WO2008156416A1

WO2008156416A1 - Positioning and tracking of animals

Info

Publication number: WO2008156416A1
Application number: PCT/SE2008/050707
Authority: WO
Inventors: Axel Elfgren
Original assignee: DeLaval Holding AB
Current assignee: DeLaval Holding AB
Priority date: 2007-06-18
Filing date: 2008-06-12
Publication date: 2008-12-24
Anticipated expiration: 2009-12-18

Abstract

The invention relates to positioning and tracking animals with respect to time instances when each animal is located at predefined positions in a farm environment. A respective tag unit (T1, T2, T3, T4) is here attached to each animal. At least one master station (M1, M2, M3) transmits downlink data messages (Ddl1, Ddl2, Ddl3) over a wireless interface. Each downlink data message includes information pertaining to an identity of the transmitting master station (MI 1 M2, M3). Each tag unit (T1, T2, T3, T4), in turn, is configured to receive the downlink data messages (Ddl1, Ddl2, Ddl3), and in response to each received downlink data message (Ddl1, Ddl2, Ddl3) store the master station identity and a time stamp (t1, t2, t3) indicating an instance at which the message (Ddl1, Ddl2, Ddl3) was received. Moreover, each tag unit (T1, T2, T3, T4) is configured to repeatedly transmit uplink data messages (DUl[M1,ID2,t1], DUl[M2,ID4,t2], DUl[M3, ID3,t3]) over another wireless interface. Each uplink data message includes information pertaining to an identity of the tag unit (T1, T2, T3, T4) from which the message is originated, the master station identity and the time stamp associated thereto. The system also includes at least one receiving station (R1 ) which is configured to receive the uplink data messages (DUl[M1,ID2,t1 ], DUl[M2, ID4,t2], DUl[M3,ID3, t3]). Hence, at each point in time, it can be determined which animal that was located proximate to which master station (M1, M2, M3).

Description

Positioning and Tracking of Animals

THE BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention relates generally to automatic identification of animals. More particularly the invention relates to a sys- term for positioning and tracking animals according to the preamble of claim 1 . Moreover, the invention relates to a method according to the preamble of claim 12, a computer program according to claim 19 and a computer readable medium according to claim 20.

The technical solutions for handling livestock in a farm environment are becoming increasingly efficient and sophisticated. For instance, the state-of-the art milking implements are now almost entirely automatic. A modern milking parlor constitutes an example wherein the involvement of a human operator can be made relatively low. However, for an optimized milking procedure and to ensure a good milk quality, this requires a reliable animal identification.

Today, various forms of systems exist for registering the animal identity at the location where the animals are milked. Typically, the identity information is here transferred wirelessly over a magnetic interface or a radio interface (e.g. based on RFID technology).

The published patent application US 2005/0145187 describes a livestock management system, wherein an electronic tag unit is capable of reading a radio frequency identification device (RFID), storing said data and transmit data to a satellite, which relays the information of an individual animal to a hub server. The hub server, in turn, appends a data record and makes that record available over a network. Hence, the livestock owner can be automati- cally notified if data from the ear tag is outside a pre-defined limit. The published patent application GB 2 353 910 discloses a system for tracking assets using local beacons. Here, objects are tracked within a building using a radio device associated with each object and an array of transmitting beacons. Each beacon transmits identification data, and based on identification data received in the radio devices attached to the objects, the location of each object is determined. The US patent 6,055,434 reveals a similar solution, although here, a mobile station is located within a mobile telecommunications network. A plurality of low powered beacons are selectively placed throughout the network. Each beacon independently transmits identification data uniquely identifying itself. By using a mapping table describing the beacon locations, a mobile station travelling within the system can determine the location of an identified beacon, and thus also estimate its own position.

Thus, previous systems exist for wirelessly locating and tracking animals as well as objects/devices. However, the known solutions are incapable of tracking and positioning animals with respect to the specific time instances when each animal is located at predefined positions, e.g. represented by milking stations, in a simple and straightforward manner.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to alleviate the problems above and thus offer an efficient, reliable and flexible solution for positioning and tracking animals in a farm environment.

According to one aspect of the invention, the object is achieved by the initially described system, wherein the system includes at least one master station which each is configured to transmit downlink data messages over a second wireless interface. Each downlink data message, in turn, contains secondary information pertaining to an identity of the master station from which the message is originated. Moreover, each of the at least one tag unit is configured to receive the downlink data messages over the second wireless interface, and in response to each received downlink data message store the secondary information along with a time stamp indicating an instance at which the message was received. The time stamp and the secondary information are then included in the uplink data message, which is transmitted over the first wireless interface to the at least one receiving station.

An important advantage attained by this arrangement is that the animal identification need not be linked to the point in time when the animal is located at a particular receiving station. Instead, tracking data may be gradually generated and stored as the animals pass various master stations. Based on forwarded tracking data, it may then be determined, with high accuracy, which ani- ma! that was located where at a given point in time. Hence, a high degree of flexibility is provided in terms of how often and at which locations the animals are identified.

According to one preferred embodiment of this aspect of the invention, each of the at least one master station is configured to transmit the downlink data messages at such respective power level that, at each position within the system, a given tag unit of the at least one tag unit is configured to receive the downlink data messages exclusively from a single master station. For example, the output power from the master stations is adjusted to such level relative to the distances between these stations and the receiver sensitivity of the tag units that the tag units may only "hear" one master station at the time irrespective of where the tags units are located. Naturally, this is advantageous in order to determine whether or not a given animal is located at a predefined position, i.e. defined by a master station.

According to another preferred embodiment of this aspect of the invention, each master station is configured to include the time stamp in the downlink data message. Thus, the time reference may be generated by a central resource. This, in turn, vouches for robustness and stability.

According to a further preferred embodiment of this aspect of the invention, each tag unit instead includes a clock module. The tag unit also includes means configured to generate the time stamp based on data produced by the clock module in connection with receiving a message from one of the at least one master station. Hence, time is handled locally in each tag unit, and the master stations only need to transmit their respective identity information over the second wireless interface.

According to yet another preferred embodiment of this aspect of the invention, each master station includes at least one radio transmitting means. Correspondingly, each tag unit includes at least one radio receiving means. The transmitting and receiving means are here configured to establish the second wireless in- terface via at least one radio resource. This enables a power- efficient data transfer, and if so desired, the communication distance between the master station and the tag unit can be made relatively long.

According to still another preferred embodiment of this aspect of the invention, each master station includes at least one means configured to transmit information via magnetic field influence. Correspondingly, each tag unit includes at least one means configured to receive information via magnetic field influence. These means are configured to establish the second wireless interface magnetically. Thereby, a short-range data exchange which is relatively immune against interference can be accomplished.

According to another preferred embodiment of this aspect of the invention, each tag unit includes at least one radio transmitting means and each receiving station includes at least one radio re- ceiving means. Here, the transmitting and receiving means are configured to establish the first wireless interface via at least one radio resource. Consequently, the uplink data messages can be transferred efficiently over relatively large distances. According to a further preferred embodiment of this aspect of the invention, it is presumed that the system includes two or more master stations. Moreover, the stations are separated from one another by a distance which substantially exceeds a res- pective transmission range of each master station. Thus, the tag units may be unambiguously linked to a given master station.

According to yet another preferred embodiment of this aspect of the invention, the tag unit also includes means for registering a physical activity of the animal to which the unit is attached. Fur- thermore, the tag unit is configured to repeatedly transmit information pertaining to the physical activity over the first wireless interface for central evaluation (e.g. to determine heat and/or health status). This is advantageous, since traditionally, ID transponders and activity sensors have been represented by two separate units attached to the animals.

According to still another preferred embodiment of this aspect of the invention, each uplink data message contains a predetermined number of data components. Each data component here represents an event, which is specified by the time stamp and inclu- des the above-mentioned primary and secondary information. Thus, based on uplink data messages received in the receiving station it may be determined, for each point in time, which animal that was located at which master station. Preferably, the tag unit is further configured to transmit the uplink data message at predefined instances (e.g. at regular intervals, or in response to a request message), and repeat the transmission of each message a predetermined number of times, say 2 to 10 times.

According to another aspect of the invention, the object is achieved by the initially described method, wherein downlink data messages are transmitted over a second wireless interface from at least one master station. Each downlink data message includes secondary information pertaining to an identity of the master station from which the message is originated. The method also involves receiving the downlink data messages in each of the at least one tag unit over the second wireless interface; storing, in response to each received message, the secondary information and a time stamp indicating an instance at which the downlink data message was received in the tag unit. The time stamp and the secondary information are included in the uplink data message which is transmitted over the first wireless interface to the receiving station.

The advantages of this method, as well as the preferred embodiments thereof, are apparent from the discussion hereinabove with reference to the proposed testing arrangement.

According to a further aspect of the invention the object is achieved by a computer program, which is loadable into the internal memory of a computer, and includes software for controlling the above proposed method when said program is run on a computer.

According to another aspect of the invention the object is achieved by a computer readable medium, having a program recorded thereon, where the program is to control a computer to perform the above-proposed method.

Further advantages, advantageous features and applications of the present invention will be apparent from the following description and the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now to be explained more closely by means of preferred embodiments, which are disclosed as examples, and with reference to the attached drawings.

Figure 1 shows a system for positioning and tracking animals according to one embodiment of the invention; Figures 2a-b show block diagrams over tag units according to different embodiments of the invention; and Figure 3 illustrates, by means of a flow diagram, the general method of controlling a computer apparatus to perform the proposed procedure implemented by the tag unit.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

We refer initially to Figure 1 which shows an example of a system for positioning and tracking animals according to one embodiment of the invention.

The proposed system includes at least one master station and at least one receiving station. Figure 1 shows one receiving station R1 and three master stations M1 , M2 and M3. Each animal that is to be positioned and tracked has a respective tag unit T1 , T2, T3 and T4 is attached to its body (e.g. around the neck). The tag unit is configured to repeatedly transmit uplink data messages D_U|[ID1 ], D_u,[M1 ,ID2,t1], D_U|[M2,ID4,t2] and D_U|[M3,ID3,t3]) over a first wireless interface. Each uplink data message includes primary information pertaining to an identity ID1 , ID2, ID3 and ID4 of the tag unit T1 , T2, T3 and T4 respectively from which the message is originated.

The receiving station R1 is configured to receive the uplink data messages D_U,[ID1], D_U,[M1 ,ID2,t1], D_u,[M2,ID4,t2] and D_U,[M3,ID3, t3] over the first wireless interface.

Each master station M1 , M2 and M3 is configured to transmit downlink data messages D_dl1 , D_d|2 and D_d|3 respectively over a second wireless interface. Each of these messages, in turn, includes secondary information pertaining to an identity of the master station M1 , M2 and M3 from which the message is originated. Moreover, each tag unit T1 , T2, T3 and T4 is configured to receive the downlink data messages D_d|1 , D_d|2 and D_d|3 over the second wireless interface, and in response thereto store the secondary information along with a time stamp t1 , t2 and t3 indi- eating an instance at which the message D_d|1 , D_d|2 and D_d|3 respectively was received. Each tag unit T1 , 12, T3 and T 4 is further configured to include the registered time stamp t1 , t2 and t3 and the secondary information in the uplink data message D_u,[M1 ,ID2,t1], D_U|[M2,ID4, t2] and D_U|[M3,ID3,t3] that is transmitted over the first wireless interface. Hence, the time instances when the tag units T1 , 12, T3 and T 4 are located in proximity to the master station M1 , M2 and M3 can be determined. However, before a tag unit T1 has received its first downlink data message no secondary information will be included in the uplink data messages D_U|[ID1 ] transmitted from this tag unit T1 . Instead, during this phase, the uplink data messages transmitted from the tag unit T1 may exclusively include the primary information pertaining to the identity ID1 of the tag unit T1.

In the embodiment of the invention illustrated in Figure 1 , we assume that each master station M1 , M2 and M3 has a respective transmission range r1 , r2 and r3 within which the master station may reach tag units T2, T4 and T3 respectively. As can be seen in Figure 1 , the master stations M1 , M2 and M3 are separated from one another by a distance d1 -2, d2-3 and d1-3, which substantially exceeds the transmission ranges r1 , r2 and r3. Consequently, a tag unit T1 , 12, T3 or T4 may never simultaneously receive downlink data messages D_d|1 , D_d[2 or D_d|3 from more than one master station M1 , M2 or M3. This renders it possible to unambiguously link a tag unit 12, T3 or T4 to a master station M1 , M2 or M3 respectively. According to one embodiment of the invention, the master stations M1 , M2 and M3 are specifically configured to transmit the downlink data messages D_dl1 , D_d]2 and D_d|3 at such respective power levels that, at each position within the system, a given tag unit tag unit T1 , 12, T3 or T4 is configured to receive the downlink data messages D_d|1 , D_d|2 and D_d|3 exclusively from a single master station M1 , M2 or M3. Moreover, on account of the time stamps t1 , t2 and t3 it is further possible to determine a respective point in time when the tag unit T2, T3 or T4 was located within the transmission ranges r1 , r2 or r3 in question.

In an automatic milking plant, it is generally advantageous to arrange master stations M1 , M2 and M3 at strategic positions rela- tive to the milking stations, such that the animals' identities can be registered well in advance of the milking place(s). Thereby, appropriate measures can be taken if a special situation arises (e.g. that the animal is found to be in heat, is known to require a particular type of milking, is found unhealthy, and/or needs me- dical treatment in connection with the milking). It may also be favorable to arrange a respective master station M1 , M2 and M3 in close proximity to the milking to verify the registered identity of the animal.

Depending on how often the downlink data messages D_d|1 , D_d|2 and D_d|3 are transmitted and/or the rate at which the time stamp data t1 , t2 and t3 is updated it may be desirable to include a predetermined number of data components in each uplink data message D_U,[M1 ,ID2,t1], D_U|[M2,ID4,t2] and D_U|[M3,ID3,t3]. Each data component here represents an event specified by the time stamp t1 , t2 or t3, and includes the primary and secondary information. For example, if a downlink data message D_d|1 , D_d|2 and D_dl3 is transmitted once every 20^th millisecond, the predetermined number may be five and one new uplink data message D_U|[M1 ,ID2,t1], D_U|[M2,ID4,t2] and D_Uι[M3,ID3,t3] per second may be generated. Thus, each uplink data message reflects five events during the preceding second.

Preferably, the tag units T1 , T2, T3 and T4 are configured to transmit their uplink data messages D_U|[1D1], D_Uι[M1 ,ID2,t1 ], D_Uι[M2,ID4, t2] and D_Uι[M3,ID3,t3] at predefined points in time, for instance at regular intervals, or in response to an incoming request message. Moreover, to improve the robustness and to reduce the interference sensitivity, the tag units T1 , T2, T3 and T4 may be configured to repeat the transmission of each message a predetermined number of times, say three. Naturally, ac- cording to the invention, any alternative numbers of repetitions and intervals other than what is stated above are equally well conceivable.

Figures 2a and 2b show block diagrams over tag units T2 accor- ding to embodiments of the invention. In Figure 2a a relatively uncomplicated tag unit T2 is illustrated.

This tag unit T2 presumes that the master station is configured to include the time stamp t1 in the downlink data message D_d!1 [t1 ] being transmitted over the second wireless interface.

The tag unit T2 includes a processing unit 210, a receiving means 220, a transmitting means 230 and a data storage means 240. The processing unit 210 is adapted to control the operation of the tag unit T2 based on a computer program, which may be stored in the data storage means 240. The processing means 210 may also be configured to store the secondary information (i.e. information pertaining to the identity of the master station M1 that originated the received downlink data message D_d|1 [t1]) and the time stamp t1 associated thereto in response to this message D_d|1 [t1 ]) in the data storage means 240. The proces- sing means 210 is further configured to cause inclusion of the time stamp t1 , the primary information (i.e. pertaining to the identity of the tag unit T2) and the secondary information M1 and the in the uplink data message D_U|[M1 ,ID2,t1 ].

The receiving means 220 is adapted to receive the downlink messages D_d|1 [t1 ] from the master stations over the second wireless interface, and the transmitting means 230 is adapted to transmit the uplink data messages D_U|1 [M2,ID4,t1 ] to the at least one receiving station over the first wireless interface. According to embodiments of the invention, the receiving means 220 may either include a radio receiver which is configured to establish the second wireless interface via at least one radio resource, or include at least one means which is configured to receive information via magnetic field influence and thus establish the second wireless interface magnetically.

Due to the potentially large distance between the tag unit T2 and the receiving station, the transmitting means 230 preferably includes at least one radio transmitting means 230 which is confi- gured to establish the first wireless interface via at least one radio resource.

Figure 2b shows a somewhat more capable tag unit T2 than the tag unit illustrated in Figure 2a. Nevertheless, all reference labels in Figure 2b which are identical to reference labels in Figu- re 2a designate the same units/entities as those described above with reference to Figure 2a.

The tag unit T2 in Figure 2b is independent from an external clock reference. This means that the downlink data messages D_d|1 [t1 ] from the master stations transmitted over the second wi- reless interface need not include any time stamps. Instead, the tag unit T2 includes a clock module 250 and means (e.g. implemented by the processing 210) configured to generate the time stamp t1 based on data produced by the clock module 250 in connection with receiving a message D_d|1 from one of the at least one master station.

According to one embodiment of the invention, the tag unit T2 includes means 260 for registering a physical activity of the animal to which the unit is attached. For example, the means 260 for registering the physical activity may include a magnetic sen- sor for detecting variations in an induced electromagnetic field. In any case, the processing 210 is configured to cause repeated transmission of information P2 pertaining to the physical activity over the first wireless interface, such that for example the receiving station R1 can process this information. Naturally, the means 260 may equally well be included in the tag unit T2 shown in Figure 2a.

In order to sum up, we will now describe the general method of controlling a computer apparatus to perform the proposed proce- dure implemented by the tag unit with reference to the flow diagram in figure 3. It should be noted that the procedure implemented by the master station(s) and the receiving station(s) is very uncomplicated, namely involving repeated transmission of down- link messages and reception of uplink messages respectively. Therefore, no corresponding flow diagrams are presented here.

In the procedure performed by the tag unit, a first step 310 investigates whether or not a downlink data message has been received from a master station. If so, a step 320 follows, and otherwise the procedure loops back to step 310. (Alternatively, as reflected by D_U|[ID1 ] in Figure, the tag unit may transmit uplink data messages also if no downlink data message has yet been received. In such a case, however, the uplink data messages are devoid of secondary information.)

Step 320 stores secondary information (i.e. information pertaining to the identity of the master station which originated the downlink message) and a time stamp indicating an instance at which the downlink data message was received in the tag unit. Thereafter, the procedure loops back to step 310.

In parallel with step 310, a step 330 investigates whether or not an uplink data message is to be transmitted (e.g. because a timing schedule prescribes such a transmission, or because a received request message orders this type of transmission). If an uplink data message should be transmitted, a step 340 follows, and otherwise the procedure continues to step 310.

Step 340 transmits an uplink data message over a first wireless interface, such that this message can be received by at least one receiving station. The uplink data message includes primary information pertaining to an identity of the tag unit from which the message is originated, and at least one pair of secondary information and associated time stamp. Subsequently, the procedure loops back to steps 310 and 330.

All of the process steps, as well as any sub-sequence of steps, described with reference to Figure 3 above may be controlled by means of a programmed computer apparatus. Moreover, although the embodiments of the invention described above with reference to the drawings comprise computer apparatus and processes performed in computer apparatus, the invention thus also extends to computer programs, particularly computer programs on or in a carrier, adapted for putting the invention into practice. The program may be in the form of source code, object code, a code intermediate source and object code such as in partially compiled form, or in any other form suitable for use in the implementation of the process according to the Invention. The program may either be a part of an operating system, or be a separate application. The carrier may be any entity or device capable of carrying the program. For example, the carrier may comprise a storage medium, such as a Flash memory, a ROM (Read Only Memory), for example a DVD (Digital Video/Versatile Disk), a CD (Compact Disc) or a semiconductor ROM, an EP- ROM (Erasable Programmable Read-Only Memory), an EEPROM (Electrically Erasable Programmable Read-Only Memory), or a magnetic recording medium, for example a floppy disc or hard disc. Further, the carrier may be a transmissible carrier such as an electrical or optical signal which may be conveyed via electrical or optical cable or by radio or by other means. When the program is embodied in a signal which may be conveyed directly by a cable or other device or means, the carrier may be constituted by such cable or device or means. Alternatively, the carrier may be an integrated circuit in which the program is embedded, the integrated circuit being adapted for performing, or for use in the performance of, the relevant processes.

Although the invention primarily is intended to be utilized in connection with cow milking, the invention is equally well adapted for testing milking machines for any other kind of mammals, such as goats, sheep or buffaloes.

The term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components. However, the term does not preclude the presence or addition of one or more additional features, integers, steps or components or groups thereof.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any suggestion that the referenced prior art forms part of the common general knowledge in Australia, or any other country.

The invention is not restricted to the described embodiments in the figures, but may be varied freely within the scope of the claims.

Claims

1. A system for positioning and tracking animals with respect to time instances when each animal is located at predefined positions, the system comprising: at least one tag unit (T1 , T2, T3, T4) adapted to be attached to a respective animal, the tag unit being configured to repeatedly transmit uplink data messages (D_U|[ID1], D_υι[M1 ,ID2,t1 ], D_U|[M2,ID4,t2], D_uI[MS, ID3,t3]) over a first wireless interface, each uplink data message including primary information pertai- ning to an identity of the tag unit (T1 , T2, T3, T4) from which the message is originated, and at least one receiving station (R1 ) configured to receive the uplink data messages (D_U,[ID1], D_uj[M1 ,ID2,t1 ], D_U|[M2,ID4,t2], D_U|[M3,ID3, t3]) over the first wireless interface, characterized in that the system comprises at least one master station (M1 , M2, M3) which each is configured to transmit downlink data messages (D_d|1 , D_d|2, D_d|3) over a second wireless interface, each downlink data message including secondary information pertaining to an identity of the master station (M1 , M2, M3) from which the message is originated, and each of the at least one tag unit (T1 , T2, T3, T4) is configured to: receive the downlink data messages (D_d|1 , D_d|2, D_d|3) over the second wireless interface, in response to each received downlink data message (D_d|1 ,

D_d|2, D_d|3) store the secondary information and a time stamp (t1 , t2, t3) indicating an instance at which the message (D_d|1 , D_d|2, D_d|3) was received, and include the time stamp (t1 , t2, t3) and the secondary infor- mation in the uplink data message (D_Uι[M1 ,ID2,t1], D_ui[M2,ID4, t2], D_U|[M3,ID3,t3]) transmitted over the first wireless interface.

2. The system according to claim 1 , wherein each of the at least one master station (M1 , M2, M3) is configured to transmit the downlink data messages (D_d|1 , D_d|2, D_d|3) at such respective power level that at each position within the system a given tag unit of the at least one tag unit (T1 , T2, T3, T4) is configured to receive the downlink data messages (D_d|1 , D_d|2, D_d|3) exclusively from a single master station of the at least one master station (M1 , M2, M3).

3. The system according to any one of the claims 1 or 2, wherein the master station is configured to include the time stamp (t1 ) in the downlink data message (D_d(1 [t1 ]).

4. The system according to any one of the claims 1 or 2, whe- rein tag unit (T2) comprises: a clock module (250), and means configured to generate the time stamp (t1 ) based on data produced by the clock module (250) in connection with receiving a message (D_d]1 ) from one of the at least one master sta- tion (M1 ).

5. The system according to any one of the preceding claims, wherein each of the at least one master station (M1 , M2, M3) comprises at least one radio transmitting means, and each of the at least one tag unit (T2) comprises at least one radio receiving means (220), the transmitting and receiving means being configured to establish the second wireless interface via at least one radio resource.

6. The system according to any one of the claims 1 to 4, wherein each of the at least one master station (M1 , M2, M3) com- prises at least one means configured to transmit information via magnetic field influence, and each of the at least one tag unit (T2) comprises at least one means (220) configured to receive information via magnetic field influence, said means being configured to establish the second wireless interface magnetically.

7. The system according to any one of the preceding claims, wherein each of the at least one tag unit (T2) comprises at least one radio transmitting means (230), and each of the at least one receiving station (R1 ) comprises at least one radio receiving means, the transmitting and receiving means being configured to establish the first wireless interface via at least one radio resource.

8. The system according to any one of the preceding claims, wherein if the system comprises two or more master stations (M1 , M2, M3), the master stations are separated from one another by a distance (d1-2, d2-3, d1 -3) substantially exceeding a respective transmission range (r1 , r2, r3) of each master station (M1 , M2, M3).

9. The system according to any one of the preceding claims, wherein the tag unit (T2) comprises means (260) for registering a physical activity of the animal to which the unit is attached, and the tag unit (T2) is configured to repeatedly transmit information (P2) pertaining to the physical activity over the first wireless interface.

10. The system according to any one of the preceding claims, wherein each uplink data message (D_U|[M1 ,ID2,t1 ], D_U|[M2,ID4, t2], D_U|[M3,ID3,t3]) comprises a predetermined number of data components, each data component representing an event specified by the time stamp (t1 , t2, t3) and comprising the primary and secondary information.

11. The system according to claimi O, wherein the tag unit (T1 , T2, T3) is configured to: transmit the uplink data message (D_U|[M1 ,ID2,t1 ], D_ul[M2, ID4, t2], D_U|[M3,ID3,t3]) at predefined instances, and repeat the transmission of each message a predetermined number of times.

12. Method of positioning and tracking animals with respect to time instances when each animal is located at predefined positions, each being associated with a unique tag unit (T1 , T2, T3, T4), the method comprising: transmitting repeatedly uplink data messages (D_U|[ID1],

D_ul[M1 ,ID2, t1], D_Uι[M2, ID4,t2], D_u,[M3,ID3,t3]) over a first wireless interface from each tag unit to at least one receiving station (R1 ), each uplink data message including primary information pertaining to an identity of the tag unit (T1 , T2, T3, T4) from which the message is originated, and receiving the uplink data messages (D_ut[ID1], D_U|[M1 ,ID2, t1], D_U|[M2, ID4,t2], D_UI[M3,ID3,t3]) over the first wireless interface in at least one receiving station (R1 ), characterized by transmitting downlink data messages (D_d|1 , D_d(2, D_d|3) over a second wireless interface from at least one master station (M1 , M2, M3), each downlink data message including secondary information pertaining to an identity of the master station (M1 , M2, M3) from which the message is originated, receiving in each of the at least one tag unit (T2, T3, T4) the downlink data messages (D_d|1 , D_d|2, D_d|3) over the second wireless interface, storing, in response to each received message (D_d|1 , D_dl2, D_d|3), the secondary information and a time stamp (t1 , t2, t3) indicating an instance at which the downlink data message (D_dl1 , D_d|2, D_d|3) was received in the tag unit (T2, T3, T4), and including the time stamp (t1 , t2, t3) and the secondary information in the uplink data message (D_U][M1 ,ID2,t1], D_U|[M2, ID4,t2], D_U|[M3,ID3,t3]) transmitted over the first wireless inter- face.

13. The method according to claim 12, comprising transmitting the downlink data messages (D_d|1 , D_d|2, D_d|3) from each of the at least one master station (M1 , M2, M3) at such respective power level that at each position within the system a given tag unit of the at least one tag unit (T1 , T2, T3, T4) is configured to receive the downlink data messages (D_di1 , D_d|2, D_d|3) exclusively from a single master station of the at least one master station (M1 , M2, M3).

14. Method according to any one of the claims 12 or 13, comp- rising including the time stamp (t1 ) in the downlink data message

(D_d,1 [t1]).

15. Method according to any one of the claims 12 or 13, comprising generating the time stamp (t1 ) based on a local time reference (250) in the tag unit (T2) produced in connection with re- ceiving a downlink data message (D_d(1 ).

16. The method according to any one of the claims 12 to 15, comprising: registering in each tag unit (T2) a physical activity of the animal to which the unit is attached, and transmitting repeatedly information pertaining to the physical activity from the tag unit (T2) over the first wireless interface.

17. The method according to any one of the claims 12 to 16, wherein each uplink data message (D_U|[M1 ,ID2,t1], D_U|[M2,ID4, t2], D_U|[M3,ID3,t3]) comprises a predetermined number of data components, each data component represents an event specified by the time stamp (t1 , t2, t3) and comprising the primary and secondary information.

18. The method according to claim 17, comprising: transmitting the uplink data message (D_U|[M1 ,ID2,t1]_I

D_U|[M2,ID4, t2], D_U|[M3,ID3,t3]) at predefined instances, and repeating the transmission of each message a predetermined number of times.

19. A computer program loadable into the internal memory (240) of a computer, comprising software for controlling the steps of any of the claims 12 to 18 when said program is run on the computer.

20. A computer readable medium (240) having a program recorded thereon, where the program is to make a computer control the steps of any of the claims 12 to 18 when the program is loaded into the computer.