Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Back~ound of_the Invention
Moving target indicator systems have generally required
the processing of signals from several successive sweeps to
produce sufficient indications of differences bet~een moving
targets particularly at low velocities. Alternatively/
expedience such as utilizing weighting factors or weighting
received echo signals as a function of range disclosed in
Patent No. 4,117,538 by Shrader et al summing compo.site
video signals from present range sweeps and predictions of
composite videos of such range sweeps have been used with
digitaL techniques for improving moving target indicating
systems.
Summary of the_ nvention
In accordance with this invention it is provided that
a moving target indicator system wherein sequences of digital
words derived from received signals which are phase related
to a reference signal are weighted with coefficients which
are functions of target velocities. Sums of the weighted
sequences may then be displayed.
More specifically, in accordance with this invention a
transmitter transmits pulses at any desired repetition rate
and produces a reference signal which preserves the phase
coherence of the transmitted signals. Received echo signals
from targets are sampled for succeeding time periods follow-
ing the transmitted signal to produce signals whose phase is
compared with the phase reference and digitized outputs are
produced and stored for succeeding transmitted pulses which
due to rotation of a directional antenna transmitting the
pulses produce slightly different echo signals. Signals from
the same range or time delay from the transmitted signal, for
three ~uccessive transmitted pulses are extracted from the
storage means or directly from the receiver to be submit-ted
with appropriate weighting. The output of the summing system
is then displayed on a display synchronied with the trans-
mitted pulses to produce any desired displày such as a planned
position indicator. This invention further discloses rejection
of clutter returns which are larger than a predetermined
threshold for each of the outputs from the summer corresponding
to a velocity channel. This invention further provides for
rejecting all signals as the comparison of the sum of the
squares of the in-phase and quadrature phase components of a
0 range sample when the magnitude is substantially different
--2--
from such a sum fox the same range of a second interpulse s~stern.
This invention further provides for storing a sequence
of summed outputs of the different velocity channels during the
Einal interpulse period of a three interpulse period group and out-
putting said sequence a plurality of times for display ~y a radar
display more than once for each interpulse.
In accordance with the present invention, there is pro-
vided a radar system processor for detecting signals in the pres-
ence of clutter comprising: means for deriving signals from re-
flections of directively radiated groups of at least three radar
pulses in which the interpulse periods in each group are equal;
detector means for extracting components of received signals phase
related to said pulses; means for sampling said components at in-
tervals in time corresponding to ranges of reflections; means for
s-toring the sequences of samples generated by said sampling means
during said interpulse periods; a plurality of velocity filters;
said filters having means for weighting each sequence of stored
samples wi-th weighting coefficients; at least some of said weight-
ing coefficients being different for diEferent velocity filters;
and means for normali~ing the ou-tputs of different ones of said
velocity filters as a function of said clutter passing through
said diEferent filters.
In accordance with the present invention, there is fur-
-ther provided a radar system comprising: means for directively
radiating groups of three radar pulses in which the interpulse
periods in each group are equal and for receiving reflections of
said pulses; means Eor extracting components of received signals
phase related to said pulses; means for sampling said componen-ts
at intervals which are a funct:ion of range; means for storing the
sequences of said samples generated during sequential in-terpulse
periods in said group; a plurality of velocity filters; each of
said filte:rs having means for weighting each sequence of s-tored
samples wi-th weighting coefficients; at least some of said coef-
ficien-ts being different for different velocity filters; and means
for normalizing the outputs of said velocity filters as a runction
of signals passing through some of said filters.
In accordance with the present invention, there is fur-
ther provided a radar system comprising: means for directively
radiating groups of three radar pulses in which the interpulse
periods in each group are equal and with each pulse in said group
radiated substantially in the same direction; means for extract-
ing components of received signal phase related to said pulses in
the presence of clutter; means for sequentially sampling said com-
ponen-ts; means for storing the sequences of said samples generated
during sequential interpulse periods in said group; a plurality
o:E velocity filters; means for weighting the outputs of said fil-
-ters with different weighting coefficients; and means for normal-
izing the outputs of different ones of said velocity filters as
a Eunction of said clutter passing through said different filters.
In accordance with the present invention, -there is fur-
ther provided a radar system comprising: means for deriving sig-
nals in the presence of clu-tter from direc-tively radiated groups
o:E three radar pulses in which the in-terpulse periods in each
grollp are equal; means for extracting components of received sig-
nals which are phase related to said radar pulses; means for samp-
ling said components at predetermined intervals; means for storing
the sequences of said samples generated during sequential in-ter-
pulse periods in said group; and a plurality of velocity :Eilters;
each of said filters having means for weighting each sequence of
stored samples with weigh-ting coefficients, some oE which are dif-
Eerent for differen-t velocity filters and for normalizing -the out-
3() puts o:E different ones of said velocity :Eilters as a function of
the portions of said clu-t-ter which pass through said different
filters; and the outputs of said velocity filters beiing logarith-
x-
~mic func-tions of saicl samples.
In accordance with the present invention, there is fl~r-
ther provided a radar system processor comprising: rneans for de-
riving reflected signals from groups of three radar pulses direct-
ively radiated substantially in the same direction; rneans for ex-
trac-ting components of received signals phase related to said
pulses and containing clutter; means for sampling said components
at predetermined intervals corresponding to predetermined ranges;
means for storing the sequences of said samples generated during
sequential interpulse periods in each group; a plurality of vel-
ocity filters; each of said filters having means for weighting
each sequence of stored samples with weighting coefficients; means
for normalizing the outputs of differenc-t ones of said velocity
filters as a function of the portions of said clutter passing
through said different filters; means for integrating said signals
on successive received pulses at different times from objects at
dif:Eerent distances producing said reflected signals; and means
for substracting different amounts of said stored clutter from
different outputs o.E said filte:rs.
~0 In accordance with -the present invention, there is fur-
ther provided a radar system comprising: means for directively
radiating a plurality of radar pulses, means for receiving and
processing reflected target and clutter echo signals of said radar
pulses comprising a plurality of filters having different bandpass
characteristics; and means for normalizing the outputs of said
Eilters as a function of a long term average of background clutter
passing through one or more of said filters.
In accordance with -the present inven-tion, there is :Eur-
ther provided a radar system comprising: means :ior deriving sig-
nals having a clutter portion from directively radiated groups of
three or more radar pulses in which interpulse periods in each
group are equal; and means for extracting components of received
- 3b -
~ a~
signals which are phase related to said radar pulses throuyh a
plurality of different velocity filters each weighted l~ith differ-
ent coefficients and having outputs which are norrnali~ed as a func-
tion of a long term average of background clutter passing through
one or more of said Eilters.
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Brief Description of the Drawings
Other and further embodiments of the invention ~
become apparent as the description thereof progresses,
reference being had to the accompanying drawings wherein:
FIG. 1 illustrates a multi-filter processor embodying
the invention;
FIG. 2 illustrates a radar system embodying the processor
of FIG. l; and
FIG. 3 is a graph illustrating the response of the filters
of the invention to stationary ground clutter.
Description of the Preferred ~mbodim
Referring now to Figures 1, 2 and 3, there is sho~l a
- three-pulse moving target detection system. Analog signal
samples from an in-phase and quadrature phase (I and Q phase~
detector 74 are sampled in sampl ~ 6 and fed to two 10 bit
analog to digital (A/D) converters 80. The input (Figure 1)
video is sampled at 1/16 nmi and sent to the three-pulse MTD
input (Figure 1)' The dynamic range of the video (noise
level to limit level) is set to 50 dB, and all signals are
preferably processed linearly over this range.
Each video input I and Q is processed in separate
sections 70 and 110 of doppler filter 24 where three ortho-
gonally weighted filters, 26, 28 and 30, are formed for I
and 3 for Q. The video signal fl from filter 26, f2 from
filter 28 and f3 from filter 30 at any sampling range are
related to the A/D output signals by the following equations:
f3 = a-2b
where a,b,c represent
f 2 = a-c the 3 sweeps within
each batch
fl = a~b+C
The responses to doppler frequencies which result are
shown in Figure 3. Curve 50 shows the frequency response F
of filter 26, curve 52 shows the frequency response F2 oE
filter 28 and curve 54 shows the frequency response F3 of
filter 30. Curve 56 shows a typical spectrum of stationary
target radar clutter.
The I and Q doppler filter outputs are combined in
each of the filters 26, 28 and 30 by conventional squaring,
sUmming, and logging. The resultant magnitude expressed as
an 8 bit logarithmic word is outputted as Fl, F2 and F3 from
filters 26, 28 and 30. Subsequent processing is performed on
these 8 bit digital words.
The zero doppler filter 26 (Fl) supplies a 65536 cell
adaptive clutter map storage 3~. The clutter map resolution
may be, for example, 1.40625 in azimuth (approximately the
3 dB radar antenna azimuth beamwidth) and 1/256 of the radar-
instrumented range. Accurate indexing of the clutter map
azimuth to the PRF is not necessary, providing the radar
transmits six or more pulses in the time the an~enna rotates
one 3 dB azimuth beamwidth.
The clutter map 32 which due to log magnitude storage
has a wide dynamic range, then provides outputs for each
range azimuth location which are the integrated value of Fl
output over several azimuth sweeps. The outputs are compared
with preset thresholds in subtraction circuits 34, 36, and
38~ The amount of each threshold is preferably adjusted to
be equal to the expected improvement factor for each filter,
26, 28 and 30. Thus, for Fl, filter 26, there is no improve-
ment so the threshold is zero and all the clutter map signals
are subtracted from the output of filter 26 in circuit 34.
For F2, the subtraction circuit 36, which is fed by F2,
preferably has a threshold of approximately 20 dB since F2,
curve 52, intersects clutter curve 56 at this level. Similarly,
the threshold of subtraction circult 38 fed by F3 is set around
40 dB since F3, curve 54, intersects clutter curve 56 at around
40 dB. Thus, in a stationary radar using this filter system,
clutter due to stationary targets may be subtracted out of
each of a plurality of different filter responses to the extent
that this clutter obscures the expected improvement provided
by the filter. This is accomplished by subtractiny that portion
of the stored clutter for each output sample from the ilters
26, 28 and 30, which exceed the threshold setting for that
filter response. Thus, false alarms passing conventional
constant false alarm rate filters 40 due to clutter exceeding
the filter clutter suppression capabilities are eliminated
and the full receiver dynamic range becomes available.
~ lthough ground clutter has been removed from the signals
at this point, weather clutter rnay still be present at each
filter output. The weather clutter strength in each filter
output is determined b~ the doppler velocity of ~he weather
itself, and by its actual velocity with respect to the radar.
If the weather is very slow moving, then the clutter map will
remove it from the Fl filter, but not the F2 or F3 filter,
if the weather has sufficient doppler velocity.
To reduce weather clutter, each threshold filter output
is passed through an averaging (range only) CFAR filter 40
which uses the average of the greatest of 8 cells on either
side of the mid cell as an estlmate of the local noise back-
ground.
The CFAR filters 40 have low loss and can remain perma-
nently in the signal path. This has the advantage that in
addition to reducing the weather clutter to noise level, the
CFAR ~ilters also tend to normalize any variations in the
noise baseline caused by the clutter map thresholding`the
doppler filter outputs.
The output averages of the CFAR filters 40 are used by
the weather contour circuit 42 to produce two levels of
weathar contours.
An interEerence editor 44 controls false alarms due to
interference and saturation limiting clutter. It measures the
sweep-to-sweep amplitude modulation of each return in each
range cell in each group. If the amplitude variation exceeds
the expected antenna scanning modulation, whether it is single
pulse or limiting clutter, the signal at that range is blanked
in that group.
The three doppler filter outputs (Yl, ~2 and F3~, after
being automatically normalized by the clutter map 32 and passed
through separate CFAR filters 40, are merged into one signal in
a combiner 46 and the resultant signal is anti-logged in a video
integrator 48 to produce a linear 8 bit signal which is then
integrated by recursive integrator 48 which integrates the
returns from successive threë pulse groups as determined by a
conventional synchronizer (not shown). Since the integrator
48 is operated in a linear fashion, the output signal dynamic
range for the 8 bit signal is about 30 dB.
~ he output of integrator 48 supplies a video regenerator
50 which repeats the processed video to raise its repetition
frequency to a suitable value for display. Regenerator 50
supplies a d/a converter 52 whose output is a video signal
~0 supplied to intensity modulate a plan position indicator 54
in Figure 2.
Figure 2 shows radar equipment in which the processor
invention of Figure 1 is incorporated. A pulse transmitter
60 generates short radio frequency pulses which are directed
by a circulator 62 to an antenna 64 which radiates them towards
a target. The signals reflected from the target are received
by antenna 64 and are directed by the circulator 62 into a
receiver 66 which amplifies them and down-converts them to an
intermediate frequency.
--8--
A reference oscillator 68 generates a continuous osci]-
lation at the intermediate frequency whose phase is referenced to
that of the transmitter. Such a system is well known and conven-
tional.
The IF signal from the receiver 66 and the reference
oscillation from the reference oscillator 68 pass into the in-
phase section 70 of the processor where they are both applied to
a phase detector 74. The output of the in-phase and quadrature-
phase detectors 74 have amplitudes which follow that of the signal
from the receiver, multiplied by the cosine and sine of the phase
angle hetween the received signal and the referenced oscillator
signal. The outputs of detector 74 are bipolar video signals which
are passed to sampling circuits 76 where, at times indicated by
a range clock 78, samples of the video signal are passed to analog-
to-digital converters 80 which convert each sample into a digital
word.
A sequence of the digital words from A/D converter 30
occurs during the interpulse period following a -transmi-tter pulse
and -this sequence is stored in a first s-tore 82 which may be a
conventional memory Eor 10 bit bites (or words) such as a random
access memory on a shift register. The sequence oE the digital
words occurring in the interpulse period following the second
transmitter pulse is stored in a second store 84 which is similar
to 82.
During the interpulse period following the last of the
three transmi-tted pulses of the group, the digital words from the
analog-to-digital converter 80 and from store 82 are fed to the
weighting networks 86, 88 and 90 of velocity filter 24, F1. Sim-
ultaneously, said digital words
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are fed respectively to weighting network3 92, 94 and 96 in
velocity filter 24, F2 and to weighting networks 98, 100 and
102 in velocity filter 24, F3.
Weighting networks 86 through 102 provide weights to
the digital words as follows:
86, 88, 90, 92, 98 and 102 are weighted +1
94 is weighted 0
96 is weighted -1
100 is weigthed -2
The digital word weighted by networX 86 through 90 are summed
in each velocity filter 24 in summing circuits 104, 106 and
108 respectively.
A quadrature phase section 110 has components 74 through
108 which are identical to those in in-phase section 70.
~eference oscillator 68 supplies the phase detector in section
110 with a reference signal which is 90 phase shifted from
the reference signal supplied to the section 70 phase detector.
Hence, the Fl, F2 and F3 outputs of 112, 114 and 116 of
section 110 are in quadrature to the outputs of summers 104,
20 106 and 108 respectively.
The filters 24 contain six sequencing circuits 12 for
squaring each of the digital outputs 104 through 116. The
respective pairs of in-phase and out-of-phase Fls, F2s and
F3s are then summed in summers 114 whose digital outputs are
logged to provide the digital outputs of filters 26, 28 and
30 which contain elements 82 ~hrough 114. The Video output
contains elements 34 through 52 o~ FIG. 1.
In operation, the returns from a group of three radar
pulses are processed coherently to produce three filtered
outputs Fl, F2 and F3. For each three-pulse group processed,
--10--
~ ~2~
there is a single output from each of the three ~iLters.
The output from a zero doppler clutter map is subtracted
from each of the three filter outputs above diffe~ent pre-
determined threshold values for each filter to remove zero
doppler return and to thus improve subclutter visibility.
Groups containing interference or clutter entering saturation
may be blanked by circuit 44.
Averaging of CFAR in the range coordinate normalizes
the signal levels in each filter before they are summed.
The CFAR normalizing signals are also used to produce weather
contours.
The three doppler filter outputs are formed after the
in-phase and quadrature-phase components of three transmitted
pùlses have been collected and the three returns ror one
range sample are summed using three different sets of weights.
The filter weights are preferably orthogonal to one another
so that output noises are uncorrelated. Output ~3 is iden-
- tical to that of a conventional three-pulse group single
filter Moving Target Indicator. Both the real and quadrature
signal components are processed identically, producing three
real channel outputs and three quadrature channel outputs
for each group of three input pulses. These signals are
rectified and combined to form a single output for each
range sample.
The clutter map 32 consists of a leaky bucket 10 pulse
integrator for each range-azimuth resolution cell in the
radar's coverage. I'he clutter map stores signals in cells
which are controlled by control codes from range clock 78
and by a standard azimuth encoder (not shown). Zero doppler
returns are integrated for preferably about 1 beamwidth of
rotating antenna 64, and the integrated value is sto^red in the
clutter map 32. This operation synchronizes the map to the an~en
na keeping the resolution cells on the map fixed in azimuth. The
clutter map leaky bucket integrator sums F1 8 to lO azirnuth scans
of antenna 64 for each cell of map 32. Map 32 then supplies the
signal to be subtracted frorn the zero doppler channel. For any
selected range gate and beam position, this signal preferably is
the larges-t map value taken from the three by three grid of points
about the cell of interest. This operation minimizes false alarms
in the vicinity of large point clutter.
The map output is also compared with subclutter visibility
thresholdst one for each doppler filter. When the map output is
larger than -the -threshold, the difference between the map and the
threshold is sub-tracted from the appropriate doppler channel.
This operation provides a means of regulating the available sub-
clut-ter visibility when radar stability has degraded.
The CFAR circuitry is a conventional range averaging CFAR.
Range samples preceding and following -the sample of interest are
summed and -the larger sum scaled and subtrac-ted from the cell of
~0 interes-t to normalize i-ts signal level. These CFAR circui-ts, which
are used in each filter channel, may also be used -to produce
weather contours. Two levels of weather contours may be generated
by comparing the largest of the three threshold signals to two
fixed -thresholds.
The particular circuit elements used herein may be simple
weighting circuits and adders. Thus, inexpensive real time pro-
cessing of radar signals can be achieved. With three pulse groups,
several groups of pulses can hit every target using a
- 12 -
high directivity radar antenna thereby improving azimuth
accuracy with high definition and reasonable antenna rotation
rates.
This completes the description of the embodiment of the
inven~ion illustrated herein. However, many modifications
thereof will be apparent to persons skilled in the art without
departing from the spirit and scope of the invention. For
example, other filter weighting value could be used and storage
structure could be used for digital words. Accordingly, it is
intended that this invention be not limited to the particular
details of the embodiments disclosed herein except as defined
by the appended claims.
