Communications: directive radio wave systems and devices (e.g. – Directive – Position indicating
Reexamination Certificate
1988-12-02
2001-02-06
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Position indicating
C342S107000, C342S418000
Reexamination Certificate
active
06184831
ABSTRACT:
TECHNICAL FIELD
The invention relates to a method and a system for determining the location and velocity of a source of electromagnetic signals by a receiver of those electromagnetic signals. More particularly, the invention relates to a method and a system for determining the location and velocity of a source of electromagnetic signals by a receiver of those electromagnetic signals, based on measurements of Doppler-shifted frequencies.
BACKGROUND ART
Combat aircraft are provided with means for detecting and locating noncooperative electromagnetic emitters, e.g., radars and communication transmitters, such means being known as “threat warning and location systems.” Presently existing threat warning and location systems utilize directive receiving antennas to provide for determining the approximate location of such emitters. In order to provide the required directivity over a wide band of frequencies, multiple antennas, each of five to ten wavelengths in dimension, are required. The installation of such sets of antennas to provide appropriate viewing angles and to satisfy other requirements on combat aircraft of advanced design creates substantial problems for aircraft designers.
The present invention is directed toward determining emitter threat location and velocity through use of signal processing rather than antenna directivity. Depending on the desired sector coverage, no more than four small-aperture antennas are required for emitter detection and location system operation. The required aperture of each of such antennas would be no more than a few inches. Such small-aperture antennas pose minimal problems in installation and can readily be made consistent with other requirements on combat aircraft of advanced design.
It has been shown that a series of measurements of the Doppler-shifted frequency of the signals from any emitter, including noncooperative emitters, when made from a moving receiver such as one installed on an aircraft, can provide a basis for calculating location and velocity of the emitter. In order to make the necessary frequency measurements with sufficient accuracy, the emitter should produce coherent signals. In particular, signals which are not continuous (e.g., pulsed radar) should be coherent on a pulse-to-pulse basis. Other methods, such as estimation of unshifted frequency, measurement of time-difference-of-arrival, or part-time use of directive antennas, must be used when the signals are not coherent.
The familiar equation for Doppler shift is:
f
D
i
=
f
o
(
1
+
v
c
cos
θ
i
)
where
f
D
i
is the value of the i
th
measurement of the Doppler-shifted frequency, f
D
f
o
is the unshifted frequency of the emitter
V is the speed of the moving receiver
c is the velocity of light
&thgr;
i
is the angle between the velocity vector of the moving receiver and the direction of arrival of the signal from the source for i
th
measurement
It might at first seem that a series of measurements of f
D
(expressed as f
D
i
) coupled with velocity and position data would allow for solution of this equation in terms of &thgr; (thus giving not only bearing but also, through additional calculations, being relational to the range to the emitter). Such, however, is not the case. The difficulty arises because the receiver lacks information concerning f
o
. The procedure outlined above leads to a consistently undetermined set of linear equations in terms of a conventional solution. The difficulty could be resolved by recording maximum and minimum values of f
D
while the receiver executes a 360° turn. However, such a method is considered to be totally unacceptable from an operational standpoint.
It is therefore desirable to provide a method for determining the range between a receiver and an emitter of electromagnetic energy of unknown frequency.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a method for determining position and velocity of a threat emitter having an unknown signal frequency. Measurements of apparent (Doppler-shifted) frequencies are used.
It is another object of the present invention to provide a method for determining the position and velocity of an emitter through use of signal processing.
It is a further object of the present invention to provide a method for determining the position of a stationary emitter using frequency measurements recorded by a single moving observer.
It is a still further object of the present invention to provide a method for determing the position and velocity of a moving emitter using frequency measurements recorded by two moving observers.
It is yet another object of the present invention to provide a method for determining the position and velocity of a moving emitter using frequency measurements recorded by one moving observer.
It is a further object of the present invention to provide a system for determing the position and velocity of a threat emitter having an unknown signal frequency. Measurements of apparent (Doppler-shifted) frequencies are used.
It is another object of the present invention to provide a system for determining the position and velocity of an emitter using signal processing.
It is a further object of the present invention to provide a system for determining the position of a stationary emitter using frequency measurements recorded by a single moving observer.
It is a still further object of the present invention is to provide a system for determining the position and velocity of a moving emitter using frequency measurements recorded by two moving observers.
It is yet another object of the present invention to provide a system for determining the position and velocity of a moving emitter using frequency measurements recorded by one moving observer.
According to one aspect, the invention is a method for determining the range to a source which emits electromagnetic energy having a substantially constant but unknown frequency. The range is determined from measurements made at at-least-three points along a trajectory having a substantially straight-line portion. the method comprises four steps. The first step is (A) moving a first receiver along the trajectory. The first receiver is responsive to a spectral band that includes a frequency that is Doppler-shifted by relative motion between the source and the first receiver. The second step is (B) measuring the Doppler-shifted frequency of the received electromagnetic energy when the receiver is located at each of the three points on the trajectory. The third step is (C) calculating at least two ratios of frequencies. The frequencies are functions of the Doppler-shifted frequencies. The fourth step is (D) calculating the range between the first receiver and the source from the frequency ratios at the at-least-three points.
In a first preferred embodiment, the invention is a method for determining the position of a stationary source which emits an electromagnetic signal having a substantially constant but unknown frequency. The method comprises four steps. The first step is (A) that of moving an observer along a trajectory. The observer's signal receiver is responsive to a spectral band that includes frequencies that are Doppler-shifted by relative motion between the emitting source and the observer. The second step is (B) measuring the Doppler-shifted frequency of the received electromagnetic signal when the observer's receiver is located at each of three points on the trajectory. Also included in step (B) is measuring the distance between observer positions at the first and second frequency measurements and measuring the distance between observer positions at the second and third frequency measurements. The third step is (C) calculating at least two distinct ratios of the apparent frequencies. One ratio cannot be the reciprocal of the other. Each ratio is a function of bearing angles, relative velocity, and the velocity of light, but not a function of the unshifted emitter frequency, f
o
. The fourth step is (D) calculating the position of the emitting source relative to the observ
Dalby Thomas G.
Kratzke Albert W.
Blum Theodore M.
Hammar John C.
The Boeing Company
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