to samples of the time-shifted first sum signal to produce an output sum signal;

(b6) subtracting samples of the time-shifted first sum signal from samples of the first difference signal to produce an output difference signal;

(b7) computing a difference/sum ratio from a ratio of a sum of the samples of the output difference signal to a sum of the samples of the output sum signal.

12. The method of claim 11, wherein values of j and k producing a minimum difference/sum ratio represent numbers of memory location shifts corresponding to phase values that indicate polarization phase and amplitude information.

13. The method of claim 11, wherein (b) includes determining the minimum difference/sum ratio by:

(b8) setting j and k to initial values;

(b9) holding j constant while incrementing k by an integer value until an initial minimum value of the difference/sum ratio is reached; and

(b10) holding k to the value producing the initial minimum value of the difference/sum ratio while incrementing j by an integer value until a minimum value of the difference/sum ratio is reached.

14. The method of claim 11, wherein j and k are initiated and successively updated in accordance with a predetermined scheme for identifying a minimum value of the difference/sum ratio.

15. The method of claim 11, wherein initial values of j and k are estimated based upon values of j and k from previous calculations of a minimum value of the difference/sum ratio.

16. The method of claim 11, wherein initial values of j and k are estimated using a phase detector.

17. An apparatus for measuring the polarization of a received signal, comprising:

an analog-to-digital converter configured to digitally sample polarization components of the received signal;

a memory device configured to store each of a number of digital samples of the polarization components in plural memory locations, thereby representing each of the digital samples as a plurality of time-stretched samples forming time-stretched digital polarization components; and

a processor configured to compute a measurement value from phase-shifted combinations of the time-stretched digital polarization components, such that phase shifts producing a minimum measurement value correspond to the polarization of the received signal.

18. The apparatus of claim 17, wherein:

said analog-to-digital converter samples the polarization components of the received signal at a sampling rate of N samples per signal cycle, where N is a positive number; and

said memory device stores each of the N samples in M memory locations, such that M×

N samples are stored for each signal cycle, where M is a positive integer.

19. The apparatus of claim 17, wherein the measurement value is a difference/sum ratio of phase-shifted combinations of the time-stretched digital polarization components, and the minimum measurement value is a null value of the difference/sum ratio.

20. The apparatus of claim 19, wherein the null value of the difference/sum ratio corresponds to phase values φ

and γ

, wherein a phase and amplitude difference between the polarization components of the received signal are a function of phase values φ and

γ

.

21. The apparatus of claim 20, wherein the phase values φ

and γ

are respectively represented by integer numbers of memory location shifts.

22. The apparatus of claim 17, wherein an effective phase resolution of the time-stretched digital polarization components is greater than that of the digital samples of the polarization components.

23. The apparatus of claim 17, wherein a phase resolution of the time-stretched digital polarization components is a function of a sampling rate of the analog-to-digital converter and a number of memory locations in which each of said number of samples is stored.

24. The apparatus of claim 17, wherein the polarization components of the received signal are horizontal and vertical polarization components.

25. The apparatus of claim 17, wherein the time-stretched digital polarization components include first and second time-stretched polarization components, and said processor is configured to:

shift samples of the first time-stretched polarization component by k sample positions to produce a time-shifted first polarization component, where k is an integer;

add samples of the time-shifted first polarization component to samples of the second time-stretched polarization component to produce a first sum signal;

shift samples of the first sum signal by j sample positions to produce a time-shifted first sum signal, where j is an integer;

subtract samples of the time-shifted first polarization component from samples of the second time-stretched polarization component to produce a first difference signal;

add samples of the first difference signal to samples of the time-shifted first sum signal to produce an output sum signal;

subtract samples of the time-shifted first sum signal from samples of the first difference signal to produce an output difference signal; and

compute a difference/sum ratio from a ratio of a sum of the samples of the output difference signal to a sum of the samples of the output sum signal.

26. The apparatus of claim 25, wherein values of j and k producing a minimum difference/sum ratio represent numbers of memory location shifts corresponding to phase values that indicate polarization phase and amplitude information.

27. The apparatus of claim 25, wherein said processor determines the minimum difference/sum ratio by:

setting j and k to initial values;

holding j constant while incrementing k by an integer value until an initial minimum value of the difference/sum ratio is reached; and

holding k to the value producing the initial minimum value of the difference/sum ratio while incrementing j by an integer value until a minimum value of the difference/sum ratio is reached.

28. The apparatus of claim 25, wherein said processor initiates and updates values of j and k in accordance with a predetermined scheme for identifying a minimum value of the difference/sum ratio.

29. The apparatus of claim 25, wherein said processor estimates initial values of j and k based upon values of j and k from previous calculations of a minimum value of the difference/sum ratio.

30. The apparatus of claim 25, further comprising a phase detector configured to measure phase information relating to the polarization components of the received signal, wherein said processor estimates initial values of j and k based on measurements of said phase detector.

31. The apparatus of claim 17, wherein said apparatus is a digital polarimeter.

32. An apparatus for measuring the polarization of a received signal, comprising:

means for digitally sampling polarization components of the received signal;

means for storing each of a number of digital samples of the polarization components in plural memory locations, thereby representing each of the digital samples as a plurality of time-stretched samples to form time-stretched digital polarization components; and

means for computing a measurement value from phase-shifted combinations of the time-stretched digital polarization components, such that phase shifts producing a minimum measurement value correspond to the polarization of the received signal.

33. The apparatus of claim 32, wherein:

said means for digitally sampling samples the polarization components of the received signal at a sampling rate of N samples per signal cycle, where N is a positive number; and

said means for storing stores each of the N samples in M memory locations, such that M×

N samples are stored for each signal cycle, where M is a positive integer.

34. The apparatus of claim 32, wherein the measurement value is a difference/sum ratio of phase-shifted combinations of the time-stretched digital polarization components, and the minimum measurement value is a null value of the difference/sum ratio.

35. The apparatus of claim 32, wherein the null value of the difference/sum ratio corresponds to phase values φ

and γ

, wherein a phase and amplitude difference between the polarization components of the received signal are a function of phase values φ and

γ

.

36. The apparatus of claim 35, wherein the phase values φ

and γ

are respectively represented by integer numbers of memory location shifts.

37. The apparatus of claim 32, wherein an effective phase resolution of the time-stretched digital polarization components is greater than that of the digital samples of the polarization components.

38. The apparatus of claim 32, wherein a phase resolution of the time-stretched digital polarization components is a function of a sampling rate of the means for sampling and a number of memory locations in which each of said number of samples is stored.

39. The apparatus of claim 32, wherein the polarization components of the received signal are horizontal and vertical polarization components.

40. The apparatus of claim 32, wherein the time-stretched digital polarization components include first and second time-stretched polarization components, and said means for computing includes:

means for shifting samples of the first time-stretched polarization component by k sample positions to produce a time-shifted first polarization component, where k is an integer;

means for adding samples of the time-shifted first polarization component to samples of the second time-stretched polarization component to produce a first sum signal;

means for shifting samples of the first sum signal by j sample positions to produce a time-shifted first sum signal, where j is an integer;

means for subtracting samples of the time-shifted first polarization component from samples of the second time-stretched polarization component to produce a first difference signal;

means for adding samples of the first difference signal to samples of the time-shifted first sum signal to produce an output sum signal;

means for subtracting samples of the time-shifted first sum signal from samples of the first difference signal to produce an output difference signal; and

means for computing a difference/sum ratio from a ratio of a sum of the samples of the output difference signal to a sum of the samples of the output sum signal.

41. The apparatus of claim 40, wherein values of j and k producing a minimum difference/sum ratio represent numbers of memory location shifts corresponding to phase values that indicate polarization phase and amplitude information.

42. The apparatus of claim 40, wherein said means for computing determines the minimum difference/sum ratio by:

setting j and k to initial values;

holding j constant while incrementing k by an integer value until an initial minimum value of the difference/sum ratio is reached; and

holding k to the value producing the initial minimum value of the difference/sum ratio while incrementing j by an integer value until a minimum value of the difference/sum ratio is reached.

43. The apparatus of claim 40, wherein said means for computing initiates and updates values of j and k in accordance with a predetermined scheme for identifying a minimum value of the difference/sum ratio.

44. The apparatus of claim 40, wherein said means for computing estimates initial values of j and k based upon values of j and k from previous calculations of a minimum value of the difference/sum ratio.

45. The apparatus of claim 32, wherein said apparatus is a digital polarimeter.

47. The digital polarimetric system of claim 40, wherein a phase resolution of the time-stretched sequence of digital samples is a function of a sampling rate of said analog-to-digital converter and a number of memory locations in which each of the digital samples is stored in said memory device.

46. A digital polarimetric system for determining the polarization of a received signal, comprising:

an analog-to-digital converter configured to digitally sample the received signal;

a memory device receiving digital samples from said analog-to-digital converter and storing each of the digital samples in a plurality of memory locations to form a time-stretched sequence of digital samples; and

a processor configured to determine the polarization of the received signal from the time-stretched sequence of digital samples.

48. The digital polarimetric system of claim 46, wherein:

said system determines the polarization of the received signal within a predetermined accuracy; and

said analog-to-digital converter samples the received signal at a sampling rate insufficient to determine the polarization of the received signal within the predetermined accuracy.

49. The digital polarimetric system of claim 46, wherein:

said analog-to-digital converter samples the received signal at a sampling rate of N samples per signal cycle, where N is a positive number; and

said memory device stores each of the N samples in M memory locations, such that M×

N samples are stored for each signal cycle, where M is a positive integer.

50. The digital polarimetric system of claim 46, wherein:

the received signal includes first and second polarization components, and said memory device stores first and second time-stretched sequences of samples corresponding to the first and second polarization components; and

said digital signal processor applies phase shifts to the first and second time-stretched sequences of samples and computes a measurement value from phase-shifted combinations of the first and second time-stretched sequences of samples, such that the phase shifts producing a minimum measurement value correspond to the polarization of the received signal.

51. The digital polarimetric system of claim 50, wherein the measurement value is a difference/sum ratio of the phase-shifted combinations of the first and second time-stretched sequences of samples, and the minimum measurement value is a null value.

52. The digital polarimetric system of claim 50, wherein said processor applies the phase shifts in increments of one sample of the first and second time-stretched sequences of samples.