referring to fig1 , in an embodiment an electronic monitoring system for offender tracking comprises a monitoring platform 2 , a residence monitoring unit ( rmu ) 7 , and a tracking device wearable by an offender 5 . the monitoring platform 2 is cloud - based and accessible by authorised persons at terminals at a monitoring centre 1 . the tracking device includes a personal identification device ( pid ) 6 . the rmu 7 is fixedly located in the place of residence / home 8 of the offender 5 . the offender 5 is an individual whose movements are restricted in accordance with home detention conditions , which specify either that the offender must remain within the area of his home during one or more predetermined period or for the duration of the monitoring . the rmu 7 is configured for communication with the pid 6 and includes a transceiver ( not shown ) to this end . the range of the communication between the rmu 7 and the pid 6 is intended to define the area over which the offender may move without breach of the home detection conditions . the rmu 7 is also configured for communication with the monitoring platform 2 via a mobile communications network 4 and a gateway 3 and the transceiver apparatus is configured for communication with the networks 4 . it will be appreciated that while fig1 illustrates only a single offender 5 , rmu 7 and home 8 , the monitoring platform 2 is configured to communicate with a plurality of rmus , which provide data on the whereabouts of a corresponding plurality of offenders . the gateway 3 is configured to receive and collect data from the rmus and is preferably configured to standardise the form of the data before providing it to the monitoring platform 2 , such that upstream software and / or hardware , that is of the monitoring platform , or downstream software and / or hardware , that is of the rmus can be changed . the rmu 7 includes a further transceiver ( not shown ) for transmitting data to and receiving data from the gateway 3 , and thus the monitoring platform 2 , via the mobile communications network 4 using a wireless protocol . the network 4 may be a gsm or umts network and the data transmitted and received via one or more base stations . the network 4 may alternatively be another kind of mobile communications network . embodiments of the invention are not limited to any particular means of communication between the gateway 3 and the rmu 7 . in alternative embodiments , the rmu 7 and the gateway 3 may communicate over a landline network , or via another kind of wireless network such a metropolitan area network , or via the internet . communication between the gateway 3 and the rmu 7 may also take place through more than one network , for example through a mobile communications network and the internet . the pid 6 is part of the tracking device that is securely attachable to an ankle of an offender 5 , for example with a locking strap that is only releasable by a field officer . alternatively , the pid 6 may be attached to another part of the offender &# 39 ; s body , for example to a wrist or waist . referring to fig2 , the pid 6 includes a processor 10 , a memory 11 , transceiver apparatus 12 and a motion tracking system 13 , for example in the form of an accelerometer , all connected to a bus 14 . the memory 11 has a computer program comprising code stored thereon , which , when run by the processor 10 , permits the pid 6 to provide the functionality described herein . the transceiver apparatus 12 is configured to communicate with the rmu 7 using a first rf technology and also the monitoring platform using a second rf technology . the motion tracking system may comprise an accelerometer , from which information on distance and direction moved by the pid 6 and thus the offender 5 can be derived . in variant embodiments , alternative means of determined direction moved by the pid 6 may be used . optionally the motion tracking system is activated , information on distance travelled and direction is also preferably collected so that location of the offender , where he or she has absconded , can be estimated . where the motion tracking system comprises an accelerometer , the accelerator may also be used in tamper detection applications . the pid 6 is programmed so that a set of parameters stored in memory 11 specific to the home detention conditions imposed upon the offender 5 . this set of parameters is known as the “ compliance rules ”. a first compliance rule is defined with values indicative of the times over which an offender must remain in the home area for compliance with the home detention conditions . the first compliance rule is met for a given time if the offender must be within his or her home area for compliance with the home detention conditions . for example , the home detention conditions may require that the offender not leave his or her home area during a defined period each day , such as from 6pm to 7am . the first compliance rule is met for a given time if that time is between 6pm and 7am . alternatively , the home detention conditions may require that the offender not leave his or her home at any time for the duration of the monitoring , an end date of which may be specified . in this case , the first compliance rule is always met . a ) a time period over which distance moved by the offender is monitored when communication between the pid 6 and the rmu 7 is determined to be lost ; and b ) a threshold distance over which the offender may move over said time period without the pid 6 determining that the home detention conditions have been breached . the second compliance rule requires that the pid 6 and thus the offender 5 not move more that the threshold distance within the specified time period . the values for the first and second compliance rules are provided to the pid 6 , preferably remotely by a person at the monitoring centre 1 using the monitoring platform 2 and via the rmu 7 . the value for the first compliance rule at least can preferably be changed remotely , such that the home detention conditions can be changed remotely . the computer program code includes a motion sensing algorithm . this enables the pid 6 to use data determined by the motion tracking system to generate data indicative of the distance travelled by offender when the pid 6 over a time period . the monitoring platform 2 is a software system usable by persons at terminals at the monitoring centre 1 . the monitoring platform 2 is configured to receive data from rmus via the gateway 3 . persons at the monitoring centre 1 may make decisions based upon the data reported by the monitoring platform 2 and coordinate activities of field officers . the monitoring platform 2 has information on the home detention conditions for each offender that is being monitored . the monitoring platform is configured to analyse the data received from the rmus and to generate an alert when it appears that an offender has breached home detention conditions . the pid 6 is configured to poll the rmu 7 periodically and to determine an alert condition if the rmu 7 does not respond to one or more polls . non - response to a poll may be indicative of the offender 5 having left his or her home , such that the pid 6 is beyond the range of the rmu 7 . however , as mentioned above , the home 8 of the offender 5 likely has blind spots in which communication between the rmu 7 and the pid 6 is not possible . generation of a false alert when the offender 5 and thus the pid 6 are located in a blind spot is to be avoided notably , homes of different offenders have different sizes , constructions , layouts and furnishings , all of which impact on number , size and location of blind spots . typically , blind spots are small in size ; for example , where a particular rf frequency is used for communication between the rmu 7 and the pid 6 , the mean size of each blind spot maybe about 300 mm in diameter . operation of the electronic monitoring system will now be described with reference to fig3 . the pid 6 polls the rmu 7 , that is , periodically pings a request to the rmu 7 and waits for a reply . one such request is indicated at a in fig3 . the mean time between sending of such requests is preferably of the order of thirty seconds , and is intended to be such as to preserve battery life of the pid 6 , while being sufficiently frequent for good monitoring of the offender 5 . the rmu 7 usually receives the requests and , in response thereto , the rmu 7 transmits a reply b to the pid 6 . one such reply is indicated at b the rmu 7 periodically sends a status update c to the monitoring platform 2 . status update indicates that the rmu 7 and the pid 6 are in communication . the status update c may be sent for each request from the pid 6 received by the rmu 7 , or alternatively may be sent after a longer time periods , for example after a predetermined time period has expired . accordingly , there may be a plurality of requests and replies a , b for each status update c sent . if the rmu 7 does not receive after a predetermined time period has expired a polling from the pid 6 , the rmu 7 sends a preliminary alert message d to the monitoring platform 2 . the monitoring platform 2 then , in response to a preliminary alert message d , initiates a preliminary alert condition . if the pid 6 does not receive a reply from the rmu 7 , the pid 6 determines that communication between the pid 6 and the rmu 7 is lost . this may be due to a reply transmitted by the rmu 7 not reaching the pid 6 , or due to the request transmitted from the pid 6 not reaching the rmu 7 with the consequence that the rmu 7 does not transmit a reply . in either case , the pid 6 does not know whether the loss of communication is due to a result of non - compliance or the offender 5 being located in a blind spot . the purpose of the next steps is for the pid 6 to determine whether the loss of communication is due to the blind spot . the pid 6 determines whether the present time is at a time over which the home detention conditions require the offender to be in the home area , that is , whether the first compliance rule is met . if the first compliance rule is not met , no further action is taken by the pid 6 . if the loss of communication is within the times of the home detention conditions , that is , the first compliance rule is met , the pid 6 activates the motion tracking system 13 at step f . the pid 6 then monitors the distance over which the offender 5 moves using the accelerometer and know algorithms , and determines whether the second compliance rule is met , that is , the pid 6 determines whether , after the predetermined time period specified in the second compliance rule , the distance moved by the offender 5 is greater than the threshold distance specified in the second compliance rule . if the distance moved is less than the threshold distance , the pid 6 determines that the loss of communication with the rmu 7 is due to the offender 5 being located in a blind spot . the pid 6 then stores in the memory data indicative of the fact that there was a loss of communication with the rmu 7 at a particular time and that it was determined that this was due to a blind spot . the pid 6 later send this data to the rmu 7 , which provides it to the monitoring platform 2 . further to the pid 6 determining that first compliance rule is met , the pid 6 also increases the frequency with which it polls the rmu 7 . if the rmu 7 receives a polling request e from the pid 6 , it sends an alert cancellation message to the monitoring centre 2 , on receipt of which the monitoring centre cancels the preliminary alert condition at step j . preferably , if communication with the rmu 7 is not re - established within a predetermined time period , the pid 6 sends a message itself including the stored data to the monitoring platform 2 using the second rf technology , on receipt of which the monitoring platform 2 cancels the preliminary alert condition . if the pid 6 determines that the distance moved by the offender 5 over the time period is greater than the threshold distance , then the pid 6 generate and sends an alert confirmation message i to the monitoring platform 2 using the second rf technology , for example over a mobile communications network such as a gsm or umts network . on receipt of the alert confirmation message , the monitoring centre 2 determines at step j to escalate the preliminary alert condition , for example by sending an alert message to a person at the monitoring centre 1 or a field officer . in a variant embodiment , the distance travelled by the pid 6 after the motion tracking system 13 has been activated is not determined in the pid 6 . instead , information from which the distance can be derived is transmitted using the second rf technology is compressed , encrypted and transmitted to the monitoring platform 2 . at the monitoring platform 2 , the information is decrypted , decompressed and processed to determine the distance that the pid 6 has travelled over the predetermined time period . this is because algorithms used to determine the distance moved maybe complex and it may be preferable to determine the distance remotely from the pid 6 on a server depending on the battery life requirements of the pid 6 and the complexity of the algorithms . in this case , if it is determined at the monitoring platform 2 that the distance moved by the offender 5 and thus the pid 6 is less than the threshold distance then the monitoring platform 2 automatically cancels the alert condition , since it can be safely assumed that the offender 5 is still within his or her home 8 and the loss of communication was caused by a blind spot . if the distance moved is greater than the threshold distance , the monitoring platform 2 determines that the alert condition should be escalated . in variant embodiments , the monitoring platform 2 and / or the pid 6 is configured to automatically adjust the values for the second compliance rules . for example , following a loss of communication , the pid 6 may determine that the distance moved by the offender is greater than the threshold distance , but the monitoring platform 2 then receives information indicating that the alert is a false alert , which may occur when the pid 6 and the rmu 7 re - establish communication after the pid 6 has transmitted an alert confirmation message . in this case , the threshold distance and / or the time period over which distance is measured may be altered . dependent upon the battery life requirements of the pid 2 , the processor 10 may analyse stored data and automatically adjust these parameters . alternatively , such analysis may take place at the monitoring platform . this is particularly useful for homes that may contain larger than average blind - spots and creates a system that learns and improves its own accuracy after every event . an example is provided in greater detail in the following . the average blind - spot size within homes is estimated to be circular and 0 . 3 m diameter . therefore by default threshold distance can be set with a distance travelled safety threshold of 1 m . the result is that the 0 . 3 m blind - spot will always be cancelled . however a blind - spot may actually be 0 . 8 m diameter , significantly larger than the average . with changes in detection accuracy , weather , objects being moved within the home , it may be assumed that there is a risk of that blind - spot increasing in diameter to over the 1 m threshold . in order to combat this an algorithm is used in which , for example , the average actual distance travelled within a blind - spot is calculated and a 50 % safety margin is added . thus in this case the measured size is 0 . 8 , so 0 . 8 × 1 . 5 = 1 . 2 m . the threshold will now be set to 1 . 2 m and is therefore less likely to falsely detect non - compliance with home detention conditions . similarly the system may learn to reduce the size threshold making the system more accurate . preferably , a a maximum and minimum threshold distance is set to prevent offenders teaching their system to provide greater freedom . a maximum size of 5 m and minimum of 0 . 2 m have been established to be sensible initial values for homes in the uk . preferably , the pid 6 is configured to disable itself outside of the times over which the curfew order is to be served , thereby to save battery power . in an embodiment , the rmu 7 optionally includes a unit by which the location of the rmu 7 can be remotely determined . the optional unit is indicated at 15 in fig2 . the unit may be in the form of a gps locator . the gps locator is capable of receiving signals in accordance with the global positioning system ( gps ), processing those signals to determine location data indicative of the location of the rmu 7 at a particular time , and providing that location data to the transceiver . the location data includes an indication of the time at which the location is indicated by the location data in the form of a time stamp . in response to receiving such data , the transceiver transmits the location data , or stores or queues it for later sending . the ability to monitor the location of the rmu 7 remotely means that the offender 5 cannot move the rmu 7 in an attempt to change the area in which the rmu 7 and pid 6 can communicate . also , inclusion of remote location determining technology facilitates self - installation of the rmu 7 , that is , installation of the rmu 7 by the offender rather than by a field officer . although the rmu 7 includes a gps locator for determining location data , there are many other ways that the location ( or approximate location ) of the positioning device might be determined . embodiments of the invention are not limited to any particular way of generating location data . the positioning device could determine location using another kind of global satellite navigation system ( gsns ). other techniques for determining location could also be used , for example where the rmu 7 is configured for communication with a mobile communications network , analysis of characteristics of signals to and / or from base stations in a surrounding mobile communications network may be performed . the location data does not have to be determined at the device ; it could be determined in a wireless communications network , or at a remote server . methods of determining the location of a device are known in the applicable art . in the above embodiments , as will be understood by the skilled person , the rmu 7 includes a memory in which computer program code is stored , and a processor , and , when the computer program code is executed by the processor , the rmu 7 performs the functionality described herein . alternatively , the functionality of the rmu 7 , and indeed the pid 6 described above , could be implemented in dedicated processing equipment or a mixture of dedicated hardware and software . the memory of the rmu 7 and the pid 6 may include volatile and non - volatile , removable and non - removable media configured for storage of information , such as ram , rom , erasable programmable read only memory ( eprom ), electrically erasable programmable read only memory ( eeprom ), flash memory or other solid state memory , cd - rom , dvd , or other optical storage , magnetic disk storage , magnetic tape or other magnetic storage devices , or any other medium which can be used to store information which can be accessed . also , in each of the rmu 7 and the pid 6 , the processor may be a single processing unit , or a plurality of connected processing units . similarly , the monitoring platform 2 is software that runs on one or more servers . as mentioned above , the monitoring platform 2 may be based in the cloud . alternatively , the monitoring platform 2 may run on one or more dedicated servers , which may be located in the monitoring centre 1 . in the field of electronic monitoring , embodiments of the invention are not limited to where an offender is monitored , but may include embodiments , in which electronic tracking devices are attached to animals and persons other than offenders . for example , persons with certain mental illnesses may desirably be monitored . further , there is no reason why tags in accordance with embodiments of the invention cannot be attached to motor vehicles , bicycles , containers , computer equipment and other physical objects . as will be appreciated by the person skilled in the art , various modifications possible to the embodiments described above . the applicant hereby discloses in isolation each individual feature and step described herein and any combination of two or more such features or steps , to the extent that such features or steps or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art , irrespective of whether such features or steps or combinations of features or steps solve any problems disclosed herein , and without limitation to the scope of the claims . the applicant indicates that aspects of the present invention may consist of any such individual feature or step or combination of features or steps . in view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention .