Communications: directive radio wave systems and devices (e.g. – Directive – Beacon or receiver
Reexamination Certificate
2001-08-30
2003-10-28
Issing, Gregory C. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Beacon or receiver
C342S424000, C342S442000, C342S465000
Reexamination Certificate
active
06639552
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to an apparatus for and method of enabling the location of a radio wave source to be determined and, more particularly, to such an apparatus and method wherein replicas of waves transmitted from a radio wave source and received at receiver sites having known locations are applied to transducers which apply acoustic waves in opposite directions to a diffraction grating for modulating an optical wave.
BACKGROUND ART
Time difference of arrival (TDOA) is a well-documented method of locating a radio frequency source. TDOA works by detecting the difference in phase of an RF wave emitted by an RF source and received by receivers having known locations. Replicas of the waves received by the receivers are supplied to an electronic phase detector. If the receivers are exactly the same distance from the RF source the phase of the RF signal at the receivers is the same, causing the phase detector to derive a null output level. If the RF source moves relative to a line equi-distant from the two receivers, one receiver is closer to the source than the other, causing a phase difference between the signals at the receivers so that the phase detector derives a signal having an amplitude and polarity indicative of the phase difference.
A problem with this typical TDOA arrangement employing phase detectors is that when the phase detector derives a null output level it is not possible to determine on which side of the two receivers the RF source is located. This problem can be resolved by including a second pair of receivers to derive a second TDOA output signal. The two pairs of receivers frequently include a common receiver. However, use of two pairs of receivers does not resolve the problems associated with relying on phase detectors deriving signals having very small amplitudes, as occurs when the two receivers are approximately equi-distant from the RF source.
I have realized that the system disclosed in the co-pending, previously mentioned application is ideally suited for detecting the time difference of arrival of an RF wave having a predetermined frequency and initial phase emitted by an RF source. This is because that system is able to derive an accurate relatively constant amplitude indication of the relative phase of the RF waves received by the two spaced receivers having known positions. In particular, the indication of the relative phase of the RF waves is determined by the spatial deflection of an optical beam, instead of the amplitude of an electronic phase detector output.
Time difference of arrival is a well-documented and studied method of locating cellular telephone handsets. TDOA is one of several techniques that may be selected by wireless carriers for cellular telephone handset emergency caller, i.e., 911, locating requirements. Additional methods include, individually and collectively: angle of arrival, pattern matching, and global positioning satellite (GPS). In these systems the cellular telephone handset emits an RF signal that is received at various receiver sites. Because each receiver has a known location and is some distance from the cellular telephone handset, the RF signal the cellular telephone handset emits arrives at a given time at each receiver. The coordinates, i.e., longitude and latitude, of each receiver are maintained in a common database and used with the associated timing information to determine a trilateral position of the cellular telephone handset relative to each receiver site. In this situation, the receivers, synchronized by an atomic clock, send to a central site timing data concerning the occurrence time of a call from the cellular telephone handset to the cellular receivers. The central site includes a processor for comparing the times and computing the coordinates, i.e., location, of the cellular telephone handset.
U.S. Pat. Nos. 5,327,144 and 5,512,908 both describe a cellular telephone handset locating system of these general types. The systems disclosed in these patents utilize the three major components of a typical prior art locator system, i.e., at least three receivers at known sites, a central processing site, and an RF source in the form of a cellular telephone handset. The systems are obviously complex and have the problem previously mentioned with regard to electronic phase detectors.
It is, accordingly, an object of the present invention to provide a new and improved method of and apparatus for determining the position of a radio frequency source, particularly a cellular telephone handset.
Another object of the invention is to provide a new and improved method of and apparatus for determining the position of a radio frequency source using time difference of arrival techniques, wherein the relative phases of a radio frequency wave at a pair of spaced receivers is detected without employing an electronic phase detector which derives a null output when the source is approximately equi-distant from the spaced receivers.
An additional object of the invention is to provide a new and improved method of and apparatus for unambiguously determining the position of a cellular telephone handset, particularly for emergency locating purposes, in a relatively simple and inexpensive manner which minimizes possible processing errors.
SUMMARY OF THE INVENTION
According to one aspect of the invention, an apparatus derives a signal indicative of a possible position of a radio wave source relative to a pair of receiver sites having known positions, wherein the radio wave source emits a radio wave to which receivers at the sites are responsive. The apparatus comprises an optically diffractive medium capable of having a moving optical grating induced therein in response to acoustic waves propagating therein. First and second electro-acoustic transducers coupled to the optically diffractive medium launch first and second acoustic waves toward each other in the optically diffractive medium in response to electric excitation of the first and second transducers by replicas of the radio waves to which the pair of receivers are responsive. The transducers are positioned and arranged so that the first and second acoustic waves meet and interact with each other in the medium. An optical source illuminates the medium. An optical detector responsive to optical energy from the optical source and modulated by the moving grating derives a signal indicative of the relative phase angle of the radio wave replicas exciting the first and second transducers.
The acoustic waves typically propagate in a direction between the transducers. The detector arrangement preferably includes plural individual detector elements positioned in an array that extends in the same direction as the direction the acoustic waves propagate. The signal indicative of the relative phase angle is derived in response to an indication of the location of the detector element which has the greatest intensity of optical energy incident on it.
The apparatus is also arranged for determining the location of the radio wave source relative to at least three receiver sites having known positions. The apparatus further comprises a second optically diffractive medium capable of having a moving optical grating induced therein in response to acoustic waves propagating therein. Third and fourth electro-acoustic transducers coupled to the second optically diffractive medium launch third and fourth acoustic waves toward each other in the second optically diffractive medium in response to electric excitation of the third and fourth transducers by replicas of the radio waves to which the receivers at another pair of the sites are responsive. The third and fourth transducers are positioned and arranged so that the third and fourth acoustic waves interact with and meet each other in the second medium. A second optical source illuminates the second medium. A second optical detector arrangement responsive to optical energy from the second optical source and modulated by the second moving grating derives a second signal indicative of the relative phase angle of
Issing Gregory C.
Lowe Hauptman & Gilman & Berner LLP
Northrop Grumman Corporation
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