WO1985001583A1

WO1985001583A1 - A method and a transponder for measuring a distance and identification

Info

Publication number: WO1985001583A1
Application number: PCT/NO1984/000039
Authority: WO
Inventors: Tore Planke; Nja^ord HESTNES; Richard Klem; Eigil SO^/RLI
Original assignee: Findit A/S
Priority date: 1983-09-30
Filing date: 1984-09-26
Publication date: 1985-04-11
Also published as: DK232585D0; JPS61500040A; IT1221130B; FI852041A0; SE8502627L; DE3490440T1; DK232585A; NL8420234A; SE8502627D0; GB2157112A; IT8412615A0; BE900729A; FI852041L; FR2552886A1; ES8600663A1; ES536367A0; GB8512308D0; NO852150L; NO833570L; AU3432084A

Abstract

A method and a transponder for measurement of distance to and identification of living creatures and/or objects/articles. Prior to reception of an interrogating signal (14-16) a transponder (23m; 38m) is listening intermittently. Upon reception of correct interrogation signal frequency (fo), the transponder will listen continuously over a time space related to the duration of the interrogation signal (14-16) in order to detect a correct enabling code (15) therein, and upon reception thereof, it will respond with a transponder identifying receipt signal (17) of duration equal to a terminal part (16) of the interrogating signal (14-16) and with a second carrier frequency (n x fo). A time-limited activation signal (18; fo) is sent to the transponder which in turn transmits a response signal (19-21; n x fo) having a duration equal to that of the activation signal (18) and may contain encoded information/status (20). The time (ts) between transmission of the activation signal and reception of said response signal is measured in order to calculate the distance between the transponder and interrogating station. Upon repeated measurement of distance from at least two locations, and accurate position of the transponder is determined through intersection point of the measured distances.

Description

AMETHODANDATRANSPONDERFORMEASURINGADISTANCEANDIDENTIFICATION

The present invention relates to a measuring- and identi¬ fication system, and in particular a method for measuring distance to and identification of living creatures, e.g. sheep, and/or objects/articles, and a transponder for use with the distance measurement and identification, and intended to communicate with at least one interrogating station as indicated in the preamble of the attached patent claims.

From German Offenlegungsschrift No. 2.919.753 there is known a stationary interrogating station and a transponder. The said interrogating station includes an energy transmitte a receiver and an evaluation unit. The transponder consists of an energy receiver and energy converter, an identificatio sign memory, a rate generator and a connecting unit includin an identification sign transmitter, and at least one antenna In the interrogating station there is located an enabling code memory and an enabling code transmitter, and the trans- ponder includes an enabling code receiver, an enabling code memory and an enabling code comparator, which is connected together with the remaining modules of the trans¬ ponder in such a manner, that an identification sign stored in the identification sign memory is only radiated from the energy transmitter of the transponder when the enabling code radiated from the interrogating station and that stored in the responding apparatus coincide. In the interrogation station, the energy transmitter and the enabling code transmitter can be comprised of the one and same transmitter. In a similar way, the energy receiver and the enabling code receiver can be comprised of one and the same receiver. In addition, the energy transmitter of the interrogating station may be provided with a directional antenna.

US-patent 4015259 discloses a transponder which upon detection of the carrier frequency of an interrogating signal transmits in return an identifying receipt of a duration equal to an end part of the interrogating signal with a second carrier frequency being a whole multiple of

O PI -% -

the interrogating signal carrier frequency. The said US- patent discloses diodes connected in series which upon radiation of microwave energy of a specific frequency c will generate in return harmonics of said frequency. In addition, the return signal is modulated with an identification code. In a particular embodiment, the transponder provides an n-bits pseudo-random interroga¬ tion code. Fundamental frequency of 3.1 GHz is proposed,

10 implying that the target which is sought will have to be within vision from the transmitter, and the range is there¬ fore substantially limited by the width of the radiation lobe. The range is indicated to be approximately 300 meters and from an aircraft at a level of approximately 300 meters, ■_jc which implies that the field of radiation will be approxi¬ mately 67 5 x 42 meters.

US-patent 4,107.675 relates to a transponder/responder system, where the interrogating station transmits time

20 limited interrogation signals on a first carrier frequency, and the transponders, when activated, transmit in return a response signal with an information code on a second carrier frequency, the time between the transmission of the interrogating signal and the receipt of the response signal

25 being used to calculate the distance between the interrogat¬ ing station and the transponders. However, coded interroga ting pulses are not transmitted. The patent in particular concerns the identification of small vessels, e.g. yachts, having problems or being in distress. In the case of

30 several vessels being in distress or having problems, or in the case where the transponder accidentally is activated on¬ board, then simultaneous return signal transmission is possible, with very heavy risk of jamming. The coding available in the transponder on board the vessel relates

35 only to identification of status (distress situation) , but the land-based station is not able to identify the vessel or the type of vessel.

As in the case of DE-OS 2,919,753, also the DE-OS 2_r736,217 and 2,508,201 disclose a technique where the object is to provide by means of the interrogating station a sufficient current supply for the connecting unit of the transponder. This represents however an unnecessary complication of the system, even though there is indicated a possibility of using a battery for the energy supply of the transponder. By using a battery in connection with the prior art system, there is the substantial disadvantage that the battery will be constantly loaded, and therefore relatively soon will be discharged. Prior art personnell calling systems, i.e. used within hospital areas and industry, are of the type where the receivers or transponder must be charged at regular intervals, usually during night- time or outside business hours. Therefore, normally a large number of apparatus will, as a general rule, be inoperative.

A substantial disadvantage of the prior art solutions, is that distance determinations are not possible, and position determination is in reality also quite difficult to exercise since the bearing possibilities are rather limited, in that the energy transmitter of the interrogating apparatus necessarily must have wide radiation lobe in order to ensure th at least one responding apparatus responds. This has in turn quite naturally an influence on the utilization ability of the system. If the radiation lobe according to the prior art system is made narrow, then the living creature or the object to be identified must generally be within vision. According to DE-OS 2,919,753, the invention disclosed therein can be used in a safety system to prevent theft. In such a case, the responding apparatus is located in a continuous field of radiation coming from the interro¬ gating apparatus.

In the said US-patent 4,107,675 there is available the possibility of a number of transponders transmitting simul¬ taneously, thereby creating error signals and mutual jamming. This prior art system also requires a rotating antenna at the interrogating station and high frequency

( _ OM signals having a very narrow radiation lobe in order to locate the vessel in distress, which means that both the antenna and transmittingequipment will be expensive. By virtue of the fact that, the _.transponder is provided with a local oscillator, the system is vulnerable to jamming and error signals. To limit the risk of jamming and error signals, the transponder equipment will have to be rather expensive.

The present invention therefore has an object to avoid the disadvantages linked to the prior art in a simple manner, simultaneously by providing a system having improved possibilities of utilization.

In breeding of sheep and catties, in particular in isolated areas, there has been an increasing problem that animals disappear from the flock/herd, either due to acci¬ dents, e.g. by falling off a cliff, dying of poisoning, foreign objects etc., being killed by other animals or for some other reason move away from the flock/herd. The problem is in particular great in connection with sheep-breeding, but also in connection with cattle being in isolated areas during a great part of the year. Further, the invention may be utilized in connection with e.g. vessels at sea for navigation or distress signal purposes.

However, it is to be stressed that the invention in no manner is limited to the locating and identification of sheep, nor only the use in connection with vessels, since these two utilizations are given as examples only in the purpose to describe in a simple manner the operation of the method and transponder according to the present invention.

The characterizing features of the method and transponder according to the present invention will appear from the attached patent claims and from the description hereinafter with reference to the enclosed drawings, illustrating the present invention by non-limitative examples.

Fig. 1 illustrates as an example an interrogating station for carrying out the method according to the invention, and Fig. 2 illustrates a modification thereof.

Fig. 3 illustrates a first embodiment of a transponder for carrying out the method according to the invention, and Fig. 4 illustrates a slight modification thereof.

Fig. 5 illustrates in schematic form, the signal type of the signals leaving the interrogating station (Fig. 5a) and the transponder (Fig. 5b) , respectively.

Fig. 6 illustrates as an example the utilization of the invention in connection with the searching for sheep.

Fig. 7 illustrates as a further example the utilization of the invention in connection with e.g. small vessels.

The present invention is based on the fundamental concept that within a system, all the transponders being incorpo¬ rated in the system, can operate on the same frequency, since only one transponder at a time responds to a coded or ciphered call from an interrogating station. This means in practice that the assignment of frequency to a system of this type is substantially simplified simultaneously with the "pollution" of the world-wide frequency spectrum being small. Simultaneously, jamming problems are avoided to a great extent.

Within the said prior art, it is in particular the power consumption which has become a great problem, even with modern electronic equipment. This is due to the fact that the transponders are constructed to listen continously for

Q^*v_PI signals from a calling- or interrogating station. The present invention is thus based on the fact that the transponders are to make use of intermittent listening and only to send a response back to the interrogating station upon selective/individual calling. This will enable the use of rather small batteries, which in turn will bring both the weight and volume of the transponders down to a minimum. For certain embodiments of the trans- ponder, solar-cell power supply may advantageously be used.

As a non-limitative example, there is according to the invention proposed that the transponder has an active/ inactive state ratio of 1/1000, i.e. that the transpon¬ der only listens 1 millisecond per second. A substantial feature of the invention is thus that it is completely unnecessary to listen in a continuous mode. Upon the detection of correct interrogating station frequency, there is over a period of approximately 100 milliseconds listened continuously for the interrogating signal trans¬ mitted from the interrogating station in order to find an enabling code which will cause activation of the trans¬ mitter part of the transponder. A long code will provide a maximum security and may e.g. contain 32 bits. This is of particular importance when the system is used for cal¬ ling in turn a large number of transponders.

A further safety feature of the present system lies in the fact that the transponder only responds to a coded/ciphered call intended for that transponder.

The response time Δ t being detected at the interrogating station will be a direct function of twice the shortest distance between the interrogating station and the respond¬ ing transponder, i.e approximately 0.6 ns/ meter or 600 ns/ kilometer. For practical applications, where there is a mutual distance of approximately 5 kilometers, the response at the transponder end will take place after approximately

^"E

OMP 1.5 micro seconds, and the response time being detected at the interrogating station will thereby be approximately 3.0 AXS , which indicates distance of 10/2 = 5 kilometers. in the response time Δ t there is also a constant time Δ t-i- ans_O representing the time delay inherently present in the transponder before it reacts. Due to filtration at the interrogating station, this time delay may be substan¬ tially greater, in the range of micro seconds. The measure- ment accuracy depends on signal noise ratio and the measurement accuracy of the time delay. With modern micro¬ electronics, it is possible to obtain circuits which with¬ out difficulty operate with nano-seconds, which renders the measurement accuracy, at least theoretically, to be in the range of 1 - 3 meter.

A further main feature of the present system is, that the transponder transmits its response signals back with a carrier frequency being a multiple n of the interrogating station frequency, n being any number greater than 0. In this manner, the transponder avoids any use of an oscillator, which renders the system automatically safe and prevents unintended transmission of signals from the transponder. Further, there is in this manner obtained a substantial frequency stability, since the carrier frequen¬ cy of the transponder always will be a pre-determined multiple of the interrogating station carrier frequency.

Since the system is considered to be used with a relatively large number of transponders, whereas the number of interrogating stations normally will be of a substantially smaller number, usually one or two, it is obvious that it will be more economical for the system to provide the equipment for frequency stability in the interrogating station itself.

A further advantage by letting the transponder use a multiple of the interrogating station transmitter frequency, is that the same type of antenna or slight modifications thereof may be used.

In fig. 1 there is illustrated as an example, an interro- 5 gating station for utilization with the present system. However, it is to be readily understood that many indi¬ vidual adaptions of the interrogating station are possible without departing from the concept and scope- of the invention. As such,fig.2 is aslightmodification of the ■JO interrogation station according to fig. 1.

In fig. 1 the interrogating station includes a combined transmitting- and receiving antenna 1 and adiplexer 2 to isolate the transmitter part and the receiver part. The

1 interrogating station is provided with a transmitter 3, a coder 6, a receiver 4 and a display 7. The display 7 may have any suitable configuration, but will in a preferred embodiment indicate e.g. which transponder that is interro¬ gated, whether the transponder responds and the distance

20 between the interrogating station and the transponder, based on the measured response time. Further, the said display can indicate a status code of the transponder.

The transmitter, the coder, the receiver and the display 25 are preferably controlled by a central control unit 5 containing a clock. A suitable carrier frequency for the transmitter part of the interrogating system could lie within the UHF-band. However, there is required a narrow band width, e.g. 15 kHz, which implies that the transmitter

—6 30 must not have a frequency deviation greater than 15 x 10

This does not represent any substantial problem, since the the power supply, the technical equipment and the weight and volume of the interrogating stations can be made substantially more complex and greater than what is

35 technical, practical and economical speaking possible in connection with transponders. It will be readily understood that the receiver 4 and the display 7 may include decoding means for decoding received information from the transponder into displayable form.

OMPI In fig. 2, representing a modification of fig. 1, the transmitter part of the interrogating station includes a ciphering unit 27 and a deciphering unit 29 providing readable input to the display 7. In order to provide a system which is less vulnerable to jamming, the interro¬ gating station may be provided with spread-spectrum modu¬ lation, and for that purpose a spread-spectrum modulator 28 is provided. Further a spread-spectrum demodulator 29 is provided at the receiving end of the interrogating station. Upon command codes from the controller unit 5, the ciphering through the unit 27 and the station trans¬ mitting frequency can be selectively controlled.

in fig. 3, there is illustrated a first embodiment of the transponder intended for use in connection with the present system. The transponders include an antenna unit 8 which may be constituted by a combined antenna or two or more antennas tuned to the frequency f_n and the response frequency n x f_n of the transponder, n being any arbitrary figure greater than 0. The antenna unit is followed by a first tuned amplifier unit 9 which can be brought into an inactive or active state by means of a microprocessor 11. The said first amplifier 9 may possibly be made of several indivudual amplifiers which are interconnected, e.g. in cascade. The first amplifier 9 is followed by a signal gate 10 which is controllable by means of the said micro¬ processor 11 and communicates with the microprocessor 11, as will be explained later on. The signal gate is followed by a frequency multiplier 12, which may, if so desired, contain several frequency multiplying stages for frequency multiplication intoamultiple of the interrogating station carrier frequency f- . At the output of the frequency multiplier 12 there will thereby appear a signal having the carrier frequency n x f , n being greater than 0. The output signal from the multiplier 12 is fed to a second tuned amplifier 13 being controlled by said microprocessor 11. The said second amplifier 13, may, like the said first amplifier 9, be comprised by several stages being mutually

OMPI interconnected, e.g^ in.cascade. _ As seen from fig. 3, the amplifier 13 is followed by the antenna unit 8, the amplifier 13 thus constituting the final transmitter stage of the transponder.

The antenna unit 8 can be comprised of a combined receiving- and transmitting unit, or seperate antennas for the receiver frequency f_n and the responding frequency n x f_Q.

The microprocessor 11 will ■ include a clock causing the amplifier 9 to be intermittently operative, e.g. in an active/inactive state ratio equal to 1/1000 during the listening mode of the transponder, and further includes a detector which upon detectionofcarrier wave signal 17 of frequency f_n from the interrogating station, see fig. 5a, causes the said first amplifier 9 to be kept continuously operative for detection of possible correct enabling code 15 for the transponder, said code being fed via the said signal gate 10 to the detector 11 in a said microprocessor. Upon detection of such enabling code, the end 16 of the received interrogation signal of frequency f_ is fed via the signal gate 10, via the frequency multiplier 12, the second ampli¬ fier 13, which is now operative by means of the micro¬ processor 11, and onto the antenna unit 8 as a receipt signal 17 from the transponder. The interrogation station will now obtain an indication that the transponder responds, and transmits an activation signal 18 of the type continuous wave and with a limited duration, see fig. 5a. Upon receipt of this signal, the transponder will feed it through the amplifier 9 via the signal gate 10 to the microprocessor 11 and back to the signal gate 10, then further to the frequenc multiplier 12, the amplifier 13 and to the antenna 8, one part of the received signal 18 being modulated with an information code 20 which is characteristic for the trans¬ ponder in question. Thus, a composite response signal 19, 20 21 is transmitted from the transponder, said response signal having a duration equal to the activation signal 18 from the interrogating station. The said signal being trans-

_/^ ^ f OMP WIF mitted from the transponder having carrier frequency n x f_Q will in a preferred embodiment consist of a first part 19 of the typecontinuous wave, followed by a code- modulated part 20, and possibly being followed by a final part 21 of the type continuous wave. However, the last part 21 may be incorporated in the part 20 in case the informa¬ tion code is to be extended.

The time lapsing from the transmission of the activating signal from the interrogaging station until the presence of the response signal 19 - 21 from the transponder to the interrogating station, has been indicated as the time Δ ts.

This response time is a function of twice the distance between the interrogating station and the transponder plus the inherent reaction time of the transponder and the interrogating station. In this manner, there is obtained an accurate measurement of distance between the interro¬ gating station and the transponder, which distance is indi- eating on the display 7.

The information code 20 in the transponder response signal can be provided by one or several external information generating means 25 connected to the microprocessor 11, if so desired. In connection with searching for sheep, the said information generator 25 may e.g. be a movement indicator in order to indicate whether the sheep is in movement or not, or e.g. a device being capable of measur¬ ing the pulse rate of the animal. It will be readily understood that this information generator can be of any configuration which is suitable for the task to be solved. If the information generator 25 despite several interroga¬ tions does not indicate any movementes, this may possibly cause a search for the sheep in order to see whether it is still alive or not.

In a further development, one could visualize the micro¬ processor being able to control an external functional device 26 being capable of carrying out some particular function, e.g. as a result of the activation signal 18 from interrogating station or a particular coded activa¬ tion signal. In this respect, it may be referred to fig.

2 indicating the ciphering device 27, which in such circum¬ stances may be advisable to use in order to increase the ^' overall safety factor of the system.

In this connection it is also referred to fig. 4, where the functional blocks 31, 32 denote ciphering and decipher¬ ing devices, respectively. Further, in order to be able to analyze signals being of the spread-spectrum type, means 32 should be provided between the signal gate 10 and the microprocessor 11 to enable analysis of signals received from the interrogating station.

The utilization of the. present system is to be further described with reference to fig. 6. In fig. 6 there is a flock of sheep m present in the area to be searched. It is in the present example possible either to have a portable interrogating station, denoted 22, which is carried from one location, labelled 22 , to a second location, labelled 22_.. However, it may be possible to have stationary interrogating stations, labelled 33 and 34, respectively in fig. 6 located at the said correspon¬ ding locations 22 , 22 . However, in order to simplify the explanation of the utilization, the following description will refer to utilization of a portable interrogating station 22. From the interrogating station 22 at the first location, denoted 22 , is transmitted interrogating signals, first of all to the transponder 23, , aftd a receipt signal generally denoted S,, , back from the transponder. Thereafter, the said activation signal is transmitted from the interrogating station at 22 and a response signal, generally indicated by S_, , is sent back to the interrogating station from the transponder after a specific time corresponding to the distance between the interrogating station at 22^,-and the transponder 23, . In those cases where the topographical transponder 23, . In those cases where the topographical conditions are such that there is e.g. a mountain 24 or the like, there may arise such conditions that the signals transmitted from the transponder are also reflected from said mountain 24 and arrive at the interrogating station as reflected signals ^s _Aι__{re i} • However, these signals will arrive in time after the signal S,, , and the interro¬ gating station at 22 will therefore ignore the signal ^S _Ά-_I_ _fi • This is due to the fact that it will always be the signal which first arrives at the interrogating station which is the one indicating the shortest distance to the transponder. After the said measurement has been made and the first distance to the transponder thereby is determined, the interrogating station 22, which in the example described is portable, is moved to a new location, denoted 22_β. As in the case of the location 22-, there is carried out corresponding signal exchanges with the transponder 23, , whereafter a second distance to the transponder will become known. Since the two locations 22,. and 22„ are known in the searching area, these A B may be indicated on a map and two circles having a radius equal to the respective measured distances can be made, whereby an accurate position of the transponder 23, and thereby the corresponding sheep can be established. If the antennas of the interrogating station 22 are directional, e.g. having a radiation diagram being less than 180 , it will be sufficient to carry out two such measurements for each transponder in order to establish the position of all of the sheep in the flock to be identified and located. Measurement regarding the sheep having the transponder 23_ is executed in the same manner as described in connection with the transponder 23, , since the signals between the transponder and the interrogating station in general is denoted by S ~ ,

S 2* ^It will also readily be understood that measurements with regard to the transponders 23_, 23., 23;-, 23, ...23 can be executed in a similar manner. It will also be

f OM readily understood that if the position in the area for the transponders 23, and 23₂ were fixed, the position of the interrogating station either at the location 22_A or 22 could be determined in a simple manner by cross-point determination. Again,it is to be pointed out that it is possible to have the two stationary interrogating stations 33, 34 instead of a portable station 22 being carried from the location 22 (same as that for 33) to the location 22 (same as for the station 34) .

Fig. 7 illustrates as an example the utilization of the present invention in connection with navigation purposes, the operation principle of the tranducer being exactly as described before. However, contrary to the example in fig.

6, the transducers are kept stationary and the interrogating station is either substantially stationay at the time of measurement or movable between locations. In the example shown, a vessel 40,e.g. a yacht,is proposed to be equipped with an interrogating station 39. The vessel 40 is at sea

35, e.g. some miles off a coast-line 36. In the waters to be travelled by vessel 40 there may be obstructions 37, e.g. a reef on which a transponder 38 has been located.

6

Further, along the coast-line 36, there has been positioned at suitable intervals transducers 38i,, 38z, 38j-., 384.,385_c, 38m

From the naval chart, the exact location of the transduc¬ ers will be known.

Interrogating and enabling signals are transmitted from the interrogating station 39 onboard the vessel 40, as described in connection with fig. 5. A similar signal exchange as described in connection with fig. 6 will then take place, in the present example between the interrogating station 39 and the transponder 38 , as well as between the station 39 and the transponder 38 . On basis of the measured distances between the station 39 and the respective transducers, two curves 41 and 42, respectively may be drawn on the naval chart, and the intersectionbetween said two curves will positively identify the exact location of the vessel. The present invention thereby also provides a useful navigation system which is simple in operation and reliable. In particular, the utilization of the invention as described as an example in connection with fig. 7 can be useful for yachts, a fisherman's fleet and in particular in areas requiring a local navigation system and also for areas having traditionally much fog. In this way more expensive navigation aids can be avoided.

It would also be readily be understood that having a trans¬ ponder onboard the vessel 40 and interrogation stations along the coast, would help to identifyand locate vessels^* travelling along the coast-line.

The present system exhibits substantial advantages relative to the prior art bearing systems, said prior art systems being based on a directional antenna being directed towards the location having the highest signal strength. Thereafter the listening apparatus is moved to a new place where a further bearing is taken towards the location which again has the largest signal strength. However, reflections may in this case cause substantial and even fatal errors in the measurement, simultaneously with the fact that it can be rather difficult to determine accurately in which direction the highest signal strength is present. According to the present system there is instead measured the distance to the signal source (the transponder) from at least two locations, whereby a completely accurate coordinate determination can be made, e.g. as described in connection with fig. 6 and 7.

The present transponder can be made in any suitable form, e.g. in the form of .integrated circuits, and the size and cost of production can thus be brought to a minimum. It is important to note that there is no frequency reference at the transponder, e.g. a local oscillator, with the

-^TREA exception of any necessary band-pass filter, which again contribute to reduced over-all cost of the transponder. The fact that the transponder does not exhibit any local 5 oscillator prevents completely that the transponder is capable of automatically and unintentionally transmitting, which is often the case with the well-known emergency beacons. Further, it is a fact that with the prior art emergency beacons, it is not possible directly to deter- 0 mine the distance thereto, as is the case with the present invention. The bearing on emergency beacons is based on a signal peak-amplitude measurements, with the inherent inaccuracies.

15 if the interrogating station makes use of a particular directional or phase sensitive antenna, e.g. with an aperture angle of 90 , it will for some applications, e.g. to find out whether the flock of sheep are in a certain limited area, only be necessary to carry out a single distance

20. measurement. In this way, it is possible to determine whether all or only some of the sheep are within a pre¬ determined circle segment.

It should be stressed that the frequency multiplication

25 factor in the transponder not necessarily need to be a whole number, but could just as well be a fraction. Although the invention has been described only in connec¬ tion with the use of one or two interrogating stations, it is evident that a plurality of interrogating stations

30 may be used without departing from the scope and concept of the invention. By the present invention, it is noted that rather subjective bearing evaluation as known in connection with signal peak-amplitude determination, is completely eliminated, since the operator of the interro¬

35 gating station is capable of reading directly on a dis¬ play the distance to the transponder.

The response signal 19 - 21 may also contain the current status of the transponder and its associated equipment.

OMPI e.g. the status of the external equipment 25, as indicated in fig. 3. Further, the receipt signal 17, see fig. 5b may contain a status code. In order to reduce the possibi- lity of jamming and easy deduction of the exchanged codes, cipher means may be provided both at the interrogating station and at the transponder with corresponding decipher¬ ing means. Further spread-spectrum modulation is avail¬ able, if necessary, said ciphering as well as spread-spectru modulation being controllable through a command code from the interrogation station or from the transponders.

Further important feature of the present invention is that it makes possible to have no signal transmission or exchange before contact with the specific transducer is required. Thus there is absolute silence before any action. Further, only the called transducer will reply.

Although the present invention has primarily been described in connection with air as transmission medium, it is readily understood by an expert in the art that the present invention is also applicable for mariieuse, by using water as trans¬ mission medium and operating with ultrasonic waves.

Claims

C l a i m s

1. A method for measurement of distance to and identi- fication of living creatures, e.g. sheep, and/or objects/ articles, where a time limited interrogation signal is transmitted from at lέast one interrogating station^*to a transpnder, said interrogation signal containing an enabling code for the transponder, said transponder upon detection of the interrogation signal carrier frequency and said enabling code sending back a receipt signal of a duration equal to a terminal part of the interrogation signal and by means of frequency multiplication is given a: second carrier frequency being a multiple of the carrier frequency of the interrogation signal (first carrier frequency) , c h a r a c t e r i z e d i n that the receipt signal

(17) activates the interrogating station (1 - 7; 1 - 5, 7, 27 - 29) to transmit a time limited activation signal

(18) of said first carrier frequency and that the trans¬ ponder (8-13) upon receiption - of the activation signal (18) sends a response signal (19 - 21) having a frequency equal to said second carrier frequency, which response signal has a duration equal to that of the activation signal, said response signal being capable of containing an information/status code (20) , and that the time between the transmission of the activation signal (18) and the receipt of the response signal (19 - 21) is measured and used to calculate the distance between the interrogating station and the transponder.

2. A method according to claim 1, c h a r a c t e r¬ i z e d i that the receipt signal (17) and the activation signal (18) is of the continuous wave (CW) type.

3. A method according to claim 1, c h a r a c t e r¬ i z e d i that the receipt signal (17) contains status information and that the activation signal (18.) is of the continuous wave (CW) tvpe.

OMPI

4. A method according to claims 1 and 2, or 1 and 3, c h a r a c t e r i z e d i n that the interrogation signal (14 - 16) and the response signal (19 - 21) during a part of their respective duration (14, 16; 19, 21) are of the type continuous wave, said enabling code (15) and said information/status code (20) appearing through pulse modulation of the respective carrier frequencies.

5. A method according to claim 4, c h a r a c t e r¬ i z e d i n that the interrogation signal (14 - 16) of the first carrier_^ frequency is composed of a contin wave (.14 - 16) with an enabling code (15) inserted therein.

6. A method according to^' claim 4, c h a r a ct e r¬ i z e d i n that the response signal (19 - 21) of the second carrier frequency (n x f_) is composed of a αontinous wave (19) followed by the information/status code (20) , the terminal part of said response signal either containing an extended information/status code or being a continous- wave (21).

7. A method according to anyone of the preceding claims, c h a r a c t e r i z e d i n that the transponder (8 - 13) prior to the receipt of the interrogation signal (14 - 16) is listening intermittently, e.g. with an active/inactive state relationship of 1/1000.

8. A method according to anyone of the preceding claims, c h a r a c t e r i z e d i n that the transponder (8 - 13) upon receipt of correct interrogation signal frequency is listening cont^'ihousry over a time space being related to duration of the interrogation signal in order to detect a possible correct enabling code (15) in the interrogation signal.

9. A method according to anyone of the preceding claims, c h a r a c t e r i z e d i n that the transponder is using parts of or the complete signal being trans- mitted from the interrogating station in order to generate a carrier-output signal (17; 19 - 21), the frequency of which is created through frequency multiplication of the interrogating station carrier frequency.

10. A method according to anyone of the preceding claims, where the position of the living creature and/or objects/ articles is to be determined, c h a r a c t e r i z e d i n that the interrogating station (1 - 7)carries/carry out identification and ^'.distance measurement repeatedly from different mutual positions relative to the transponder, and that the transponder position is determined through determination of the intersection point of the distances measured.

11. A method according to anyone of the preceding claims, c h a r a c t e r i z e d_'! i n that the carrier frequencies are spread-spectrum modulated.

12. A method according to anyone of the preceding claims, c h a r a c t e r i z e d i n that the coded signals are of ciphered type.

13. A method according to anyone of the preceding claims, c h a r a c t e r i z e d i n that the transmission medium is air and that the operating frequencies are within the UHF frequency band.

14. A method according to anyone of the claims 1 - 1Q, c h a r a c t e r i z e d i n that the transmission is water, and that operation is based on ultrasonic wave propagation.

15. A transponder for measuring a distance to and identi¬ fication of living creatures, e.g. sheep, and/or objects, and intended to communicatewithatleastoneinterrogating statio c h a r a c t e r i z e d i n that the transponder include a) a receiver- and transmitter antenna unit (8) , b) a first controllable signal amplifier means (9) connected to said antenna unit (8) , c) a controllable signal gate (10) connected to the output of said first signal amplifier means (9) , and having a two-way connection with a detector- and control unit (11) , e.g. a microprocessor, and in addition being connected t a frequency multiplier (13) , said multiplier multiplying the interrogating station carrier frequency with a multip of said frequency, d) . second controllable amplifier means (13) connected to the output of said frequency multiplier for amplification of the transponder output signal, the output of said seco controllable amplifier means (13) being connected to the antenna unit (8) .

16. A transponder according to claim 15, c h a r a c t e¬ r i z e d i n that the said amplifier means (9, 13) are controlled by said detector- and control unit (11) .

17. A transponder according to claim 15 or claim 16, c h a r a c t e r i z e d i n that the detector- and con¬ trol unit (11) contains a clock causing said first amplifier means (9) to become intermittently operative, e.g. in an ac¬ tive/inactive state relationship of 1/1000, during the listen ing mode of the transponder, and contains a detector which upon detection of the interrogating station carrier wave (14) of a first carrier frequency (f ) causes the first ampli¬ fier means (9) to be kept continuously operative for detectio of a possible correct enabling code (15) , the terminal part (16) of the received interrogation signal (14-16) upon detec- tion of such enabling code being fed via the signal gate (10), the frequency multiplier (12) and said second amplifier means (13) to the antenna unit.

,_. O PI

18. A transponder according to claim 17, c h a r a c t e¬ i z e d i n that the detector- and control unit (11) uponthe receptionof an activation signal (18) in the trans- ponder, controls this signal via the signal gate and further to the frequency multiplier (12) and therefrom _v via said second amplifier means (13) to the antenna unit (8) as a response signal of the said second carrier frequency

(n x f_Q) .

19. A transponder according to anyone of the claims 15 - 18 c h a r a c t e r i z e d i n that the detector- and contro unit (11) is provided with at least one external, connectable information generator (25) .

20. A transponder as indicated in one or more of the. claims 15 -19, c h a r a c t e r i z e d i n that the detector- and control unit (1^*1) is connected with an external functiona device (26) which can be activated by said control unit (11) .

21. A transponder according to anyone of the claims 15 - 20, c h a r a c t e r i z e d i n that the detector part of the detector- and control unit (11) contains a memory for the enabling code (15) being characteristic of said trans- ponder.

22. Atransponder according to claim 20, c h a r a c t e ¬ r i z e d i that said functional device (26) can be activated by a coded activation signal (18) from said inter- rogating station.

23. Atransponder according to anyone of the preceding claims c h a r a c t e r i z e d i n that the antenna unit consist of a combined receiver- and transmitter antenna or at least one antenna tunedto the receiving frequency and at least an antenna tuned to the transponder response frequency.

24. A transponder according to anyone of the preceding claims, c a r a c t e r i z e d i n that the said first and second carrier frequencies are within the UHF band.

25. A transponder according to anyone of the preceding claims, c h a r a c t e r i z e d i n that the transponder is operable with spread-spectrum modulation.

26. A transponder according to anyone of the claims 15 - 23, c h a r a c t e r i z e d i n that said receiver- and antenna unit (8) is an ultrasonic trans¬ ducer.