GB2259227A

GB2259227A - Improvements in or relating to transponders

Info

Publication number: GB2259227A
Application number: GB9118675A
Authority: GB
Inventors: Christopher Ralph Pescod; David John Gunton; Stanley Frederick Clarke; Robert Charles Hunt
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1991-08-30
Filing date: 1991-08-30
Publication date: 1993-03-03
Anticipated expiration: 2011-08-30
Also published as: GB9118675D0; GB2259227B

Abstract

An identification system comprises a transmitter, a receiver and a plurality of tags, each with its own transponder. The transmitter transmits an interrogating signal which is received by the transponders and used to initiate an on-tag time delay. At the end of its respective time delay each transponder transmits a digital reply code to the receiver, the system then using these transponder replies appearing in the receiver to identify, following only one interrogation by the transmitter, each tag on the basis of its own specific combination of time delay and digital code.

Description

IMPROVEMENTS IN OR RELATING TO TRANSPONDERS This invention relates to transponders, and more particularly transponders which are equipped with their own identifying signature and which may be used in groups within the context of an identification system.

Identification systems using transponders are known, in which a transmitter transmits an interrogating signal to a transponder, which in turn transmits its own reply signal in the form of a unique code to a receiver. Facilities exist in the receiver for identifying the transponder, and hence the object or person to which it is attached as a "tag", on the basis of its own unique code. Such systems are known from, for example, GB-A-2,077,556 and GB-A-2,203,313.

A disadvantage with the known identification systems is that they are unable to identify a number of tags following only one interrogation signal, the usual procedure being to transmit one, or even more than one, interrogation signal per tag to be detected. There are many instances where identification of a large number of objects or personnel within a short period of time is desirable, one of the most critical being the monitoring, in an emergency, of the mass exodus of personnel through fire exits, a procedure which has proved to be very difficult using the known techniques.

The invention seeks to overcome the disadvantages presented by the known systems, and describes in particular a multiple-access identification scheme versatile enough to identify individuals and able, if implemented in the required configuration; to determine the whereabouts of those individuals.

According to the invention there is provided a plurality of tags, for identifying objects with which said tags are associated, each tag including: a first receiving means, for receiving and responding to an interrogation signal; a time delay means, for providing a time delay, said delay being initiated by response of said first receiving means to said interrogation signal; and a first transmitting means, for transmitting an output signal upon termination of said time delay; wherein each tag has an identifying signature based on said time delay.

The identifying signature may be unique to each tag.

The first transmitting means may be arranged to transmit the output signal in the form of an output digital code, which code may be either the same for all of the tags or different for at least some of the tags.

The first receiving means may be arranged to respond to at least one of a plurality of interrogation signals, each interrogation signal being in the form of an interrogating digital code.

The first receiving means may include a storage means, for storing at least one reference digital code, and a comparison means, said response of said first receiving means to said at least one of a plurality of interrogation signals being by comparison within said comparison means of said interrogating digital codes with said at least one reference digital code.

Any or all of the time delay, the reference digital code and the output digital code may be variable and set by means of software control.

The objects to be identified may be personnel.

According to another aspect of the invention an identification system is provided, comprising: a plurality of tags, for identifying objects with which said tags are associated, each tag including: a first receiving means, for receiving and responding to an interrogation signal; a time delay means, for providing a time delay, said delay being initiated by response of said first receiving means to said interrogation signal; and a first transmitting means, for transmitting an output signal upon termination of said time delay; wherein each tag has an identifying signature based on said time delay; said identification system further comprising: a second transmitting means, for transmitting said interrogation signal, and a second receiving means, for receiving said output signal; said system being arranged to identify each tag, on the basis of said identifying signature, following only a single interrogation of said plurality of tags by said second transmitting means.

The identifying signature may be based also on said output signal.

According to yet another aspect of the invention an identification system is provided, comprising: a plurality of tags, for identifying objects with which said tags are associated, each tag including: a first receiving means, for receiving and responding to an interrogation signal; a time delay means, for providing a time delay, said delay being initiated by response of said first receiving means to said interrogation signal; and a first transmitting means, for transmitting an output signal upon termination of said time delay; wherein each tag has an identifying signature based on said output signal; said identification system further comprising: a second transmitting means, for transmitting said interrogation signal, and a second receiving means, for receiving said output signal; said system being arranged to identify each tag, on the basis of said identifying signature, following only a single interrogation of said plurality of tags by said second transmitting means.

The identifying signature may be unique to each tag.

The second transmitting means may be arranged to illuminate a given space by means of said interrogation signal, such that only those tags found within said space are identified by said system.

The first transmitting means may be arranged to transmit the output signal in the form of an output digital code.

The second transmitting means may be arranged to transmit the interrogating signal in the form of a digital interrogating code.

The second transmitting means may be arranged to transmit a plurality of different interrogating codes, said first receiving means including means for responding to at least one of said plurality of different interrogating codes.

The identification system may be arranged so that a number of spaces are illuminated, each space by its own transmitting means, response of a tag in any one illuminated space being determined by the transmission within that space of an interrogating code to which said tag is arranged to respond.

Each tag may respond uniquely to its own interrogating code, in which case the identification system may also function as a paging system.

The second receiving means may include correlating means for distinguishing between each different output digital code.

The second transmitting means may transmit a periodic interrogation signal.

At least one guard period may be inserted into the period between interrogations, said guard period serving to allow the transmission of non-interrogating data from said second transmitting means to said tags.

The identification system may be adapted to provide information on the location and/or direction of travel of said tags.

At least one illuminated space may be divided into two overlapping zones, each zone being illuminated by its own second transmitting means to receive different but related interrogation signals, said first receiving means being adapted to respond to said related signals by modifying said output signal.

Said modification of said output signal may take the form of an inversion of said output digital code.

The plurality of tags and the second transmitting means and second receiving means may be free to move with respect each other. Alternatively, the second transmitting means and second receiving means may be in a fixed station, each tag being attached to a movable object or person, or said plurality of tags may be substantially stationary, said second transmitting means being movable relative to said plurality of tags.

The objects to be identified may be personnel.

Applications of the invention include "audience" voting and automatic checking of people and/or objects in transit, e.g. entering or leaving a train, plane or bus.

Such a system might also be of benefit to parcel sorting offices, in which the diverse shape and size of the objects to be identified preclude the use of more conventional methods, such as bar-code reading.

Embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which: - Figure 1 shows a prior art interrogation system; Figure 2 is a timing diagram showing several interrogation access schemes in accordance with the invention; Figure 3 is an explanatory diagram of the TDMA-withoutput-code and CDMA/TDMA schemes in accordance with the invention; Figure 4 is a timing diagram showing a general hybrid CDMA/TDMA access scheme in accordance with the invention; Figure 5 shows a typical coverage scenario for an identification system in accordance with the invention; Figure 6 is an illustration of a location/motion direction detection system in accordance with the invention; and Figure 7 is a schematic diagram of an interrogator and a transponder for an identification system in accordance with the invention.

Figure la shows a typical prior art transponder, as used in conjunction with a prior art interrogator, shown in Figure lb. The transponder 10, which is mounted on a tag 20, itself attached to an object or person which is desired to be identified, receives a 132KHz interrogating signal at receiver 21 and this signal, after detection, amplification and division by 2 in detector 23, amplifier 24 and divider 25, respectively, is fed as a 66KHz re-transmission carrier 27 to modulator 28 at input 29.

A code memory register 32, in which an N-Bit identifying code has been programmed, has the output 33 of each of its stages taken to a data selector 34, the input lines of which are connected to the corresponding output lines of an index counter 36. The input 35 of index counter 36 is supplied by the re-transmission carrier 27, after division of that carrier by 384 in divider 38. The output of data selector 34 is fed to the modulating input 39 of modulator 28, and the output of modulator 28 is first amplified in amplifier 40, then transmitted as a modulated carrier 41 to the interrogator 50.

Thus in operation, assuming all counters and dividers to be initially reset, transponder 10 starts to be interrogated by interrogator 50, receiving a 132KHz interrogating signal at the input of receiver 21 and using a detected and divided version of this - the 66KHz re-transmission carrier - to form both the carrier input of modulator 28 and, after further division in divider 38, the clock input of index counter 36. Thus, at the start of interrogation, since the index counter 36 is reset, the data selector 34 will select bit 1 of the code memory register 32 and this will be fed via line 37 to modulator 28, thereby modulating re-transmission carrier 27 with bit 1 of the identifying code, the modulated carrier 41 then being transmitted.

After 2 x 384 cycles of the 132KHz interrogating carrier, index counter 36 receives another clock signal at input 35, causing the data selector 34 to select the next bit, bit 2, of the N-bit code in memory register 32. This bit is then taken via line 37 to the modulating input 39 of modulator 28, and as a consequence bit 2 of the identifying code is transmitted as a modulated carrier 41 to interrogator 50. This occurs every 768 cycles of the interrogating carrier, i.e. every 5.82mS, until all N bits have been selected by index counter 36.

Figure lb shows a prior art interrogator 50, in which a master oscillator 52, operating at 132KHz, transmits an interrogating signal of the same frequency, via transmitter 54, to a coded tag 20 incorporating the transponder 10. The coded output of the transponder, transmitted by the transponder at 41, is received by receiver 53 in the interrogator and, after amplification in amplifier 56 and decoding in phase locked loop decoder 57, is fed to a shift register 58.

When all N bits have been accessed in transponder memory register 32, the central timing unit 60 in interrogator 50 instructs a first N-bit latch 62 to accept the contents of shift register 58. A second interrogation of the transponder then takes place, during which shift register 58 accepts the N-bit code a second time, and upon transmission of all N bits by the transponder, the central timing unit 60 issues a further instruction to transfer the contents of latch 62 to a second N-bit latch 63 and then load the new contents of shift register 58 into latch 62. A comparator 66 compares the contents of these two latches and, upon perfect matching, sends a validation signal 67 to the central timing unit 60. Finally, the validated code is then displayed and sent to a bus interface for further processing.

Although this known system is capable of identifying any one of a number of transponders, based on an on-tag digital code, the total number being dependent on the length of the code itself, it is capable of identifying a plurality of transponders simultaneously, or within a short time period following only one interrogation of the transponders by the interrogating transmitter, this process being hereinafter referred to as "multiple access". In fact, the prior art system requires at least two interrogations to identify one transponder. It is this main drawback that the present invention seeks to overcome.

In the embodiments of the invention to be described fixed terminals ("interrogators") illuminate, on a given command, the areas in which identification, counting, etc is to be effected. Any transponders within the illuminated area will respond to the interrogation signal, resulting after processing - in a set of return signals in the interrogator. The timing of each signal and/or its code will be representative of the particular transponders interrogated, and in a preferred embodiment of the invention will be uniquely representative of each of these transponders.

Signals will be received for the duration that the transponder is within the illuminated area and use can be made of this redundancy to improve, e.g. by validation of response by means of multiple interrogations, the integrity of the system, thus reducing false alarms and the probability of missed detection.

The choice of the method and medium of transmission is not particularly critical, the main criterion being that the necessary bandwidth be able to be accommodated. This is determined by the maximum number of "objects" to be identified, passing through the illuminated area in a given minimum time. Consequently, in principle, r.f.

electromagnetic, optical or acoustic radiation could be used. In some circumstances magnetic coupling might also be involved.

In practice, multipath effects and mechanical, safety and regulatory considerations will play an important part in determining the most suitable mode of transmission for particular situations.

In a preferred embodiment of the invention the interrogator transmits the interrogation signal in the form of a digital code word every frame period, the duration of which period is determined by the application and is a function of the maximum rate of travel of the transponders through an illuminated area or, put another way, the minimum time spent by a transponder within that illuminated area.

This code word, often called a "unique word" (UW), has the property in a noisy environment of low probability of false alarm and missed detection, and is received by the object to be identified. In its simplest form this word could be a single pulse, but this is not preferred because of its lack of protection, particularly in a noisy environment. This unique word is correlated with a matching pattern in the transponder receiver, to provide a timing pulse for the subsequent transmission of the transponder's code word.

Figure 2 of the drawings illustrates the various multiple-access schemes which form embodiments of the invention, in which the frame period (time between interrogations) is assumed the same in each case. The bandwidth required for a given scheme depends on the number of elements transmitted in each frame period. Figure 2a shows a scheme whereby identification of each of a plurality of tags is performed purely by means of code words in the tags themselves. In this system the interrogator transmitter transmits a unique word, which, after detection in each interrogated transponder, gives rise to the transmission by each transponder of its own unique code word. Thus, in a system of N tags, N code words would be simultaneously received by the interrogator and thereby identified.

This scheme, termed "code division multiple access" (CDMA), requires that each code have the property of high auto-correlation and low cross-correlation, thereby providing a high probability of detection even when a number of such patterns overlap. Typical patterns which may be used include, but are not restricted to, Maximum Length Sequences and Gold Codes. The output pattern is transmitted back to the interrogator where, after reception, it is correlated with a set of binary code patterns in the interrogator receiver, one of which matches the output pattern. An output pulse is consequently produced in the interrogator, to indicate that matching has occurred.

A property of the CDMA approach is that the number of usable discrete codes is less than the total number of elements making up the code, which consequently means that, if a large number of tags is to be identified at the same time, a code of an even larger number of elements is required. The coding gain provided by the autocorrelation process does permit a reduction in the power required to transmit the information back to the interrogator. A further advantage is that, since identification is based solely on coding, slight uncertainties in the timing of the tag's coded reply back to the interrogator are insignificant.

This scheme requires in the interrogator receiver one correlation detector per different code used.

A second form of multiple access, "time division multiple access" (TDMA) is illustrated in Figure 2b. In this system each tag has its own time delay, unique to that tag. Here, when a tag has detected transmission of the unique word by the interrogator, it triggers a particular time delay, at the end of which an output signal is transmitted back to the interrogator. As shown in Figure 2b this may be a single element. Each tag transmits its output pulse in its own "time slot", one tag after the other. As with the CDMA system, the unique word may be repeated by the interrogating transmitter at intervals (the "frame period").

From the frame period may be derived, by reference to the total number of transponders required, the timing slot period for the TDMA system.

This form of access technique described above is simple in concept and realisation and has the advantage of requiring a narrow bandwidth than the CDMA case. It does, however, have the disadvantages of higher transmission power requirement in the transponder, and greater sensitivity to uncertainty of delays through the system.

A third form of multiple access, illustrated in Figure 2c, is TDMA with output code. This scheme is essentially the same as TDMA, except that the signal that is output from each tag now consists of a multiple-element code word. This code word is the same for all the tags and is autocorrelated, as in CDMA, in the interrogator receiver.

The uniqueness of each tag is - as with the simple TDMA scheme - determined purely from the timing. This combination requires more bandwidth than the simple TDMA scheme and is susceptible to timing uncertainties; however, it has the advantage of a reduced peak transmission power requirement in the transponder.

The complexity of realisation of the interrogator is not much higher than that of the TDMA scheme, only one correlator being required in the interrogator receiver.

The final multiple-access embodiment is the preferred embodiment involving a combination of both CDMA and TDMA (Figure 2d). This hybrid scheme differs from the previous one in that a selected number of transponder code words is used in combination with a selected number of delays, the uniqueness or "identifying signature" of each tag then being a function of both delay and coding. This preferred scheme increases the degree of flexibility of the identification system and can reduce the bandwidth requirement substantially, at the expense of only moderately increased receiver complexity in the interrogator. The delay sensitivity compared with simple TDMA or TDMA with output code is also much reduced.

Figure 3 gives a graphical representation of both the TDMA with output code scheme (Figure 3a) and the CDMA/TDMA scheme (Figure 3b).

In Figure 3a the number of different time delays is, purely for the sake of illustration, taken to be 8, whereas the number of different codes is 1 only. At time t=0 an interrogation pulse is received by each of the eight transponders, upon which the unique time delay in each tag is triggered. Tag 1 has, for simplicity of illustration, a delay of lmS, tag 2 a delay of 2mS, tag 3 a delay of 3mS, and so on. Upon expiry of its time delay each tag transmits its own code, in this case the common code, code 1. The frame period can be seen to equal, in time slots, the number of different delays plus the time required for the common code, i.e. in this example 8 + 3 = 11 slots.

Figure 3b shows the situation with the hybrid CDMA/TDMA scheme, and for the sake of illustration assumes a system involving 8 different time delays and 8 different output codes. In the general case there may be M different time delays and N different codes, the total number of tags which may be uniquely identified being M x N. In an extreme case M may equal 1, to give purely a CDMA system, or N may equal 1, giving a pure TDMA system.

Figure 3b can be seen to correspond largely with Figure 3a, the only difference being the use of more than one output code among the tags; specifically, in this case, N=8. At time t=0 the interrogator transmitter transmits an interrogation pulse. All tags within the zone or space illuminated by that pulse respond by triggering their own particular time delay, at the termination of which each tag transmits back to the interrogator its own particular code.

In the example of Figure 3b, 8 tags transmit digital codes 1 to 8 respectively at the end of the first time slot, another 8 tags transmit the same codes respectively at the end of the next time slot, and so on, these codes then being then received by the interrogator receiver and, in one embodiment of the receiver, compared in parallel with a bank of similar codes in the receiver, i.e. an autocorrelation process takes place. Those codes that find a match in the interrogator receiver may then pass through to a man/machine interface connected to the system, where identification of the tags concerned may be displayed, logged etc.

To illustrate the advantages of this hybrid CDMA/TDMA scheme over the others previously mentioned, a more realistic case may be taken in which the maximum number of transponders (M x N) is 5000. If we assume that a total (N) of 18 unique transponder output codes is employed, and that each code is a maximum length sequence of 127 elements, we can say that the minimum number of time slots required is, referring to Figure 3b, M + 127, i.e. 5000/18 + 127 = 405.

If we further assume that a number of unique interrogating words are transmitted by the interrogator each frame and that also a certain amount of "guard time" (see later) is to be allowed for each frame period, the total equivalent number of time slots required for these extra components being, say, 107, then the total number of time slots per frame will be 405 + 107 = 512. Contrasting this with the 5000 time slots required for a pure TDMA system in this particular example yields an approximately ten-fold reduction in bandwidth required, given that the frame period to be the same in both cases. There will also be an approximately 1270-fold saving in peak power, since there is a ten fold reduction in bandwidth and a coding gain of 127.

The frame period is effectively determined by the dimensions of the illuminated space (often known also as the "footprint") of the interrogating transmitter, and the maximum expected speed of a tag through that space.

Assuming the maximum speed of a tag to be 1 metre/sec., and the width of a particular illuminated area to be, say, 1 metre, the frame period for an ensured minimum of two replies within the illuminated area is 1/2 sec. i.e. two interrogations per second, and therefore for a total of 512 time slots in each frame the length of each time slot is 0.977ms, leading to an occupied bandwidth of approximately lkHz. In practice this bandwidth would probably have to be doubled, in order to increase the resolution between the time slots.

As already mentioned, addressing or interrogation of each tag may be by either a single pulse or, preferably, a unique word. However, if instead of the use of a single unique word in the basic system a selected number of address words is sent, interrogation of each transponder may proceed in an orderly fashion. With suitable codes a hierarchical organisational response can be obtained. Such an arrangement would permit interrogation of particular classes of transponders (representing, for example, managers, clerical workers, labourers, etc) as and when required.

This would have the feature of signalling the presence of It is also possible to effect hierarchical identification by arranging for tags in a certain class or subclass to output variations on a particular output code.

Thus, for example, Figure 4 shows tags with output codes 1 and 11 and 3 and 31, where 11 and 33 could represent, for example, inversion of codes 1 and 3, respectively. This would increase the maximum number of users possible in a system and would have the advantage or reducing the required system bandwidth at the expense of complicating, or at least increasing the demand on, the processing power of the interrogator receiver, by virtue of an increased correlation requirement.

In the scheme shown in Figure 4 a series of words is transmitted by the interrogator, which, following a "guard time" to be later explained, triggers a number of transponders recognising those words. At the first time slot a tag, in this case tag 1, can respond either to word A with output code 1, or to word B with output code 11. At the same time slot tag 3 responds to either word A or word B, with output code 2, and so on for the rest of the tags shown.

In practice an identification system according to the invention could well be required to monitor a number of zones simultaneously, in which case each zone would require its own transmitter. Figure 5 shows such an arrangement with four separate zones A-D, being monitored. Coverage A represents areas in which head counts might be required in an emergency (e.g. fire exits), and in which all transponders are expected to respond. Area B is shown to have larger coverage but the number of transponders expected to respond here is smaller, and further discrimination of authorised personnel is provided by arranging for the system to recognise only the transponder output code words 1, 11, 2 and 21. Likewise also C selects output code words 13 and 31, and so on.

It is also possible to have, within each zone, the addressing of more than one class of personnel, i.e. the arrangement of Figure 4 may be combined with that of Figure 5.

Further enhancements of the basic system are provided for by the invention, one of which involves the allocation of one unique address word to each transponder. Such an arrangement would permit a simple paging facility to be realised, or could even allow short messages to be built up in a tag over a number of frames, the length of the messages being determined largely by the size of the illuminated area and the mobility of the person within that area.

A second enhancement of the basic system is the use of so-called "guard times" within each frame. Two guard time periods are shown as an example in Figure 4, the first occurring between the end of interrogation transmission and the point in time at which the first transponder reply is expected, the second occurring between the end of reception of the final output code and the start of the next interrogation. It is to be understood, however, that the guard times may, in general terms, be distributed as desired throughout the frame. Inclusion of a guard time enables the transmission of packets of data to be made by the interrogator, which could take the form of digital packet speech.

A third enhancement enables direction of travel of a transponder within an area to be determined. The scheme employed by the invention is illustrated in Figure 6 and involves the splitting of an illuminating beam into two zones, A and B, each with its own transmitted signal, the zones being illuminated by different but synchronised interrogation signatures. Zone A receives the interrogating code word shown at line (a), zone B that shown at line (b). The transponder receiver registers a waveform whose exact shape is a function of the tag's position in the total illuminated area. If the tag is situated at the extreme left of zone A it will receive a waveform of essentially the same shape as waveform (a). If now it moves towards zone B it will receive a waveform consisting of the waveform at (a) plus a proportion of that at (b), i.e. something in the order of waveform (c).When the tag is central between the two zones the waveform will change to that shown at (d), whereas continuation into zone B above will accentuate receipt of waveform (b), resulting in the waveform shown at (e).

The transponder can be arranged to detect the difference between these signals and send back to the interrogator the corresponding information on position and direction of travel, deduced from the above waveforms. This can be done by suitable means, one method being to invert the transponder output code word. Simple logic in the interrogator receiver could then determine the position, direction of travel and speed of the transponder.

A typical basic system architecture is shown in Figure 7. In the interrogator 100 (Figure 7a) all basic timing information and clock generation is determined from oscillator 110, which feeds clock generator 112, which in turn synchronises all timing via the timing generator 111.

System controller 113 controls the unique word(s) to be sent at times determined by the timing generator 111.

Interrogating words from the unique word generator 115 subsequently modulate - preferably by amplitude shift-keying in ASK modulator 117 - a carrier oscillator 116 situated in the interrogator, the output of modulator 117 being then sent for transmission via a transmit/receive switching means 120.

Transponder 150, when illuminated, receives the transmitted interrogating signal via diplexer 152 and, after detecting and amplifying this signal in detector 153 and amplifier 154, respectively, correlates it in unique word detector 158. Unique word detector 158 effectively consists of a storage means for storing a code pattern to which the tag is to respond, and a comparator means for comparing that pattern with the incoming interrogation code pattern.When the comparator means has established a match between the two, a pulse is generated, which firstly switches the tag from a low-power-consumption standby state to an active state via line 157 and standby/active switch 159, and secondly initiates, in a time delay means, namely code word and delay generator 162, the generation of the time delay peculiar to that tag and - upon termination of that time delay - instructs the code word and delay generator 162 to output the output code peculiar to that tag. This output code is then used to modulate a carrier oscillator 165 via secondary modulator 164 at input 163, the resulting output signal being then transmitted back to interrogator 100 via transmitting means 152, which may take the form of a diplexer.

Interrogator 100 receives this output signal and directs it, via transmit/receive switch means 120, to a filtering, amplifying and frequency translating section consisting of filters 121 and 122, amplifiers 123 and 124 and mixer 126, respectively. After removal of any aliasing signal in filter 128 and analogue-to-digital conversion in A/D converter 129, the received signal is demodulated and correlated in processing means 130, and the result of the correlation is sent to system controller 113, which subsequently initiates the logging of entries and exits made by personnel, etc. This takes place in man/machine interface 127, which may take the form of a VDU, printer, etc. The system is also capable of logging the simple presence of objects or personnel within an area, in cases where these are not in motion.

The generation of the unique word, transponder output code and time delay can be software controlled by the use of electrically erasable programmable memories (EEPROM's), enabling the system to be changed to meet different requirements and applications.

One known method of transmitting the information between the fixed terminal and the transponder and back is to transmit a carrier essentially continuously between the periods of unique words. This enables the transponder to use this signal and to modulate it with the necessary information on the return path. The method is sometimes called passive generation and reduces the energy demands on the transponder. However, care has to be taken that upper and lower sidebands generated by the process do not destructively interfere. If sideband filtering is impractical, a means of reducing this problem is to use an image reject mixer in the receive path of the fixed terminal.

Of course the interrogator does not have to be in a permanently fixed station. In some circumstances it could be preferable to pass the interrogator over the 'objects' to be counted, one such instance being the identifying of parcels in a post office sorting room.

Claims

1. A plurality of tags, for identifying objects with which said tags are associated, each tag including: - a first receiving means, for receiving and responding to an interrogation signal; - a time delay means, for providing a time delay, said delay being initiated by response of said first receiving means to said interrogation signal; and - a first transmitting means, for transmitting an output signal upon termination of said time delay; wherein each tag has an identifying signature based on said time delay.

2. A plurality of tags, according to claim 1, in which said identifying signature is unique to each tag.

3. A plurality of tags, according to claim 1 or 2, in which the first transmitting means is arranged to transmit the output signal in the form of an output digital code.

4. A plurality of tags, according to claim 3, in which the output digital code is the same for all the tags.

5. A plurality of tags, according to claim 3, in which the output digital code is different for at least some of the tags.

6. A plurality of tags, according to any of the preceding claims, in which said first receiving means is arranged to respond to at least one of a plurality of interrogation signals, each interrogation signal being in the form of an interrogating digital code.

7. A plurality of tags, according to claim 6, in which said first receiving means includes a storage means, for storing at least one reference digital code, and a comparison means, said response of said first receiving means to said at least one of a plurality of interrogation signals being by comparison within said comparison means of said interrogating digital codes with said at least one reference digital code.

8. A plurality of tags, according to any of the preceding claims, in which said time delay is variable and is set by means of software control.

9. A plurality of tags, according to claim 7 or claims 7 and 8, in which said reference digital code is variable and is set by means of software control.

10. A plurality of tags, according to claim 3 or claims 3 and 8, in which said output code is variable and is set by means of software control.

11. A plurality of tags, according to any of the preceding claims, in which said objects to be identified are personnel.

12. A plurality of tags substantially as hereinbefore described with reference to Figures 2 to 7 of the accompanying drawings.

13. An identification system, comprising: - a plurality of tags, for identifying objects with which said tags are associated, each tag including: - a first receiving means, for receiving and responding to an interrogation signal; - a time delay means, for providing a time delay, said delay being initiated by response of said first receiving means to said interrogation signal; and - a first transmitting means, for transmitting an output signal upon termination of said time delay; wherein each tag has an identifying signature based on said time delay; - a second transmitting means, for transmitting said interrogation signal; and - a second receiving means, for receiving said output signal; said system being arranged to identify each tag, on the basis of said identifying signature, following only a single interrogation of said plurality of tags by said second transmitting means.

14. An identification system, according to claim 13, in which said identifying signature is based also on said output signal.

15. An identification system, comprising: - a plurality of tags, for identifying objects with which said tags are associated, each tag including: - a first receiving means, for receiving and responding to an interrogation signal; and - a first transmitting means, for transmitting an output signal upon response of said first receiving means to said interrogation signal; wherein each tag has an identifying signature based on said output signal; - a second transmitting means, for transmitting said interrogation signal; and - a second receiving means, for receiving said output signal; said system being arranged to identify each tag, on the basis of said identifying signature, following only a single interrogation of said plurality of tags by said second transmitting means.

16. An identification system according to any of claims 13 to 15, in which said identifying signature is unique to each tag.

17. An identification system, according to any of claims 13 to 16, in which said second transmitting means is arranged to illuminate a given space by means of said interrogation signal, such that only those tags found within said space are identified by said system.

18. An identification system, according to claim 17, in which said first transmitting means is arranged to transmit said output signal in the form of an output digital code.

19. An identification system, according to claim 17 or 18, in which said second transmitting means is arranged to transmit said interrogation signal in the form of a digital interrogating code.

20. An identification system, according to claim 19, in which said second transmitting means is arranged to transmit a plurality of different interrogating codes, said first receiving means including means for responding to at least one of said plurality of different interrogating codes.

21. An identification system, according to claim 19 or 20, in which a number of spaces is illuminated, each space by its own transmitting means, response of a tag in any one illuminated space being determined by the transmission within that space of an interrogating code to which said tag is arranged to respond.

22. An identification system, according to claim 20 or 21, in which each tag responds uniquely to its own interrogating code.

23. An identification system, according to claim 22, in which said system functions as a paging system.

24. An identification system, according to any of claims 18 to 23, in which the second receiving means includes correlating means for distinguishing between each different output digital code.

25. An identification system, according to any of claims 17 to 24, in which the second transmitting means transmits a series of interrogation signals.

26. An identification system, according to claim 25, in which at least one guard period is inserted into the period between interrogations, said guard period serving to allow the transmission of non-interrogating data from said second transmitting means to said tags.

27. An identification system, according to any of claims 17 to 26, in which said system is adapted to provide information on the location and/or direction of travel and/or

of said tags.

28. An identification system, according to claim 27, in which at least one illuminated space is divided into two overlapping zones, each zone being illuminated by its own second transmitting means to receive different but synchronised interrogation signals, said first receiving means being adapted to respond to said related signals by modifying said output signal.

29. An identification system, according to claim 28, in which said modification of said output signal takes the form of an inversion of said output digital code.

30. An identification system, according to any of claims 17 to 29, in which said plurality of tags and said second transmitting means and second receiving means are free to move with respect to each other.

31. An identification system, according to any of claims 17 to 29, in which said second transmitting means and second receiving means are in a fixed station, each tag being attached to a movable object.

32. An identification system substantially as hereinbefore described with reference to Figures 2 to 7 of the accompanying drawings.

32. An identification system, according to any of claims 17 to 29, in which said plurality of tags is substantially stationary, said second transmitting means being movable relative to said plurality of tags.

33. An identification system, according to any of claims 13 to 32, in which said objects to be identified are personnel.

34. An identification system substantially as hereinbefore described with reference to Figures 2 to 7 of the accompanying drawings.

Amendments to the claims have been fled as follows 1. A plurality of tags, for identifying objects with which said tags are associated, each tag including: - a first receiving means, for receiving and responding to an interrogation signal; - a time delay means, for providing a time delay, said delay being initiated by response of said first receiving means to said interrogation signal; and - a first transmitting means, for transmitting an output signal upon termination of said time delay; wherein each tag has an identifying signature based on said time delay and said output signal.

4. A plurality of tags, according to Claim 3, in which the output digital code is the same for some but not all of the tags.

5. A plurality of tags, according to Claim 3, in which the time delay is the same for some but not all of the tags.

10. A plurality of tags, according to Claim 3 or 4, in which said output code is variable and is set by means of software control.

13. An identification system, comprising: - a plurality of tags, according to any of the preceding claims; - a second transmitting means, for transmitting said interrogation signal; and - a second receiving means, for receiving said output signal; said system being arranged to identify each tag, on the basis of said identifying signature, following only a single interrogation of said plurality of tags by said second transmitting means.

14. An identification system according to Claim 13 in which said identifying signature is unique to each tag.

15. An identification system, according to Claim 13 or 14, in which said second transmitting means is arranged to illuminate a given space by means of said interrogation signal, . such that only those tags found within said space are identified by said system.

16. An identification system, according to Claim 15, in which said first transmitting means is arranged to transmit said output signal in the form of an output digital code.

17. An identification system, according to Claim 15 or 16, in which said second transmitting means is arranged to transmit said interrogation signal in the form of a digital interrogating code.

18. An identification system, according to Claim 17, in which said second transmitting means is arranged to transmit a plurality of different interrogating codes, said first receiving means including means for responding to at least one of said plurality of different interrogating codes.

19. An identification system, according to Claim 17 or 18, in which a number of spaces is illuminated, each space by its own transmitting means, response of a tag in any one illuminated space being determined by the transmission within that space of an interrogating code to which said tag is arranged to respond.

20. An identification system, according to Claim 18 or 19, in which each tag responds uniquely to its own interrogating code.

21. An identification system, according to Claim 20, in which said system functions as a paging system.

22. An identification system, according to any of Claims 16 to 21, in which the second receiving means includes correlating means for distinguishing between each different output digital code.

23. An identification system, according to any of Claims 15 to 22, in which the second transmitting means transmits a series of interrogation signals.

24. An identification system, according to Claim 23, in which at least one guard period is inserted into the period between interrogations, said guard period serving to allow the transmission of non-interrogating data from said second transmitting means to said tags.

25. An identification system, according to any of Claims 15 to 24, in which said system is adapted to provide information on the location and/or direction of travel and/or speed of said tags.

26. An identification system, according to Claim 25, in which at least one illuminated space is divided into two overlapping zones, each zone being illuminated by its own second transmitting means to receive different but synchronised interrogation signals, said first receiving means being adapted to respond to said related signals by modifying said output signal.

27. An identification system, according to Claim 26, in which said modification of said output signal takes the form of an inversion of said output digital code.

28. An identification system, according to any of Claims 15 to 27, in which said plurality of tags and said second transmitting means and second receiving means are free to move with respect to each other.

29. An identification system, according to any of Claims 15 to 27, in which said second transmitting means and second receiving means are in a fixed station, each tag being attached to a movable object.

30. An identification system, according to any of Claims 15 to 27, in which said plurality of tags is substantially stationary, said second transmitting means being movable relative to said plurality of tags.

31. An identification system, according to any of Claims 13 to 30, in which said objects to be identified are personnel.