U.S. patent number 3,794,992 [Application Number 05/224,130] was granted by the patent office on 1974-02-26 for radio frequency intrusion detection system.
This patent grant is currently assigned to General Dynamics Corporation. Invention is credited to John B. Gehman.
United States Patent |
3,794,992 |
Gehman |
February 26, 1974 |
RADIO FREQUENCY INTRUSION DETECTION SYSTEM
Abstract
In order to detect personnel and other intruding objects in open
terrain, one or more long wire antennas are buried in the ground
and located orthogonal to a vertically polarized souce of VHF
radiation, such as a continuous wave transmitter which feeds an
elevated vertical dipole. The intruder couples the radiation to the
buried antenna which feeds a receiver. The receiver produces an
output responsive to the characteristics of the radiation coupled
to the antenna by the intruder to produce an alarm indicative of
the presence of the intruder.
Inventors: |
Gehman; John B. (La Jolla,
CA) |
Assignee: |
General Dynamics Corporation
(St. Louis, MO)
|
Family
ID: |
22839381 |
Appl.
No.: |
05/224,130 |
Filed: |
February 7, 1972 |
Current U.S.
Class: |
342/28;
340/539.23; 340/552; 342/103; 340/539.1 |
Current CPC
Class: |
G08B
13/2497 (20130101); G01S 1/02 (20130101) |
Current International
Class: |
G08B
13/24 (20060101); G01S 1/00 (20060101); G01S
1/02 (20060101); G01s 009/02 (); G08b 013/26 () |
Field of
Search: |
;340/258B,258C
;343/5PD |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hubler; Malcolm F.
Attorney, Agent or Firm: Lukacher; Martin
Claims
What is claimed is:
1. A radio frequency intrusion detection system for detecting the
presence of an intruding object in a radio frequency radiation
field adapted to be produced by a radio frequency transmitting
source, which comprises a receiving antenna disposed remotely from
the transmitting source of said radiation field at such a distance
that the capacitative and inductive coupling from the source is
insignificant at the radiation field frequency, said antenna being
disposed below the surface of the terrain upon which said radiation
field is incident and to which said field is coupled when said
object is in proximity to said antenna, and receiver means
connected to said antenna and responsive to radio frequency signals
generated in said antenna by said radiation field when coupled to
said antenna by said object for providing an output indicative of
the presence of said object.
2. The invention as set forth in claim 1 wherein said terrain is
the ground, and said antenna is buried below the surface of said
ground a distance less than the penetration depth for the frequency
of said radiation field.
3. The invention as set forth in claim 2 wherein said radiation
field is of a frequency in the VHF band and said distance is less
than 1 foot.
4. The invention as set forth in claim 1 wherein said antenna is a
long wire.
5. The invention as set forth in claim 4 wherein said receiver is
connected to one end of said wire.
6. The invention as set forth in claim 4 wherein said receiver is
connected to the center of said wire.
7. The invention as set forth in claim 4 wherein said wire is at
least 50 feet in length.
8. The invention as set forth in claim 5 wherein said wire is from
50 to 400 feet in length from the point where it is connected to
said receiver to an end thereof.
9. The invention as set forth in claim 4 wherein said wire is
disposed orthogonally to the direction of propagation of said radio
frequency radiation field.
10. The invention as set forth in claim 1 wherein said system is
adapted to secure an area and wherein a plurality of said antennas
are disposed in end to end relationship about the perimeter of said
area.
11. The invention as set forth in claim 10 including a plurality of
receivers separately connected to different ones of said
antennas.
12. The invention as set forth in claim 11 including common
monitoring means connected to all of said receivers for detecting
the outputs of said receivers indicating the presence of an object
in the proximity of each of said antennas.
13. The invention as set forth in claim 1 wherein said antenna is a
long straight wire, and including a transmitting antenna for
radiating waves polarized vertically with respect to said wire to
establish said radiation field.
14. The invention as set forth in claim 13 wherein said
transmitting antenna is a whip antenna disposed vertically upon
said surface and located remotely from said wire antenna.
15. The invention as set forth in claim 1 including a second
antenna disposed in side-by-side relationship with said first named
antenna means for providing said second antenna with a length
approximately one-quarter wavelength different in length from said
first antenna, and means for connecting said first and second
antenna to said receiver.
16. The invention as set forth in claim 15 wherein said connecting
means comprises switch means for connecting said first and second
antennas in rapid succession to said receiver.
17. The invention as set forth in claim 15 wherein said receiver
comprises first and second receivers connected respectively to said
first and second antennas, each of said receivers providing a
separate output and means responsive to the sum of said separate
outputs for providing said indication of the presence of said
object.
18. The invention as set forth in claim 1 wherein said receiver
includes a phase lock loop having a narrow band pass.
19. The invention as set forth in claim 18 wherein said phase lock
loop comprises a phase discriminator to which a signal
corresponding to the radiation detected by said antenna is applied
and a variable frequency oscillator and a narrow band pass filter
for applying the output of said discriminator to said oscillator
for controlling the frequency thereof, a synchronous detector,
responsive to said signal, and means for shifting the phase of the
oscillations from said oscillator and applying said shifted
oscillation to said synchronous detector, and means responsive to
the output of said synchronous detector for providing said output
indicative of the presence of said output.
20. The invention as set forth in claim 18 wherein said receiver
further includes a frequency translator for heterodyning the signal
detected by said antenna to an intermediate frequency signal and
means for applying said intermediate frequency signal to said phase
lock loop.
21. The invention as set forth in claim 20 wherein said frequency
translator includes a mixer, first and second local oscillators
connected to said mixer, a direct current power supply line for
providing operating voltage to said receiver and means for
selectively reversing the polarity of direct current applied to
said line for selectively enabling said first and second local
oscillators.
22. The invention as set forth in claim 18 wherein said receiver
further includes means responsive to said receiver output for
translating said output into an identification burst of
predetermined frequency and duration which corresponds to the
reception of said output.
23. The invention as set forth in claim 22 wherein said system
comprises monitoring means responsive to said tone bursts which
have different frequencies over a band of frequencies, said
monitoring means including a wide band amplifier for amplifying
said band of frequencies, a mixer coupled to the output of said
amplifier, a variable frequency oscillator also coupled to said
mixer, detector means coupled to said mixer for providing signals
corresponding to said tone bursts which are converted by said mixer
to a given frequency, a recycling counter providing a plurality of
outputs corresponding to different counts, means for applying clock
pulses to said counter causing it to count, a digital to analog
converter connected to said counter for converting the counts
stored in said counter into an analog signal and applying said
analog signals to said oscillator for varying the frequency thereof
correspondingly with said counter, and means operated by said
counter for separately displaying said detector means output
signals which occur simultaneously with different ones of said
counts.
24. The invention as set forth in claim 6 wherein said wire is from
50 to 400 feet in length from the point where it is connected to
said receiver to an end thereof.
Description
The present invention relates to security systems, and particularly
to a system for sensing personnel and other intruding objects
through the use of radio frequency electromagnetic wave
radiation.
The invention is especially suitable for use in security systems
for protecting the boundaries of a large area in open terrain by
sensing the presence of personnel and other intruding objects
crossing these boundaries.
Electromagnetic personnel sensing systems of various types have
been suggested. For the most part they operate in accordance with
principles of capacitive and inductive coupling between the
personnel or other object to be detected and a sensing device (see
for example U.S. Pat. Nos. 3,439,358 and 3,462,692). Other systems
operate on the basis of the disturbance in a standing wave pattern
produced by intruding personnel (see U.S. Pat. No. 2,038,878).
Still others operate in a manner similar to radar systems and
detect reflections from the intruding personnel (see U.S. Pat.
3,163,861). While it may be feasible to utilize such systems to
secure relatively limited areas, they are not particularly adapted
for securing a large area, say in open terrain, such as the
perimeter of a factory or other large installation. In addition
such known systems may be difficult to install, particularly in a
manner such that they are not obtrusive to intruding personnel who
may be interested in avoiding or disabling the sensing system.
It is therefore, an object of the present invention to provide an
improved electronic security system.
It is a still further object of the invention to provide an
improved security system which utilizes radio frequency
electromagnetic radiation.
It is a still further object of the invention to provide an
improved radio frequency security system which is sensitive to
moving objects such as personnel.
It is a still further object of the invention to provide an
improved radio frequency security system which is especially
adapted for securing large areas, out of doors and in open
terrain.
It is a still further object of the invention to provide an
improved radio frequency security system which may readily be
installed at low cost.
It is a still further object of the present invention to provide an
improved radio frequency security system which is unobtrusive and
the operation of which is not readily ascertainable by intruding
personnel.
Briefly described, the present invention affords a system for
detecting the presence of an object, such as personnel, in a radio
frequency, radiation field, through the use of an antenna disposed
below the surface of the terrain upon which the radiation field is
incident. The antenna may be a long wire, say 50 to 400 feet in
length, connected to a receiver which detects the radio frequency
signal which is coupled to the antenna by intruding personnel. The
intruding personnel act as a receiving antenna or reflector and
provide the necessary coupling to the buried sensing antenna. The
intruder will then cause a variation in the field strength of the
signal monitored by the receiver, such that the receiver produces
an output indicative of the presence of the intruder.
The invention itself, both as to its organization and method of
operation, as well as additional objects and advantages thereof
will become more readily apparent from a reading of the following
description in connection with the accompanying drawings in
which:
FIG. 1 is a schematic diagram of a radio frequency object detection
system in accordance with the invention, which system is especially
adapted for securing the perimeter of a large area;
FIG. 2 is a perspective view, schematically illustrating the
operation of the system in sensing intruding personnel;
FIGS. 3a, 3b, and 3c are schematic diagrams illustrative of the
operation of radio frequency security systems embodying the
invention;
FIG. 4 is a wave form illustrative of the output from a receiver of
the system shown in FIGS. 1 and 2 which results when intruding
personnel pass over the buried sensing antenna;
FIG. 5 is a block diagram of the receiver portion of a system
embodying the invention;
FIG. 6 is a schematic diagram of circuits used in the receiver
portion of a system embodying the invention;
FIG. 7 is a block diagram illustrating a radio frequency object
detection system in accordance with another embodiment of the
invention;
FIG. 8 is a block diagram of a system in accordance with still
another embodiment of the invention;
FIG. 9 is a more detailed diagram, partially in block form and
partially in schematic form, which illustrates a receiver system in
accordance with an embodiment of the invention; and
FIG. 10 is a block diagram of a monitoring system suitable for use
with a number of receivers of the type illustrated in FIG. 9, which
monitoring system is provided in accordance with features of the
present invention.
The system provided by the invention, unlike systems requiring
electromagnetic (either inductive or capacitive) coupling to a
sensing device or direct reflection of signals to a pick-up
antenna, is especially adapted to protecting large areas in open
terrain. Thus, when a large area, such as several acres is to be
secured, several buried sensing antennas, e.g., six antennas 10,
12, 14, 16, 18 and 20, may be disposed about the perimeter of the
area to be secured.
Each of these antennas may be a length of insulated wire (No. 10 to
No. 30, wire being suitable) which are disposed in linear paths
about the perimeter of the area and buried below the surface of the
ground. Perferably, the sensing antennas are buried to a depth of
one to three inches. They may, however, be buried up to 18 inches
below the ground. The antennas are connected in the center thereof,
via coaxial cables, 22, 24, 26, 28, 30 and 32, to receivers 34, 36,
38, 40, 42 and 44. The center connection reduces losses due to
attenuation in the wire of the antennas. The receivers may,
however, be connected to one end of their respective antenna wires.
The receivers are preferably sensitive narrow bandwidth radio
receivers of the type to be described hereinafter in connection
with FIG. 9. However, any receiver capable of detecting signals in
the frequency range of the radio frequency radiation field
established in the vicinity of the antennas may be used. The
receivers, 34, 36, 38, 40, 42 and 44 may also be buried
underground, and receive power and transmit alarm signals through a
power distribution and alarm line or cable, 46. This cable, 46, is
connected to a monitoring and power distribution system 48 which
translates the alarm outputs from the receivers into visual or
aural indications, say an array of lights, which designates which
antenna 10 through 20, has sensed the presence of an intruding
object or person.
The radio frequency radiation is established by a transmitting
antenna 50 which preferably is a vertical dipole which transmits a
vertically polarized radiation field. The antenna 50 is driven by a
VHF continuous wave transmitter 52. Frequencies between 40 MHz and
150 MHz are preferable. For personnel detection a frequency of 60
MHz is especially preferred since a man is approximately one
quarter wavelength long at about 60 MHz, and thus acts as a good
receiving antenna or reflector to translate the radio frequency
radiation from the transmitting antenna into the buried sensing
antennas.
It will be noted that the buried sensing antennas are disposed
orthogonally to the transmitting antenna 50 rejecting the direct
vertically polarized signal. However, the vertically polarized
signal from the transmitting antenna reflected or coupled to a
vertical target such as a person, is especially well picked up by
the buried horizontal sensing antenna. The vertical target is
believed to cause ground current to flow near the buried sensing
antenna wire, which in turn induces the signal into the wire. It
may also reflect a signal along the axis of the antenna which has a
maximum response. In other words, the intruding personnel provides
coupling to the wire of the buried sensing antenna and will
increase the magnitude of the signal picked up from the remote
transmitter by reflection. Thus the receivers 34 through 44 coupled
to their respective buried sensing antennas, produce outputs which
vary in accordance with the field strength of the signal picked up
by the buried sensing antennas. When an intruder approaches a
buried sensing antenna, this field strength increases and is
detected in the receivers to indicate the presence of the
intruder.
The operation of the system may be better understood from FIG. 2.
The buried sensing antenna, 10, is shown by way of example. It is
indicated as being 100 feet long. With normal ground conditions
(the ground not being highly conductive) the buried wire of the
antenna 10 can be 400 feet long. The antenna is indicated as being
buried up to a depth of 1.5 feet and is connected by way of the
buried coaxial cable to the receiver 34 which is also buried. The
cable to the power and monitoring station 46 is also shown as being
buried.
Very little signal may normally be received by the buried sensing
antenna 10 due to its orthogonal relationship with respect to the
transmitting antenna and because the transmitting radiation is
vertically polarized, while the buried antenna is disposed
horizontally. The intruder 4 acts like a vertical reflector and
directly couples the radio frequency signal directly into the
antenna 10. In the event that the antenna 10 picks up some of the
incident signal, the signal coupled to the antenna by the intruder
will add vectorially to the signal which is normally picked up. The
approach of the intruder will then cause a cyclical variation in
the output from the receiver 34 as is illustrated by way of example
in FIG. 4. In effect, a doppler signal is generated as the intruder
moves toward the sensing wire 10 if he moves in the direction
toward the signal source (viz., the antenna 50). A cycle is
generated for every half wavelength of motion at the frequency of
the radiation. When the intruder crosses the sensing wire still in
the direction toward the rf source a much lower frequency doppler
signal is generated. This signal characteristic is also illustrated
in FIG. 4. Thus, by viewing the signal from the receiver, say on
the chart recorder, the presence of an intruder can be visually
ascertained. The receiver may also have a threshold detector, as
will be discussed in detail in connection with FIG. 5 through 9 for
automatically indicating the presence of the intruder.
The radiation pattern of the antenna 10 is also illustrated in FIG.
3a. It will be noted that the antenna has a bidirectional pattern
centered about the center point 56 of the antenna wire to which the
receiver 34 is connected. The antenna pattern has a multiplicity of
lobes and favors vertically polarized signals coming at an angle
above the ground and along the axis of the wire. Accordingly,
vertically polarized signals from the transmitter reflected from
the intruder are picked up by the horizontal wire antenna 10.
In FIG. 3, one of the lobes (shown in the dash line) of an antenna
wire, 58, which is connected to a receiver 60 at one end thereof,
is shown. It will be understood that a plurality of such lobes, the
number of which depends on the length of the wire, will exist. The
radiation pattern favors somewhat an intruder near either end of
the sensing wire antenna 58. Pickup of radiation signals by a
buried horizontal sensing antenna, such as shown at 62 in FIG. 3c,
is also effected by the passage of the intruder above the center of
the antenna causing ground currents (illustrated schematically by
the lines, 64 and 66) to flow near the antenna 62 and induce
signals therein. It is believed that signals are caused to be
picked up due to the presence of an intruding object by one or more
of the mechanisms discussed, in connection with FIGS. 2 and 3,
although the invention is not limited to any theory by which pickup
of radiation signals may be explained, which explanation is offered
herein for purposes of elucidation.
As illustrated in FIG. 5 automatic alarm indication may be afforded
by connecting the output of a receiver 66 which receives input
signals from the buried sensing antenna by way of a capacitor 68 to
a comparator amplifier 70. In the receiver the signal, such as
illustrated in FIG. 4, which is picked up by the buried sensing
antenna and applied to the input of the receiver, is detected to
produce a slowly varying analog signal corresponding thereto which
is coupled to the comparator 70 via the capacitor 68. The
comparator receives a threshold level from a potentiometer, 72.
When this threshold is exceeded by the amplitude of the signal from
the receiver output, the comparator produces a pulse which triggers
an alarm indicator 74. This indicator may, for example, by a
latching relay which illuminates a lamp or sounds a buzzer or other
audible alarm. The alarm may be reset manually.
Inasmuch as the output from the receiver may vary cyclically, it
may be coupled, as shown in FIG. 6, through a capacitor 76 and
oppositely polarized diode rectifiers 78 and 80 to the direct and
inverting inputs of an operational amplifier 82. The rectifier and
operational amplifier act as a full wave rectifier and amplifier
translating the cyclic input from the receiver into a unipolar
output which is coupled through a capacitor 84 to a comparator 86.
The comparator 86 has a threshold voltage applied to another input
thereof from a potentiometer 88, thus the full wave rectified
output exceeds the threshold. The comparator 86 provides an output
pulse to an alarm indicator, such as discussed above in connection
with FIG. 5.
The motion of the intruder produces a cyclical response when moving
toward the buried sensor antenna or on a parallel path. This
response is due to the vector addition of direct signal leakage or
miscellaneous scattered reflections from fixed objects and the
reflected signal from the moving intruder. The positive or negative
peaks are in or out-of-phase conditions, zero responses are
quadrative additions. To reduce the possibility of a missed
detection due to a chance path giving a quadrature addition, zero
response, two buried sensor antennas BSA.sub.1 and BSA.sub.2 are
used as indicated in FIGS. 7 and 8. One of the antennas has an
added quarter-wavelength section, or a delay line 90 which provides
a quadrative shift of the received signal with respect to the other
buried sensing antenna. An electronic switch 92 which may be a pair
of circuit gates alternately enabled by the output of a free
running multivibrator is used to switch the outputs from the
antenna alternatively to the input of a receiver 94. One or the
other or both will provide the receiver with a peak signal
regardless of any path the intruder takes.
In FIG. 8 instead of an electronic switching circuit 22, a pair of
receivers 100 and 102 are separately connected to the buried
sensing antennas BSA.sub.1 and BSA.sub.2, the receiver 102 being
connected by way of the delay line 90, from BSA.sub.2. The outputs
of the receivers are capacitive coupled by capacitors 103, and 105
and are additively combined by way of a diode network 104 and 106.
The rectified signals from receivers 100 and 102 are connected to
the input of a comparator 108. Positive output from 104 and 106 is
applied to the non-inverting input of 108 while the negative output
from 104 and 106 is applied to the inverting input of comparator
108. The connection via the resistor pads 111 is additive into
comparator 108. A threshold voltage for the comparator 108 is
obtained from a potentiometer 110. The change in output from the
comparator which results when the threshold is exceeded, operates
an alarm indicator 112. Again, in the case of FIG. 8, the
quadrature and in-phase signals resulting from the intruder are
additively combined in the circuits 104 and 106 which are connected
to the outputs of the receivers 101 and 102, thus providing an
output from an intrusion regardless of the path taken by the
intruder.
Since the signal expected to be produced when an intruder is
present has a very slow variation (viz., of the order of 1 Hz or
less), the bandwidth of the receiver is desirably narrow. Such
narrow bandwidth aids in increasing sensitivity and reducing the
adverse effects of noise and interference. The receiver shown in
FIG. 9 provides the necessary narrow bandwidth and noise
discriminating characteristics as well as other features. The
signal from the buried sensing antenna is first amplified in a
radio frequency amplifier 114, and then is applied to a frequency
translator including a first mixer 116. This first mixer may be
connected to two local oscillators 118 and 120. These two local
oscillators may be alternatively selected so as to provide two
channels, say for radiation at two different frequencies, e.g., 100
MHz and 105 MHz. The channels may be selected by alternating the
polarity of the supply voltage from the monitoring station by means
of a double poled switch 122. Since the lines of the power
distribution cable, which are of opposite polarity are connected
separately to the local oscillators 118 and 120 through different
diodes 124 and 126 by way of chokes 128 and 130, a different one of
the oscillators will be operating at any one time depending upon
the position of the switch 122 and the polarization of the diodes.
Thus, either channel may be remotely selected from the monitoring
station.
After amplification in an intermediate frequency amplifier 140, a
second stage of frequency translation is provided in a second mixer
142 which receives injection signals from a second local oscillator
144. In order to afford a degree of narrow bandwidth the
intermediate frequency amplifier 146 which selects the mixer
product at the mixer 142 may have a narrow bandwidth of
approximately 500 Hz. The intermediate frequency signals from the
amplifier 146 may be detected in an automatic gain control
amplifier 148 and applied by way of a gain control potentiometer
150 to the RF and IF amplifiers 114 and 140. The output of the IF
amplifier 146 is applied to a phase lock loop 152 including a phase
discriminator 154 and a low-pass filter 156 having a 1Hz bandwidth
which controls a voltage controlled variable frequency oscillator
158. The loop 152 thus remains locked to the frequency of the
radiation received by the sensing antenna. The received signal is
applied to a synchronous detector 160 together with a signal from
the voltage control oscillator 158, shifted 90.degree. by a phase
shift circuit 162. The phase lock loop, phase shift, synchronous
detector and integrator combination affords an additional degree of
filtering which narrows the bandwidth to 1Hz reducing the
possibility of external interference, and/or audio modulation on
the sensing carrier. The double conversion in the receiver
intermediate frequency stages also aids in increasing the
sensitivity of the system. The output of the synchronous detector
which is integrated in the integrator circuit 164 is applied to a
rectifier and threshold detector circuit 166 such as shown in FIG.
6, which changes its level and triggers a one-shot multivibrator
168. The multivibrator 168 enables an oscillator to produce a burst
of oscillation for the period of the pulse in the one-shot
multivibrator, say two seconds in duration, the frequency of the
oscillations from the oscillator 170 is indicated as f.sub.a.
Suitably, this frequency may be any frequency in a band say, from
100 to 500 KHz. The tone burst from the oscillator, 170, is coupled
via capacitors 172 and 174 to the power distribution and alarm
line, whence it is delivered to terminals 176 and 178 at the
monitoring station.
The power distributed along the line is coupled by way of chokes
180 and 182 to a bridge rectifier 184. The output of the bridge
rectifier is voltage regulated in a voltage regulator 186 which may
be a zener diode and produces operating voltages indicated at +
V.sub.P and -V.sub.P which are applied to the circuit components of
the receiver for operating these components and the entire receiver
from voltages generated at the monitoring station and distributed
along the line.
A system for displaying the alarm output from several receivers is
illustrated in FIG. 10. The signals are applied to terminals 176
and 178 and capacitively coupled to a wideband amplifier 188. The
wideband amplifier can thus handle and amplify various
identification frequencies produced by oscillators, such as the
oscillator 170 in the receiver shown in FIG. 9. The output of the
amplifier is applied to a mixer 190 which receives injection
signals from a variable frequency voltage controlled oscillator
192. The frequency at which this oscillator operates is controlled
by a digital to analog converter 194 which receives a digital
signal from a recycling counter 196. The counter is driven by a
clock oscillator 198 which causes the counter to count, say to 10,
recycling every second, for example. Since the number reached by
the counter varies as each new clock pulse arrives at the
oscillator 198, the voltage developed by the converter 194 changes
correspondingly as does the injection from the voltage controlled
oscillator 192. Accordingly, the frequency at which input signals
will provide mixer products which will pass through the bandwidth
of an intermediate frequency amplifier 200 will vary with each
count. Thus, during each cycle of the counter operation, each of
the possible identification frequencies produced by oscillators,
such as the oscillator 170 in the several receivers which may be
included in the system, will be sampled. The output of the
amplifier 200 is detected and amplified in a detector and amplifier
circuit 202 which may drive a speaker to produce an audible alarm
if any of the receivers detect an intruder. A visual alarm will be
provided at an indicator lamp A through J, each of which
corresponds to a different tone burst frequency. Thus, AND gates
206 to 208, each corresponding to a different one of the lamps A
through J, will be successively enabled by the counter 196. These
gates will be enabled in time relationship with the different
expected identification bursts. If an AND gate 206 to 208 is
enabled, a flip-flop 210 associated therewith will be set and
current will flow through one of the lamps A through J. The
flip-flops may be reset manually through a pushbutton switch 212.
Since there will be a corresponding time relationship for the
enabling of the gates 206 to 208 and the arrival of detected
outputs corresponding to different frequency tone bursts, each of
the indicator lamps will correspond to a different identification
burst and therefore to a different receiver and its associate
sensing antenna. A panoramic display is therefore provided which
facilitates the monitoring of several sensing antennas in the
system.
From the foregoing description it will be apparent that there has
been provided an improved object detection system which utilizes
radio frequency radiation for the detection of personnel and other
intruding objects. While the system has been described in
connection with a transmitter and antenna for providing the
radiation which is picked up by the sensing antenna, it will be
appreciated that the system may also utilize existing sources of
signals as may be broadcast by broadcast stations in the vicinity,
especially FM broadcast stations. Other variations and
modifications within the scope of the invention will undoubtedly
suggest themselves to those skilled in the art. Accordingly, the
foregoing description should be taken merely as illustrative and
not in any limiting sense.
* * * * *