Electronic Surveillance System

Abstract

A passive label is interrogated by transmitting electromagnetic energy to the label and receiving electromagnetic energy from the label. Time delay means are provided in the label, preferably by utilization of surface acoustic waves, so that the returned energy is transmitted from the label after the interrogation energy has ceased. The label includes a substrate of piezo-electric material having coded information thereon, and serving to receive electromagnetic energy, convert it to acoustic energy, store the converted energy for a suitable time, reconvert the stored energy to electromagnetic energy and to transmit the electromagnetic energy to the receiver.

Description

BACKGROUND OF THE INVENTION

The basic principle of operation of any interrogating system for passive labels, is as follows: Energy in some form is transmitted to the label by a transmitter and transmitting antenna unit. This energy is then processed in some way by the label, and the resulting energy retransmitted by the label as a "reply" signal. This "reply" energy is then detected, suitably processed and information extracted therefrom by a sensitive receiver and receiving antenna unit. It is basic to all interrogation systems that the very small reply energy from the label be distinguished from the very much larger transmitter or "interrogation" energy. This distinction can be obtained by various methods.

SUMMARY OF THE INVENTION

The present invention utilizes a method which achieves the desired result, incorporating time delay in the label preferably by utilization of surface acoustic waves so that the reply energy is transmitted after the interrogation energy has died away.

The facilities offered by the present invention provide for the open or secret interrogation by radio waves of coded information from prepared passive labels by a remote sensing apparatus. Some of the many applications are: a. Automatic sorting of passengers' luggage in airline terminals. b. Sorting and routing of letters and parcels in postal services. c. Identification, accreditation and location of personnel in security installations, factories, hospitals or military theatres. d. Ticketing of passengers in transportation systems. e. Prevention of theft of merchandise from shops or warehouses, of books in libraries or of appropriate items in factories or other places, by tagging such items with a label and locating a receiver covering each exit, so that the unauthorized passage of such tagged items through each exit will be detected.

A system according to the invention may be set up to provide the following features:

A. The system returns several, or even many, binary digits ("bits") of information to the interrogator. A social security number for example requires 30 bits of information.

B. The labels containing the coded information are passive, with indefinitely long storage life, can be read non-destructively, are durable under various environmental and handling conditions, are small and have low manufacturing cost.

C. The labels can have any orientation relative to and considerable distance from the sensing apparatus, can be in motion, and can be separated from the sensor by optically opaque barriers.

D. The coded signal is distinguishable from background clutter signals accidently produced by the environment of the label being interrogated.

This distinction from clutter signals is made by the incorporated time delay and, where necessary, by pulsetime coding of the reply signal.

e. The encoding of the information on the label can be performed by simple means at the time the label is put into service. Users of the system need stock only blank labels rather than a complete set of labels with all possible codes.

To better illustrate the principles involved, there is described below one possible design or embodiment for a particular system, namely a system for the encoding of 10 bits of information in a plastic card 5 cm .times. 8 cm .times. 1 mm, the card to be sensed from a distance of 3 meters. The card may have any orientation and can be moving at a speed of up to 1 meter per sec. in any direction, as may be required for example in a baggage sorting operation.

The general principle of a system according to this form of the invention is to provide in the label card a means of receiving electromagnetic energy, converting it to acoustic form, storing it for a suitable time, reconverting it to electromagnetic energy for retransmission in a coded form which then contains the information encoded in the label.

A further form of the invention is also described in which a carrier frequency of 10 MHz is used.

BRIEF DESCRIPTION OF THE DRAWINGS:

In order to assist in an understanding of the system it is described with reference to the accompanying drawings in which:

FIG. 1 is a block diagram of the system as a whole,

FIG. 2 is a similar diagram of the transmitter unit,

FIG. 3 is a curve showing the passband of the output filter,

FIG. 4 is an isometric view of a label for use with the system,

FIG. 5 is a view of a portion of the label to an enlarged scale,

FIG. 6 shows details of one of the array elements of the label,

FIG. 7 is a diagram showing the sequence of pulses arriving at the receiver,

FIG. 8 is a block diagram of the receiver,

FIG. 9 is a block diagram of the signal processor, and

FIGS. 10 and 11 show modified forms of array structures for labels intended for simplified applications,

FIG. 12 is a block diagram of the system as a whole,

FIG. 13 is a diagram of the antennas used in the transmitter and receiver units,

FIG. 14 is a circuit diagram of the master oscillator for the transmitter unit (and also of the local oscillator for the receiver unit),

FIG. 15 is a circuit diagram of a gated amplifier used in the transmitter unit (and also of a gated amplifier used in the receiver unit),

FIG. 16 is a circuit diagram of one of two low power amplifiers used in the transmitter unit,

FIG. 17 is a circuit diagram of a dynamic range expanding and power level setting unit used in the transmitter,

FIG. 18 is a circuit diagram of a medium power amplifier used in the transmitter unit,

FIG. 19 is a circuit diagram of the transmitter output amplifier,

FIG. 20 is a diagram of the coded label,

FIG. 21 is a diagram of the surface acoustic delay line,

FIG. 22 is a circuit diagram of the receiver preamplifier,

FIG. 23 is a circuit diagram of a gated rf amplifier used in the receiver,

FIG. 24 is a circuit diagram of a balanced mixer and balance to unbalance amplifier used in the receiver,

FIG. 25 is a circuit diagram of a narrow band amplifier in the output of the receiver.

The basic components of the system are shown in block diagram in FIG. 1. The system contains a transmitter of electromagnetic waves 1, an information carrying label 2, a receiver of electromagnetic waves 3, all of which are operated simultaneously. There is also a customer encoding device 4, which is used to encode the desired information on to previously blank stock labels prior to their use in the system.

Further details of the transmitter unit appear in FIG. 2. The transmitter employs standard UHF and microwave technology. The principle components and specifications are:

a. A master oscillator 5 operating at (in this example) 897.5 MHZ, with a main output 6 and reference output 7 as shown.

b. A low frequency pulse oscillator 8 producing rectangular pulses of duration 100 nanoseconds, rise time 5 nanoseconds, pulse repetition frequency 10 KHZ, provided with a main output 9 and a reference output 10, as shown.

c. A pulsed power amplifier 11, with center frequency 915 MHZ, band width 50 MHZ, peak output power 100 watts, pulse length 100 nanoseconds, pulse repetition frequency 10 KHZ, and on/off ratio in excess of 150 decibels.

d. An output filter 12, with a passband shown in FIG. 3, to restrict the frequency components of the output radiation to those allowed by the statutory authority. The position of the carrier in relation to the passband of FIG. 3 has been chosen to provide vestigial sideband modulation.

e. A microwave antenna system 13, which illuminates the area containing the information label to be read. An antenna gain of 6 decibels is chosen in this design. Higher figures can be used to advantage and without difficulty.

f. Under some circumstances it is useful to employ microwave adsorbing materials 14, in the main lobe of the transmitter antenna to avoid electromagnetic echo signals from distant objects.

The construction of a suitable information carrying passive label is shown in the isometric drawing FIG. 4. The outer section is in the form of a plastic or cardboard card 15, which serves as a protection for the inner sensitive elements 16. Printed or punched information 17 can be included on the card if this is convenient for other purposes. The part of the card which interacts with the sensing system is a microwave antenna system 18, one form of which might be a lumped loaded loop for omni directional response as shown. This antenna is connected via a transmission line 19, to the part of the card on which the information is encoded. This latter element is shown in more detail in FIG. 5.

The coding portion of the card consists first of a piezoelectric substrate 20, in this example a plate of single crystal quartz is used. Other materials can be used providing that they singly or in combination provided a high piezoelectrics co-efficient transducer region and a low acoustic loss propogation region.

The information is encoded on the substrate in the form of the spatial pattern formed by the conducting electrode array deposited on the substrate surface. Details of the space pattern appropriate to the ten bit binary code 1101001111 are shown in FIG. 5, and the details of one of the array elements are shown in FIG. 6. The array contains an end element 22, consisting of 26 electrodes and a set of coding elements 23, consisting of 16 electrodes. Alternate electrodes are connected to different conductors of the transmission line 19, from the microwave antenna 18. The spacing of the electrodes in this example is approximately 2.mu.m, the precise distance is adjusted to be one-half of a wave length of a surface electroacoustic wave at the operating center frequency of 915 MHZ.

The precise manner in which the desired code is carried by the array is that the connection or disconnection of an array element 23, at a given point on the main transmission line 19, signifies respectively a one or a zero binary digit. In practice all cards are manufactured with a full sequency of ones by having all array elements present. The required code is impressed on the card by the user by severing the connections of an appropriate number of array elements from the main transmission line. Labels may, however, be coded during manufacture by omitting the electrode structure from one or more elements or by not connecting them to the transmission line.

In operation, the card receives the pulsed electromagnetic energy via its antenna 18, and energizes the entire array along the transmission line 19. The various elements of the array launch surface electroacoustic waves along the piezoelectric substrate in the direction of the transmission line. After a time equal to the propogation time for such waves along the blank portion 24, of the transmission line, the electroacoustic waves are reconverted to electromagnetic energy and reradiate electromagnetic waves via the antenna 18. This reradiated energy is picked up and processed by the receiver 3.

A diagram of the sequence of pulses which arrive at the receiver is shown in FIG. 7. The sequence consists of a large amplitude pulse 25, arriving directly from the transmitter a series of unwanted interference pulses 26, resulting from propogation of electroacoustic pulses between various elements of the coding array, followed by the wanted set of pulses 27, which result from propogation of electroacoustic pulses between the end element 22, and the set of coding elements 23. It is this last group of pulses which are free of interference and contain the coded information, which are processed by the receiver in the manner described below:

The various components of the receiver 3, are shown in block diagram form in FIG. 8. The directional antenna 28 is similar in design to the transmitter antenna 13. A band pass filter 29 serves to reject possible radio frequency interference from sources unrelated to this system. A limiting device 30 protects the receiver from saturation or overload from the large amplitude transmitted pulse. A low noise (noise figure less than 6 db) pre-amplifier 31, and post-amplifier fitted with automatic gain control 32, provide an amplified received pulse sequence to the signal processing unit 33. Details of the design of the signal processing unit capable of providing for maximum sensitivity, using the technique of synchronous detection, are given below.

A block diagram of the signal processor appears in FIG. 9. The amplified pulse sequence from the receiver enters at 34, is divided into two signals, fed via buffer amplifiers 35, to the synchronous detectors 36 and 37. The reference drive for detector 36 is obtained from the transmitter master oscillator signal which enters at 7. The reference drive for detector 37, is derived from the same reference via the .pi./2 phase shift network 38. As a result the detectors 36 and 37 perform respectively in phase and quadrature phase detection of the received signal with respect to the transmitter master oscillator. The detected signals are fed to buffer amplifiers 39, from each of which 10 outputs, in the present example, are available. Each of the 10 outputs from these buffer amplifiers is then fed to one of 20 gating circuits 40, only two of which are shown. These gates are controlled by a count down circuit 41, which is synchronized with the transmitted pulse via a signal brought from the transmitter through the reference line 10. The count down circuit has ten output pulses each with a width equal to the transmitter pulse, 100 nsec in this example. The various output pulses have different time delays from the transmitted pulse, each adjusted to the delay expected from one or another of the pulses in the received pulse train 27. The outputs of the various gates 40 are filtered in low pass filters 42 which set the effective noise band width of the system. The outputs of these filters are fed through buffer amplifiers (not shown) to the square law devices 43, which produce a (unidirectional) output proportional to the square of the input signal over the designed operating range. The design of such a unit presents only a simple problem requiring for solution an operational amplifier and a semiconductor diode network. As a final step in the signal processor the outputs of corresponding pairs of square law devices 43, are added and fed to the set of ten output terminals 44, (only one shown) which provide the 10 bits of information. A reference signal from each of these bits is returned to the receiver via line 45, to provide automatic gain control. The presence of an automatic gain control signal requires at least one non zero bit in the coded sequence. Inclusion of odd parity check in the code ensures the presence of this required bit. The inclusion of this check bit provides an additional safeguard against false triggering of the system by spurious objects.

The outputs of the above described signal processor can be fed to a wide range of logic circuits, not shown in FIG. 9, to perform the various command identification and sorting tasks required of the overall system. The design of such logic circuits follows well established procedures.

Calculations show that the power losses occurring in various parts of the overall transmission path from transmitter to receiver are:

a. Electromagnetic propogation loss from transmitter antenna to label antenna: 33 db.

b. Electromagnetic to electroacoustic conversion loss: 38 db.

c. Electroacoustic propogation loss: 2db.

d. Electroacoustic to electromagnetic conversion loss: 31 db.

e. Electromagnetic propogation loss from label antenna to receiver antenna: 33 db.

The overall transmission path loss is 137 db.

The noise band width of the receiver is determined by the low pass filter 42, which follows the synchronous demodulation and is set to 1 KHZ. The input noise level of the receiver, allowing for 6 db. noise figure and 1 db loss in the band pass filter 29, is -167 db W. The input signal level at the receiver is -107 db W. The signal to noise ratio at the receiver is thus 60 db and the system is not receiver noise limited.

The system depends for its success on distinguishing the acoustically delayed echos from background clutter produced by direct electromagnetic echo. Since the acoustic time delay before retransmission of the coded pulse sequence is, in this example, in excess of 3 microseconds, the relevant electromagnetic echos will be via propogation paths of lengths in excess of 900 meters, and will in most circumstances be suitably small. Problems which may arise can be eliminated by proper use of the antenna patterns of the transmitter and receiver, in conjunction with suitably placed natural or artificial microwave adsorbers. Calculations have shown that direct echo can be reduced well below the acoustic echo level if the resultant enclosure has a Q factor of less than 100, and the system is then not limited by background clutter.

There are certain obvious variations from the design example described in detail which may be made to suit particular applications. In particular some of them are:

a. Change of carrier frequency from 897.5 MHZ. The dimensions of the electroacoustic conversion array may be changed to lower or higher values as required by the technology to be employed in their manufacture.

b. Pulse length and pulse repetition rate may be varied to make a longer or more compact code possible.

c. Changes may be made in transmitter power level and the characteristics of transmitter, receiver and label antennas, including use of duplexing, to provide various microwave propogation systems.

d. A range of substrate materials can be used for the acoustic propogation, including piezoelectric materials whose acoustic loss is not necessarily low, deposited on low acoustic loss substrates. Magnetostriction devices may also be used in place of or in conjunction with the piezoelectric materials to accomplish the electroacoustic conversion.

e. The transducer array structure can be modified in number and shape of elements, and in the manner of its interconnection to the transmission line.

f. Coding methods other than the simple binary, such as pulse height, width or position, can be used.

g. The disposition of the various elements in the card, and the size shape and nature of the card can be varied to suit particular applications. In particular it may be advantageous to employ the edge rather than the surface of the acoustic substrate for the propogation of the acoustic waves.

h. The code and array structure can be simplified to fewer, or even one, element for simplified applications such as object surveillance as described in application (e) of section 1. Use is made of a surface wave reflector, or a rat race propogation path, in this case. Two such possible simplified structures are shown diagrammatically in FIG. 10 and FIG. 11. In the structure of FIG. 10, use is made of a surface wave reflector 46, which returns the acoustic pulse to the single acoustic-wave launching and receiving array 47. In the structure of FIG. 11, the surface wave is constrained by suitable groves 48, etched in the surface of the quartz substrate to propogate around a circular, or "rat-race", propogation path so as to again return to the single acoustic-wave launching and receiving array after a suitable time delay.

i. As an alternative to using a pulse code in simple surveillance applications it is in fact sufficient to couple a resonant acoustic structure of sufficiently high Q such that it will continue to ring after the termination of the transmitter pulse. If the ringing time be long enough the resulting echo is easily distinguished from background clutter caused by direct electromagnetic echo.

To provide further assistance in understanding the system there is described below with reference to FIGS. 12 to 21, a version which operates at a carrier frequency of 10 MHz and provides for a return signal carrying five bits of information.

The basic components of this realization of the system are shown in FIG. 12. The system is controlled by a pulse repetition frequency generator 50, (of which no schematic is given because it is a commercial instrument), which sends pulses at a 50 KPPS rate to the pulse width generator 51 (another commercial instrument); the output pulses being 0.5 .mu. sec. long. The gated amplifier 52, shown in FIG. 15 is controlled by the pulse width generator, and feeds 0.5 .mu. sec. pulses of radio frequency energy which have been generated by master oscillator 53, shown in FIG. 14, to the series of low power amplifiers 54 and 55 shown in FIG. 16. The amplified gated radio-frequency pulses are expanded in dynamic range by range expander and level setting circuits 56, shown in FIG. 17, and, after further amplification in medium power amplifier 57 shown in FIG. 18 and output amplifier 58, shown in FIG. 19 are fed to the transmitter antenna 59. The transmitter antenna is in the form of a shielded square magnetic dipole of scale 12 inches of a form of construction well known for aircraft direction finder loops and is loaded to a Q factor of 5. Details of the transmitter antenna are shown in FIG. 13, and schematic diagrams of many of the transmitter circuits are provided in FIGS. 14 to 19 inclusive.

The signal from the transmitter antenna travels by nearfield electromagnetic propogation to the coded label 60, the detailed construction of which is shown in FIGS. 20 and 21. The figures show a label suitable for a reply signal returning the particular five-bit code 11111. FIG. 20 shows the label used, consisting of a printed-circuit magnetic loop antenna 71 on a 6 inches .times. 4 inches epoxy-glass card 72 tuned to resonate at 10 Mc/s by a fixed capacitor 73 and loaded by a fixed resistor 74 to a Q factor of 5. The coding portion of the label is again a quartz substrate 5 cm .times. 2.5 cm .times. 2 cm thickness, carrying the conducting electrode pattern 75 shown in FIG. 21, which returns the five bit binary code 11111, connected to the antenna 71 as shown in FIG. 20.

The electrode pattern 75 is constructed and operates in a manner similar to that described in connection with the first embodiment of the invention. It will be noted however that the dimensions are quite different due to the use of a lower carrier frequency. In the electrode pattern 75 the spacing between individual electrodes of the pattern is 0.1625 mm. After a delay of approximately 6 .mu. sec. following the completion of the transmitter pulse, the coded reply signal is retransmitted by the label and a portion of the reply energy is received by the receiver antenna 61. The output signal for the receiver antenna is initially amplified by low-noise receiver amplifier 62, shown in FIG. 22, which has been specially designed to provide rapid recovery from overload, and is then passed to gated amplifier 63, shown in FIG. 23.

The gated amplifier 63 is one of two gated amplifiers 63 and 64 which are controlled in the receiver from the pulse repetition frequency generator 50 via delay generator 65 and pulse width generator 66. The function of delay generator 65 and pulse width generator 66 is to open the receiver gate at a period of time so delayed with respect to the time of the transmitter pulse as to correspond to the detection of a particular bit in the reply code. Varying the amount of delay provided by delay generator 65 allows various bits in the reply coded to be detected separately.

The further operation of the receiver is concerned with the balanced mixer 67, shown in FIG. 24, which receives the gated signals both from the low noise amplifier 62 and a highly stable local oscillator 68 and produces at its output the difference frequency resulting from the mixing of the two signals. This frequency is equal to the difference in frequency between master oscillator 53 and local oscillator 68. It is important to the operation of the overall system that master oscillator 53 and local oscillator 68 should have a closely controlled frequency to maintain the difference frequency within the passband of the narrow band tuned amplifier 68a, shown in FIG. 25. This difference frequency must be suitably chosen and must lie sufficiently about zero frequency (that is DC) to avoid 1/f or flicker noise, but below the pulse repetition frequency generated by pulse repetition frequency generator 50, such that no mixing products of the gate transients of gates 63 and 64 will contaminate or add noise to the system output. The output from tuned amplifier 68a is fed to a non-linear detector 69 which measures the magnitude of the difference frequency signal from tuned amplifier 68a and registers a "1" as being received from label 60 for the particular bit position then under examination if this signal suitably exceeds the system noise level.

Claims

We claim:

1. An electronic surveillance system, comprising transmitter means for transmitting electromagnetic signals, label means adapted for attachment to an article under surveillance for receiving a signal from said transmitter means and for retransmitting a reply, and receiver means for receiving and processing the reply; said label means including signal propagating means responsive to the signal from the transmitter for propagating the transmitted signal along a path at a rate slower than electromagnetic propogation, a plurality of sensing means mounted at coded locations along the path of said propagating means each for sensing the presence of a propagated signal at the location so that said sensing means together sense each signal sequentially in a given coded time order, an energy carrier means coupled to each of said sensing means for retransmitting the sequence of signals in the order they are sensed by said sensing means as the reply, said carrier means forming a signal path between said sensing means so that signals can move faster than the propagation rate of said propagating means.

2. A system as in claim 1, wherein said carrier means includes a conductive medium.

3. A system as in claim 1, wherein said sensing means each includes conductive transducing means coupled to said propagating means.

4. An apparatus as in claim 3, wherein said carrier means includes a conductive medium.

5. An apparatus as in claim 1, wherein said propagating medium comprises a piezoelectric material.

6. A system as in claim 1, wherein said propagating means forms a delay line on part of the path free of sensing means, a plurality of said sensing means being located beyond the delay line along the path.

7. A system as in claim 6, wherein said sensing means located beyond the delay line are bunched within a distance less than the length of the line, said path beyond the distance being substantially free of sensing means.

8. A system as in claim 7, wherein one of said sensing means is located at the beginning of the delay line opposite the other of said sensing means.

9. A system as in claim 6, wherein one of said sensing means is located at the beginning of the delay line opposite the others of said sensing means.

10. An electronic surveillance system comprising transmitter means for transmitting electromagnetic signals, label means attachable to an article under surveillance for receiving an interrogation from the transmitter and for retransmitting a reply, and receiver means for receiving and processing the reply; said label means including acoustic signal propagating means responsive to the interrogation from the transmitter for acoustically propagating signals corresponding to the interrogation along a path, a plurality of energy return means mounted on said propogating means at coded locations for removing a portion of the energy of each signal as it passes the location and making the removed portion of the signals available for retransmission to said receiver means at the reply, said energy return means including a first plurality of interconnected parallel transducers mounted on said propagating means and a second plurality of interconnected parallel transducers interleaved between said first plurality of transducers, said transducers being spaced so as to correspond with the wave length of the acoustic signals at the frequency of excitation of the signals.

11. A system as in claim 10, wherein said propagating means includes a piezoelectric crystal.

12. A system as in claim 11, wherein said energy return means are mounted on the surface of the crystal and the energy is propagated along that surface of the crystal.

13. An apparatus for responding to an electromagnetic interrogation and producing an electronic reply, comprising signal propagating means responsive to the interrogation for propagating a signal corresponding to the interrogation along a path, a plurality of sensing means respectively mounted at coded locations along the path of said propagating means each for sensing the presence of a propagated signal at the location so that said sensing means together sense each propagated signal sequentially in a given coded time order, and energy carrier means coupled to each of said sensing means for forming the reply from the sequence of signals in the order that they are formed by said sensing means, said carrier means forming a signal line between said sensing means along which the signals can move substantially faster than the propagation rate of said propagation means.

14. An apparatus as in claim 13, wherein said carrier means includes a conductive medium.

15. An apparatus as in claim 13, further comprising antenna means for responding to the interrogation and applying it to said propagating means, said antenna means being connected to said carrier means for retransmitting the sequence of signals formed as the reply.

16. An apparatus as in claim 15, wherein said propagating medium comprises a piezoelectric material, said piezoelectric material being connected to said antenna means.

17. An apparatus as in claim 13, wherein said sensing means each includes conductive transducing means coupled to said propagating means.

18. An apparatus as in claim 13, wherein said propagating medium comprises a piezoelectric material.

19. An apparatus as in claim 13, wherein part of said propagating means forms a delay line free of sensing means, a plurality of said sensing means being located beyond the delay line.

20. An apparatus as in claim 19, wherein said sensing means located beyond the delay line are bunched within a distance less than the length of the line, said path beyond the distance being substantially free of sensing means.

21. An apparatus as in claim 20, wherein one of said sensing means is located at the beginning of the delay line opposite the other of said sensing means.

22. An apparatus as in claim 19, wherein one of said sensing means is located at the beginning of the delay line opposite the others of said sensing means.

23. An apparatus as in claim 13, further comprising antenna means responsive to the interrogation and coupled to said propogating means, said antenna means being connected to said sensing means for transmitting the reply electromagnetically.

24. An apparatus as in claim 23, wherein said propagating means includes a piezoelectric crystal coupled to said antenna means so as to propagate the signals acoustically, said sensing means each including a first plurality of interconnected electrodes mounted on the surface of said crystal parallel to each other and transverse to the direction of propagation of the signals and a second plurality of interconnected electrodes parallel to each other and extending transverse to the direction of propagation of the signals and interleaved between said first plurality of said electrodes, said crystal being tuned to propagate signals at a predetermined expected received frequency, said electrodes being spaced from each other at one half the wavelength of the predetermined frequency, said carrier means being mounted on said crystal and connecting said sensing means.

25. An apparatus as in claim 13, wherein each of said sensing means are removable from the locations for changing the coding of the reply.

26. An apparatus for responding to an electromagnetic interrogation and producing an electronic reply, comprising acoustic signal propagating means responsive to the interrogation for propagating signals corresponding to the interrogation acoustically along a path, a plurality of energy return means mounted on said propagating means at coded locations for removing a portion of the energy from each signal as it passes the location and for making the removed portion of the signals available for retransmission as the reply, said energy return means including a plurality of interconnected parallel transducers mounted on said propagating means and a second plurality of interconnected parallel transducers interleaved between said first transducers.

27. An apparatus as in claim 26, wherein said propagating means is adapted to respond to signals of a given frequency, said transducers being spaced from each other one half wavelength of the frequency.