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Claims  |
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We claim:
1. A radar system for operating in an electromagnetic environment including
in-band and out-of-band interferences, comprising:
means for sampling the environment to provide a spectral electromagnetic
profile representative of the environment;
processor means for obtaining from the profile an optimal channel for the
environment and for generating a receiver matched filter corresponding to
the channel and equalizer coefficients from responsive pulses, the optimal
channel having an optimum center frequency and maximum channel bandwidth
with fewest in-band interferences;
means for receiving an echo of a target signal transmitted to the
environment;
means for multiplying the frequency spectrum of the echo with the matched
filter and for notching out the interferences in the frequency spectrum;
means for transforming the interference free frequency spectrum into a
responsive pulse to be transmitted to the processor means;
wherein the processor means, using the responsive pulse, calculates
transversal equalizer filter coefficients for reducing temporal sidelobes
of the responsive pulse to effect an interference free signal having a
desirable main lobe to sidelobe ratio.
2. The apparatus according to claim 1, wherein the sampling means comprises
an automatic channel monitor that continually examines the environment for
updating the spectral electromagnetic profile representative thereof.
3. The apparatus according to claim 1, wherein the multiplying means
comprises a matched filter having frequency coefficients for eliminating
the interferences from the frequency spectrum of the received echo.
4. The apparatus according to claim 1, wherein the transforming means
comprises an inverse Fast Fourier Transformer.
5. The apparatus according to claim 1, further comprising:
a Fast Fourier Transformer for converting the echo of the target signal
into its corresponding frequency spectrum.
6. The apparatus according to claim 1, wherein the processor means further
generates from the profile waveform parameters to be used for generating
test target signals.
7. The apparatus according to claim 6, further
means using the waveform parameters for synthesizing the test target
signals;
wherein the test target signals are transmitted to the environment when the
radar system is activated to transmit signals thereto and are fed to the
receiving means for recursive calculations to generate an optimal main
lobe to sidelobe ratio for the interference free signal.
8. In a radar system operating in an electromagnetic environment including
in-band and out-of-band interferences, a method of maintaining signal
detectability, optimizing resolution and restoring non-ambiguous
performance for the system, comprising the steps of:
sampling the environment;
estimating from the sampled environment an electromagnetic profile
including the interferences;
determining from the profile an optimal channel for the environment and
channel transmit waveform parameters, the channel having an optimum center
frequency and maximum channel bandwidth with fewest in-band interferences;
generating a receiver matched filter corresponding to the determined
optimum channel;
transmitting a test target signal based on the waveform parameters into the
environment;
receiving an echo of the target signal and converting the same into a
corresponding frequency spectrum;
multiplying the frequency spectrum with the matched filter for notching out
the interferences in the signal spectrum;
compressing the multiplied frequency spectrum into a responsive pulse; and
utilizing the responsive pulse to calculate transversal equalizer filter
coefficients for reducing temporal sidelobes of the responsive pulse to
realize an uncontaminated pulse signal with a desirable main lobe to
sidelobe ratio.
9. The method of claim 8, further comprising the step of:
repeating the utilizing step until an optimal main lobe to sidelobe ratio
is obtained.
10. The method of claim 8, wherein the sampling step comprises the step of:
continually updating the sampling of the environment.
11. The method of claim 10, wherein the generating step comprises the steps
of:
generating different matched filters in response to different
electromagnetic profiles effected from environmental sample updates; and
utilizing responsive pulses from the compressing step to generate
respective transversal equalizer filter coefficient.
12. A radar system for operating in an electromagnetic environment
including in-band and out-of-band interferences, comprising:
means for sampling the environment to provide a spectral electromagnetic
profile representative of the environment;
processor means for obtaining from the profile an optimal channel for the
environment and for generating a receiver matched filter corresponding to
the channel, notch filter coefficients and equalizer coefficients from
responsive pulses, the optimal channel having an optimal center frequency
and maximum channel bandwidth with fewest in-band interferences;
means including an antenna having an array of elements for receiving a
plurality of reflected signals representing echoes of target signals
transmitted by the antenna to the environment;
a plurality of filter means each for notching out the interferences in the
frequency spectrum of a corresponding one of the reflection signals;
beam forming means for accepting all of the interference notched out
reflection signals and for automatically combining the reflection signals
to form a beam signal having pattern nulls in the direction of the
interferences, the beam signal being substantially devoid of sidelobe
interferences;
compressor and equalizer means for utilizing information from the receiver
matched filter and the equalizer coefficients to compress the beam signal
and to perform equalization to compensate for possible temporal sidelobes
introduced thereto by the notch filter means;
whereby a substantially interference free signal having a desirable main
lobe to sidelobe ratio is generated
13. The radar system according to claim 12, further comprising:
second filter means for receiving the main beam signal and for removing any
residual interferences from the main beam signal, the second filter means
further providing feedback to the beam forming means to insure the removal
of the residual interferences.
14. The radar system according to claim 12, wherein the sampling means
comprises an automatic channel monitor that continually examines the
environment for updating the spectral electromagnetic profile
representative thereof.
15. The radar system according to claim 12, wherein each of the notch
filter means comprises:
a Fast Fourier Transformer for converting the reflection signal to its
corresponding frequency spectrum;
a filter connected to the output of the Fast Fourier Transformer and being
supplied by the processor means with data relating to the notch filter
means for correlating the notch filter mean data with the corresponding
frequency spectrum; and
an inverse Fast Fourier Transformer connected to the output of the filter
for inverse transforming the correlated frequency spectrum to a
corresponding time domain signal.
16. The radar system according to claim 12, wherein the compressor and
equalizer means comprises:
a Fast Fourier Transformer for converting the beam signal from the beam
forming means to its corresponding frequency spectrum;
a filter connected to the output of the Fast Fourier Transformer and being
supplied by the processor means with data relating to the receiver matched
filter and equalizer coefficients for correlating the data with the
corresponding frequency spectrum; and
an inverse Fast Fourier Transformer connected to the output of the filter
for inverse transforming the correlated frequency spectrum to a
corresponding time domain signal.
17. The radar system according to claim 12, wherein each of the notch
filter means comprises:
at least one IMS A100 type, programmable transversal filter integrated
circuit.
18. In a radar system operating in an electromagnetic environment including
in-band and out-of-band interferences, a method of maintaining signal
detectability, optimizing resolution and restoring non-ambiguous
performance for the system, comprising the steps of:
sampling the environment;
estimating from the sampled environment an electromagnetic profile
including the interferences;
determining from the profile an optimal channel for the environment and
channel transmit waveform parameters, the channel having an optimum center
frequency and maximum channel bandwidth with fewest in-band interferences;
generating a receiver matched filter corresponding to the determined
optimum channel, notch filter and equalizer coefficients;
transmitting a plurality of target signals based on the waveform parameters
to the environment;
utilizing an array antenna to receive a plurality of reflection signals
representative of echoes of the target signals, each array of the antenna
receiving one of the reflection signals and any interference superposed
thereon;
suppressing the interference in each of the reflection signals and
converting the reflection signals into corresponding frequency spectrums;
combining the corresponding frequency spectrums to form a beam signal
having pattern nulls in the direction of interferences and substantially
eliminating sidelobe interferences from the beam signal;
utilizing data relating to the receiver matched filter and the equalizer
coefficients to compress and to equalize the beam signal for compensating
possible temporal sidelobe interference introduced thereto during the
suppressing step;
thereby generating a substantially interference free signal having a
desireable main lobe to sidelobe ratio.
19. The method according to claim 18, wherein the combining and eliminating
step comprises the step of:
utilizing an adaptive array digital beam-former for receiving the
interference suppressed reflection signals and for outputting the beam
signal.
20. The method according to claim 19, further comprising the step of:
feedbacking data to the digital beam-former to ensure that no interference
exists in the main beam and that the beam-former suppresses only the
sidelobe interferences. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention relates to radar systems and more particularly to a
technique and apparatus for operating radar and narrow band
communications-type emitters in the same frequency band such that the
radar transmitter and receiver maintain good signal detectability,
optimized resolution and non-ambiguous performance.
BACKGROUND OF THE INVENTION
It is well known by those versed in the art of radar pulse waveform design
that radar target resolution is inversely related to radar waveform
bandwidth. It is also well known that the optimum theoretical detection
performance of any radar system is dependent only upon the pulse waveform
energy and the receiver noise configuration. In an effort to increase
waveform energy using peak power limited radar transmitters, long pulse
duration, constant envelope and wide bandwidth signals are used. These
efforts are covered by the general category of large time-bandwidth
product or pulse compression waveforms and are disclosed by the following
references: Huttman German Patent Ser. No. 768,068; Cauer German Pat. No.
892,772; Sproule, et al. British Patent Ser. No. 604,429; Dicke U.S. Pat.
No. 2,624,876; and Darlington U.S. Pat. No. 2,678,997.
In general, the long duration, high energy radar pulse in the
above-mentioned systems is phase (or frequency) modulated (or coded) to
realize a bandwidth that is orders of magnitude greater than that
predicted by its pulse width alone. And it is this phase (or frequency)
modulation (or code) that must be removed by a receiver of the system when
its received echoes are processed. To do so, the received long duration
pulse is compressed by the receiver into a narrow, high amplitude pulse.
In most existing systems, this compression is performed in a fixed analog
dispersive delay line. However, in accordance with copending application
Ser. No. 196,579 entitled "FM Modulation Technique for Producing Frequency
Rejection Bands" by Cermignani, et al., and also copending application
Ser. No. 196,578 entitled "Narrow Band Interference Suppressor for Pulse
Compression Radar," by Schreiber, et al., both applications having been
assigned to the same assignee as the instant invention, it may presently
be performed digitally, using a real time programmable discrete Fourier
transform/inverse Fourier transform device. The discrete Fourier transform
of the received time waveform is taken in real time, conjugate phase
weighted to cancel the phase modulation (or code), amplitude weighted to
control temporal sidelobes or ambiguity, and then transformed back into
the time domain.
There exists, however, electromagnetic environments under which such radar
systems must operate where narrow bandwidth, high-power interference
sources are active at frequencies within the same bandwidth as that of the
pulse compression radar. If, as presently done, the combination of the
received interference and the desired, small echo signal is processed by
an analog dispersive delay line, the resulting compressed time pulse may
become distorted and undetectable, due to the presence of the much larger
interference.
One approach to correct this problem is to design and implement narrow,
fixed bandwidth, band eliminate filters in the radar receiver, prior to
pulse recompression, so that the unwanted interference frequencies are
attenuated prior to passing the received echo signal through the
dispersive delay line. Yet because the interference changes its center
frequency and bandwidth as a function of time and radar antenna azimuth
angle, the narrow band eliminate filters must track the interference.
Consequently, the radar must perform a spectral analysis of the
environment; that is, precisely locating the interference emitters in the
radar band of operation and tuning the band eliminate filters to the
undesired emitter center frequency.
In practice, however, since the narrow, fixed bandwidth, band eliminate
filters would attenuate, besides the interference and noise, the signal
itself, there is a significant net loss in the signal-to-noise ratio,
especially if multiple narrow band cancellers are needed to remove
multiple in-band interferences. As is well known in the signal processing
art, this result follows directly from the fact that the resulting
receiver transfer function is not the "matched filter" for the transmitted
signal; hence, there is the degradation in post-detection signal-to-noise
ratio caused by the mismatch filter loss.
Degradation in post-detection signal-to-noise ratio notwithstanding, the
narrowed, fixed bandwidth, band eliminate filters also introduced
intolerable increases in the compressed pulse temporal sidelobes. As is
well known in the signal analysis art, this increase in the compressed
pulse temporal sidelobes follows directly from the fact that receiver
transfer function causes "paired echo" distortion of the recompressed
pulse, i.e., the amplitudes of the paired echoes are proportional to the
relative bandwidth of the band eliminate filters, and their locations
relative to the main pulse of the signal are determined by their
displacement from the center frequency of the original signal. Oftentimes
this degradation of main lobe to sidelobe ratio is referred to as
ambiguity, since there would appear to be many targets when in fact there
is only one. Thus, present systems do not adapt the transmitted signals to
avoid the interference bands but only filter the unadapted receiver signal
to eliminate the interference, with resulting loss in detectability and
distortion that causes loss of resolution and increases ambiguity.
BRIEF DESCRIPTION OF THE INVENTION
The present invention is a technique and the apparatus thereof for
restoring radar detectability, optimizing radar resolution and restoring
non-ambiguous performance when the radar is required to operate in an
electromagnetic environment that includes many simultaneous in-band and
out-of-band narrow bandwidth interferences.
In particular, the present invention uses an automatic channel monitor to
sample the electromagnetic environment in which the radar system is to
operate. From the sampling of the environment, an electromagnetic
interference (EMI) profile, which serves as a data base for calculation of
the necessary bandwidth and waveforms later on, is obtained. Next, the
appropriate bandwidth and the center frequency for a signal is determined.
And with these pieces of information, a radar waveform is designed by
means of an adaptive waveform processor, the waveform having a spectrum
which contains no energy in-band with the interferences. At the same time,
the processor also designs a matched filter that notches out the
interferences and reduces their levels below receiver noise. The receiver
is next equalized, by certain equalizer coefficients, so that when the
radar pulse is received, an appropriate proper main lobe to sidelobe ratio
is obtained in the compressed pulse. As a result, since there is no energy
in-band with the interferences and an acceptable main lobe to sidelobe
ratio is present, an optimum detection, i.e. a matched filter solution, is
obtained. Thus, the present invention permits a radar system to operate in
an electromagnetic environment which includes many simultaneous in-band
and out-of-band narrow bandwidth interferences.
Therefore, it is an objective of the present invention to allow both radar
and narrow band communications-type emitters to operate in the same
frequency band.
It is another objective of the present invention to provide a radar system
that has optimal resolution, non-ambiguous performance and good signal
detectability.
It is yet another objective of the present invention to provide for a radar
system that does not need to increase its transmitter power to restore
detection range nor increase its data processing load to work around range
ambiguity in target tracking.
The above-mentioned objectives and advantages of the present invention will
become more apparent and the invention itself will be best understood by
references to the following description of an embodiment of the invention
taken in conjunction with the accompanying drawings, wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram indicating the different components needed for
the present invention adaptive waveform radar system;
FIG. 2 shows an EMI profile, an idealized transmitter signal spectrum for a
linear FM, fixed bandwidth and center frequency radar for a nonadaptive
waveform;
FIG. 3 illustrates the deficiencies of linear FM radar signals operating in
the presence of interferences;
FIG. 4 illustrates the situation for an adaptive waveform operating in an
EMI environment identical to that shown in FIG. 2;
FIG. 5 shows pulse compression with the adaptive waveform, both without EMI
and with EMI and narrow band cancellers;
FIG. 6 is a block diagram of the real time Fast Fourier Transformer
simulator used in the present invention;
FIG. 7 is a table that demonstrates the matched filter detection
performance with no interference;
FIG. 8 is a table illustrating the simulation results with interference;
FIG. 9 is a table demonstrating that matched filter detection performance
can nominally be achieved in an environment with interference by using the
present invention technique and apparatus; and
FIG. 10 is a diagram illustrating the difference between a waveform
obtained as a result of the instant invention and a waveform similar to
that obtained in FIG. 5, and
FIG. 11 is a block diagram illustrating a second embodiment of the
invention which uses an antenna array instead of a corporate feed antenna.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
It is well known that the spectrum of a pulse compression waveform is
determined by its phase (or frequency) modulation (or code). It is further
well known that the optimum detection of such a pulse is achieved using a
matched filter, i.e., a filter having an amplitude response identical to
the pulse's Fourier transform amplitude and with phase response conjugate
to the pulse's Fourier transform phase.
It has also been well documented that any band limited signal may be
represented by its sampled values taken at a rate greater than twice the
bandwidth of the signal, i.e., the Nyquist rate. Furthermore, it has been
well documented that the discrete Fast Fourier Transform (FFT) of blocks
of those samples is a discrete (sampled) representation in the frequency
domain of the pulse's true Fourier transform and that complex
multiplication of the FFT and subsequent inverse discrete Fast Fourier
Transformation (IFFT) under nonrestrictive conditions constitute a linear
filtering, i.e., a linear convolution.
In copending application by Schreiber, et al. entitled "Narrow Band
Interference Suppressor for Pulse Compression Radar", having application
Ser. No. 196,578, assigned to the same assignee as the instant invention
and incorporated herein by reference, it is disclosed that the relevant
information carried by any received pulse compression signal is in its
phase only and that the amplitude information is irrelevant. Conventional
linear FM, pulse compression radar receivers are tacit acknowledgement of
this because the pulse compression dispersive delay line operates on the
received radar pulse to cancel its phase. The receiver amplitude tapering
is conventionally invoked to control compression pulse temporal sidelobes
and is independent of pulse spectrum amplitude, usually being selected
from several known tapers such as Hamming, Hanning, Tschebyscheff, Taylor,
etc.
Referring to FIG. 1, there is shown a functional block diagram of the
present invention adaptive waveform radar. As shown, the adaptive waveform
radar system uses a conventional antenna such as rotodome antenna 2
connected to a duplexer 4. It should be appreciated that an antenna array
may also be used. In fact, such will be discussed as a second embodiment
of the invention, infra.
The output of duplexer 4 is connected to an automatic channel monitor 6 and
a radar receiver 8. As is well known, pulses may both be received and sent
by duplexer 4. Connected and providing input to duplexer 4 is a radar
transmitter 10, which also has an output line 24 connected to radar
receiver 8. The radar transmitter is connected to a radar pulse waveform
synthesizer and is being fed thereby. An adaptive waveform processor 14,
which receives its input data base from automatic channel monitor 6,
provides the input for waveform synthesizer 12 via line 13. Processor 14
also provides an input to a filter 16 via line 15 which has as its input
and output a Fast Fourier Transform circuit (FFT) 18 and an inverse
Fourier transform circuit (IFFT) 20, respectively. For ease of discussion,
FFT 18, filter 16 and IFFT 20 may be combined and refer to as a real time
Fast Fourier Transformer (FFT) 22.
Automatic channel monitor 6, which is made by The General Electric Company,
continually samples the electromagnetic environment in which the system is
being operated to provide a spectral estimation thereof. The estimation is
constantly updated to provide in real time the interference spectrum and
the best channel for the radar system to operate in. The best channel
would be that which has the widest bandwidth with the least number of
interferences. The interference spectrum provides the emitter center
frequency, the bandwidth and amplitude as a function of radar azimuth
angle. From these pieces of information, and electromagnetic interference
(EMI) data base for the system is formed.
The EMI profile is provided as an input to adaptive waveform processor 14,
which is a special purpose computer employing a microprocessor and a
memory, which are available from Motorola, INTEL, and Cypress
Semiconductor Corporations. In essence, processor 14 is an algorithmically
specialized computational system having a design reflecting the
requirement of the specific algorithms for the system. It is programmable
in the sense that it can solve the same algorithms for different initial
conditions and coefficient sets. This programmability is a non-real time
overhead function and is not a reconfiguration of the processor. And the
algorithms that the processor must execute, in terms of Fast Fourier
Transforms and inverse Fast Fourier Transforms, are well known and
described, for example, by Bowen, et al. in "VLSI Systems Design for
Digital Signal Processing", Prentice-Hall, Englewood Cliffs, New York
(1982).
For the instant invention system, adaptive waveform processor 14 analyses
the EMI data base (or profile) provided by monitor 6 and determines the
best available channel for each azimuth sector. In other words, the center
frequency, the maximum channel bandwidth for fewest in-band interferences,
the interference center frequencies and the bandwidth are all determined,
in view as the best available channel, by processor 14. Thereafter, using
signal design techniques well known to those versed in the art and
discussed in aforenoted copending application Ser. No. 196,579 by
Cermignani, et al. entitled "FM Modulation Technique for Producing
Frequency Rejection Bands", incorporated herein by reference, a signal is
designed to match, in the "matched filter" sense, the best available
channel as defined by the interferences, as measured by monitor 6.
Processor 14 also designs the matched filter (for the waveform from radar
receiver 8, to be discussed more in-depth later) by supplying equalizer
coefficients to filter 16 of real time FFT 22, which comprises FFT 18,
filter 16 and IFFT 20, all of which have been exhaustively discussed in
the aforenoted incorporated Schreiber, et al. copending application.
The above-mentioned signal designed to match the best available channel is
synthesized in synthesizer 12, which is manufactured by The Hewlett
Packard Company, and fed to a conventional pulse radar transmitter 10,
manufactured for example by The General Electric Company. The signal may
be provided to conventional duplexer 4, before being fed to a fan-beam
rotodome antenna 2, manufactured for example by The Randtron Company.
The signal designed by synthesizer 12 has no energy in-band with the EMI
profile. In radar transmitter 10, the power of the signal is amplified
before being radiated out at some preferred direction into the environment
by means of antenna 2. And when the radiated signal impinges on a target,
a reflection is obtained. And by means of control circuits well known in
radar systems, the transmitter is shut down by the time the signal hits
the target. Accordingly, the receiver is turned on by the duplexer and the
reflection of the sent signal, i.e., the echo of the target, is received
through rotodome antenna 2 and directed by duplexer 4 to radar receiver 8,
which is a conventional type of radar receiver. Receiver 8 includes an
analog to digital converter and simply amplifies the received echo and
converts the same to a complex, digital signal, i.e., in-phase and
quadrature, before providing it as a complex input to FFT 18. FFT 18 takes
blocks of the echo signal and converts the same to its frequency spectrum
as indicated by the brackets underneath FFT 18 in FIG. 1.
Once the return signal has been converted to its frequency domain
representation, it is then multiplied by the receiver matched filter and
the equalizer coefficients in filter 16, supplied thereto by processor 14.
From this multiplication, the interferences on the reflection signal are
suppressed. IFFT 20 next converts the frequency spectrum back into the
time domain and presents the reflection radar signal, as an uncontaminated
compressed pulse, for further processing via line 23. It should be
appreciated that the operation performed by real time FFT 22 involves
digital signal processing, as described in the aforenoted Schreiber, et
al. copending application. It should further be appreciated that the
signal sent via line 23 may have a very low main lobe to sidelobe ratio,
as illustrated in FIG. 10.
Referring to FIG. 2, an EMI profile for a non-adaptive waveform and an
idealized transmit signal spectrum for a linear FM, fixed bandwidth at
center frequency radar is shown. It can be seen that within the transmit
signal spectrum there are three in-band interferences The filtering
provided by the receiver imposes the nominal cos.sup.4 spectral amplitude
weighting for the recompressed pulse temporal sidelobe control and for the
out-of-band interference suppression. Three band eliminate filters are
used to suppress the in-band interferences. Thus, it should be clear to
those skilled in the art of signal analysis that the receiver filter is
not a matched filter for the transmit signal. Consequently, severe loss of
pulse detection signal-to-noise ratio occurs.
In addition, the three spectral holes, as indicated by the waveform at the
bottom portion of FIG. 2, indicates tha | | |
