Communications: directive radio wave systems and devices (e.g. – Directive – Beacon or receiver
Reexamination Certificate
2000-01-27
2001-09-04
Issing, Gregory C. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Beacon or receiver
C342S451000
Reexamination Certificate
active
06285319
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a method of geolocating an emitter, and more particularly, to the use of observer motion and observer heading change to reduce the error in geolocating an emitter when associating bearing differences with circles passing through both the observer and the emitter.
BACKGROUND OF THE INVENTION
The association of circular lines-of-position with horizontal bearing differences is a technique well known in navigation (see, for example, G. P. Clark, “Simplified Determination of the Ellipse of Uncertainty”, Navigation: Journal of the Institute of Navigation, Vol. 21, No. 4, 1974). Royal, in U.S. Pat. No. 3,922,533 describes its use in multi-platform RF emitter location. In U.S. Pat. No. 5,526,001, the instant inventor presented a method to generate the circles employing phase difference measurements from a two antenna, uncalibrated, long-baseline-interferometer (LBI). When deriving the bearing difference from LBI phase differences, measurement precision is proportional to the interferometer baseline length, and inversely proportional to the emitter's frequency. Since the circular lines-of-position are generated from LBI phase measurements they are called phase circles.
FIG. 1
illustrates the unambiguous phase circles generated for arbitrary emitter locations relative to the observer. Because the LBI baseline d
108
(
FIG. 1
) is typically hundreds of wavelengths long, the phase measurements
109
give extremely high spatial angle resolution
103
, but are highly ambiguous. The phase difference
107
measured between points
111
m
1
and
112
m
2
is associated with the phase circle
100
through the angle change measurement
106
. But a consequence of the ambiguities
101
on the individual phase measurements is that the angle difference
106
is also ambiguous
105
, and hence a family of phases circles
102
results. Each member of this family passes through the fixed points m
1
and m
2
marking the beginning and end of the phase change measurement. The track
104
the observer flies between these two points is arbitrary, and does not directly affect the generation of the family of phase circles. To obtain the emitter location a second set of phase circles must intercept this first family, and also the phase ambiguities must be correctly resolved. A second observer typically generates the second set of phase circles.
Note that
FIG. 1
alternatively illustrates, for unambiguous phase measurements, the phase circles generated for arbitrary emitter locations relative to the observer. That is, for all specific ambiguity integers possible for all emitter locations, it illustrates the resulting phase circles through the emitters. Both interpretations of the figure are important for understanding the improvements introduced by the present invention because ambiguous phase circle interpretation is germane to one of the key features of the invention: optimal performance independent of the eventual ambiguity resolution process, i.e., independent of the actual values of
105
N
1
-N
2
corresponding to the emitter position
113
.
In generating the second family of possible COP, the only information available is the start position (
111
FIG. 1
) and predicted end point
112
of the observer making the initial; emitter detection.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a method and apparatus for determining emitter location independent of the eventual phase ambiguity resolution process.
A further object of the present invention is to provide optimal performance in the resolution of emitter location, independent of emitter relative bearing and range.
Yet a further object of the present invention is to provide a method of determining emitter location that is independent of the ambiguity resolution process and independent of emitter relative bearing and range.
It is another object of the present invention to implement an approximation to an exact solution that has the desired property of forming two sets of circles from bearing difference measurements, these two sets intersecting everywhere at nearly right angles for any operationally significant emitter location.
Therefore, it is a further object of the present invention to determine the second data collection track that must be flown to generate the second family of circles, intersecting all members of the first family orthogonally, utilizing only knowledge of the start and end points of the first data collection.
It is yet another object of the present invention to employ transitions between common tactical formations when implementing a second data collection track.
Still another object of the present invention is to require only short data collection intervals, typically ten seconds or less, when generating the COP families.
It is also an object of the present invention that the COP intersect nearly orthogonally at the emitter, no matter what the initial emitter relative bearing, for every range beyond a certain minimum range from the observers.
Another object of the present invention is to provide a method for best approximating the theoretical solution so that the approximation most closely solves
These and other objects of the present invention are achieved by a method for determining the geolocation of an emitter, using at least one observer measuring signal change while moving on at least two observation tracks. The measurement of emitter signal change is begun between a first observer position and a second observer position. The second observer position is predicted at the time the last measurement from which signal change will be derived is made. The method utilizes knowledge of the measurement start and end points to determine a second pair of observer signal-change measurement start and end positions such that the emitter line-of-position (LOP) determined by the second signal change measurement will intersect the LOP associated with the first signal change measurement at possibly multiple points, but at each of these point intercept orthogonally to within the signal change measurement errors. The method determines the intersection points of the LOPs and assigns a likelihood to each intersection point for measuring the probability for each of the multiple intersection points that it is the emitter location, determines a correct emitter position from these likelihood weights, and generates an error associated with the emitter position estimate.
Advantageously, the method of this invention can be used with any means for generating the COP that produces those COP over a suitably short time interval. In particular, the COP can be unique and not a family such as those produces ab initio from ambiguous LBI phase measurements. Then the present invention assures that, without any advance knowledge of the emitter position, the two COP will intersect almost orthogonally at its location. But, most significantly, if the COP are generated from ambiguous LBI phase measurements, the present invention assures that when the ambiguity is resolved, the COP at the true emitter position will intersect nearly orthogonally since all members of the ambiguous sets of phase circles are orthogonal.
REFERENCES:
patent: 3922533 (1975-11-01), Royal
patent: 4734702 (1988-03-01), Kaplan
patent: 5343212 (1994-08-01), Rose et al.
patent: 5526001 (1996-06-01), Rose et al.
patent: 5652590 (1997-07-01), Deaton
patent: 5835060 (1998-11-01), Czarnecki et al.
patent: 5914687 (1999-06-01), Rose
Oshman, Y. et al, “Optimization of Observer Trajectories for Bearings-Only Localization”, IEEE Trans. on Aerospace and Electronics Systems, Jul. 1999, pp. 892-902.*
Koteswara Rao, S. “Comments on ‘Discrete-Time Observability and estimability Analysis for Bearings-Only Target Motion Analysis’”, IEEE Trans. on Aerospace and Electronics Systems, Oct. 1998, pp. 1361-1367.*
Yaowei, Xu et al, “Passive Location of Fixed Emitter Using Additional Navigation Information”, Proc. of the IEEE 1997 National Aerospace and Electronics Conf., Jul. 1997, pp. 1034-1038.*
G.P. Cl
Issing Gregory C.
Litton Systems Inc.
Lowe Hauptman & Gilman & Berner LLP
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