GB2491332A

GB2491332A - An ultra low frequency navigation system

Info

Publication number: GB2491332A
Application number: GB201011895A
Authority: GB
Inventors: Trevor Ajan Reginald; Michael Kiernan; Daniele Doneddu; Jason Perkins
Original assignee: PEEPO GPS Ltd
Current assignee: PEEPO GPS Ltd
Priority date: 2010-07-15
Filing date: 2010-07-15
Publication date: 2012-12-05
Also published as: GB201011895D0

Abstract

A navigation system for a pedestrian, the system comprising a plurality of transmitters 12a, 12b, 12c each having a location, and each of which is operable to transmit an identification signal being unique to the transmitter concerned, and having a frequency and phase combination unique to the transmitter concerned, the frequency being less than 300kHz, i.e. within a LF, VLF or ULF band; a mobile receiver 18 operable to receive the respective identification signals from the transmitters, and to determine a relative position of the receiver with respect to the plurality of transmitters; and a guidance unit associated with the receiver, and operable to receive relative position information from the receiver, and to output direction information derived from such relative position information in combination with stored mapping information. This type of system could be used by pedestrians that are blind or visually impaired over a range of environments.

Description

NAVIGATION SYSTEM

The presented invention relates to a navigation system and in particular relates to a navigation system which uses low frequency signalling.

Whilst aircraft, ships and military vehicles have been provided with navigation systems for many years, recent technological development have meant that such navigation systems have become more user friendly and widely available. As a result, the use of personal navigation systems has become more commonplace over the past few years.

Currently, many navigation systems use Global Positioning System (GPS). GPS is a space-based global satellite system that provides three dimensional location and time information to a personal navigation system receiver anywhere on the Earth provided the receiver has an unobstructed line of sight to four or more GPS satellites. GPS navigation systems are widely used in private vehicles to help with road navigation. GPS navigation systems are also used by, for example, outdoor sports enthusiasts as an aid to navigation in conjunction with a detailed map and are of particular use in poor weather. However, whilst GPS navigation aids are useful and reliable, they have two significant limitations. The first limitation is that the GPS satellite system provides the receiver with a location position having an accuracy of between 1 to 10 meters. Whilst such accuracy is acceptable in most instances, for some users, such as a visually impaired pedestrian a higher precision of pinpointing location would be necessary for the pedestrian to navigate safely.

The second limitation of the GPS system is the need for the navigation system receiver to have a clear line of sight to at least four satellites. In many circumstances this is not always possible, for example if the user is underground, inside or surrounded by tall buildings. Again, whilst this is not an issue for many users, in the case of a visually impaired user, a more reliable and consistent provision of navigation information is required as they will still require navigation aid when walking, for example, within an underground shopping mall.

Alternative navigation systems are available which utilise mobile phone network cell transmission terminals to provide location information. The mobile phone navigation systems are limited in their reliability by the requirement of a connection to a cell terminal to obtain a reliable high quality signal in order for a location position to be obtained. As many mobile phone users are aware, a high quality connection is not possible in all environments, with poor quality signal reception common in both rural and urban areas. Navigation aids which rely on the mobile phone network also have limited utility in a multi-path environment, such as a densely routed area of a city, or within an office block, as it becomes difficult for the navigation system to accurately measure and pinpoint the exact location.

The determining of the location currently requires a complex algorithm which includes pre-measured positions. in view of this, while mobile phone navigation systems are useful as a general navigation tool, in the case of a visually impaired user, a more reliable and accurate provision of navigation information would be required.

The third type of navigation system, which is more commonly used in an enclosed environment such as an office building, is a locally implemented navigation system, for example one which uses fingerprinting based locations and/or other technology, including Zigbee. A system relying on fingerprinting based location requires the use of a distributed network of sensor modules capable of recognising the characteristic uniquc features present in each individual's finger prints. Each sensor module can communicate with each other module, and a base unit, using a wired or wireless network and any appropriate protocol. The dynamics of the users as they move within the environment, checking their fingerprint through the sensors as they go, are therefore monitored. This type of navigation system has the benefit of not requiring connection with an external source and is of particular use in meeting the challenge posed with multi-path environments such as office environments. However, the system is limited by the need to be able to determine the users position as they move into different environments as sensor modules are required in each environment to enable continued navigation assistance.

Therefore, none of the navigational systems detailed can operate consistently and reliably in all environments. The currently available navigation systems which use GPS and mobile terminals for position location must be able to communicate with one or several reference points to enable a position to be located. As has been described above, this is not always possible. In addition, whilst locally implemented navigation systems can be more accurate, they are limited by their local nature. As a result, a user of these navigation systems needs alternative means, such as for example, a map, to support navigation which, if the user is congenitally or environmentally visually impaired, is not always possible.

Therefore, there is a need for a navigation system which can provide reliable and accurate location and navigation information to a user throughout all types of environments.

It is an object of the present invention to obviate or mitigate at least one of the aforementioned problems.

According to a first aspect of the invention there is provided a navigation system for a pedestrian, the system comprising: a plurality of transmitters each having a location, and each of which is operable to transmit an identification signal being unique to the transmitter concerned, and having a frequency and phase combination unique to the transmitter concerned, the frequency being less than 300kHz; a mobile receiver operable to receive the respective identification signals from the transmitters, and to determine a relative position of the receiver with respect to the plurality of transmitters; and a guidance unit associated with the receiver, and operable to receive relative position information from the receiver, and to output direction information derived from such relative position information in combination with stored mapping information.

The provision of a navigation system for a pedestrian which uses transmission of signals from a plurality of transmitters, each of which operates at a frequency of less than 300kHz enables a receiver to provide a guidance unit with precise position information and thus act as a navigation aid throughout different environments and spaces, including underground.

Conveniently, the frequency may be less than 30kHz or may be less than 3kHz.

Use of a frequency of less than 30kHz will enable the navigation system increased signal penetration and navigation aid coverage across a wider range of environments and spaces. Use of a frequency of less than 3kHz will enable the navigation system increased signal penetration and navigation aid coverage across a yet larger range of environments and spaces.

Each transmitter may further be operable to transmit an identification signal at prcdefined intervals, for example. between one and five times per second. The identification signal is preferably a periodic signal.

Preferably, the receiver and the transmitters may include respective clock signal generators, which are operable to synchronise with one another.

The provision of clock signal generators in the receiver and transmitters which are operable to synchronise with one another facilitates accurate location determination by the receiver using the signals transmitted by the transmitters.

According to a second aspect of the invention there is provided a personal guidance device, for example for a pedestrian, the device comprising: a receiver operable to receive the respective identification signals from a plurality of transmitters each having a location, and each of which is operable to transmit an identification signal being unique to the transmitter concerned, and having a frequency and phase combination unique to the transmitter concerned, the frequency being less than 300kHz, the receiver also being operable to determine a relative position of the receiver with respect to the plurality of transmitters; and a guidance unit operable to receive relative position information from the receiver, and to output direction information derived from such relative position information in combination with stored mapping information.

The provision of a personal guidance device which uses transmission of signals from a plurality of transmitters, each of which operates at a frequency of less than 300kHz enables a receiver to provide a guidance unit with precise position information and thus act as a navigation aid throughout different environments and spaces including underground.

Conveniently, the frequency may be less than 30kHz or may be less than 3kHz.

Use of a frequency of less than 30kHz will enable the guidance device increased signal penetration and navigation aid coverage across a wider range of environments and spaces. Use of a frequency of less than 3kHz will enable the guidance device increased signal penetration and utility as a guide across a yet larger range of environments and spaces.

Preferably, the receiver is operable to receive identification signals at predefined intervals, for example, between one and five times per second.

The receiver may further comprise a clock signal generator operable to synchronise with clock signal generators in such transmitters.

The receiver may be further provided with a compass such as, but not limited to an n-axis magnetometer, to determine directional information.

The position and direction information available to the system may be stored, for example, at the receiver unit, such that in the event of loss of fixed identification signals, the apparatus is operable to output destination information derived from the stored position and direction information in combination with the stored mapping information to guide a user to a defined location despite the absence of identification signals.

The defined location may comprise a location already encountered during the course of the journey or may comprises, for example, the nearest example of a selected group of locations such as, but not limited to, police stations, hospitals, government offices or other predefined safe location.

Furthermore, the buffer of stored position information and direction information may comprise any desired size, but may typically include information and/or data stored during the five to ten minutes immediately preceding identification signal loss.

A further safety feature of the system may comprise the ability to send, on request, an alert to a predefined address, such as but not limited to, a police station or security center or to a carer, to notify the recipient of the alert that the user of the system requires assistance. It will be understood that the alert may also be sent to a mobile location e.g. to a carer via a mobile device such as a mobile handset, pager or the like.

Embodiments of the present invention will now be provided, by way of example only, and with reference to the following figures, in which: Figure 1 is a schematic diagram of a navigation system according to a first embodiment of the present invention; Figure 2 is a schematic diagram of navigation system receiver unit of the navigation system of Figure 1; Figure 3 is a schematic diagram of a navigation system receiver unit according to a second embodiment of the present invention; Figure 4 is a schematic diagram of a portion of the navigation unit of Figure 3;and Figure 5 is a schematic diagram of another portion of the navigation unit of Figure 3.

With reference to Figure 1 there is shown a navigation system 10 having three Low Frequency (LF) transmitters 12a, 12b, 12c each of which has a corresponding transmitting signal range 13a, 13b, 13c. The transmitters 12a-c are located such as to surround a geographic area 16 so that the transmitting signal range 13a-c includes the geographic area i6 which the navigation system 10 is to cover.

However, it is to be noted that the transmitters may be located outside of the geographic area and still provide coverage to the desired geographic area. The system 10 is also provided with a navigation unit 18 which is operable to receive the signals generated by each transmitter 12a-c when it is within the signal range 13a-c of the transmitters 12a-c. The transmitters l2a-c are synchronised by means of a synchronous clock signal 20 which in this case is provided by radio 22. The navigation unit 18 is also synchronised by means of the same clock signal 20 resulting in the transmitters 12a-c and the navigation unit 18 being synchronous.

Each of transmitters 12a-c transmits a periodic signal s with duration T and phase ti, i,rjth i=1,2,...,n corresponding to a frequency v belonging to the LF band frequency being less than 300kHz. The frequency may be within the Very Low Frequency band (VLF) of less than 30kHz or the Ultra Low Frequency (ULF) band of less than 3kHz depending on the environment of the location of the navigation system 10, with the frequency chosen as to be appropriate to provide a reasonable accuracy of location. Each transmitter 12i generates its own transmitted signal s of phase q which corresponds to a precise location point of the transmitter l2 with geographic co-ordinates latitude lat. and longitude long.

An embodiment of the navigation unit 18, is shown in more detail with reference to Figure 2. The navigation unit 18 comprises a LF receiver unit 30 which is provided with an antenna 31, a signal analyser 32, a microprocessor 34, Flash ROM 36 connected to the microprocessor 34 via memory controller 38 and RAM connected to the microprocessor 34 via memory controller 42. The navigation unit 18 further includes a wireless module, in this case Bluetooth module 44 and a digital compass 46, both of which are connected to the microprocessor 34. The navigation unit 18 is also provided with a connection unit 48, in this case serial bus 48 and a power unit 50. In this embodiment, an input unit 52, which is a keyboard is removably connected to the navigation unit 18 via the connection unit 48 on a temporary basis.

The navigation unit 18 receives transmitted signals s, in this case signals 5a, 5b and s which are captured by antenna 31, through receiver unit 30. The waveform of each signal 5a s captured is compared by the signal analyser 32 which in this case is an on-chip spectrum analyser. The spectrum analyser 32 compares the received signals s by computing the phase difference Aqab between the signal a received from transmitter 2a and the signal 5b received from transmitter 12h* This process is repeated for each of the signals s received. From the phase difference APah between pair of transmitters l2a and l2b along with the remaining phase differences Api for each remaining combination of pairs of transmitters from transmitters 12a-c, it is possible to obtain the relative distance Dab, Dac, and Dhc of the navigation unit 18 from each of the transmitters 12a-c. The computed distance Dac will identify a plane perpendicular to the line of sight between transmitter 12a and transmitter 12c and the navigation unit 18 lies within this plane. The plane is established for each pair of transmitters 12a, 12b and 12c. intersection between these planes will identify a single point which identifies the location of the navigation unit 18. This process is carried out on an ongoing iterative basis. The navigation unit must be in the signal range of three transmitters or more in order to allow for triangulation.

In an alternative embodiment of the navigation unit 18, a reference synchronised signal is made available to compare to the waveforms. The reference synchronised signal is made available either by means of an internally synthesized signal with a certain phase of i jref or by utilizing one of the transmitter signals as the reference one. The comparison carried out by the signal analyser 32 in this case examines phase shift between the reference waveform and signal s.

In a further embodiment of the navigation system 10, each of the transmitters 12a-c transmit periodic signals s of different frequencies v, i=1,2,.. .,n. Each of the signals s are, when the navigation unit 18 is within range, received by the navigation unit 18 and compared, using suitable frequency analysis, to establish a single point of location for the navigation unit 18 within the navigation system 10 on an ongoing iterative basis.

To achieve navigation using the navigation unit 18, the Flash ROM 36 is provided with digital mapping information which enables a journey start point and a journey end point to be selected and a route between the two to be computed. The route selection is, in this case, input into the navigation unit 18 using keypad 52 which is removably connected temporarily to the navigation unit 18 via serial bus 48. The receiver 30 receives the signals 5a-c from each transmitter and using the process detailed above, the microprocessor 34 extracts the location point for the navigation unit 18. The microprocessor 34 also determines which direction the navigation unit 18 is pointing by means of the data received from the digital compass 46. The latitude and longitude of the extracted location point is compared to the selected route which is intended to be followed; the selected route makes use of the digital map stored in the Flash ROM 36. As the user of the navigation unit 18 moves along the selected route, the extracted location point for the navigation unit 18 identified from the received transmitter signals 5ac is compared with the previous point on the mapped route generated for the navigation unit 18 and with the current direction in which the navigation unit 18 is facing which is provided by the output of digital compass 46. The output from the microprocessor 34 as a result of these features being compared generates the next location point. The navigation unit 18 then compares the next location point with the next predicted point in the mapped route. The microprocessor 34 within the navigation unit 18 will then send the user, by way of an output unit (not shown) a signal indicating the direction in which to travel to reach the next point on the intended route. It will be appreciated that in alternative embodiments, the output unit may be embedded within receiver 30. If, as the user starts to travel, the angle between the direction from the current point and the next point, and the direction from the current point to the expected point is higher than a certain predetermined critical value, the navigation unit 18 sends a correction signal to the user by way of an output unit (not shown). If, as the user starts to travel, the angle between the direction from the current point and the next point, and the direction from the current point to the expected point is within a predetermined acceptable range, the navigation unit 18 sends a reassurance signal, by way of output unit (not shown) to the user that the correct navigation direction is being taken. ii

The use of low frequency radio signals by the navigation system 10 enables the transmitted signals 5a-c to be able to penetrate most materials and thus be able to penetrate through structures or geographical features which the user may encounter during travel such as pedestrian travel. The increased penetration of the transmitted signals 5a-c enables the receiver 30 to receive the transmitted signals 5a-c even when the user is in an environment such as an underground shopping mall thus providing the navigation unit 18 with a reliable signal s on which to establish precise location information relating to the navigation unit 18, The navigation unit 18, can further be provided with an assessment unit (not shown) which enables the continual assessment and monitoring of the direction of and the speed of travel of the navigation unit 18. Determining the direction of and speed of travel of the navigation unit 18 provides an additional reference measure when determining the position of the navigation unit 18. The reference measure can be established as the starting point of the navigation unit 18 is known and the motion progress can be detennined and tracked using the direction of and speed of travel data thus enabling location points after a period of travel to be determined by extrapolation.

The combination of the low frequency signal transmission with additional monitoring carried out by the assessment unit (not shown) enables navigation unit 18 to determine location points with high precision.

In use the navigation unit 18 may be provided as a portable or handheld device which can be carried or worn by a user. The user may be further assisted in the guidance process by the device via motion sensors with which they are in contact with or by an audio command which can be provided directly to the user or via one or two headphones. The navigation unit 18 may alternatively be formed integrally within mobility equipment such as wheelchairs or scooters.

With reference to Figures 3, 4 and 5, another embodiment of the navigation system is shown. in Figure 3 it can be seen that navigation unit 18 is, in this case, formed of an input unit, which in this case is reception unit 11 8a, shown in more detail with reference to Figure 4 and an output unit, which in this case is tactile unit 1 18b shown in more detail with reference to Figure 5. This embodiment of the navigation unit 18 enables a user to transport reception unit 1 18a for example, by carrying reception unit Ii 8a in a backpack, whilst carrying or wearing tactile unit 1 i8b for example, as a small unit held in their hand, so as to enable navigation to take place discreetly.

As can be seen in Figure 4, the reception unit 118a includes a microprocessor 134, Flash ROM 136 connected to the microprocessor 134 via memory controller 138 and RAM 140 connected to the microprocessor 134 via memory controller 142.

The reception unit 118a further includes a wireless module, in this case a radio frequency wireless communication transceiver 144 and a digital compass 146 which in this case is a n-axis magnetometer, both of which are connected to the microprocessor 134. The reception unit ii 8a is also provided with a connection unit 148, in this case serial bus and a power unit 150. The reception unit 118a is further provided with an input unit 152 which provides input to microprocessor 134; output module 154 with its provided data output from microprocessor 134; motion detection module 156; navigation module 158, in this case a satellite navigation module and plug in module 159, each of which provide data input to the microprocessor 134. In this embodiment, the navigation module 158 is a space-based global navigation satellite system module which operates depending upon, for example, the standard time signal (radio signal). In addition, in this embodiment, the input unit 152 comprises an input pad with an alphanumeric input pad which is integrally formed within the reception unit 1 l8a. The motion detecting module 156 incorporates sensors (not shown) which provide the microprocessor 134 with data confirming if the reception unit 1 18a is in motion and thus if the iterative process of route detection and provision should be implemented. It will be understood that the navigation unit 18 may function effectively without the inclusion of a motion detection module 156. in this case, the plug-in module 159 is a secure digital card which is inserted in a secure digital card slot (not shown) provided in the reception unit 1 18a whereby the plug in module 159 is removably attachable to the reception unit liSa. The removably attachable plug in module 159 provides means for downloading data to the reception unit 1 ISa.

In this embodiment, the Flash ROM 136 stores control software which in this case is the firmware of the navigation unit 10. In addition, in this case, the Flash ROM 136 stores suitable software so as to provide navigation capability including, but not limited to, databases with geographic coordinates such as latitude, longitude, altitude and the like. It will be appreciated that whilst the finnware is in this case stored within the same Flash ROM 136 modules as the other navigation software,it may. for example, be stored in a separate ROM module, such as, but not limited to, a dedicated BEPROM module.

The tactile unit 1 18b, shown in more detail with reference to Figure 5, comprises a microprocessor 234, a connection unit 248, in this case a serial bus; a power unit 250 and a wireless module, in this case Bluetooth module 244 which is in communication with wireless module 144 of reception unit 11 8a. The microprocessor 234 is also in communication with tactile elements 264a, b, c via respective driver elements 262a, b and c. When the tactile unit 1 18b receives a data signal from the reception unit 1 l8a this is provided to microprocessor 234 which then outputs a signal corresponding to the tactile element 264a to be activated. In this case, the signal is provided to the tactile element 264a via driver 262a which drives the intended element 264a to produce an output signal.

In an alternative embodiment of the reception unit 11 8b, the signal from microprocessors 264a-c may be sent directly to tactile elements 262a-c thus activating them, as desired, to produce output signals.

To achieve navigation with the navigation unit 18 shown in Figures 3, 4 and 5 the Flash ROM 136 of the reception unit 1 l8a is provided with digital mapping information which enables a journey start point, a journey end point and a route between the two to be selected. The selection is, in this case, carried out using input unit 152. The receiver (not shown) receives the signals 5ac from each transmitter within the navigation system 10 and using the process detailed above, extracts the location point for the navigation unit 18. The latitude and longitude of the extracted location point is compared to the selected route which is intended to be followed; the selected route makes use of the digital map stored in the Flash ROM 136. As the user of the navigation unit 18 moves along the selected route, the extracted location point for the navigation unit 18 identified from the received transmitter signals s is compared with the previous point on the mapped route generated for the navigation unit 18 and with the current direction in which the navigation unit 18 is facing which is provided by the output of digital compass 146.

The output from the microprocessor 134 as a result of these features being compared generates the next location point. The reception unit 11 8a then compares the next location point with the next predicted point in the mapped route, The microprocessor 134 within the reception unit 1 iSa will then send the tactile unit 1 18b a signal via wireless module 144 which is received by wireless module 244.

The signal sent indicates the direction in which the user must travel to reach the next point on the intended route and is processed by microprocessor 234 which provides appropriate signals to tactile elements 264a-c which are then activated to provide a tactile signal to the user to indicate the direction in which they should move, if, as the user starts to travel, the angle between the direction from the current point and the next point, and the direction from the current point to the expected point is higher than a certain predetermined critical value, the reception unit 1 18a sends a correction signal to the user by way of tactile unit ll8b. If, as the user starts to travel, the angle between the direction from the current point and the next point, and the direction from the current point to the expected point is within a predetermined acceptable range, the reception unit 1 18a sends a reassurance signal, by way of tactile unit 1 lSb to the user that the correct navigation direction is being taken.

As LF signals by their nature can be susceptible to several sources of disturbing noises including, but not limited to, noise from electronic devices, motors and computer systems each of which can cause interference in the LF signal. However, in a further embodiment of the navigation unit 18, the navigation unit is provided with high selectivity filters (not show. n) which filter with relative accuracy any such disturbing noises thus reducing the interference in the LF signal.

It is also noted that the low transmission speed associated with LE signals does not adversely affect the operation of the navigation system 10 as transmitters 12a-c are not, in the embodiments detailed above, required to send specific data instead sending periodic signals 5a s having a predetermined frequency and phase and with the determination of the location point being performed on the navigation unit.

Various modifications may be made to the embodiments hereinbefore described without departing from the scope of the invention. For example, it will be clearly understood that whilst the navigation system illustrate in Figure 1 comprises three transmitters l2a, 12b and l2c, the navigation system may include any number of transmitters l2a -12n. Also, it will be clearly understood that whilst the clock signal which synchroni.ses the transmitters l2a-n and receiver unit 18 is described as being provided by radio 20, it could also be generated by any suitable means including, but not limited to, by GPS receivers linked to the transmitters 12a-n and receiver unit 18.

The route selection process has been detailed as being carried out using a keypad 52 temporarily connected to the serial bus 48 or using a speech recognition module 54. However route selection may be carried out using any one of a variety of methods including, but not limited to a temporary or permanent keypad 52; a temporary or permanent input pad (not shown); vocal command, whereby the vocal commands are captured by a speech recognition module (not shown) which is in communication with the microprocessor of the navigation unit or by using a touch pad screen (not shown). in another embodiment the input module 52 can incorporate a speech recognition module used in addition to the keypad. The speech recognition module and the keypad unit may or may not coexist within the same embodiment. Data may also be input into the navigation unit 18 via the wireless module 44 from any compatible wireless device as desired.

It will be understood that whilst the RAM memory 40 is detailed as being connected to the microprocessor 34 it could alternatively be embedded within the microprocessor 34, with further external RAM memory (not shown) being interfaced to extend the capabilities of the microprocessor 34 in order to support more resource-intensive operations, such as for example speech recognition. The amount of RAM required by the navigation unit 10 will depend upon the maximum addressable memory capacity of the microprocessor 34. The microprocessor 34 is complimented by read-only memory (ROM) 36 of a suitable amount, which in this case is Flash ROM but may alternatively be EEPROM or any suitable ROM type.

The Flash memory ROM 36 can be organised in one or more modules of adequate capacity and whilst detailed as being connected to the microprocessor 34 via memory controller 38, it may alternatively be formed in a single unit with the microprocessor 34. The Flash ROM 36 is described as storing relevant navigation software data modules and it will be appreciated that these can be operable to make use of stored or otherwise accessible databases including geographic coordinates such as latitude, longitude, altitude; additional information on locations, points of interest, and other information which may be utilised by the unit and the user to refine the navigation.

Furthermore, whilst the navigation unit 10 is detailed as including a serial bus 48, the navigation unit 18 may usefully be provided with more than one communication interface including, but not limited to, a serial link such as an RS- 232; an USB, mini-USB or micro-USB port or any other suitable communication port, to enable communication and data exchange between the navigation unit 18 and any other suitable device. Such connection capability will provide the navigation unit with mechanisms for expanding the capability of the navigation unit 18, updating the firmware of the navigation unit 18 or performing other navigation system maintenance operations to the navigation unit 18; as well as to provide updates to the software, provide new additional software modules or provide updated maps to the navigation unit 10.

It will be clearly understood that each of these variations may be implemented in any of the described embodiments and includes but is not limited to the embodiment of Figures 1 and 2. It will appreciated that with reference to the embodiment detailed with reference to Figures 3, 4 and 5, whilst the output unit 118b has been described as a tactile unit, the output unit 118b may alternatively provide an output to a user in a any suitable way including but not limited to audio commands and visual commands. It will also be understood that whilst the embodiment described with reference to Figures 4, 5 and 6 details communication between the reception unit 118a and tactile unit 118b as being wireless through wireless Bluctooth modules 144 and 244 respectively, any suitable communication may be implemented including but not limited to by way of wire connections, using InfraRed communication or any suitable radio wave frequency. In addition, plug-in module 159 has been detailed as a secure digital card. However, the plug in module may be any suitable additional module including, but not limited to, a multi-standard in which case the reception unit 1 18a would require to be provided with an appropriate multi format memory slot

Claims

CLAIMS1. A navigation system for a pedestrian, the system comprising: a plurality of transmitters each having a location, and each of which is operable to transmit an identification signal being unique to the transmitter concerned, and having a frequency and phase combination unique to the transmitter concerned, the frequency being less than 300kHz; a mobile receiver operable to receive the respective identification signals from the transmitters, and to determine a relative position of the receiver with respect to the plurality of transmitters; and a guidance unit associated with the receiver, and operable to receive relative position information from the receiver, and to output direction information derived from such relative position information in combination with stored mapping information.
2. A system as claimed in claim 1, wherein the frequency is less than 30kHz.
3. A system as claimed in claim 1, wherein the frequency is less than 3kHz.
4. A system as claimed in any one of the preceding claims, wherein each transmitter is operable to transmit its identification signal at a predetermined signal frequency.
5. A system as claimed in claim 4, wherein the signal is transmitted at predefined intervals.
6. A system as claimed in claim 4 or 5, wherein the identification signal is a periodic signal.
7. A system as claimed in any one of the preceding claims, wherein each transmitter is operable to transmit an identification signal at a predetermined signal frequency which is different from the predetermined signal frequency of each other transmitter.
8. A system as claimed in any one of the preceding claims, wherein the receiver and the transmitters include respective clock signal generators, which are operable to synchronise with one another.
9. A guidance device for a pedestrian, the device comprising: a receiver operable to receive the respective identification signals from a plurality of transmitters each having a location, and each of which is operable to transmit an identification signal being unique to the transmitter concerned, and having a frequency and phase combination unique to the transmitter concerned, the frequency being less than 300kHz, the receiver also being operable to determine a relative position of the receiver with respect to the plurality of transmitters; and a guidance unit operable to receive relative position information from the receiver, and to output direction information derived from such relative position information in combination with stored mapping information.
10. A device as claimed in claim 9, wherein the frequency is less than 30kHz,
11. A device as claimed in claim 9, wherein the frequency is less than 3kHz.
12. A device as claimed in claim 9. 10 or 11 wherein the receiver is operable to receive identification signals at predetermined signal frequencies from such transmitters.
13. A device as claimed in claim 12, wherein the respective signal frequencies are in the range l to 5Hz.
14. A device as claimed in any one of claims 9 to 13, wherein the identification signals are periodic signals.
15. A device as claimed in any one of claims 9 to 14, further comprising a clock signal generator operable to synchronise with clock signal generators in such transmitters.