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BACKGROUND OF THE INVENTION
This invention relates to signal processors for pulse doppler radars, and
more specifically to such processors providing Moving Target Indicator
(MTI) operation and Constant False Alarm Ratio (CFAR) capability.
One of the common problems with conventional moving target indicator (MTI)
processing is its inability to completely eliminate false alarms. This
problem has drawn increased attention of late because of the disabling
effects which excessive false alarms can have an automatic detection and
tracking systems. Even well designed, coherent MTIs, capable of providing
40dB or more of signal-to-clutter ratio improvement, suffer in this
regard. The problem is due, in major part, to very large clutter
scatterers such as water towers, cliff faces and the like. Even after
suppression by MTI processing, echoes from such scatterers often are
strong enough to exceed detection thresholds.
Approaches to solution of this problem commonly have involved some form of
automatic gain control (AGC) or normalization which attenuates very strong
input signals to a level such that the MTI suppression will suffice to
prevent false alarms. Probably the most extreme example of such prior
approaches is the "hard limited" MTI processor, in which the processor
chain comprises a hard limiter followed by MTI and pulse compression.
The hard limiter output contains only phase information and is independent
of the level of its input. Hence, low-doppler returns of all amplitudes
will be suppressed by the MTI to a well defined level below a detection
threshold. In-the-clear target returns, on the other hand, will not be
suppressed by the MTI, and will be detected after integration via pulse
compression.
Thus, the hard limited MTI does provide the desired CFAR capability as well
as an "in-the-clear" target detection capability. Unfortunately, it does
not also provide a capability to detect targets immersed in clutter. Such
target returns are suppressed, at the hard limiter output, according to
the clutter-to-signal ratio at its input, and the pulse compression
integration will be inadequate for reliable detectability.
The hard limited MTI accordingly provides CFAR and intra-clutter
visibility, but fails to provide adequate subclutter visibility. In very
patchy or spiky clutter, intra-clutter visibility will permit automatic
target tracking even though returns are occasionally lost in the clutter.
However, in heavier, more homogeneous clutter situations, subclutter
visibility, which is attainable only by linearly processing the
signal-plus-clutter return, is required.
Another known approach, which achieves CFAR operation without the attendant
loss of subclutter visibility, involves an AGC function incorporated prior
to the MTI. The attenuation as a function of range must, of course, be
periodic so as not to destroy the cancellation. This approach, however,
suffers from a serious shortcoming (beyond the practical difficulties
associated with generating a periodic attenuation function) when employed
with coded pulses, especially those with high time-bandwidth products.
The AGC circuit precedes the MTI which, in turn, precedes the pulse
compression network in order to limit the required dynamic range of the
latter. Hence, the attenuation which accompanies the return from a large
clutter scatterer lasts for at least the uncompressed signal pulse width.
This means that any signals of interest, separated from the strong clutter
scatterer by something less than the range extent of the uncompressed
pulse, will be quieted by the presence of the clutter. This phenomenon can
seriously degrade the detection capability of the radar.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, CFAR operation of an MTI
processor is attained even in the presence of very large clutter returns,
and this CFAR capability is achieved while preserving a useful degree of
subclutter visibility. Briefly, such operation is accomplished by
paralleling an MTI processor chain, which may otherwise be conventional in
arrangement, with a second "blanking" channel which provides the CFAR
capability. The first or primary processor channel includes an MTI
cancellation circuit, pulse compressor, detector and threshold comparator;
the blanking channel contains an attenuator or other gain control means, a
pulse compressor and a detector, the latter two elements being similar to
the corresponding elements in the primary channel and the attenuator being
included for control of relative signal levels in the two channels. The
output of the detector in the blanking channel provides a second detection
threshold for the output of the primary channel, permitting target signal
output therefrom only when the primary channel signal exceeds both a fixed
detection threshold and the threshold provided by the blanking channel
output. The addition of this second threshold effectively raises the
output threshold transiently when strong fixed target returns are being
processed, thus reducing false alarms which might otherwise be caused
thereby. In preferred implementations of the invention the pulse
compression network may be time-shared between the primary and blanking
channels, by operating the MTI in an "integrate-and-dump" mode, and for
better linearity in the presence of very high level signals complementary
AGC adjustments may be added to the MTI processor input and output.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features and advantages of the invention will be more
readily apparent from the following detailed description when read in
conjunction with the accompanying drawings, in which:
FIG. 1 is a block diagram of a conventional MTI processor;
FIG. 2 is a block diagram of an MTI processor in accordance with the
invention;
FIG. 3 illustrates the frequency response of the two channels in the MTI
processor of FIG. 2;
FIG. 4 is a block diagram of an alternative embodiment of the invention
incorporating an integrate-and-dump MTI; and
FIG. 5 is a block diagram of another form of the invention including AGC
across the MTI circuit.
DESCRIPTION OF PREFERRED EMBODIMENTS
With continued reference to the drawings, FIG. 1 illustrates a conventional
MTI processor of the kind incorporating a hard limiter 11 at its input,
with pulse compression as at 13 following the MTI cancellation circuit 15.
The pulse compressor is followed by a detector 17 the video from which is
compared against a fixed threshold as at 19, and passed only if it exceeds
that threshold. The operation of the hard limiter 11 is to discard
amplitude information and output only the phase information, so that low
doppler returns even of large amplitude will be suppressed in the MTI
circuit 15 which follows the limiter, to a well defined level below the
detection threshold introduced at 19. In-the-clear target returns, on the
other hand, will not be suppressed by the MTI, and will be detected, after
integration through pulse compression as at 13, to thus yield a video
output signal indicating the target presence.
The thresholding function introduced by comparator 19, together with hard
limiting at 11 of the input to the MTI cancellation circuit 15, provides
the desired CFAR capability while maintaining in-the-clear target
detection capability as also desired. Unfortunately, it does not as well
provide the capability to detect targets immersed in clutter, since such
target returns are suppressed by the hard limiter according to the
clutter-to-signal ratio at its input, and the pulse compression
integration at 13 will be inadequate for reliable detectability.
Subclutter visibility is thus severely impaired if not wholly absent in
this known system.
To achieve subclutter visibility while maintaining CFAR operation, the MTI
processor of the invention as illustrated in FIG. 2 comprises a second or
blanking channel providing a second detection threshold which the primary
channel video must also exceed in order to output a target signal. This
second channel includes a pulse compressor 23 and detector 25, which may
be similar in circuitry and function to the corresponding elements in the
primary channel, and an attenuator 27 the function of which is to
compensate for the attenuation introduced into the primary channel by the
MTI circuitry 15, to thus establish the particular interrelationship of
signal levels in the two channels hereinafter described.
The operation of the processor of FIG. 2 may best be understood by
reference to FIG. 3, which illustrates the frequency response ot the
primary (MTI) and blanking channels. As shown, the MTI response displays
the usual zero-doppler frequency notch effective to sharply attenuate low
doppler clutter returns, while the blanking channel response is
essentially flat through the frequency range of interest. Since the
blanking channel signal input to comparator 21 defines a threshold with
must be exceeded by the MTI channel signal in order that a target be
indicated, and since as shown in FIG. 3 the blanking channel signal level
exceeds the MTI channel level across the frequency band of the MTI notch,
the blanking channel signal operates to block any target signal output
through this frequency band. Further, since any high level clutter signal
content which is present in the blanking channel at any given time will be
effective transiently to raise the threshold which it defines, the
correspondingly high level signal content of the MTI channel still will
not exceed the threshold as thus raised.
This avoids the false alarms which otherwise might result from the presence
of such high level clutter signal content in the primary channel, which
would exceed the fixed threshold at 19 and so would yield a target signal
output in the absence of a blanking channel threshold. In other words,
valid targets are declared by the system of FIG. 2 only when the processed
primary channel signal exceeds both its fixed detection threshold and the
blanking channel output simultaneously, i.e., at the same range cell, and
since the blanking channel output varies directly with clutter level any
signals of magnitude representing large clutter scatterers are prevented
by this blanking channel threshold from being indicated as a target.
To further explain the operation of the MTI processor of FIG. 2, it may be
helpful to analyze its operation in terms of the MTI channel gain factor
(.alpha.) applied to low doppler clutter returns by the low frequency
notch of the MTI, as compared against the blanking channel gain factor
(.beta.) introduced by the attenuator 27 in the blanking channel, the gain
factors .alpha. and .beta. being both less than unity and measured as
indicated in FIG. 2. In a typical processor in accordance with the
invention the MTI channel gain at the MTI notch center frequency may be of
the order of -50dB and the blanking channel gain .beta. about twice that,
i.e., about -47dB.
If the clutter power at the MTI input is P.sub.c, the notch introduced by
the MTI at zero doppler will reduce the clutter power in the MTI output to
.alpha. P.sub.c. Away from this zero doppler or notch frequency, at the
higher doppler frequencies which represent targets of interest, the MTI is
assumed to have essentially unity response so that a signal of power
P.sub.s at the MTI input has approximately the same power as at its
output. This assumption is not strictly correct, but in practical systems
the MTI response sufficiently approximates unity that the assumption may
be taken as valid for present purposes.
The blanking channel has a substantially flat frequency response at a level
corresponding to the gain factor, .beta., introduced by attenuator 27.
Thus, signal will an average power P.sub.s at the attenuator input and
clutter with an average power P.sub.c at the attenuator's input exit at
the power levels .beta.P.sub.s and .beta.P.sub.c respectively. Pn
indicates the thermal noise power at the channel outputs. Its value is
reduced, relative to the signal and clutter power, to reflect the
integration gain via pulse compression undergone by the signal and
clutter.
In order to achieve the desired CFAR operation it is necessary that the
blanking channel gain factor, .beta., always be greater than the MTI
channel gain factor, .alpha., at MTI notch frequencies; that is:
.beta.>.alpha. (1)
As indicated in the foregoing, targets are declared only when
Pn + Ps + .alpha.Pc > T (2)
where T is the MTI channel fixed threshold, and
Pn + Ps + .alpha.Pc > .beta. (Pn + Ps + Pc) (3)
Inequalities (1), (2) and (3) will now be examined in three different
situations; first, when only clutter is present; second, when in-the-clear
signal is present; and third, when signal and strong clutter are present
simultaneously.
Case 1 -- Clutter Only
In this case the inequalities (2) and (3) which must be satisfied if a
target is to be declared reduce to
Pn + .alpha.Pc > T (4)
and
Pn + .alpha.Pc > .beta. (Pn + Pc) (5)
With a well designed MTI, inequality (4) will be satisfied only by very
large clutter returns. But in such cases, the inequality .beta.>.alpha.
insures, with a very high probability, that inequality (5) will not be
satisfied. Thus, so long as the clutter is suppressed more by the MTI than
by the blanking channel attenuator, it will not be falsely detected. And
the CFAR operation is independent of the clutter lever (at least for high
clutter-to-noise ratio situations).
Case 2 -- Signal In-The-Clear
In this situation, inequalities (2) and (3) reduce to
Pn + Ps > T (6)
and
Pn + Ps > .beta. (Pn + Ps) (7)
Now so long as
.alpha. < .beta. < < 1 (8)
inequality (7) will always be satisfied and the detection criterion reduces
to the classical one of signal-plus-noise exceeding a fixed threshold.
Thus, the proposed technique does not degrade the detectability of targets
in the clear.
Case 3 -- Signal Immersed in Clutter
In strong clutter situations (Pc > > Ps, Pc > > Rn) the detection criteria
become
Pn + Ps + .alpha.Pc > T (9)
and
Pn + Ps + .alpha.Pc > .beta.Pc (10)
Assuming that the MTI attenuation is sufficient to provide a high processed
signal-to-clutter ratio (i.e. Ps > .alpha.Pc) and that the signal return
is strong enough to exceed the threshold T with a sufficiently high
probability (i.e., Pn + Ps > T), detection criteria (9) and (10) then
become simply
Pn + Ps > .beta.Pc (11)
Thus, as a consequence of adding the blanking channel to the conventional
processor, the signal-to-clutter ratio improvement capability of the
system has been reduced from .alpha..sup.-.sup.1 to .beta..sup.-.sup.1,
i.e., a reduction of about one-half where .alpha. and .beta. have the
approximate 1:2 relationship mentioned above as typical. This small
sacrifice in subclutter visibility enables the assurance of CFAR
operation, however, and accordingly represents a very desirable trade-off
particularly in automatic detection and reaction systems in which the need
for CFAR is critical.
With reference now to FIG. 4, an alternative embodiment of the invention is
shown requiring only a single pulse compression network for both the
primary and blanking channels, this being significantly advantageous
because conventional pulse compression networks are relatively complex and
because difficulty sometimes is experienced in precisely matching two such
networks as is desirable where the two channels are separate as
illustrated in FIG. 2. Use of a single pulse compressor in both channels
is made possible by time-sharing the pulse compression network between the
two channels and use of an "integrate-and-dump" type MTI processor which
may itself be of known type.
In FIG. 4, the "integrate-and-dump" MTI 31 comprises a recirculation loop
including a delay element 33 which introduces a time delay equal to one
pulse repetition period (PRP), and a summer 35 in which the recirculated
signal is algebraically summed with the input signal in a multiplier 37 to
which amplitude and phase weighting signals, designated "complex weights"
in FIG. 4, are introduced on a pulse-to-pulse basis. In conventional
integrate-and-dump MTI systems, these complex weights normally are
generated on a look-to-look or beam direction basis in the beam direction
control system, and applied to the radar IF signal after first converting
it to digital in-phase and quadrature form. The weights are introduced by
complex weight multipliers which conveniently may take the form of a
digital multiplier quad, and the necessary delay and recirculation loop
components of the MTI may likewise be digitally implemented.
After the MTI has thus individually weighted a series of pulses, say N
pulses, and completed their integration by recirculation and addition, the
integration product is gated out through a gate element 39 responsive to
suitable control means (not shown) and the recirculation loop then is
"dumped" or emptied preparatory to the start of the next such integration
cycle. The advantage of such known integrate-and-dump MIT operation is
that it enables selective weighting of successive returns as a function of
beam look angle, to thus enable compensation for fixed clutter pattern
variations and other radar response anomalies dependent on azimuthal
angle.
In the system of FIG. 4 this integrate-and-dump MTI affords the further
advantage that it provides an output into the processor chain, through
gate 39, only during one interpulse period, the Nth interpulse period.
Accordingly, during the next earlier or N-1st interpulse period, gate 39
may be switched to pass the receiver IF signal directly, with attenuation
as at 41 for purposes of blanking channel level control as previously
explained. This attenuated but otherwise unprocessed signal becomes the
blanking channel signal and when passed by gate 39 during the N-1st
interpulse period is processed through the pulse compressor 43 and
detector 45, and compared to the detection threshold introduced through
comparator 47. If above this threshold, the signal is stored in a memory
for one interpulse period, which as indicated in FIG. 4 may be
accomplished most simply by passing the signal through a one-PRP delay
line 49, which thus shifts this signal into the Nth pulst interpulse
period.
The MTI output is gated at 39 through the pulse compressor 43 during this
Nth interpulse period; it is then detected, compared to the detection
threshold at 47, and, if larger, compared to the delayed blanking channel
output in comparator 51. A target is declared only if the primary channel
output is the larger in both comparisons, just as in the case of the
embodiment of FIG. 2.
As previously mentioned, the processor of this invention can be made to
achieve CFAR operation at the cost of only a small degree of subclutter
visibility so long as the MTI suppresses the clutter returns by more than
the attenuation in the blanking channel, provided of course that the
clutter signal frequencies fall within the MTI response notch. While this
usually is the case it is possible that certain forms of chaff and other
relatively rapidly moving clutter may include high frequency components
outside the notch. Difficulties may also be experienced in situations in
which the clutter level is so high that it drives either the radar
receiver or the MTI circuit, or both, into saturation, as may result from
spreading of the clutter spectrum by nonlinear devices in the receiver or
MTI chain. To avoid these difficulties it may be desirable to include, in
addition to the usual reciever AGC circuitry, means for enhancing the
linearity of the processor chain including the MTI to better assure
linearity even in the presence of very high level signals.
An alternative embodiment of the invention affording such linearity is
shown in FIG. 5, in which an AGC circuit 61 is responsive to the radar IF
signal level to apply gain adjustments, through gain control circuits 63
and 65, respectively, both prior to and following the MTI circuit 67. The
gain adjustment applied following the MTI is the inverse, i.e., is equal
in magnitude and opposite in direction, to that applied before the MTI;
hence the two adjustments are complementary and their only effect is to
attenuate otherwise troublesome signals within the MTI. The remainder of
the MTI and blanking channel circuitry is the same as described in
reference to FIG. 2, and functions in essentially the same manner.
As will be obvious to those skilled in the art, the relative gains .beta.
of the blanking channel and .alpha. of the MTI channel could equivalently
be maintained, if preferred, by omitting the blanking channel attenuator
and adding an operational amplifier to the MTI channel to restore the same
interrelationship of levels of the MTI and blocking channel signals as
hereinbefore described. As will also be understood, the threshold signal
input to the first threshold comparator may either be fixed relative to
the noise level or variable with a derived and running estimate of
background level, or both such thesholds could be provided with these and
the blanking signal threshold arranged in any preferred sequence.
If desired, other features may be added to MTI processors in accordance
with the invention. Post-detection processing, for example, employing
diversity combining to achieve signal-to-clutter ratio improvements beyond
that provided by the MTI alone, may be advantageous for some applications
of systems in accordance with the invention, and if preferred such
post-detection processor may be time shared between the primary and
blanking channels in a manner similar to that described for the pulse
compressor in FIG. 4.
In summary, the invention as described in the foregoing presents a simple,
effective solution to an important and frequently encountered problem --
the detection of targets in clutter, particularly strong ground clutter,
while maintaining a low and constant false alarm rate.
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