Apparatus and method for the monopulse linking of frequency agile emitter pulses intercepted in on single interferometer baseline

• US 6,411,249 B1
• Filed: 07/19/2000
• Issued: 06/25/2002
• Est. Priority Date: 07/19/2000
• Status: Expired due to Term

• Current Assignee: Litton Systems Incorporated, Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Northrop Grumman Corporation
• Inventors: Rose, Conrad
• Primary Examiner(s): Sotomayor, John B.
• Application Number:
  US09/619,474
• Time in Patent Office: 706 Days
• Field of Search: 342/13, 342/89, 342/98, 342/102, 342/147, 342/149, 342/156, 342/162, 342/195
• US Class Current: 342/13
• CPC Class Codes: G01S 3/46 using antennas spaced apart and measuring phase or time difference between signals therefrom, i.e. path-difference systems
• CPC Class Codes: G01S 7/021 Auxiliary means for detecting or identifying radar signals or the like, e.g. radar jamming signals

Abstract

Disclosed is a method of associating a single pulse from an agile emitter with previously detected pulses from that emitter in a time interval less than the pulse repetition interval (PRI) of the radar. Ambiguous phases from the previously detected pulses from the same agile emitter are stored. A single cos(aoa) from a subset of the stored ambiguous phases is estimated. A new ambiguous phase φ_m at frequency f_m, is detected. This frequency is different from at least one of the frequencies associated with the phases in the stored set. The phase measurement is made between two antennas spatially separated by distance d. A set of differenced phases is formed and corresponding differenced frequencies from the stored set, with at least one member of this set being the difference of the new ambiguous phase and frequency with one of the stored phases and its associated frequency. The phase cycle measurement ambiguity integer is measured resolving the phase difference formed from the new ambiguous phase utilizing this set of phase and frequency differences. The phase cycle measurement ambiguity integer is computed resolving the new ambiguous phase difference if the new pulse is from the same emitter as the stored set by utilizing the previously estimated cos(aoa) and newly measured frequency f_m. The measured and computed ambiguity integers are compared. The newly detected pulse is associated with the previously stored pulses as being from the frequency agile emitter if the integers are equal.

Claims

1. A method of associating a single pulse from an agile emitter with previously detected pulses from that emitter in a time interval less than the pulse repetition interval (PRI) of the radar comprising:

storing ambiguous phases from the previously detected pulses from the same agile emitter;

estimating a single cos(aoa) from a subset of the stored ambiguous phases;

detecting a new ambiguous phase φ

_m at frequency f_m which frequency is different from at least one of the frequencies associated with the phases in the stored set, the phase measurement made between two antennas spatially separated by distance d, forming a set of differenced phases and corresponding differenced frequencies from the stored set, with at least one member of this set being the difference of the new ambiguous phase and frequency with one of the stored phases and its associated frequency;

measuring the phase cycle measurement ambiguity integer resolving the phase difference formed from the new ambiguous phase utilizing this set of phase and frequency differences;

computing the phase cycle measurement ambiguity integer resolving the new at ambiguous phase difference if the new pulse is from the same emitter as the stored set by utilizing the previously estimated cos(aoa) and newly measured frequency f_m; and

first comparing the measured and computed ambiguity integers, and associating the newly detected pulse with the previously stored pulses as being from the frequency agile emitter if the integers are equal.

2. The method of claim 1 where the integer measuring, computing and comparing steps occur before detecting a new ambiguous phase φ

_m+1 at frequency f_m+1 different from f_m.

3. The method of claim 1 where the integer measuring, computing and comparing steps are applied sequentially to a set of previously stored pulses not yet associated with the emitter.

4. The method of claim 1 comprising:

predicting the new ambiguous phase difference, correct if the new pulse is from the same emitter as the stored set, this prediction done utilizing the previously estimated cos(aoa) and newly measured frequency f_m;

second comparing the detected ambiguous phase with the predicted phase and

deleting the newly detected pulse from the stored set of pulses if the predicted and measured phase differences are not equal to within measurement and estimation tolerances.

5. The method of claim 4, wherein said predicting second comparing and deleting steps occur after said first comparing step is successfully completed.

6. The method of claim 2, comprising:

predicting the new ambiguous phase difference, correct if the new pulse is from the same emitter as the stored set, this prediction done utilizing the previously estimated cos(aoa) and newly measured frequency f_m;

second comparing the detected ambiguous phase with the predicted phase; and

deleting the newly detected pulse from the stored set of pulses if the predicted and measured phase differences are not equal to within measurement and estimation tolerances.

7. The method of claim 6, wherein said predicting second comparing and deleting steps occur after said first comparing step is successfully completed.

8. The method of claim 1, comprising:

estimating a set of pulse parameters for each pulse previously stored in the set, with frequency and pulse repetition interval being possible members of the parameter set, these parameters being invariant from pulse-to-pulse within measurement and estimation tolerances;

estimating these pulse parameters for the newly associated pulse,

comparing the newly estimated pulse parameters with the previously estimated pulse parameters; and

deleting the newly associated pulse from the set if the parameter match does not satisfy conventional Electronic Surveillance Measures (ESM) criteria for identifying the new pulse as being generated by the same emitter as those in the previously stored set.

9. The method of claim 8, wherein said estimating steps, comparing and deleting steps occur after said comparing step is successfully completed.

10. The method of claim 1, wherein for each phase ψ

_i measured between two antennas spatially separated by distance d, with bias error b, random error ε

_i, at angle-of-arrival aoa₁ having ambiguity integer n_i, the phase and frequency f_i are related according to the equation ${\mathrm{\varphi <br><br>}}_i=\frac{f_i}{c}<br><br><br><br>\text{\hspace{1em}<br><br>}<br><br>\cos <br><br>\left({\mathrm{aoa}}_i\right)-n_i+b+{\mathrm{\varepsilon <br><br>}}_i.$

11. The method of claim 10, comprising:

differencing pairs of the stored phases and generating a set of phase differences Δ

ψ

_k=ψ

_p−

ψ

_m each with associated ambiguity integer n_p−

n_m; and

estimating a single cos(aoa) for this set of phase differences such that the single cos(aoa) replaces the true cos(aoa_i) in the set of Δ

ψ

_k with minimum error in the approximate relation $\left[\begin{array}{c}{\mathrm{\Delta <br><br>\varphi <br><br>}}_1\\ \mathrm{⋮<br><br>}\\ {\mathrm{\Delta <br><br>\varphi <br><br>}}_k\end{array}\right]\cong <br><br>\left[\begin{array}{c}{f}_s-f_t\\ \mathrm{⋮<br><br>}\\ f_q-f_r\end{array}\right]<br><br>\frac{d}{c}<br><br>\cos <br><br>\left(\mathrm{aoa}\right)-\left[\begin{array}{c}{n}_s-n_t\\ \mathrm{⋮<br><br>}\\ n_q-n_r\end{array}\right]+\left[\begin{array}{c}{\mathrm{\varepsilon <br><br>}}_s-{\mathrm{\varepsilon <br><br>}}_t\\ \mathrm{⋮<br><br>}\\ {\mathrm{\varepsilon <br><br>}}_q-{\mathrm{\varepsilon <br><br>}}_r\end{array}\right].$

12. The method of claim 10, wherein the estimating and approximating the cos(aoa) steps comprises determining exactly the ambiguity integers N_i where $[\begin{array}{c}{N}_1\\ ⋮\end{array}$

Nk]=[ns-nt⋮

nq-nr].

13. The method of claim 12, comprising:

generating an array or matrix of numbers G from the measured frequency differences and LBI baseline such that $G<br><br>\stackrel{\to <br><br>}{D}\cong <br><br>\left[\begin{array}{ccc}0& \mathrm{\dots <br><br>}& 0\\ \mathrm{⋮<br><br>}& \mathrm{⋰<br><br>}& \mathrm{⋮<br><br>}\\ 0& \mathrm{\dots <br><br>}& 0\end{array}\right]$

 

where $\stackrel{\to <br><br>}{D}\cong <br><br>\left[\begin{array}{c}{f}_s-f_t\\ \mathrm{⋮<br><br>}\\ f_q-f_r\end{array}\right]<br><br>\frac{d}{c},$

 

and

estimating the ambiguity integers N_i according to the relation between the known measured phase differences and unknown integers $G<br><br>\left[\begin{array}{c}{\mathrm{\Delta <br><br>\varphi <br><br>}}_1\\ \mathrm{⋮<br><br>}\\ {\mathrm{\Delta <br><br>\varphi <br><br>}}_{h-1}\\ {\mathrm{\Delta <br><br>\varphi <br><br>}}_h\end{array}\right]=G<br><br>\left[\begin{array}{c}{N}_1\\ \mathrm{⋮<br><br>}\\ N_{h-1}\\ N_h\end{array}\right].$

14. The method of claim 13 wherein estimating the ambiguity integers step, and subsequently estimating the cos(aoa) step are both done using a maximum likelihood estimator.

15. The method of claim 12, wherein estimating the ambiguity integers step, and subsequently estimating the cos(aoa) step are both done using a maximum likelihood estimator.

16. The method of claim 1, comprising computing the phase cycle measurement ambiguity integer N_new by resolving the new ambiguous phase difference Δ

ψ

=ψ

_i−

ψ

_new, if the new pulse is from the same emitter as the stored set, comprising;

predicting the resolved phase utilizing the estimated cos(aoa) and measured frequencies according to the approximation ${\mathrm{\Delta <br><br>\varphi <br><br>}}_{\mathrm{pred}}=\frac{f_i-f_{\mathrm{new}}}{c}<br><br><br><br>\text{\hspace{1em}<br><br>}<br><br>\cos <br><br>\left(\mathrm{aoa}\right),\mathrm{and}$

predicting the ambiguity integer N_new from this computed phase by means of the relation N_new=Δ

ψ

_predmodulo(1).

17. The method of claim 1, comprising:

generating G from the measured frequency differences and LBI baseline length such that $G<br><br>\stackrel{\to <br><br>}{D}\cong <br><br>\left[\begin{array}{ccc}0& \mathrm{\dots <br><br>}& 0\\ \mathrm{⋮<br><br>}& \mathrm{⋰<br><br>}& \mathrm{⋮<br><br>}\\ 0& \mathrm{\dots <br><br>}& 0\end{array}\right]$

 

where $\stackrel{\to <br><br>}{D}\cong <br><br>\left[\begin{array}{c}{f}_s-f_t\\ \mathrm{⋮<br><br>}\\ f_q-f_r\\ f_i-f_{\mathrm{new}}\end{array}\right]<br><br>\frac{d}{c},$

partitioning the array of numbers G to create two weighting sets {right arrow over (g)}_m, a single column of numbers, and G<sub>m−

1</sub>, where

18. The method of claim 17 wherein the ambiguity integers and phase measurements are made to depend on only a single previously resolved stored phase, allowing measurement of the new ambiguity integer in μ

secs, resulting in the reduction of {right arrow over (g)}_m to $\begin{array}{c}\left[\begin{array}{c}0\\ \mathrm{⋮<br><br>}\\ 1\end{array}\right]\end{array}$

and comprising;

computational processing of G to the form

19. The method of claim 1, wherein the phase detection occurs on one of a set of calibrated conventional interferometer baselines, where the particular baseline used from the set may be different at each pulse update.

20. The method of claim 1 where the phase detection occurs on an LBI baseline, where the baseline is frequency calibrated but the two antennas forming the baseline may have different signal polarization responses.