U.S. patent number RE37,877 [Application Number 09/894,925] was granted by the patent office on 2002-10-15 for electronic counter measures in radar.
Invention is credited to Dorothy Ghose, Rabindra N. Ghose, Walter A. Sauter.
United States Patent |
RE37,877 |
Ghose , et al. |
October 15, 2002 |
Electronic counter measures in radar
Abstract
Apparatus and method for eliminating an interference signal from
a desired signal where a difference of polarization exists or can
be made to exist. The desired signal is received on a first antenna
of appropriate polarization and the interference signal is received
via another port of the same antenna or a second antenna polarized
ninety degrees away from the first. The signal received via the
other polarization is adjusted for amplitude and phase so that all
interference will be cancelled when both signals are subtracted in
a summing junction. In the case where the interference and desired
signals do not have polarization differences, provision is made to
change the polarization of the desired signal.
Inventors: |
Ghose; Rabindra N. (late of Los
Angeles, CA), Ghose; Dorothy (Los Angeles, CA), Sauter;
Walter A. (Malibu, CA) |
Family
ID: |
25264819 |
Appl.
No.: |
09/894,925 |
Filed: |
June 28, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
833590 |
Sep 15, 1977 |
06114983 |
Sep 5, 2000 |
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Current U.S.
Class: |
342/16; 342/159;
342/17; 342/188 |
Current CPC
Class: |
G01S
7/024 (20130101); G01S 7/36 (20130101) |
Current International
Class: |
G01S
7/02 (20060101); G01S 7/36 (20060101); G01S
007/36 () |
Field of
Search: |
;342/16,17,18,13,89,159,188 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Sotomayor; John B.
Claims
We claim:
1. Apparatus for receiving a wanted radio frequency signal in the
presence of interference comprising: a first antenna polarized in
the same mode as the wanted radio frequency signal; a second
antenna polarized orthoginally with respect to said first antenna;
means coupling said first antenna substantially directly to a radio
receiver for receiving said wanted radio frequency signal; a signal
controller coupled to said second antenna for varying the amplitude
and phase of any signal received by said second antenna; means
subtractively combining the output of said signal controller with
the signals present in said first antenna to radio receiver
coupling means; means for sampling the signals present between said
subtractively combining means and said radio receiver; a signal
processor coupled to said sampling means and to said signal
controller to receive signal samples in the polarization of said
first and second antennas; said signal processor including means
for comparing the phase of signals derived from said first and
second antennas.Iadd., said means for comparing the phase of
signals deriving phase correction constants.Iaddend.; said signal
processor including means for comparing the amplitude of signals
derived from said first and second antennas.Iadd., said means for
comparing the amplitude of signals deriving amplitude correction
constants.Iaddend.; .[.said signal processor including means for
deriving the amplitude and phase correction constants for said
signal controller from said comparing means;.]. and means coupling
the output of said signal processor to said signal controller for
controlling the amount of phase and amplitude variation of said
signal controller to produce a minimum interference signal at said
sampling means.
2. Apparatus for receiving a wanted radio frequency signal in the
presence of interference comprising; a first antenna polarized in
the same mode as the wanted radio frequency signal; a second
antenna polarized orthoginally with respect to said first antenna;
means coupling said first antenna substantially directly to a radio
receiver for receiving said wanted radio frequency signal; a signal
controller coupled to said second antenna for varying the amplitude
and phase of any signal received by said second antenna; means
subtractively combining the output of said signal controller with
the signals present in said first antenna to radio receiver
coupling means; first and second sampling means, said first
sampling means sampling first signals present between said
subtractively combining means and said radio receiver; a local
oscillator; a first mixer coupled to said local oscillator; means
introducing the output of said first sampling means into said first
mixer whereby the signals sampled thereby are converted to an
intermediate frequency; said second sampling means sampling signals
from said second antenna, a second mixer coupled to said local
oscillator; said second sampling means coupled to said second mixer
to convert signals sampled from said second antenna to the same
intermediate frequency as the output of said first sampling means;
a signal processor coupled to said first and second mixers and to
said signal controller to receive signal samples in the
polarization of said first and second antennas; said signal
processor including means for comparing the phase of signals
derived from said first and second antennas; said signal processor
including means for comparing the amplitude of signals derived from
said first and second antennas at said intermediate frequency.
Description
PRIOR ART STATEMENT
The three patents described below relate to the interference
cancellation techniques described in this application.
U.S. Pat. No. 3,699,444, INTERFERENCE CANCELLATION SYSTEM,
describes a radar system circuit which uses a portion of the
transmitted signal, after phase shifting and attenuation, to cancel
the transmitted signal received at the receiver antenna.
U.S. Pat. No. 3,716,863, INSTRUMENT LANDING ERROR CORRECTING SYSTEM
described apparatus for cancelling an interfering signal coherent
with the desired signal, but varying in amplitude and phase.
U.S. Pat. No. 4,016,516, REFLECTIVE SIGNAL CONTROLLER, describes a
circuit for varying the amplitude and polarity of an rf signal.
Parts of said circuit may be of use in this inventive
application.
None of the above references teaches or suggests a technique for
cancelling interference based on a difference of polarization
between it and the desired signal.
BACKGROUND OF THE INVENTION
Described herein is apparatus for preventing the jamming of radar
by an electronic countermeasure (ECM) jamming or interference
signal; and more particularly, apparatus for receiving two or more
different polarizations of the radar return signal and the
interference signal, and using that different polarization received
signal to cancel the interference signal without also cancelling
the desired radar return signal.
It is common for radar systems to be jammed by hostile electronic
countermeasure interference signal sources. These electronic
countermeasure systems typically monitor the radar band, accurately
determine the frequency, pulse repetition rate another radar system
characteristics, and transmit interfering signals of sufficient
power and of appropriate timing to render the radar inoperative.
These countermeasure systems operate in real time so that a change
of radar frequency will instantaneously be followed by a change of
interference frequency. Therefore, some method of interference
avoidance is required what will act to cancel interference, both
pulsed and continuous, even at the exact frequency of the radar
system.
The prior art includes U.S. Pat. No. 3,716,863, INSTRUMENT LANDING
ERROR CORRECTION SYSTEM, commonly assigned, which cancels an
interference signal by producing a correction signal of equal
frequency and amplitude but of appropriate polarity. When the
interference, correction and desired signals are received and
summed, the interference signal is cancelled and the desired signal
remains. This system is useful where the desired and interference
signals are of the same frequency and are coherent but differ in
phase. An example is an aircraft receiving an ILS signal directly
from a transmitter and simultaneously receiving a reflected ILS
signal from a nearby structure.
A system employing this principle may employ a circuit for varying
the basic signal phase and amplitude to produce a correction
signal. Such a circuit is described in U.S. Pat. No. 4,016,516,
REFLECTIVE SIGNAL CONTROLLER, commonly assigned. This signal
controller is designed to be inserted into the path between a
source and the utilization device to allow the control of signal
amplitude ratio and polarity.
The prior art thus recognizes the problem of interfering signals of
the same frequency, and describes the generation of a correction
signal of appropriate phase to cancel said interference. This is
possible since the correction signal generator is coupled to, and
therefore is coherent with the interference signal source.
In the case where the interference signal is produced by an
electronic counter measure source (ECM), however, there can be no
coupling to the source to generate a coherent correction signal,
and cancellation of a jamming signal is not feasible by this
method.
An alternative is to cancel such interference by taking advantage
of the difference in signal polarization between desired and
interference signals. This technique uses circuits equivalent to
those required by prior cancellation systems, but requires a
different antenna installation. In the case where the interference
and desired signals do not have polarization differences, provision
is made to change the polarization of the desired signal.
SUMMARY OF THE INVENTION
The proposed system comprises a receiver equipped with two
receiving antenna ports, each configured to receive waves at a
particular polarization, the two polarizations being ninety degrees
out of phase with each other. For example, one could be configured
to receive vertically (V) polarized waves and the other, horizontal
(H). Then, to the extent that the ratios in the interference and
radar return signals are different at the V and H receive ports, a
cancellation signal can be obtained by adjusting the amplitude
ratio and electrical phase angle of the V or H receive port signal
that does not normally receive the desired signal. Therefore,
cancellation of the interference signals only can be provided even
if the desired and interference signals are at the same
frequency.
This system can function wherever the desired versus interference
ratios are not the same at the two polarization ports. This would
be true if the interference were vertically polarized and the
desired signals horizontally, for instance. Also, the interference
may be circularly polarized and the desired signal, vertically. In
the latter case, the horizontally polarized antenna would receive
the interference while the vertically polarized antenna would
receive both interference and desired signals. A signal received
via one channel (port) could be used on the other channel for
interference cancellation.
In the case where the interference signals polarization angle is
matched to that of a desired radar signal, the radar polarization
angle may be changed by the operator to avoid cancellation of the
desired radar signals. In the case where the energy is radiated
from a radar antenna, the polarization of the radiated signal could
be rotated through the use of the appropriate waveguide "plumbing".
The system will then again operate as stated above.
One variation that the system has to compensate for is the ratio of
received interference signal power received by each antenna port.
For instance, circularly polarized interference has equal amounts
of horizontally and vertically polarized signal power. Therefore,
the same amount of interference power received at one antenna port
has to be subtracted from the signal received at the other antenna
port. On the other hand, horizontally polarized interference would
provide a relatively small amount of interference at the vertical
antenna port. This system variable is the amplitude ratio between
the channels. The system automatically reduces (or increases) the
amplitude ratio required to provide interference cancellation.
Another system variable is the amount of time or phase delay
between channels. The same interference signal may be received by
one receiving antenna port a fraction of a wavelength ahead of the
other. To compensate, an electrical phase angle and/or a variable
delay in the system is provided, so that cancellation signals are
produced with the proper electrical phase angle to cancel the
interference optimally.
Both the phase and amplitude ratio control circuits are monitored
and, to the extent that perfect cancellation was not produced,
error signals are generated. These error signals are then fed back
to the phase and amplitude control circuits, closing the loops to
make automatic system corrections.
Therefore, an object of this invention is to protect a radar
receiver from interference by differentiating between normal
returns and interference, based on differences between their
polarizations and to cancel the interference thus detected.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified block diagram of a portion of a receiver
system incorporating this interference cancellation circuit;
FIG. 2 is a detailed diagram of this system, where the amplitude
ratio and phase corrections are processed at the intermediate
frequency stage; and
FIG. 3 is a schematic diagram of one signal processor
embodiment.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 contains the main components of the interference
cancellation system, including vertically polarized 10 and
horizontally polarized 11 antennas or antenna ports. The invention
utilizes a discrimination concept for the desired and interference
signals based on their polarization and is, therefore, not
constrained by the directions of arrival of the desired signal and
the interference. Consequently, the discrimination means
encompassed by the invention accomodates friendly and hostile
interference arriving through both mainlobes and sidelobes.
Another aspect of this invention based in its polarization
discrimination capabilities is that the adaptive nature of the
counter-countermeasure provision makes the interference suppression
in the receiver useful even for moving receivers and interference
sources. This is true for two reasons. First, the ratio of received
interference power between antennas and the interference delay
between antennas will not vary rapidly. Moreover, to the extent
that there may be a variation, the closed loop error detecting
circuits will automatically compensate for the variation.
If the interference source is a circularly polarized signal in the
same frequency band as the receiver, and if the desired received
signal is vertically polarized; there will be a strong horizontal
component of the interference while the horizontal component of the
desired signal will be negligible. Even if there is de-polarization
in the medium, the relative magnitude of the ratio of the desired
signal appearing as a horizontally polarized field component will
be significantly lower than the ratio of the horizontal and
vertical fields of a circularly polarized jamming signal or
interference. In the invention, means are provided to receive the
orthoginal (horizontal) component of the incoming field in addition
to the normal (vertical) component such that the antenna aperture
available to both vertical and horizontal ports is the same. In
other words, the effective receive antenna gains for the vertical
and horizontal polarizations are the same or nearly the same.
The signal or interference as received by the horizontal port 10 of
the antenna feed is the source for the synthesis of the
interference as it appears at the vertical port 11 of the receive
antenna. This is due to the fact that the horizontal and vertical
components of the jamming signal are correlated, except perhaps by
an amplitude ratio factor and a phase or time delay. The amplitude
ratio factor could be due to the differential proportion
characteristics of the interference through the medium and the
relative antenna orientations. The delay between the vertical and
horizontal component of the interference appearing at the radar
receive antenna could also be due to similar reasons.
It is evident, therefore, that if an appropriate amplitude ratio
factor and a phase or time delay are introduced as transfer
functions of the signal controller 12, as shown in FIG. 1, the
output of the signal controller will be a cancellation signal which
is identically the same as the interference appearing at the
vertical port of the receive antenna. The subtraction of these two
signals, shown in FIG. 1, then will yield an output where the
interference is cancelled but the desired signal is not, since the
cancellation of the desired signal cannot occur due to its
"negligible" level at the output of the signal controller. If the
synthesized transfer function characteristic of the signal
controller, that is the amplitude ratio factor K and time (or
phase) delay T, are not exactly what are required, the difference
between the synthesized cancelling signal and the interference will
not be zero. This nonzero residual interference signal then can be
used as the error signal of a high-gain servo-loop that drives the
factors K and T until the error signal vanishes. The equilibrium
condition for the loop then assures the absence of the interference
at the receive line.
These functions are accomplished by the apparatus of FIG. 1 as
follows: Both horizontally and vertically polarized antenna ports
10 and 11 receive rf energy. One antenna port (the horizontally
polarized antenna in FIG. 1) supplies its output to an amplifier 14
and signal controller 12, the latter being used to generate the
appropriate power and delay parameters described above.
A signal controller capable of receiving an rf signal, or either
attenuating or amplifying it, and of varying the amount of delay
between the controller input and output are old in the art. See,
for example, U.S. Pat. No. 4,016,516 commonly assigned, for a
detailed description of the signal controller for this application,
and which is incorporated by reference herein.
The signal controller output is adjusted for amplitude ratio and
delay. This is then subtracted from the vertical port signals in
the summing junction 13.
The output of the summing junction 13 is the correct signal with
the interference deleted. For example, if the interference was
received mainly at the horizontal antenna, and the desired signal
at the vertical; the output of the signal controller would be
adjusted to output the proper amplitude and phase to cancel the
interference in the vertical component.
An error sampling determination is made at a sampler at the output
of the summing junction 13. Uncompensated errors in the form of rf
signal levels are used as inputs to a signal processor 16. These
signals are analyzed for their amplitude and delay relationship to
the received interference signals, and correction signals would be
applied to the signal controller 12 to more accurately cancel the
interference, thus closing the error detection loops. An example of
a circuit used for cancelling interference not identical to the
desired signal in amplitude and phase is described in U.S. Pat. No.
3,716,863, which is incorporated by reference herein.
FIG. 2 shows an actual implementation plan for a vertically
polarized radar. In this figure, the signal processing part of the
servoloop, referred to above, is effective at some "intermediate
frequency" instead of at the radar receive frequency.
Two probes are mounted in the antenna, one with horizontal
polarization and the other, vertical. The horizontally polarized
component 20 is coupled through an rf limiter 22 and amplifier 23
to a signal controller 24 which varies its amplitude ratio and
phase angle (or time delay). The adjusted output is then injected
into the vertically polarized channel through an rf amplifier 25
and coupler 26 to cancel out the interference signal received. The
resultant signal is transmitted to the receiver where it is used as
a regular received signal.
The error signal feedback loop in this embodyment comprises the
remainder of the circuit. The orthogonally polarized component is
summed through coupler 34 with the local oscillator 27 output at
junction 30 and the difference resulting is amplified in an IF
amplifier 28. Similarly, a sampling of the signal being transmitted
to the receiver, already corrected for interference, is summed with
the same local oscillator 27 output at junction 31 and is applied
to another IF amplifier 29. Finally, both IF outputs are compared
in the signal processor 32. To the extent that a residual
interference signal remains in the corrected signal from the
sampling coupler 33, an additional correction is generated by the
signal processor 32, and applied to the signal controller 24 to
correct for the uncompensated error.
The signal processor 32 of FIG. 2 may be implemented in any of
several well known ways to produce the amplitude (K) and time delay
(T) output signals. One technique for generating these factors is
to do so at a time when the desired signal is known to be absent.
By definition, the only remaining signal is the undesired one. At
this time, the outputs of both ports could be applied to an
amplitude comparator for producing an amplitude correction signal
K, and simultaneously both signals could be applied to a phase
comparator generating a phase correction signal T. One time when
the desired signal is normally absent in a radar is before the
pulse transmission during which time reflected pulses are no longer
being received. Additional circuits could be added to disable the
transmitter and enable the signal processor to any time under
operator control.
A manual control may also be provided so that the operator could
adjust K and T while observing the radar scanner, manually tuning
for minimum interference.
FIG. 3 is a simplified schematic diagram of one automatic signal
processor embodyment. Both channels comprise a noise limiter 34 and
35 so that the circuit will not react to random noise, but will
react only to received signals large enough to overcome a
predetermined threshold. Both channel signals are compared on the
basis of amplitude and phase, the outputs being K and T correction
signals of appropriate polarity and amplitude which, when applied
through the signal controller, will result in the complete
cancellation of the interference signal. Sample and hold circuits
38 and 39 are provided so that the K and T levels can be changed
only at times when the noise threshold has been exceeded by an
actual interference signal. The input to the horizontal (H) channel
is multiplied by the constant K in amplifier 40 so that, for proper
values of K, there will be an exact amplitude match at the
amplitude comparator 36. The resultant amplitude and phase factors
are then coupled to the signal controller and used as described
above.
Although both FIGS. 1 and 2 cite the problem of a circularly
polarized jammer or interference source, the invention is not
constrained to such specific situations only. If for example, the
interference source is horizontally polarized, it is readily seen
from both FIGS. 1 and 2 and the same operational principle
discussed above, that the interference at the receive line will be
cancelled by the process described above. If, however, the jamming
signal and the desired signal are vertically polarized, the
simplified schematic arrangement shown in FIG. 1 will not be
adequate for the cancellation of the jamming signal. One can remedy
such situation by changing the polarization of the desired signal.
Since a radar, provided with means of employing both linear and
circular polarizations, can always react to select the polarization
most suitable for its purpose in a jamming environment, a change of
polarization of a radar signal usually does not pose any
operational problems. For the circularly polarized radar case, it
is seen that one can establish a port in the receive antenna line
where there will be a predominant interference, in comparison with
the desired signal. The signal from this port then could always be
utilized to synthesize the appropriate cancellation interference as
shown in FIG. 1.
From the foregoing, it may be seen that we have invented a system
where an interfering signal may be cancelled provided that there is
a difference of polarization between interference and desired
signals. Further, where the interference and desired signals are
similarly polarized, provision has been made to change the desired
signal polarization to create said polarization differences.
This discussion has used radar as an illustrative embodyment.
However, it is clear that this system can be used to cancel
interference in any rf receiving system; including any kind of
radio communication link.
The above described embodyments and methods are furnished as
illustrations of the principle of this invention and are not
intended to define the only embodyments possible in accordance with
our teachings. Rather, protection under the United Stated Patent
Law shall be afforded to us not only to the specific embodyment
alone, but to those falling within the spirit and terms of the
invention as defined in the following claims.
* * * * *