Communications: directive radio wave systems and devices (e.g. – Directive – Utilizing correlation techniques
Reexamination Certificate
2002-08-15
2003-12-30
Issing, Gregory C (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Utilizing correlation techniques
C342S444000, C342S465000
Reexamination Certificate
active
06670920
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the determination of the geographical location of a target emitter and, in particular, to the determination of the geographical location of a target emitter by the coherent, time integrated measurement of received signal wavefront phase differences through a synthetic aperture defined by the path of a single mobile receiving platform and the reconstruction of the wavefront of the received signal.
BACKGROUND OF THE INVENTION
There are many circumstances wherein it is necessary or desirable to determine the geographic location of an emitter of electromagnetic radiation, such as a radar system, a communications facility or device or an emergency beacon or transmitter. Typical applications may include, for example, military signal intelligence (SIGINT) and electronic intelligence (ELINT) operations for locating radar or communications facilities, and air, land and sea rescue operations wherein it is necessary to locate an emergency beacon or transmitter, such as used in aircraft and vessels, or communications devices ranging from conventional or emergency radio devices to cell phones.
Such applications and operations are characterized by common requirements that are, in turn, imposed by general, common characteristics of the target emitters to be located and the situations or circumstances under which the target emitters are to be located. For example, the signal transmitted by a target emitter may be of relatively low power, as in the case of emergency beacons or emergency radios, or may be masked, distorted or effectively reduced by terrain or weather conditions, and such conditions may be intentionally imposed in, for example, military or otherwise hostile situations. In addition, the time available or permissible for locating a target emitter may be limited in both military and civil situations, that is, and for example, in military counter-measures operations or in search and rescue operations, and the resources available for target emitter location may be limited.
As such, it is generally necessary or desirable for a system for locating target emitters to be mobile, that is, to be readily transportable into the general geographical location of a target emitter on an aircraft, vehicle or vessel, both to bring the locator system into range of the target emitter and to allow the locator system to search as large an area as possible in the minimum time. It is also desirable that a locator system be transported and employed in and from a single platform, whether an aircraft, vessel or vehicle, as the use of a single platform reduces the system cost, reduces demand on frequently limited resources and allows a greater area or number of areas to be searched when multiple platforms are available. A single platform system also eliminates the complexity and time delays inherent in deploying and coordinating multiple cooperatively operating platforms.
Related problems are that locator system should be capable of determining the geographic location of a target emitter at the greatest possible range, both to reduce the search time and to reduce risk to the locator system in hostile environments, whether due to weather or terrain factors or otherwise hostile factors. The locator system must also be capable of identifying the geographic location of a target emitter with the greatest possible accuracy as insufficient accuracy in locating a target emitter may render counter-measures ineffective in military situations and may unacceptably delay locating or reaching the target emitter in civil situations, such as search and rescue operations, particularly in difficult terrain or weather conditions. In addition, the locator system should be capable of locating as wide a range of target emitter types as possible, and correspondingly over as wide a range of the electromagnetic spectrum as possible, to allow a given locator system to be employed in as wide a range of applications and situations as possible.
The factors and system elements that determine and limit the characteristics and capabilities of an emitter location system, and in particular a single platform, mobile emitter location system, are numerous and inter-related. Two of the primary elements, however, are the receiving element through which target emitter signals are received and the method by which the received signals are used to identify the geographic location of a target emitter.
For example, current methods for single platform emitter location are based upon determining multiple direction finding (DF) bearings, often referred to as DF “cuts”, to the target emitter at points along a path traversed by the locator platform, such as the flight path of an aircraft. Each “cut” is a determination the gradient, that is, the directional spatial derivative, of the wavefront of a signal emitted by the target emitter and, in theory, indicates the direction of the emitter relative to the locator platform at the point the “cut” is taken. Successive DF cuts are used to determine a Line of Bearing (LOB) “fan” of DF cuts and the location of the target emitter is taken as the point of intersection of the DF cuts, that is, of the bearings forming the LOB fan. This method has been found to provide reasonable results within certain limitations, but is subject to significant limitations and problems. For example, signal propagation factors between the emitter and the locator system path at various points, such as variations in propagation conditions, local multipath distortions, multiple propagation paths and reflections, will result in significant errors in the measured gradients of the wavefront and this significant errors in the measured bearings between the locator system and the target emitter. In addition, the accuracy of conventional DF/bearing systems is dependent upon the accuracy with which the associated antenna or other signal receiving element can determine a bearing to an emitter, which in turn is dependent upon the characteristics of the antenna, such as the size of the antenna. For such reasons, it has been found that for reasonable and acceptable accuracy the ratio of the distance between target emitter and the locator system and the length of the path traversed by the locator system between bearings must be on the order of 1:1, thereby severely limiting either the accuracy of the method or the range at which locations can be accurately determined, or both.
The limitations of conventional DF methods and basic problems in determining the location of a signal emitter through conventional DF methods may be more clearly understood and illustrated by briefly considering the principles of operation of conventional direction finding methods. Referring therefore to
FIG. 1A
, a signal emitted by a Signal Source
2
may be viewed as comprised of a series of curved Wavefronts
4
of wavelength &lgr; radiating from the Signal Source
2
. It is well known and understood that the Gradients
6
of Wavefronts
4
, that is, the spatial derivatives of the Wavefronts
4
, will be essentially normal to the Wavefronts
4
at each point along each Wavefront
4
and, under relatively ideal transmission conditions, will thereby point to the Signal Source
2
. A conventional direction finding (DF) system accordingly attempts to determine two or more Lines Of Bearing
8
to the Signal Source
2
by determining the Gradients
6
of Wavefronts
4
at two or more points along any Wavefront
4
, as illustrated by Gradients
6
A,
6
B and
6
C. Gradients
6
A,
6
B and
6
C then determine corresponding Lines of Bearing
8
A,
8
B and
8
C to the Signal Source
2
and the crossing point of Lines of Bearing
8
in turn identifies the location of Signal Source
2
. As illustrated in
FIG. 1B
, however, more normal non-ideal transmission conditions, such as multi-path affects, distort Wavefronts
4
so that while the Gradients
6
at various points along any of Wavefronts
4
are normal to a Wavefront
4
at each point, the Gradients
6
, as illustrated by Gradients
6
D and
6
E, are erratic and inconsistent
BAE Systems Information and Electronic Systems Integration Inc.
Issing Gregory C
Long Daniel J.
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