GB2448743A

GB2448743A - Bi-directional communication in an asset tracking system

Info

Publication number: GB2448743A
Application number: GB0708118A
Authority: GB
Inventors: Bruce William Henry Harbon; Christopher James French
Original assignee: Airbus DS Ltd
Current assignee: Airbus DS Ltd
Priority date: 2007-04-26
Filing date: 2007-04-26
Publication date: 2008-10-29
Also published as: GB0708118D0

Abstract

An asset tracking system 2 is disclosed comprising a plurality of mobile devices 10,12,14,16,18 to be tracked and a central server 4 that is able to communicate with each of the mobile devices over a communications network 8. In use, each mobile device 10,12,14,16,18 sends position data to the central server 4 over the communications network 8. The central server 4 further sends position data regarding the said mobile devices 10,12,14,16,18 to each of the mobile devices 10,12,14,16,18 over the communications network 8. The position data may be a difference between a previous position and a current position of each mobile device to reduce the amount of data being transferred. Each of the mobile devices may comprise a GPS receiver to determine the location of each device.

Description

-1-2448743 Asset Tracking System This invention relates to asset

tracking systems. In particular, the invention relates to the provision of bi-directional communications in asset tracking systems.

The present invention provides a method of tracking a plurality of mobile devices, the method comprising the seeps of: providing a central server able to communicate with each of said plurality of mobile devices over a communications network; arranging for each mobile device to send position data to said central server over said network; and sending position data of said mobile devices to each of said mobile devices over said communications network.

The present invention also provides an asset tracking system comprising: a plurality of remote devices to be tracked; a central server; and a communications network, wherein: the central server is adapted to communicate with each remote device over said communications network; each mobile device is adapted to send position data to the central server over said network; and the central server is adapted to send position data regarding said mobile devices to each of said mobile devices over said communications network.

The present invention further provides a server for use in an asset tracking system, wherein the system further comprises a plurality of remote devices to be tracked, wherein the server is adapted to: communicate with each remote device over a communications network; receive position data from each mobile device; and send position data regarding the position of said mobile devices to S -2-each of said mobile devices over said communications network.

The present invention yet further provides a mobile device for use in an asset tracking system, wherein the system further comprises a central server, wherein the mobile device is adapted to: communicate with the central server over a communications network; send position data to the central server over said network; and receive position data regarding the position of other mobile devices in communication with the central server over said communications network. In some forms of the invention, the position data includes data regarding the positions of all mobile devices with the exception of the mobile device to which the data is being sent. In other forms of the invention, the position data includes data regarding the positions of all mobile devices except the mobile device to which the data is being sent.

In one form of the invention, the position data takes the form of a difference between a previous position and a current position of the mobile device (referred to herein as a position delta). This can be used to enable the cumulative application of position deltas. The transmission of changes in position rather than absolute position also has the advantages of reducing the amount of data being transferred and increasing the security of the data.

The position data may take the form of a first signal indicative of a change in latitude and a second signal indicative of a change in longitude.

In one form of the invention, the nuniber of bits used to transmit the position data is variable in dependence on the magnitude of the change in position. For example, small changes in position may require, say, three bits of data to indicate the change, whereas larger changes of position may require more data bits. By allowing small changes to be represented by small numbers of data bits, the amount of data transfer can be kept to a minimum.

The present invention may also allow an absolute position to be transmitted. In one form of the invention, this requires 26-bits of latitude data and 26-bits of longitude data to be sent.

The said position data may be obtained using a satellite positioning system, such as the well-known Global Positioning System. In some forms of the invention, all mobile devices obtain position data in this way.

However, other means for obtaining position data could be used. Furthermore, in some forms of the invention, different mobile devices may obtain position data in different ways.

The data may be transferred using one or more data packets. In one exemplary form of the invention, the data packets are UDP packets.

Each data packet may include a code identifying that packet. This feature may be used to enable the server and/or a mobile device to determine when it has missed a packet of data.

The server may resend data packets if a mobile device indicates that it has not received a particular packet.

Similarly, a mobile device may resend a data packet if the server indicates that it has riot received a particular packet.

In some forms of the invention, data may be encrypted before it is sent from said server to said mobile device.

Similarly, in some forms of the invention, data may be encrypted before it is sent from a mobile device to said central server. In an exemplary form of the invention, the said encryption takes the form of cipher-block to chaining; however, other forms of the invention may be used.

In one form of the invention, position data for all data channels is sent in order of increasing channel number, such that the channel numbers are implied. This reduces the amount of data transferred over the network.

The server may transmit data to said clients after the expiry of a predetermined time since the last transmission of data; in one form of the invention, data is sent every 4 seconds.

Each mobile device may send position data to said server on receipt of position data from the server. This sending of data may take place immediately on receipt of position data from the server.

In one form of the invention, a separate message including position data is sent to each of said mobile devices. That message may include position data of all mobile devices, with the exception of the mobile device to which it is being transmitted. Alternatively, the message may include position data of all mobile devices, including the mobile device to which it is being transmitted.

The mobile devices could take many difference forms. By way of example, in one form of the invention, the mobile devices are hand-held, wireless devices.

Devices and methods in accordance with the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which: Fig. 1 is a block diagram of an asset tracking system in accordance with an aspect of the present invention; Fig. 2 is a schematic representation of the transmission of data from a moving client device in accordance with an aspect of the present invention; Fig. 3 shows the format of a UDP message sent from the server to client devices in an application of the present invention; Fig. 4 shows the format of a UDP message sent from a client to the server in an application of the present invention; Fig. 5 shows the format of a UDP message sent from a client to the server in an application of the present invention; and Fig. 6 shows an exemplary image provided on a client device in accordance with an aspect of the present invention.

Figure 1 shows an asset tracking system, indicated generally by the reference numeral 2, comprising a server 4 connected to a display 6 and to a communications network 8. The system 2 also comprises a number of mobile devices 10, 12, 14, 16 and 18, each connected to the communications network 8. Each of the mobile devices is termed a client in the system 2.

Each client 10, 12, 14, 16 and 18 comprises a GPS receiver used to identify the precise location of the device. Each client transmits positional data to the central server 4 via the communications network 8. In this way, the central server is able to provide a visual overview of the location of each of the clients on the display 6.

In addition, the server 4 is arranged to transmit data to each of said clients so that each client is informed of the position of the other clients. Each client is provided with a display so that a visual indication of the position of each of the mobile units can be given to a user of the client device.

In one form of the invention, the communications network is a GPRS network. In one form of the invention, the mobile devices are personal digital assistants (PDAs).

Of course, the skilled person would be aware of many alternative communication networks and mobile devices that could be used.

The system 2 of Figure 1 transfers data over the network 8 using User Datagrarn Protocol (UDP) messages (that protocol is sometimes referred to by other names, such as Universal Datagrarn Protocol).

The UDP messages contain delta information for members of the group and positions of current members (Server to Client) and a position delta for the Client (Client to Server).

The UDP messages use sequence numbers and retransmissions for reliable in order receipt of successive information deltas. This arrangement is shown in Figure 2, which shows a client device 20 moving in the direction shown by arrow 22. At time t1, the client device 20 transmits data including a sequence number (exemplary sequence number 32 is shown in Figure 2) and a position delta indicating the change in position since the last transmission of data (position delta l is shown in Figure 2). At time t2, the client device 20 transmits a new sequence number (sequence number 33) and a new position delta (L2). As shown in Figure 2, the position delta L2 represents the change in position (in terms of longitude and latitude) since time t1.

Assuming a spherical earth of radius 6372795 metres, five decimal places of degree gives an accuracy of 2*PI*6372795/360/1e5 = 1.1 metres per hundred thousandth of a degree. Hence a full latitude or longitude can be represented in LOG(360*100000, 2) 26 bits.

The use of position deltas avoids the need for periodic retransmission of full positions, which has both bandwidth and security advantages, and enables the use of cipher block chaining for the encryption of position information to enhance security. The design is simplified as there is no need to synchronise the deltas with absolute positions that would otherwise be transmitted on a parallel TCP stream. Finally, the reliable transmission of deltas means that the deltas can be applied cumulatively at the receiver. This is achieved by using fixed precision 5 decimal place arithmetic for latitude and longitude values so there is no drift due to floating point rounding.

Figure 3 is a block diagram showing the format of a UDP message sent from the server 4 to update the client devices 10, 12, 14, 16 and 18. The client update message, indicated generally by the reference numeral 30, comprises a message type indicator 31, a receiver sequence number 32, a transmit sequence number 34, a set of group delta descriptors 36, a set of position delta descriptors 38 and padding bits 39.

The message type indicator 31 has a value 0 to indicate that the particular message is an initial message arid a value 1 to indicate that the particular message is part of an ongoing sequence of messages.

The receiver sequence number 32 is a 3-bit number indicating the sequence number of the next message that the sender (i.e. the server) is expecting to receive from the client concerned. Similarly, the transmit sequence number 34 is a 3-bit number of the current message.

The group delta descriptors 36 are used to inform client devices whether any new clients have been added to the network and/or whether any previous clients have been removed. Each group delta descriptor starts with one of the following bit sequences: 0 ("DiffAdd"), 10 ("DiffDeleted") or 11 ("DiffGroupDeltaEnd"). The transmission of the bit sequence 0 (the DiffAdd signal) is followed by a 10-bit channel number of a channel that has been added to the group; position deltas are supplied for this channel until further notice. The transmission of the bit sequence 10 (the DiffDeleted signal) is followed by a 10-bit channel number of a channel that has been deleted from the group; position deltas are no longer supplied for this channel. The transmission of the bit sequence 1]. (the DiffGroupDeltaEnd signal) marks the end of the group delta descriptor.

By way of example, consider the following bit sequence: o 1010101010 0 1100110011 1110001110 11.

The first sequence indicates that the channel number 1010101010 has been added to the group, the second sequence indicated that the channel 1100110011 has been added to the group, the third sequence indicates that channel 1110001110 has been deleted from the group and the final sequence terminates that group delta descriptors.

Once a set group delta descriptors (if any) have been transmitted, a set of position delta descriptors 38 are transmitted. The delta descriptors transmit position information for each client device in the group. The delta descriptors are provided for each channel in the group, in order of increasing channel number.

Accordingly, it is not necessary to transmit the channel number in order to identify the channel to which the data refers.

Each position delta descriptor starts with one of the following bit sequences: 00 (DiffNoFurther"), 01 (DiffDe1ta1"), 10 ("DiffDelta2"), 110 (DiffDe1ta3"), 1110 ("DiffFull"), 1111 ("DiffPosDeltaEnd").

The bit sequence 00 ("DiffNoFurther") indicates that no update to the client position has been received since the last value supplied. There are therefore no following bits for that channel. It should be noted that there may be no data to transmit either because the client concerned has not moved, or because no data has been received from that client.

The bit sequence 01 ("DiffDeltal") indicates that the position delta for the channel is small, i.e. that the change in position of the client device is small. The bit sequence 01 is followed by a 1-bit value indicating whether the new latitude is bigger (bit 1 transmitted) or smaller (bit 0 transmitted) followed by a 3-bit representation of the change in latitude. This is followed by a 1-bit value indicating whether the new longitude is bigger (bit 1 transmitted) or smaller (bit 0 transmitted) followed by a 3-bit representation of the change in longitude.

The bit sequence 10 (DiffDelta2") indicates a medium position delta size, i.e. that the change in position of the client device is of a medium level. The bit sequence is followed by a 1-bit value indicating whether the new latitude is bigger (bit 1 transmitted) or smaller (bit 0 transmitted) followed by a 6-bit representation of the change in latitude. This is followed by a 1-bit value indicating whether the new longitude is bigger (bit 1 transmitted) or smaller (bit 0 transmitted) followed by a 6-bit representation of the change in longitude.

The bit sequence 110 ("DiffDelta3") indicates that the position delta for the channel is large, i.e. that the change in position of the client device is large. The -11 -bit sequence 110 is followed by a 1-bit value indicating whether the new latitude is bigger (bit 1 transmitted) or smaller (bit 0 transmitted) followed by a 9-bit representation of the change in latitude. This is followed by a 1-bit value indicating whether the new longitude is bigger (bit 1 transmitted) or smaller (bit 0 transmitted) followed by a 9-bit representation of the change in longitude.

The bit sequence 1110 ("DiffFull") indicates that an absolute position is being transmitted. The bit sequence 1110 is followed by a 26-bit value representative of the absolute latitude. This is followed by a 26-bit representation of the absolute longitude.

The set of position delta descriptors ends with the bit sequence 1111 ("DiffPosDeltaEnd").

By way of example, consider the following data sequence: 01 1011 1101 01 0111 1001 1001101 011000001 00 0101011100 1001111000 The first sequence indicates that the latitude of the first channel has increased by the data value 011 and the longitude of the channel has increased by the data value 101.

The second sequence indicates that there is no change to the position of the second channel.

The third sequence indicates that the latitude of the third channel has decreased by the data value 111 and the longitude of that channel has increased by the data value 001.

The fourth sequence indicates that the latitude of the fourth channel has increased by the data value 001101 and the longitude of that channel has decreased by the data value 11000001.

The fifth sequence indicates that there is no change to the position of the fifth channel.

The sixth data sequence indicates that the latitude of the sixth channel has decreased by the data value 101011100 and that the longitude of that channel has increased by the data value 001111000.

The final sequence terminates the position delta descriptors.

Finally, padding bits 39 are added so that the overall message length is equal to a whole number of bytes.

Figure 4 is a block diagram showing the format of an initial UDP message sent from a client to the server.

The initial UDP message, indicated generally by the reference numeral 40, comprises a message type indicator 42, a client channel number 44, a connected sequence number 46 and zero padding 48.

The message type indicator 42 has a value 1 to indicate -13 -that the message is an initial message (as described below with reference to Figure 5, a value 0 indicates that the message is a sequenced message).

The client channel 44 is a 10-bit signal indicating the channel number of the client.

The connected sequence number 46 is a 16-bit number that the server has passed to this client during an initial PCP connection exchange. The connected sequence number 46 allows the server to correlate the UDP packet with the connection. In one form of the invention, the connected sequence number is used as the GPRS network port maps UDP ports so the server cannot determine the originator from the source IP port number.

Finally, padding bits 48 are added so that the overall message length is equal to a whole number of bytes.

Figure 5 is a block diagram showing the format of an sequenced UDP message sent from a client to the server.

The sequenced UDP message, indicated generally by the reference numeral 50, comprises a message type indicator 51, a receiver sequence number 52, a transmitter sequence number 54, a position delta descriptor 56 and zero padding 58.

The message type indicator 51 has a value 0 to indicate that the message is an initial message (as described above with reference to Figure 4, a value 1 indicates that the message is an initial message).

The receiver sequence number 52 is a 3-bit signal indicating the sequence number of the next message the sender is expecting to receive.

The transmitter sequence number 54 is a 3-bit number indicating the sequence number of this particular S message.

The position delta descriptor 56 sends a single position delta indicating the change in position (if any) of the mobile device since the last message. The position delta descriptor 56 starts with one of the following bit sequences: 00 ("DiffNoFurther"), 01 ("DiffDeltal"), 10 ("DiffDelta2"), 110 (DiffDelta3"), 111 ("DiffDeltaFull").

(It should be noted that there is no need for a DiffposDeltaEnd code to terminate the delta descriptor as only one delta descriptor is sent.) As described above with reference to Figure 3, the bit sequence 00 (DiffNoFurther") indicates that there is no change of position since the last message, the bit sequence 01 ("DiffDeltal") indicates that the position delta is small, the bit sequence 10 ("DiffDelta2") indicates a medium position delta size, the bit sequence ("DiffDelta3") indicates that the position delta for the channel is large, and the bit sequence 111 (DiffFull") indicates that an absolute position is being transmitted.

The form of the position deltas is the sante as the position deltas described above with reference to Figure 3. Thus, each position delta sends separate longitude and latitude data having a number of bits dependent on the position delta size.

Finally, padding bits 58 are added so that the overall message length is equal to a whole number of bytes.

In order to centralise control of the positional updates and allow possible future compensation for Server to S Client network delays, the following protocol is used.

The Server maintains a 4-second timer for each Client.

When the timer expires for a given Client, the server formulates a new GroupUpdate message containing delta information relative to the previous message transmission and sends it to the Client (such messages are described in detail above). Upon receipt, the Client decodes the message and applies deltas. The Client then immediately replies with the latest positional delta information for its own position. This simplifies the implementation at is the Client and ensures that each end is transmitting its most up to date status every 4 seconds. Internally a Client maintains a list of group members ordered by channel number. Upon receiving a GroupUpdate message, it adjusts the list (adding and deleting members) before applying the ordered positional deltas.

The Server retransmits the most recent packet at the next expiry if the Client indicates that it has not received it. Likewise the Client retransmits to maintain the reliability of the stream. Both ends maintain timers to cause connection cleanup in the event that the TCP streams do not trigger shutdown of the connection (e.g. if the PDA is cut off from GPRS suddenly).

The reference model for costing is a 5 Client system with a single user group. With continual movement of all members of the group equivalent to driving in a built up zone, a single positional update is sent to each Client every 4 seconds of approximately 15 bytes in size. In addition, each Client sends a positional i..ipdate to the server of approximately 5 bytes. This gives a total data transfer per client of (15+5)*(60/4)*60 = 18 Kbyte/hour.

At 80p/Mbyte this is 18e3*80/1e6 = 1.4 pence per hour per Client as required.

By way of example, Figure 6 shows an exemplary image, indicated generally by the reference numeral 60, that might be displayed by a client device in an application of the present invention. The image 60 shows a map. The location of the client device is indicated by the reference numeral 62. The locations of two other client devices are indicated by the reference numerals 64 and 66.

The present invention can also be used to transfer other data, in addition to positional data, from client devices to the central server and from the central server to all client devices. By way of example, the image 60 also shows an area marking 68, together with points of interest indicated by markers 70 and 72. The area marking 68 and points of interest markers 70 and 72 may have been input at any of the client devices in a group.

Claims

CLAIMS: 1. A method of tracking a plurality of mobile devices, the

method comprising the steps of: providing a central server able to communicate with each of said plurality of mobile devices over a communications network; arranging for each mobile device to send position data to said central server over said network; and sending position data of said mobile devices to each of said mobile devices over said communications network.
2. A method as claimed in claim 1, wherein said position data takes the form of a difference between a previous position and a current position of the mobile device.
3. A method as claimed in claim 2, wherein said position data takes the form of a first signal indicative of a change in latitude and a second signal indicative of a change in longitude.
4. A method as claimed in any one of claims 1 to 3, wherein the number of bits used to transmit the position data is variable in dependence on the magnitude of the change in position.
5. A method as claimed in any preceding claim, wherein said position data is obtained using a satellite positioning system.
6. A method as claimed in any preceding claim, wherein said data is transferred using one or more data packets.
7. A method as claimed in claim 6, wherein said data packets are UDP packets.
8. A method as claimed in claim 6 or claim 7, wherein each packet include a code identifying that packet.
9. A method as claimed in any one of claims 6 to 8, wherein the server resends data packets if a mobile device indicates that it has not received a particular packet.
10. A method as claimed in any one of claims 6 to 9, wherein a mobile device resends a data packet if the server indicates that it has not received a particular packet.
11. A method as claimed in any preceding claim further comprising the step of encrypting data before it is sent from said server to said mobile device.
12. A method as claimed in any preceding claim, further comprising the step of encrypting data before it is sent from a mobile device to said central server.
13. A method as claimed in claim 11 or claim 12, wherein said encryption takes the form of cipher-block chaining.
14. A method as claimed in any preceding claim, wherein position data for all data channels is sent in order of increasing channel number, such that the channel numbers are implied.
15. A method as claimed in any preceding claim, wherein the server transmits data to said clients after the expiry of a predetermined time since the last transmission of data.
16. A method as claimed in any preceding claim, wherein each mobile device sends position data to said server on receipt of position data from the server.
17. A method as claimed in any preceding claim, wherein a separate message including position data is sent to each of said mobile devices.
18. An asset tracking system comprising: a plurality of remote devices to be tracked; a central server; and a cormnunicatjons network, wherein: the central server is adapted to communicate with each remote device over said communications network; each mobile device is adapted to send position data to the central server over said network; and the central server is adapted to send position data regarding said mobile devices to each of said mobile devices over said communications network.
19. A system as claimed in claim 18, wherein said position data takes the form of a difference between a previous position and a current position of the mobile device.
20. A system as claimed in claim 18 or claim 19, further comprising means for varying the number of bits used to transmit the position data in dependence on the magnitude of the change in position.
21. A system as claimed in any one of claims 18 to 20, wherein each mobile device further comprises a satellite positioning receiver for use in obtaining said position data.
22. A system as claimed in any one of claims 18 to 21, wherein the central server sends a separate message including position data to each of said mobile devices. to
23. A system as claimed in claim 22, wherein the central server further comprises a timer means and wherein the central server sends a message to a mobile device each time the timer means indicates that a predetermined period has elapsed.
24. A server for use in an asset tracking system, wherein the system further comprises a plurality of remote devices to be tracked, wherein the server is adapted to: communicate with each remote device over a communications network; receive position data from each mobile device; and send position data regarding the position of said mobile devices to each of said mobile devices over said communications network.
25. A mobile device for use in an asset tracking system, wherein the system further comprises a central server, wherein the mobile device is adapted to: communicate with the central server over a communications network; send position data to the central server over said network; and receive position data regarding the position of other mobile devices in communication with the central server over said communications network.