WO2007049011A1

WO2007049011A1 - Tracking device

Info

Publication number: WO2007049011A1
Application number: PCT/GB2006/003914
Authority: WO
Inventors: Julian Ivings; Kester Holmes; Iraklis Hatziathanasiou
Original assignee: Oxloc Limited
Priority date: 2005-10-27
Filing date: 2006-10-20
Publication date: 2007-05-03
Also published as: EP1941297A1

Abstract

A tracking device includes a tracking unit (21) and a power supply unit (31) for supplying power to the tracking unit. The tracking unit (21) includes a locating system (24,26) for acquiring location data from received location signals and a communications system (28,30) for transmitting acquired location data to a remote server. The power supply unit (31) includes a primary storage device (32) comprising an electrolytic cell and a secondary storage device (34), the secondary storage device being constructed and arranged to be charged by the primary storage device (32) when the power demand is low and to supply power to the tracking unit (21) when the power demand is high.

Description

TRACKING DEVICE

The present invention relates to a tracking device and in particular, but not exclusively, to a tracking device that may be used for tracking the movement of assets that do not have an available power supply, for example lorry trailers, goods containers and so on. The invention also relates to a tracking system.

Various tracking devices are known that use the global positioning system (GPS) to obtain geographical location data and the GSM (global system for mobile communications) network to transmit the location data to a base station, for example as a SMS (short message service) text message. Such tracking devices may be used for example to track vehicles. In this case they can be powered by the vehicle's electrical system and so can operate virtually indefinitely. Other tracking devices have been developed for tracking the movement of people or animals. These tend to be small, self-contained units having an onboard battery. Generally, they can only be used for a few days or weeks before the battery has to be recharged or replaced.

There is a growing need for a device that can be used to track assets such as lorry trailers for which the known tracking devices described above may not be suitable, because an external power supply is not readily available and the asset has an extended service interval (for example a year or more), during which it may be impractical to recharge or replace the batteries. Such a system requires a self-contained tracking device with an onboard power supply that is capable of operating without attention for an extended period, for example a year or more. Designing a device that meets this requirement and that is also reasonably compact and inexpensive is however extremely difficult.

Ih order to design such a device, numerous problems have to be overcome. For example, the GPS and GSM systems both have inherent problems that make it extremely difficult to design a tracking device that is highly energy efficient. Ih particular, both systems have a relatively high power consumption when operated continuously. One well-known way of reducing power consumption is by scheduling: i.e. by switching the device on and off at regular intervals. However, this in turn leads to additional problems with both the GPS and GSM systems. In the GPS system, after turning the GPS receiver on it can take an extended period of time to obtain a set of reliable location data, particularly if the geographical location of the device has changed greatly since the last GPS fix. If a fix cannot be obtained within a predeterminedperiod (for example because GPS signals cannot be received), the GPS detector may retry many times, causing additional energy consumption.

Further problems can be encountered in a GSM system that is not always switched on. When the GSM system is switched on, it polls the GSM network in order to log on-. This requires high power use. If for any reason the system is unable to log on, the polling sequence may be tried numerous times, leading to high energy consumption.

Self-contained tracking devices may use lithium batteries as a power supply, as they have a very high energy capacity for their size and weight. However, the efficiency at which the battery converts chemical energy into electrical energy depends on the current drawn from the battery. For example, a lithium battery with a rated capacity of 19Ah (at a current of 6mA) may have an actual capacity of only 6Ah at a current of 20OmA. As the GPS and GSM systems both intermittently draw quite high currents, efficient use is not made of the lithium battery.

A further problem with lithium batteries is that supplying a large load can cause the battery voltage to drop significantly. This can cause a connected micro-controller to reset, thus preventing proper operation of the product. In addition, excessive loads may sometimes be required for short periods of time, which the battery is incapable of supplying. This can cause the tracking device to fail, even though a significant amount of energy may still potentially be available within the battery.

It is an object of the present invention to provide a tracking device that mitigates at least some of the aforesaid problems. According to the present invention there is provided a tracking device including a tracking unit and a power supply unit for supplying power to the tracking unit, wherein the tracking unit includes a locating system for acquiring location data from received location signals and a communications system for transmitting acquired location data to a remote server, and the power supply unit includes a primary storage device comprising an electrolytic cell and a secondary storage device, the secondary storage device being constructed and arranged to be charged by the primary storage device when the power demand is low and to supply power to the tracking unit when the power demand is high.

The provision of a secondary storage device that is charged by the primary storage device when the power demand is low and supplies power to the tracking unit when the power demand is high avoids the need for large currents to be drawn from the primary storage device. That device is therefore able to operate under low current conditions, which are ideal for maximum energy conversion efficiency. The secondary storage device also helps to prevent excessive voltage drop when a large current is drawn, which could cause the tracking device to reset or to drop a communications connection.

The primary storage device preferably has a rated capacity in the range 20-100Ah. Advantageously, the primary storage device comprises a lithium thionyl chloride cell.

The secondary storage device preferably comprises a hybrid layer capacitor.

Advantageously, the power supply unit includes a voltage regulator, which is constructed and arranged to maintain the output voltage of the power supply unit at or above a predetermined value. This also prevents an excessive voltage drop when a large current is drawn, so preventing the tracking device from resetting or dropping a communications connection. It also ensures that as much energy as possible is extracted from the storage device before it has to be replaced.

The locating system preferably includes a GPS receiver. Advantageously, the locating system is reset if the elapsed time since receiving valid location data exceeds a predetermined value. The locating system is preferably constructed and arranged to switch between an active mode and a low power sleep mode according to a power management process. Preferably, the locating system is constructed and arranged to revert to the sleep mode if valid location data cannot be obtained within a predetermined timeout period.

As the tracking device may be used internationally, it is possible that its location may vary by thousands of miles between fix intervals on a 24 hour reporting schedule. When the GPS system is switched off it remembers its last location. Once switched on again the GPS system acquires the time and, using its almanac and last location, expects to receive location data from certain satellites that should be visible by 'line of sight' . If the current location is very different from the previous location, the GPS system will expect to see satellites that are not currently in view, owing to the location change around the globe. This increases the time required to obtain a fix, so consuming extra energy. Typically, a GPS system will fail to fix when it is constrained to a short timeout period, wasting battery consumption. It has been discovered that sending a 'cold start' command (effectively a hard reset) enables the GPS system to obtain a fix more quickly, particularly when its geographical position has changed significantly since the last fix.

Typically a GPS fix should take approximately 45 seconds from cold start conditions. However, this period may be affected by environmental conditions such as sky view (the ability of the GPS antenna to have an unimpeded view to any point in the sky). Buildings, canyons, vehicles, trees, foliage and so on can reduce the visible angle of the sky, reducing or blocking reception of the satellite signals to the GPS system. This means the cold start period of 45 seconds is a minimum and it may need to be much longer in less than ideal conditions. As the GPS system draws a current of approximately 22OmA, this consumes battery life a thousand times faster than when it is in low power mode.

By providing a timeout mechanism to abort GPS activity after typically 90 seconds, the battery lifetime can be protected under arduous conditions. This timeout period can be configured remotely by SMS messaging so that a custom trade off can be set against battery lifetime and effort to achieve a GPS fix. The timeout range may for example be 0-255 seconds.

Advantageously, the communications system is constructed and arranged to communicate with an external communications network. The communications system is preferably constructed and arranged to switch between an active mode and a low power sleep mode according to a power management process. Advantageously, the communications system is constructed and arranged to attempt to establish a connection with the external communications network upon entering the active mode. Advantageously, the communications system is constructed and arranged to revert to sleep mode if a network connection cannot be established within a predetermined number of attempts. The communications system preferably includes a GSM system.

With the GSM system, significant problems maybe encountered if the system is not always on. To minimum wasted power the following features may be provided.

Log on to the GSM network may be polled for every 25 seconds. On the third failure, log on may be aborted to save power.

To save power wastage under poor GSM coverage areas whilst on fast tracking rates (below one hour) a 'no GSM mode' may be implemented. If three consecutive GSM log on failures occur the tracking device will enter 'no GSM' mode, where all attempts to log onto the GSM network are barred for an hour, preventing energy being wasted trying to log on when it is unlikely to be successful. After one hour the system exits the 'no GSM' mode to allow reconnection, if GSM coverage has improved.

Advantageously, the tracking device is constructed and arranged to respond to control signals transmitted to the tracking device from the remote server.

The tracking device may include data acquisition means for acquiring data relating to the condition of the primary storage device and transmitting that data to the remote server.

Preferably, every time a GPS fix is attempted the battery condition is assessed. The condition is reported each time a SMS message is sent. Battery readings may be taken representing the battery voltage under the following conditions: no load connected (initial conditions), load connected (under load), load removed (recovery conditions). The time interval since the last significant power draw from the battery (for GPS or GSM activity) may also be reported, so that the readings can be filtered for validity, as the battery can only be assessed once it has had significant recovery time since moderate current draw. According to another aspect of the invention there is provided a tracking system including a tracking device according to any one of the preceding statements of invention and a remote server, wherein the tracking device and the remote server are constructed and arranged to communicate via the communications network.

The remote server is preferably constructed and arranged to assess the validity of data relating to the condition of the primary storage device and to discard data that is likely to be invalid.

An embodiment of the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a block diagram of a tracking system;

Figure 2 is an isometric view of a tracking device;

Figure 3 is a top view of the tracking device;

Figure 4 is a side view of the tracking device;

Figure 5 is a functional block diagram of a tracking device;

Figure 6 is a flow chart illustrating operation of the GPS power management system;

Figure 7 is a flow chart illustrating operation of the GSM power management system; and

Figure 8 is a flow chart illustrating operation of a battery monitoring system.

The tracking device of the present invention forms part of a tracking system, which is illustrated schematically in Figure 1. The system includes a set of GPS satellites 2 positioned in orbits around the earth. The satellites transmit location signals, which are detected by the tracking device 4. The tracking device 4 then uses the received signals to determine its geographical location. Geographical location data (and optionally other data) is transmitted by the tracking device 4 to a computer server 6 at a base station via the GSM network 8 and the internet 10. The server 6 assembles the received data and compiles a set of tracking data that includes location fixes, times and possibly other data. This set of tracking data is stored on the server. The tracking data maybe accessed by a user 12 by any suitable communication means (for example, via the internet, or by telephone or mail etc).

The tracking system and all the components of that system, in so far as they are described above, are conventional and will not therefore be described in detail.

The tracking device 4 is shown in Figures 2 to 4. The tracking device includes a waterproof housing 13 that is formed as a plastics moulding, having a first compartment 14 for a tracking unit, a second compartment 16 for a set of batteries, and a set of brackets 18 for securing the tracking device to an asset such as a lorry trailer. A connector 20 for a sleep key is provided at one end of the housing 13. A sleep key may be fitted into the connector to force the tracking device into a low power sleep mode, to conserve energy.

The electrical and electronic components that make up the tracking device 4 are shown schematically in Figure 5. These include a tracking unit 21 comprising a micro-controller 22, a GPS receiving module 24 having a GPS antenna 26 and a GSM communications module 28 having a GSM antenna 30. The tracking unit 21 is powered by a power supply unit 31 that includes a primary storage device (a lithium battery) 32, a secondary storage device 34 and a step-up voltage regulator 36. The micro-controller 22 is also connected via a service connector 38 to the sleep key 20. AU of the above components apart from the sleep key 20 are fully enclosed within the housing 13.

The primary storage device 32 consists of a lithium battery comprising two lithium thionyl chloride "D" cells connected in parallel, each cell having a rated capacity of 19Ah and a nominal output voltage of 3.67V. The lithium battery therefore has a total rated capacity of 38Ah.

The secondary storage device 34 comprises a rechargeable hybrid layer capacitor (or "HLC") having a rated capacity of 0.15Ah. The HLC is arranged to be trickle charged from the lithium battery whenever the power demand is low, for example when the GPS system and the GSM system are in a low power "sleep" mode. When the power demand is high, for example when the GPS system or the GSM system are active, power is drawn primarily from the HLC, thus avoiding a large drain on the lithium battery. The voltage regulator 36 is powered by the lithium battery 32 and the HLC 34, and is designed to provide a constant output voltage of 3.67V regardless of the input voltage. In practice, the voltage regulator 36 is able to operate down to an input voltage of approximately 1.8V. This enables it to extract charge from the lithium battery and provide a constant output voltage for powering the tracking unit 21 , even when the lithium battery is almost completed exhausted.

In use, the tracking device 4 periodically acquires location data via the GPS system 2 and then transmits that data to the server 6 as an SMS text message via the GSM network 8. The tracking data is stored on the server 6 and can be accessed by a user 12 via the internet or any other suitable communications medium. The tracking device 4 can also receive and respond to operational commands transmitted to the tracking device from the server 6, again as SMS text messages via the GSM network 8.

The tracking device has three core functional modes of operation, as follows:

Scheduled Tracking - GPS fixes are taken at regular intervals and reported to the server via SMS, thereby providing a trail tracking the asset's journey.

Position on Request - The tracking device polls the server at regular intervals via the GSM system to see if a request has been made for the device to reveal its location. If a request has been made, the tracking device obtains a GPS fix and reports its position to the server. If no request has been made, a GPS fix is not obtained.

Alerting - The tracking device may optionally be connected to one or more sensors for detecting problematic environmental conditions (for example temperature or humidity), or other potential problems (for example impact). If an undesirable condition is sensed (as determined by predefined criteria), a GPS fix is obtained and reported to the server, together with the time and an indication of the undesirable condition that has been sensed. This maybe useful during the transportation of sensitive goods, which may be damaged by exposure to unsuitable conditions.

The tracking device 4 is designed specifically for use in situations where no external power is available and it is required to operate for extended periods without recharging or replacing the batteries. For example, one typical application of the tracking device is for tracking lorry trailers. These may have service intervals of one or two years and it is important for practical reasons that the tracking device can operate without attention for a similar period. This places severe demands on the power supply unit 31.

The tracking device has been designed to ensure maximum service life. This has been achieved through two main design features: the choice of a suitable power supply unit and the use of software or firmware to control power consumption and avoid wasted energy. These features of the tracking device are discussed below.

The power supply unit 31 includes a battery pack comprising two lithium thionyl chloride "D" cells 32, each having a rated capacity of 19Ah. The battery pack therefore has a total rated capacity of 38Ah. However, the efficiency with which the cells convert chemical energy into electricity depends on the instantaneous current draw, as illustrated in the following table.

It can be seen that the capacity of the lithium cell drops significantly from 19 Ah at a current of about 6mA, to just 6Ah at a current of 200mA.

During operation, the current drawn by the tracking unit 21 varies considerably, depending on which components are active. When the GPS module 24 is active it draws a current of approximately 22OmA. If this current were drawn directly from the lithium battery, the efficiency of energy conversion would be very low, leading to reduced battery life. In the power supply unit of the tracking device, this problem is avoided by providing a hybrid layer capacitor (HLC) 34. The HLC does not have a large capacity or such a high energy density as a lithium battery, but it is capable of supplying a relatively large current for a short duration. In use, the HLC 34 is trickle charged from the lithium battery 32 during periods of low power demand, for example when the GPS and GSM systems are inactive. Then, when the power demand is high (for example when the GPS and GSM systems are active), the HLC supplies the necessary power to the tracking unit 21, without placing a large load on the lithium battery 32. The lithium battery 32 thus converts chemical energy to electricity at a low current, ensuring high efficiency and greatly increasing the useful lifetime of the lithium battery.

This advantage is illustrated by the following examples, which are based on a 20 minute scheduled tracking interval, consisting of 19 minutes of sleep mode (with negligible current drawn) followed by 1 minute of GPS activity at a current of 110mA per cell).

In a system with no HLC, the charge supplied per 20 minute cycle will be: l min @ HOmA = 1.83mAh 19 min @ OmA = OmAh Total 1.83mAh

At a current of 110mA, the lithium cell has an available capacity of approximately 8.8Ah. This would yield 8.8/0.00183 = 4808 GPS operations.

In a system with a HLC, the total charge supplied per 20 minute cycle is the same (1.83mAh). However, this is averaged over the whole of the 20 minute period, giving an average current of 5.5mA. At a current of 5.5mA, the lithium cell has an available capacity of 19Ah. This Would yield 19/0.00183 = 10382 GPS operations.

It can be seen that using a HLC yields over twice the expected lifetime.

A further problem with battery powered devices that use microcontrollers is that switching on a large load can cause the battery voltage to drop to a point where the microcontroller is forced to reset, thus preventing proper functional operation of the device. In addition to this, demand spikes can occur placing an excessive load on the battery for a short period of time. For example, the GSM modem may make brief demands on the battery supply of up to 2 amps, which are outside the practical limits of lithium thionyl chloride batteries.

Lithium thionyl chloride batteries struggle to deliver currents over 5OmA efficiently, and at this current level the output voltage (nominally 3.67V) drops significantly. A modem current demand of 60 mA per cell typically reduces the battery voltage to around 3.35V, which is very close to the drop out level of the modem. The minimum operating voltage of the modem is 3.2V, so only a small drop from the battery voltage of 3.67V can be tolerated before network connectivity will be dropped. This can create the situation where there is significant energy left in the battery but it cannot be extracted, leaving the device inoperable.

To avoid these problems, a switching voltage regulator 36 is used to enable the tracking device to work down to an external battery voltage of 1.8V, which is much below the minimum operating voltage of the GPS receiver and GSM modem. The voltage regulator 36 maintains an output voltage of 3.67V by converting a lower input voltage into a higher output voltage in exchange for increasing input current. The HLC 34 also helps to prevent significant input voltage drop.

Provided that the lithium battery 32 and the HLC 34 can supply enough power to meet the load requirements of the GPS system 24 and GSM modem 28, the battery voltage can drop significantly without causing either a reset of the microcontroller 22 or drop out of the GSM connection. Without this step up regulator and the HLC, the tracking device would not be operable, owing to unwanted resetting of the controller and drop out of the GSM connection as the battery output voltage drops.

The firmware features that are implemented in the microcontroller 22 to control power consumption and avoid wastage will now be discussed. These include routines for controlling operation of the GPS system 24 and the GSM system 28.

The power management routine for controlling operation of the GPS system 24 during scheduled tracking is illustrated schematically in figure 6. During scheduled tracking, the GPS system obtains location fixes at regular predetermined intervals, by receiving location signals from orbiting satellites. To conserve power, the GPS system is designed to be active only when obtaining a GPS fix, and to switch to a low power "sleep" mode at all other times.

In the first step 50 of the routine, the microcontroller checks whether it is time for the next scheduled GPS fix. If the answer is no, the routine goes to step 52 and remains in sleep mode until the next four second clock tick, then returns to step 50 and again checks whether it is time for the next fix. If the answer is yes, the routine goes to step 54 which instigates a battery assessment cycle, and then to step 56 which switches on the GPS system. In step 58, the system checks how much time has elapsed since the last GPS fix. If the elapsed time is less than the predetermined timeout period, for example 3 hours, the routine goes to step 62 and awaits a location signal (a "navigation message") from the GPS satellite system. If the elapsed time is greater than the timeout period, the routine goes to step 60 and issues a "cold start" command, which forces the GPS system to reset before going to step 62 and awaiting a location signal. In this case, the GPS system does not attempt to use previously stored position data to establish its current position. This leads to significant energy savings if the tracking device has moved a long way from its position at the previous GPS fix.

In step 64 of the routine, the system waits for a predetermined timeout period to obtain a GPS fix. If a fix is obtained within the timeout period, the routine goes to step 66 and checks the validity of the fix. If the fix is valid, the position data is logged to memory in step 68 and then in step 70 the GPS system is switched to the low power sleep mode, until the next scheduled GPS fix, as determined by steps 52 and 50. If the GPS fix is invalid, the routine returns to step 62 and awaits the next location message.

If in step 64 a GPS fix is not obtained within the predetermined period, the routine times out and jumps to step 70 in which the GPS system is switched to the low power sleep mode. This ensures that power is not wasted by repeatedly trying to obtain a GPS fix when location messages cannot be received (for example, when the tracking device is inside a building, tunnel, ship or aircraft, which blocks GPS signals). The routine then waits until the next scheduled GPS fix, as set by steps 52 and 50. The GPS fix timeout period in step 64 can be configured remotely by sending a SMS message to the tracking device 4 from the server .6. This allows it to be set as appropriate, according to the anticipated movement of the tracking device.

The routine for controlling operation of the GSM system is shown in figure 7. This routine is designed to prevent energy being wasted by repeatedly trying to log on to a GSM network when GSM signals cannot be received (for example, when the tracking device is inside a building, tunnel, ship or aircraft). This is achieved by switching the system to "no GSM" mode at such times.

In step 80, the routine checks whether it is time for the next scheduled GSM activity. If the answer is yes, the routine goes to step 82 and checks whether the system is in "no GSM" mode. If the system is in "no GSM" mode, it goes to step 84 and waits in sleep mode until the next four second clock tick, then goes to step 86 in which it checks whether the system has been in "no GSM" mode for more than one hour. If it has been in "no GSM" mode for less than one hour, the routine returns to step 80 and continues to cycle through steps 82, 84 and 86.

If the system has been in "no GSM" mode for more than one hour, the routine goes to step 88 and clears the "no GSM" mode. Then, when the routine reaches step 82 it goes to step 90 in which the GSM modem is turned on. At step 92 the routine waits 25 seconds for the modem to establish a network connection then at step 94 the routine checks whether the modem has logged on to the GSM network. If not, the routine goes to step 96, which returns the process to step 92, providing that there have been no more than three attempts to log on. If there have been more than three attempts the routine goes to step 98 which checks whether log on has failed consecutively more than twice. If so, the system is set to "no GSM" mode in step 100 and the modem is turned off (i.e. returned to sleep mode) in step 102, before returning to step 84. The system then waits an hour before attempting to log on again in step S6. This ensures that energy is not wasted by repeatedly trying to log on when the tracking device is in a location where there is no GSM signal.

If in step 98 log on has not failed consecutively more than twice, the routine goes to step 102 and the modem is turned off (without being set to "no GSM" mode) before returning to step 84. The routine then cycles through steps 86, 80, 90 to 98 for a second log on attempt.

Ih step 94, if Hie modem successfully logs on to the GSM network, the routine goes to step

104, which initiates GSM activity and sends an SMS message containing stored location data. In step 106, the routines checks whether an SMS message is being sent. If so, in step

108 battery assessment data is included in the message. The routine then goes to step 110 and logs off the GSM network. If no SMS message is being sent in step 106, the routine goes to step 110 and logs off the GSM network. The routine then turns the modem off in step 102 and cycles through steps 86, 80 and 84 until it is time for the next scheduled GSM activity.

A routine for monitoring the condition of the battery is illustrated in figure 8. In step 120, the server 6 receives a SMS message from the tracking device 4. The message will contain both location data and battery assessment data. In step 122, the server decides whether the battery assessment data is likely to be valid by checking how much time had elapsed at the time the data was obtained since the last significant power drain on the battery. If the elapsed time exceeds a predetermined value (for example 4 hours), the data is likely to be valid and the process goes to step 124 and the battery assessment readings are used to update the server 6. An indication of current battery status may be made available to an end user 12, for example via the internet.

If the elapsed time does not exceed the predetermined value, the data is unlikely to be valid. Ih that case, the process goes to step 126 and the data is discarded. The previous valid battery assessment readings are retained on the server 6.

In summary, the addition of a HLC can approximately double the battery lifetime, and the combination of a HLC, a step up regulator and firmware to minimize energy wastage can yield battery lifetimes of up to five years.

Claims

1. A tracking device including a tracking unit and a power supply unit for supplying power to the tracking unit, wherein the tracking unit includes a locating system for acquiring location data from received location signals and a communications system for transmitting acquired location data to a remote server, and the power supply unit includes a primary storage device comprising an electrolytic cell and a secondary storage device, the secondary storage device being constructed and arranged to be charged by the primary storage device when the power demand is low and to supply power to the tracking unit when the power demand is high.

2. A tracking device according to claim 1, in which the primary storage device has a rated capacity in the range 20-100Ah.

3. A tracking device according to claim 1 or claim 2, in which the primary storage device comprises a lithium thionyl chloride cell.

4. A tracking device according to any one of the preceding claims, in which the secondary storage device comprises a hybrid layer capacitor.

5. A tracking device according to any one of the preceding claims, in which the power supply unit includes a voltage regulator, which is constructed and arranged to maintain the output voltage of the power supply unit at or above a predetermined value.

6. A tracking device according to any one of the preceding claims, in which the locating system includes a GPS receiver.

7. A tracking device according to any one of the preceding claims, in which the locating system is reset if the elapsed time since receiving valid location data exceeds a predetermined value.

8. A tracking device according to any one of the preceding claims, in which the locating system is constructed and arranged to switch between an active mode and a low power sleep mode according to a power management process.

9. A tracking device according to claim 8 , in which the locating system is constructed and arranged to revert to the sleep mode if valid location data cannot be obtained within a predetermined timeout period.

10. A tracking device according to any one of the preceding claims, in which the communications system is constructed and arranged to communicate with an external communications network.

11. A tracking device according to claim 10, in which the communications system is constructed and arranged to switch between an active mode and a low power sleep mode according to a power management process.

12. A tracking device according to claim 11 when dependent on claim 10, in which the communications system is constructed and arranged to attempt to establish a connection with the external communications network upon entering the active mode.

13. A tracking device according to claim 12, in which the communications system is constructed and arranged to revert to sleep mode if a network connection cannot be established within a predetermined number of attempts.

14. A tracking device according to any one of the preceding claims, in which the communications system includes a GSM system.

15. A tracking device according to any one of the preceding claims, wherein the tracking device is constructed and arranged to respond to control signals transmitted to the tracking device from the remote server.

16. A tracking device according to any one of the preceding claims, including data acquisition means for acquiring data relating to the condition of the primary storage device and transmitting that data to the remote server.

17. A tracking system including a tracking device according to any one of the preceding claims and a remote server, wherein the tracking device and the remote server are constructed and arranged to communicate via the communications network.

18. A tracking system according to claim 17 when dependent on claim 16, in which the remote server is constructed and arranged to assess the validity of data relating to the condition of the primary storage device and to discard data that is likely to be invalid.