Note: Descriptions are shown in the official language in which they were submitted.
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The presen-t invention relates to intrusion detection
sys-tems and, specifically -to a system using bistatic radar and
Doppler radar processing techniques in conjunction with
phase-coded pulse compre~sion methods to achieve a high resolution
detection ~one "window".
The term "pulse compression" is used in the sense given
in the I.E.E.E. Standard Dictionary, namely: "The coding and
processing of a signal pulse of long time dura-tion to one of short
time duration and high range resolution, while maintaining the
benefits of high pulse energy."
Intrusion detection systems can be of a line sensor type
or volumetric sensor type~ Line sensors provide perimeter
security. In ~uch systems, targets passing between the antennas
cause partial or complete blockage of the transmit signal,
resulting in the declaration of an alarm. Such systems cannot,
however, provide true Doppler detection since there is no net
radial movement. Leaky cables have also been used to provide
perimeter coverage but are difficult to deploy rapidly. The
operation of a leaky cable sensor above ground is troubled by
moving foliage, sunlight, temperature drift and moisture around
and on the cables. The result can be a high false alarm rate and
an unsatisfactory detection performance.
The present invention relates to a volumetric sensor.
There is a requirement for such a volumetric security sensor to
detect intruder3 and vehicles. For example, weapons stockpiles,
mobile C3I resources and garrisons must be alerted to an intrusion
well before the intruder can wreak havoc on the resource. The
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requirement typically ie for a detection zone which i3 at leacJt a
few ten~ of meters away from the reeource to allow unre~trictsd
movement about the re~ource itcelf. It may be impractical to
poeition -the intru~ion detection ~y~tem in or near the detection
zone. Consequently, there i~ a requirement for a oecurity ~en~or
that can provide a detection zone far enough away from the
re~ource and the seneor it~elf to allow ~ecurity forcee to react.
A well-defined detection zone centered 20-200m away from the
re~ource ic cufficient for a number of ~ecurity applications. The
cecurity sensor should be able to detect and track intrusion~ to
allow eecurity forcee to quickly locate and intercept the
potential intruder.
Any intrueion detection radar cystem ehould have a well
defined detection zone. Movement outcide the detection zone
ohould not reault in an alarm. Personnel movement in the vicinity
of the antennas, ~uch ao within an encampment, must be tolerated
by the eystem. Since the detection zone iB uaually ~ome di~tance
from the antennas, radar range-gating techniquee are necessary.
The eensor must provide detection of a variety of ground
level intrucionc, euch ae vehicles and both a crawling and on-foot
intruder. Airborne intrueion (e.g., hang glider, parachute) mu~t
also be detected by the radar syctem. The detection procese for
euch targete can be optimized by u~ing coherent or ~ynchronous
detection techniquee, thuc enabling Doppler signal procesaing.
Thi~ allowc both magnitude and pha3e information of the returned
eignal to be uced in procescing the data.
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Foliage penetration capabilities are required for a
number of security applications. The sensor must aloo be ab]e to
diecrimina-te between the eignal returned from blowing foliage and
a legitimate targe-t, Signal processing techniques, for example
Fourier analysis and Kalmus filtering, are commonly uc3ed to help
"unmask" an intruder's scattered signal from the clutter return
signal. The ~3ensor must be able to maintain a high detection
capability/low false alarm rate over all weather conditions.
There are a variety of techniques that can be used to
determine the range of the received aignal. The traditional means
to achieve this has been pulse-type radar; that is, a pulse or
burst of RF energy i8 transmitted, with the received signal
sampled once or a number of times. Each successive sample
corresponds to a more di~tant range cell. The depth of the
detection zone is approximatsly equal to the length of the pulse
multiplied by the speed of light. Eor example, a 200nsec pulse
can yield a range resolution of 60m or better. As a result, fine
resolution in range requires a short pulse and therefore a higher
peak power. In an effort to reduce the peak power requirement
while still maintaining the same range resolution, radar designers
utilize pulse compression signals. The more common pulse
compression signals used are the frequency chirp and phase-coded
waveforms. A chirp waveform is accomplished by a gradual (or
otep-wise) increase or decrease in the rate of change of phase of
the transmitted signal. Phaee-coded signalc3 are obtained by
changing the phase, at instante determined by a code ~equence, in
a smooth or abrupt faehion of an otherwise continuous wave
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~ignal. There are a variety of code sequences that may be
sui-table fvr radar pulse compression. Pseudo-noise (PN)
sequences, Barker codes and complementary series are examples of
code sequences -that have favorable characteristics in terms of
radar detection. The means to compress the returned pulse
compression signal so as to achieve the same integrated response
as a single pulse having the same total duration as the pulse
compression signal (without sacrificing range resolution) may also
take a variety of forms. Some of the more common techniques and
technologies include digital correlation, active correlation, SAW
devices, CCD correlators, and acousto-optic devices.
A line sensor using pseudo-random codes is disclosed in
U.S. Patent No. 4,605,922. This patent teaches a microwave motion
sensor system using spaced transmitting and receiving antennas.
The transmitted signal is modulated by a pseudo-random code to
cause a spreading of the transmitted signal over a wide frequency
band. This renders any jamming techniques ineffective. The
receiver has a similar pseudo-random code generator to that in the
tran.smitter and locks on to the transmitted code. The random code
sequence is not used for range gating as is done in the invention
of this application.
U.S. Patent No. 4,458,240, i~sued July 3, 1984 (issued
on a divisional application of U.S. Patent No. 4,187,501) shows a
system using transmission line ,sensors in which the starting phase
of the transmitted signal i8 switched by 0 or 180 from pulse to
pulse under the control of a pseudo-random code generator. As in
U.S. Patent No. 4,605,912, this spreads the spectral energy and
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greatly reduces the effect of any interfering signa]. The ranrlom
code sequence is no-t used for range gating as it is in the
invention of the present application.
A Continuous wave (CW) or multi-CW radar, though capable
of providing a simple receiver design because of a high transmit
signal duty-cycle at or near unity~ cannot satisfy many of the
above requirements The lower bound on the sampling rate for a CW
radar, given by the Nyquist sampling theorem, can be as low as a
few tens of llertz for the targe-ts of interest. Movement around
the antennas can overwhelm the return signal from targets just a
few tens of meters away since there is no range-gating capability
with such a signal. Similarly, large targets which are past the
desired detection area cannot be suppressed; consequently,
railways and roadways near the desired detection range can result
in an unacceptably high nuisance alarm rate. Because of the
inability to provide range-gating with CW signals, the
non-fluctuating portion of the received signal, also referred to
as the profile or stationary clutter, is usually quite large,
often placing limits on the receiver sensitivity. U.S. Patent No.
4,595,924 describes a Very High Frequency (VHF) CW ~oppler radar.
A more conventional pulse-type radar, while capable of
providing one or more "range cells", has numerous drawbacks.
These include a much faster, more complex and therefor more costly
data collection process, susceptibility to intentional or
unintentional interference, ease of targeting by hostile forces,
and an increased peak transmit power because of the transmit
signal's low duty-cycle (typically well under 10 %). As well,
sampling in excess of 10 MHz is required for a detection zone
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resolution of 30 m or less. U.S. patent No. 3,6()3,996 describes such a pulsed
Dopp]er radar.
SUMMARY OF THE INVENTION
The present invention pertains to a personnel intrusion detection
radar that achieves a range-gated detection zone with a high duty-cycle phase-
coded pu!se compression signal. The range to the detection zone and its depth
or width are programmable. By using a high duty-cycle pulse compression signal
the more favorable attributes of CW and pulse-type radar systems are combined.
The result is a radar system with the range resolution of a pulse-type radar and
the sampling/preprocessing sirnplicity of a CW radar. Though designed primarily
as an intrusion detection system, the sensor may also be used for object
detection (e.g., railcars).
This result is achieved in accordance with the invention by using a
pulse compressed signal containing a pseudo-random code to establish both the
range and range window for targets of interest.
Specifically the invention relates to an intrusion detection system
comprising: means transmitting an r.f. signal formed from a continuous wave
modified by phase changes at selected instants; means providing a code se~uence
to control the selected instants; means receiving a portion of the transmitted
signal which may have been modified by the presence of a target; and means
mixing the received signal with a delayed replica of the transmitted signal to
establish a detection zone external to the space between the antennas, the delay
establishing the range of the detection zone; whereby the system provides an
enhanced response relating to objects in the detection zone.
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An embodiment of the lnvention will now be desc-ribed in
conjunction with the accompanying drawings in which:
Figure 1 is a schematic diagram ~howing the apparatus
and detection zone of the system of the invention;
Figure ~ shows the layout of the apparatus of the
invention;
Figure 3 is a diagram illustrating the configuration of
the detection zone;
Figure 4 is a schematic diagram of the transmitter
portion of the sy~tem;
Figure 5 is a schematic diagram of the receiver portion
of the system; and
Figure 6 i~ a schematic diagram of the signal proce~sing
portion of the ~ystem.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS
The intrusion detection system providing a
three-dimensional range-gated detection cell is shown in Figure
1. The combined antenna beam patterns of the transmit and receive
antennas shape the detection zone in azimuth and eleva-tion. The
detection zone may exist over a complete hemisphere, as
illustrated in Figure 1, or a portion thereof. The depth and
range of the detection zone are programmable. An intruder
approaching the antenna~ is illuminated with a phase-coded VHF
signal. A portion of the scattered signal i9 received a-t the
receive antenna. ~nen the intruder enters the detection zone his
return signal causes a deviation from the nominal or quiescent
received signal spectrum. The radial movement of an intruder in
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the detection zone changes the phase and magnitude of the returned
.~ignal, due to the Doppler effect. A ~ubstan-tial change in the
signal spectrum result~ ln the declaration of an alarm. The
signal received at the receive antenna from objects not in the
detection zone is negligible following synchronous detection
because of the autocorrelation properties of the code 3equence,
thereby providing a range-gated detection capability.
Consequently, the system achieves a range resolution equal to that
of a pulse-type radar, but because the sy~tem transmits and
receives a high duty-cycle signal, requires only the sampling and
preprocessing speed of a CW-type radar.
The system consists of a power source, two antennas,
lead-in cable for the antennas, a termina~ and an electronics
unit, as shown in Figure 2. The unit is powered using ac or dc
power. A terminal allows the operator to set the detection zone
range, the detection zone depth, the receiver gain, and other
system selections, and to receive the results of the processed
receive signal such as alarm information. The transmit signal
generated in the electronics unit 18 transferred to the transmit
antenna by coaxial lead-in cable. Similarly, the signal received
at the receive antenna propagates down another lead-in cable to
the receiver section of the electronics unit.
Referring again to Figure l, the electronics unit
generates the transmit signal and sends this signal via lead-in
cable 2 to one of the antennas 4. The other antenna 5 is used to
receive the reflected ~ignal. The received signal is transferred
to the receiver portion of the electronics unit by way of lead-in
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cable 3, The properties of the tran.smit waveform are such that a
range-gated detection zone ie establi~hed about the antennas. By
u~ing omni-directional antenna.~, omni-directional coverage is
obtained. Directional antennas will reduce the azimuth and/or
elevation coverage
The detection ~one is ellipsoidally-shaped with the
antennas repre~enting the foci of the ellipsoid. Omni-directional
antennas are as3umed. Referring now to Figure 3, the operator
makes two selections:
(i) antenna separation : the antennas are separated by a
distance of 2xo, with a straight line between the antennas
defining the x-axis.
(ii) nominal range : a nominal range of b is selected,
3pecifying the range along the y and z-axes.
The detection zone range along -the x-axis, a, is
determined by the two selections:
a = (xo2+b2)l/2
The detection zone exist3 for any (x,y,z) such that
(x-xo)2 y2 + z2
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a2 b2
Typical detection zone value~ for the inventive system
are: an antenna separation of 30m, a nominal range of 80m, a
detection zone depth of 20m. The approximate volume for these
detection zone parameter3 is 1.6x106 cubic meters.
The operational and performance benefit~ of an intruder
detection radar in the low VHF band and, in particular, near 60
~z, are well under3tood. The radar cro3s-section (RCS) for
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humans is a maximum in the low Vl{F band. This occurs because the
effective length of a human while standing on ground is
approximately a quarter-wavelength. Small animals aad birds, by
contrast, have a minimal RCS at this frequency range because they
are much smaller than the 5m wavelength of a 60 M}l~ signal; these
smaller taIgets are, in fact, referred to as Rayleigh scatters.
Similar results are also applicable for propagation through the
forest; leaves, pine needles and branches are Rayleigh scatterers
as well. As a result, the propagation 1088 through the forest and
its clu-tter return are substantially less than that experienced by
sensors operating at higher frequencies.
Figure 4 illustrates the transmitter portion of an
intrusion detection system of this invention. A con-tinuous wave
source 15 supplies a VHF signal to a modulator 17 through a power
splitter 16. The VHF signal is preferably at 60 MHz. Modulator
17 is also supplied with a code sequence from code generator 13
which has been filtered by lowpass filter 14 to remove the higher
frequency components of the code. This results in the output from
modulator 17, which is supplied to transmitting antenna 4 through
a wideband amplifier 18, being a continuous wave wi-th smooth phase
changes at eaGh change of the code from generator 13. The rate at
which the code is generated is controlled by a clock 12 which, in
turn, is controlled by a control unit 11. By changing the clock
rate the code sequence rate i8 changed and, as will be shown
below, the depth of the range window altered.
A digital delay unit 19 is coupled to the output of the
code sequence generator 13 and supplies a delayed version of the
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code eequence -to a lowpae~ filter 20 and subcaeqllently to
modulator 21 substantially identical to modulator 17. This
provide~ a reference signal for use in the receiver for
demodulating any received signals from a target. So that both
in-phase and quadrature-phase received signal~ may be detected,
quadrature power splitter 22 supplie~ appropriate signala on lines
23 and 24 to the receiver unit. The amount of delay in -the
digital delay circuit 19 determines the nominal range at which the
detection zone will be located and the clock rate determines the
depth of the detection ~one.
The pre~ence of a target in the detection ~one causea a
reflection of some of the transmitted signal. The reflected
~ignal, together with some signal received directly from the
transmitting antenna, i3 picked up by receiving antenna 5 (see
Figure 5). The received signal i~ bandpa~s filtered and amplified
in amplifier 34 and divided u~ing power splitter 35 into two
channels. These signal~ are applied to modulators 36 and 37,
where they are modulated by the in-phase 23 and quadrature-pha~e
24 reference signals, respectively. Referring to the in-phase
channel for example, the received signal is mixed with the delayed
replica of the transmitted signal in modulator 36. Only those
signal component~ which are correla-ted with the delayed code
sequence are detected and, hence, an enhanced signal representing
any reflection at the particular range defined by this delay is
produced. These in-pha~e and quadrature-phase detected ~ignal~
are then processed in the normal manner through the remaining
signal proces~ing channels ~hown in Figure~ 5 and 6.
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A typical ayctem operate~ at a -tran~mitted power of
20 mW with a code length of between 100,000 and 1 million bit~ and
at a clock rate of 10 M}lz. The longer the code length i~ the
~maller i~ the probability of any ambiguity in the returned
~ignalc. There i~ a limit to code length, however, becau~e the
spacing between -the ~pectral component~ mu3t be greater than the
Doppler bandwidth to avoid other ambiguitiec in the received
cignal. Typically, for a maximum Doppler bandwidth of lOHz the
spectral components are spaced at leact 20 Hz apart. Ac
previou~ly mentioned, a variety of code ~equences cuch a~
pseudo-noi~e, Barlcer code~ and complementary ~erie~ can be u~ed.
Specifically, the in-pha~e detected signal goe~ through
a low pa~s filter 38, an amplifier 39, and a ~ample-and-hold
circuit 42. Both oignals are then multiplexed in multiplexer 44,
converted to digital ~ignals in converter 45, and procecsed by
proce~eor 46 for eub~equent Doppler frequency re~pon~e. Signale
exceeding a given thre~hold produce an alarm warning eignal.
Although a particular embodiment hao been described, it
will be clear that variations are poeeible while remaining within
the ecope of the inventive concept. A3 the size of the detection
zone increaseo, it becomee more important to locats the target in
range, azimuth and elevation. Thie additional direction finding
requirement can take a variety of form~. A high gain receive
antenna can be ~canned over the desired detection zone, with the
azimuth/elevation reeolution being determined by the antenna
beamwidth. Alternatively, target azimuth and/or elevation can be
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de-termined by using two or more receive antennas to compare the
relative pha~e of the return signale.
As a related embodiment, either the tran~mit or receive
element could be replaced by a leaky coaxial cable. ~or example,
the cable could encircle the antenna at a fixed radius. The
effect of the pulse compression ~ignal is to achieve a plurality
of detection ~one~ along the cable. Consequently, the perimeter
along the cable i9 effectively divided into a number of sectors,
thereby providing an indication of the intrueion location.
Although the apparatus of the preferred embodiment
described above change~ phase by 180~; the equipment will function
with different amounts of pha~e change, 45 or 90~ for example.
The effect of the different angle of phase change is to modify the
spectrum of the tran~mitted signal slightly by reducing it~ higher
frequency content. Further, it is not necessary that there be
only switching between two phase angles. Instead, the transmitted
waveform could be switched by three or more different phase
shifters, provided only that a delayed replica of the transmitted
signal is used in the receiver.
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