GB2203313A

GB2203313A - Transponder surveillance using radar

Info

Publication number: GB2203313A
Application number: GB08704529A
Authority: GB
Inventors: Bryan Stephen Rickett
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1987-01-03
Filing date: 1987-02-26
Publication date: 1988-10-12
Anticipated expiration: 2007-02-26
Also published as: GB8700050D0; GB2203313B; GB8704529D0

Abstract

A radar pulse transmitter and receiver remotely interrogate personnel and/or objects, and one or more transponders are carried by or attached to authorised or other personnel or objects required to be identified by the radar pulse receiver. Each transponder includes a radar-receiver antenna (4), a radar-pulse detector (10) and a high frequency high-gain return-signal transmitter (18, 19, 30) including transmit antenna. The return-signal transmitter is only connected to the transponder power source (17) for the duration of a received and detected radar pulse to allow transmission to the radar receiver of a high-frequency return signal which forms part of a coded signal for identifying one of the personnel or objects. The transponder may be credit card sized and have a continuously powered frequency synthesiser (13, 14). <IMAGE>

Description

Improvements relating to Radar Systems This invention relates to radar systems of the kind for obtaining information (e.g. location, condition, identification etc) relating to personnel or objects (e.g. vehicles) at or llcar a particular location or within a predetermined area under surveillance.

The invention has especial application to radar systems for identifying authorisedlunauthorised personnel - when located within a particular surveillance region (e.g. airport) or when crossing a secure fence or when approaching the entrance to a secure site.

Such radar systems of the timid described require the personnel or objects under surveillance to be interrogated by a radar bansmitter/receiver which may be stationary or portable (e.g. handheld). Such interrogation may cause transponder means carried or otherwise associated with personnel or objects under surveillance to respond to receive radar pulses and to transmit information to the radar transmitterfteceiver for identifying the personnel or objects concerned.

The transponder means for such radar systems have hitherto required undesirably bulky receive and transmit high-gain antennas (e.g. horn antennas) especially in the absence of high-gain amplifiers having relatively high power consumption. Known transponder means have produced HF and VHF return signals in response to interrogating radar pulses. These known undesirablity bulky transponder means have suffered from relatively short battery life and also from poor resolution resulting in low positional accuracy and the obscuring of unauthorised personnel or objects by the transponder HF or VHF return signal to the radar tran smitterireceiver.

According to the present invention there is provided a radar system for detecting and obtaining information relating to personnel andlor objects at or within a predetermined range of a particular location or within a predetermined area under surveillance, comprising a radar pulse transmitter and receiver for the remote interrogation of personnel and/or objects, and one or more transponders which are adapted to be carried by or attached to authorised or other personnel or objects required to be identified by the radar pulse receiver and each of which includes a radar pulse receive antenna, a radar pulse detector and a high frequency highgain return signal transmitter including transmit antenna, said highgain transmitter only being connected to power supply means for the transponder for the duration of a received and detected radar pulse in order to facilitate the transmission to the radar receiver of a high frequency radar return signal which forms part of a coded signal for identifying one of said personnel or objects.

In carrying out the present invention the radar system may comprise a plurality of transponders each of which comprises small dipole receive and transmit antennas mounted at the respective ends of a small substrate of about creditvard size which also carries the entire circuitry of the transponder which may include a plurality of intergrated circuits.

The transponder of the radar system may comprise a continuously powered frequency synthesiser for producing under the control of frequency shifting means relatively low frequency signals of different frequencies which are mixed by mixer means with successive received and detected radar pulses in order to provide a frequencycoded signal output which is amplified by higbain amplifier means of the return signal transmitter before being transmitted to the radar receiver for identifying one of said personnel or objects.

Alternatively, the transponder may comprise a continuously powered relatively low fixed frequency oscillator producing fixed frequency signals which are fixed with selected received and detected radar pulses to define a binary-coded return signal to identify one of the personnel or objects concerned for transmission to the radar receiver after amplification by high-gain amplifier means of the return signal transmitter.

In Carrying out the invention the radar transmitter may be arranged to produce a rotating scanning beam and the complete coded return signals for identifying personnel or objects may be transmitted to the radar receiver during a single scan cycle.

Alternatively, the radar transmitter may be arranged to transmit a fixed radar beam.

By way of example the present invention will now be described with reference to the single-figure accompanying drawing which shows a radar system including a personnel or object identifying transponder.

The radar system illustrated in the draping comprises a pulse radar transmitter 1 and receiver 2. The transmitter 1 may be adapted to provide a rotating scanning beam for sweeping a secure ground area (e.g. airport) to detect the presence of authoriscdlunauthorised personnel and objects. in the present example for the pulse radar transmitter may comprise a 10 Watt X-Band radar transmitter having a 30 db gain antenna which will produce a sufficiently large signal at a radius of 1 km to be detected by miniaturised transponder means 3 of the system which is accommodated on a small substrate about the size of the usual credit card and which will be carried by or attached to authorised personnel or objects within the secure area being surveilled by the radar scanning beam.

More specifically, the transponder 3 comprises a small dipole receive antenna 4 which may be located at one end of the transponder substrate. A radar pulse signal received by the receive antenna 4 is initially routed by means of a field-effect transistor 5 to a high-frequency low-power pulse detector including inductors 6 and 7 capacitors 8 and 9 and a diode 10 together with resistive and capacitive time-constant components 11 and 12 across the output of the detector and connected to the input of a video amplifier 13 the output from which is fed to a variable threshold comparator 14 baving a variable resistor 15 associated with it. The threshold comparator produces an output which causes operation of a low resistance transistor switch 16. In operation, the switch 16 applies battery power from a battery power source 17 which supplies power continuously to the detector circuitry in order to energise high frequency high-gain amplifiers 18, 19 and 30 of a return signal transmitter for the duration of a received and detected radar pulse.

The output from the threshold comparator 14 also causes the fieldeffect transistor 5 at the input of the detector to route the received and detected pulse to the bigh-gain amplifiers 18 and 19 of the return signal transmitter. It may have be mentioned that the transistor 5 could be replaced by a passive splitter device which routes parts of the received ndar pulse to the detector and the highgain amplifier 18, simultaneously.

In the present example the return signal transmitter includes a frequency synthesiser 20 comprising a voltage-controlled oscillator 21 which is controlled by a fixed frequency signal from a standard oscillator 22 which is mixed by mixer 23 with the output from a frequency divider 24 itself controlled by means of a frequency shifting device 25 (e.g. stepping switch) arranged to operate to produce a predetermined shift or change in frequency of the voltage.

controlled oscillator 21 in rcsponse to successive radar pulses received by the detector. The change in frequency is effected when the transistor switch 16 restores to normal at the end of a received radar pulse which then allows the synthesiser 20 to settle dovn to the new frequency in the period between successive radar pulses.

The output from the voltage-controlled oscillator which may vary between 20 to 50 Mhz, for example is applied to a mixer 26 where it is mixed with the amplified received and detected radar pulse signal before being amplified further by the amplifier 30 and then transmitted by a small dipole transmit antenna 27 which is fitted to the other end of the transponder substrate remote from the receive antenna 4.It will thus be appreciated that as a succession of radar pulses (e.g. 100) are received by the transponder during each scanning cycle by the scanning beam the frequencies of the corresponding successive return signals to the radar receiver by the return signal transmitter shown will be predetermined in accordance with a particular code for identifying the particular transponder and thus the authorised person carrying or the object to which the substrate is attached. These radar return signals will be detected by the radar receiver as normal target return signals but will occur in a number of separate intermediate frequency bands so as to the isolated from any of the normal radar clutter signals. Any target alarm will be witheld until the complete different frequency coded signal is received and identified.The number of different frequency pulses making op the coded signal may vary according to requirements that will clearly be dependent upon the total number of authorised personnel or different objects that need to be identified by the interrogating radar system.

In other contemplated embodiments of the present invention the identifying frequency-coded signals for authorised personnel or objects may be spread over two or more cycles of the radar scanning beam or the frequency synthesiser may be dispensed with and a fixed frequency signal derived from a fixed frequency oscillator and mixed with selected received and detected radar pulses of a succession of radar pulses in order to provide a binarywoded pattern of radar return signals for identifying authorised personnel or objects to be identified.

The radar transmitter/rectiver, instead of being at a fixed location which would normally be the case in continuous radar scanning or surveillance systems for airports or the like, could be of portable fonn (e.g. hand-held) providing a fixed radar beam which could be directed in any direction for the detection and/or identification of personnel or objects along the line of tbe fixed beam.

Such an arrangement might be of use for scanning a group of objects (e.g. gas bottles) for the selection of specific objects (e.g. bottles containing a particular gas) of the group.

Alternatively, the radar transmitterlreceiver may be fixed and provided with a fixed radar beam. Such arrangements may, for example, be used for interrogating personnel or vehicles approaching a security site entrance or crossing a secure fence.

As will be fully appreciated the present invention may have many other applications including search and/or rescue applications for locating and identifying missing personnel on land or at sea.

Whatever form the embodiments of the invention take bowever, the invention provides the advantage of reduced power consumption due to the fact that the transponder return signal transmitter only needs to be energised from the transponder power supply for the duration of a radar pulse at the duty cycle of the radar (typically 2%). This feature of reduced power consumption by the transponder enables high-gain amplifiers (e.g. 5mA each) to be employed in the transponder return signal transmitter which, in combination with the high frequency of the radar system enables both receive and transmit antennas of the transponder to be very small (e.g. dipole antennas of about 1 inch long).These characteristics of the transponder in the radar system of the present invention allow the entire transponder to be mounted on a small substrate about the sizt of a credit card. The battery power source 17 may comprise a 100mA battery which would keep the transponder operating for many weeks.

Claims

CI AT'4 S

1. A radar system for detecting and obtaining information relating to personnel and or objects at or within a predeiermined range of a particular location or .within a predetermined area under sur eillancc, comprising a radar pulse transmitter and receiver for the remote interrogation of personnel and or objects and one or more bansponders which are adapted to be carried by or attached to authorised or other personnel or objects required to be identified by the radar pulse receiver and each of which includes a radar pulse receive antenna, a radar pulse detector, and a high frequency highgain return signal transmitter including transmit antenna, said high-gain transmitter only being connected to power supply means for the transponder for the duration of a received and detected radar pulse to facilitate the transmission to the radar receiver of a high frequency radar return signal which forms part of a coded signal for identifying one or said personnel or objects.

2. A radar system as claimed in 'claim 1, in which each transponder comprises small dipole receive and transmit antennas mounted at the respective ends of a small substrate of about creditcard size which also carries the entire circuitry of the transponder.

3. A radar system as claimed in claim 1 or claim 2, in which the transponder comprises a continuoosly-powered frequency synffiesiser for producing under the control of frequency shifting means relatively low frequency signals of different frequencies which are mixed by mixer means with successive received and detected radar pulses to provide a frequency coded output which is amplified by high-gain amplifier means of the return signal transmitter and transmitted to the radar receiver for identifying one of said personnel or objects.

4. A radar system as claimed in claim I or claim 2, in which the transponder comprises a continuously-powered relatively low fixed frequency oscillator producing fixed frequency signals which are mixed with sclected received and detected radar pulses to define a binary-coded signal to identify one of the personnel or objects for transmission to the radar receiver after amplification by higb-gain amplifier means of the return signal transmitter.

5. A radar system as claimed in any preceding claim, in which the radar transmitter produces a rotating scanning beam and in which complete coded return signals for identifying personnel or objects are transmitted to the radar receiver during a single scan cycle.

6. A radar system as claimed in any of claims 1 to 4, in which the radar transmitter produces a fixed beam.

7. A radar system as claimed in any preceding claim, in which the radar transmitter and receiver define a portable (e.g. hand-held) equipment.

8. A radar system as claimed in any of claims 1 to 7, in which the transponder detector comprises a threshold circuit which produces an output signal when a radar pulse is received and detected, this output signal being applied to a solid-state switch which responds to connect power to the high-gain amplifying means of the transponder return signal transmitter.

9. A ndar system as claimed in any preceding claim, in which the input to the detector of the transponder comprises a solid-state switch (e.g. field-effect transister) which initially routes a received radar pulse to the detector circuit which includes a signal threshold circuit and in which the solid-state switch operates in response to an output from the threshold circuit to route the received radar pulse to high-gain amplifier means of the return signal transmitter.

10. A radar system as claimed in any or claims 1 to 8, in which a passive splitter device is connected in the input to the detector for simultaneously routing parts of the receive radar pulse to the detector and to high-gain unplifying means of the return signal transmitter respectively.

11. A radar system substantially as hereinbefore described with reference to the single-figure accompanying drawing.