GB2553349A

GB2553349A - Method and system for calibrating one or more sensors of an inertial measurement unit and/or initialising an intertial measurement unit

Info

Publication number: GB2553349A
Application number: GB1614983.3A
Authority: GB
Inventors: Barnes Matthew; Carr Conor
Original assignee: Draeger Safety UK Ltd
Current assignee: Draeger Safety UK Ltd
Priority date: 2016-09-04
Filing date: 2016-09-04
Publication date: 2018-03-07
Anticipated expiration: 2036-09-04
Also published as: GB2553349B; WO2018042200A2; GB201614983D0; WO2018042200A3

Abstract

A method of calibrating one or more sensors (e.g. a gyroscope and/or accelerometer) of an inertial measurement unit (IMU) 28 in a portable module 20. The module is docked to a dock 14 and calibration carried out after determining that the portable module has been docked. The dock may be attached to a static structure such as a wall or a moving object such as a vehicle. The calibration may be automatically stopped if the dock starts to move. The dock may also charge the battery 24 of the portable module. The module may include a docking determining module 26. Also disclosed is a method of initialising an inertial measurement unit in a portable module. The module is docked to a dock and the initial position and heading of the IMU is set to a position and orientation based on that of the dock. The dock is fixed within a vehicle, and the IMU may be initialised to the initial absolute position and heading of the vehicle. The initial position and heading may be registered when it has been determined that the portable module has been undocked. The methods may relate to a personnel location system, e.g. firefighters (figure 1, 1) with the dock being situated within a fire engine (figure 1, 16).

Description

(71) Applicant(s):

Draeger Safety UK Ltd (Incorporated in the United Kingdom)

Ullswater Close, Blyth Riverside Business Park, BLYTH, Northumberland, NE24 4RG, United Kingdom (72) Inventor(s):

Matthew Barnes

Conor Carr

1614983.3 (5η [_NT ci_:

G01C 25/00 (2006.01)

04.09.2016

G01C 21/16 (2006.01) (56) Documents Cited:

EP 2589927 A2 WO 2014/100093 A1

US 20120029811 A1 US 20120010812 A1

US 20110098921 A1 US 20080234935 A1

US 20060106558 A1 (58) Field of Search:

INT CL G01C, G06F Other: EPODOC, WPI, TXTE (74) Agent and/or Address for Service:

Haseltine Lake LLP

Redcliff Quay, 120 Redd iff Street, BRISTOL, BS1 6HU, United Kingdom (54) Title of the Invention: Method and system for calibrating one or more sensors of an inertial measurement unit and/or initialising an intertial measurement unit

Abstract Title: Method and system for calibrating one or more sensors of an inertial measurement unit and/ or initialising an inertial measurement unit (57) A method of calibrating one or more sensors (e.g. a gyroscope and/or accelerometer) of an inertial measurement unit (IMU) 28 in a portable module 20. The module is docked to a dock 14 and calibration carried out after determining that the portable module has been docked. The dock may be attached to a static structure such as a wall or a moving object such as a vehicle. The calibration may be automatically stopped if the dock starts to move. The dock may also charge the battery 24 of the portable module. The module may include a docking determining module 26. Also disclosed is a method of initialising an inertial measurement unit in a portable module. The module is docked to a dock and the initial position and heading of the IMU is set to a position and orientation based on that of the dock. The dock is fixed within a vehicle, and the IMU may be initialised to the initial absolute position and heading of the vehicle. The initial position and heading may be registered when it has been determined that the portable module has been undocked. The methods may relate to a personnel location system, e.g. firefighters (figure 1, 1) with the dock being situated within a fire engine (figure 1, 16).

INITIALISATION

WIRELESS

TRANSMITTER

CALIBRATION

MODULE

DOCKING

DETERMINING

MODULE

MOVEMENT

DETERMINING

MODULE

DOCKING

DETERMINING

MODULE

VEHICLE

-MONITORING

UNIT

FIGURE 4

At least one drawing originally filed was informal and the print reproduced here is taken from a later filed formal copy.

/4

CN in

t s

3/4

60

FIGURES

4/4 a:

FIGURE 4

METHOD AND SYSTEM FOR CALIBRATING ONE OR MORE SENSORS OF AN

INERTIAL MEASUREMENT UNIT AND/OR INITIALISING AN INTERTIAL

MEASUREMENT UNIT

The invention relates to a method of calibrating one or more sensors, such as a gyroscope or accelerometer, of an inertial measurement unit. The invention also relates to a method of initialising an inertial measurement unit. In particular, although not exclusively, the invention relates to calibrating an initialising an inertial measurement unit of a personnel location system.

In various situations it may be desirable to monitor the position of a person and display their location on a map. For example, it is desirable to be able to display the position of a firefighter working at an incident on a map displayed on an electronic display.

Previous methods and systems have been considered for monitoring the position of a person, such as a firefighter. In one previously considered arrangement a firefighter is provided with a satellite navigation receiver (such as a GPS receiver) which transmits back to an operator the location of the firefighter. However, in built-up areas it can be difficult, if not impossible, to quickly pick-up a signal, and the accuracy may not be sufficient for the purpose required.

In another previously considered arrangement, the firefighter is provided with an inertial measurement unit (IMU) which tracks the movement of the firefighter to determine the position. However, in order to determine the position of the person, it is necessary to enter the firefighter’s starting position and orientation. Further, previously considered systems that heavily rely on IMU may be inaccurate.

Embodiments of the invention aim to address at least some of the above problems to some extent.

According to an aspect there is provided a method of calibrating the output of one or more sensors of an inertial measurement unit, the inertial measurement unit being at least a part of a portable module, the method comprising: docking the portable module to a dock; determining that the portable module has been docked; and in response to determining that the portable module has been docked, performing a calibration process to calibrate the output of at least one sensor of the inertial measurement unit.

Determining that the portable module has been docked may be done in any suitable way, such as by using a wired (e.g. contact between terminals) or a wireless connection (e.g. inductive coupling, magnetic coupling etc.).

Docking the portable module to the dock may fix the position of the portable module relative to the dock. The dock may have a cavity for receiving the portable module that corresponds to the external profile of the portable module. In another arrangement the portable module may have a cavity for receiving a portion of the dock.

The calibration process may be performed whilst the dock is stationary. The calibration process may be continually performed whilst the portable module is docked and whilst the dock is stationary. The calibration process comprises a plurality of sequentially performed calibration steps. The calibration steps may be discrete steps. There may be a period of time (e.g. a wait time) between each calibration step. The calibration step may not be implemented until the end of the calibration step.

When the portable module is docked, the dock may supply power to the portable module. The power may be supplied by a physical connection or by a wireless connection. The portable module may automatically determine that it has been docked.

The dock may be attached to a static structure such as a wall or a fixed frame. The dock may be attached to a moveable object. The moveable object may be a vehicle, such as an emergency response vehicle such as a fire engine.

The method may further comprise determining that the dock is stationary. The calibration process may be performed in response to determining that that the portable module has been docked and that the dock is stationary. The method may further comprising halting the calibration process when it is determined that the dock is no longer stationary. Determining that the dock is stationary may be based on movement data relating to the movement of the moveable object. The moveable object may be provided with one or more sensors (e.g. accelerometers, gyroscopes) which generate movement data relating to the movement of the object. The movement data may be provided by an engine control unit (ECU) of the vehicle.

The inertial measurement unit may comprise at least one gyroscope sensor. The calibration process may calibrate the output of the at least one gyroscope sensor.

There may be three mutually orthogonally arranged gyroscope sensors. The calibration process may calibrate the outputs of the three gyroscope sensors.

The inertial measurement unit may comprise at least one accelerometer. The calibration process may calibrate the output of the at least one accelerometer. There may be three mutually orthogonally arranged accelerometers. The calibration process may calibrate the outputs of the three accelerometers.

The portable module may be arranged to form part of a personnel location system.

The portable module may be arranged to be carried by a person, such as an emergency response, such as a firefighter.

According to an aspect there is provided a system for calibrating a portable module; comprising: a portable module comprising an inertial measurement unit comprising one or more sensors; a dock to which the portable module can be docked; a docking determining module arranged to determine whether the portable module has been docked; and a calibration module arranged to perform a calibration process to calibrate the output of at least one sensor of the inertial measurement unit; wherein the system is arranged such that calibration module performs the calibration process in response to determining that the portable module has been docked.

The portable module and dock may be arranged such that the position of the portable module is fixed relative to the dock when the portable module is docked.

The system may be arranged such that the calibration module performs the calibration process whilst the dock is stationary. The system may be arranged such that the calibration module continually performs the calibration process whilst the portable module is docked and whilst the dock is stationary. The calibration module may be arranged to perform a calibration process comprising a plurality of sequentially performed calibration steps. The dock may be a powered dock arranged to supply power to the portable module when it is docked. The portable module may comprise the docking determining module.

The dock may be attached to a static structure. The dock may be attached to a moveable object. The moveable object may be a vehicle.

The system may further comprise a movement determining module arranged to determine whether the dock is stationary. The system may be arranged such that the calibration module performs the calibration process in response to determining that that the portable module has been docked and that the dock is stationary. The system may be arranged such that the calibration module halts the calibration process if it is determined that the dock is no longer stationary. The movement determining module may be arranged to determine whether the dock is stationary based on movement data relating to the movement of the moveable object. The moveable object may be provided with one or more sensors which are arranged to generate movement data relating to the movement of the object. The movement data may be provided by an engine control unit of the vehicle.

The portable module may be arranged to be carried by a person, such as an emergency responder, such as a firefighter.

According to an aspect there is provided a method of initialising an inertial measurement unit which is at least a part of a portable module, the method comprising: docking the portable module to a dock which is fixed to a vehicle, such as an emergency response vehicle (e.g. a fire engine); defining the initial position of the inertial measurement unit based on at least the position of the dock; and defining the initial heading of the inertial measurement unit based on at least the orientation of the dock.

The dock may have a fixed relative position relative to the position of the vehicle and a fixed relative orientation relative to the orientation of the vehicle. The initial position of the inertial measurement unit may be defined based on at least the fixed relative position of the dock. The initial heading of the inertial measurement unit may be based on at least the fixed relative orientation of the dock.

The method may further comprise determining the absolute position and the absolute orientation of the vehicle. The initial absolute position of the inertial measurement unit may be based on at least the position of the dock and the absolute position of the vehicle. The initial heading of the inertial measurement unit may be based on at least the orientation of the dock and the absolute orientation of the vehicle.

The method may further comprise registering the initial position and the initial heading of the inertial measurement unit. The method may further comprise determining that the portable module has been undocked. The initial position and the initial heading of the inertial measurement unit may be registered in response to determining that the portable module has been undocked.

According to another aspect there is provided a system for initialising a portable module, comprising: a portable module comprising an inertial measurement unit; a dock fixed to a vehicle and to which the portable module can be docked; and an initialisation module arranged to: define the initial position of the inertial measurement unit based on at least the position of the dock; and define the initial heading of the inertial measurement unit based on at least the orientation of the dock.

The system may further comprise a determining module arranged to determine the absolute position and the absolute orientation of the vehicle. The initial absolute position of the inertial measurement unit may be based on at least the position of the dock and the absolute position of the vehicle. The initial heading of the inertial measurement unit may be based on at least the orientation of the dock and the absolute orientation of the vehicle.

The system may further comprise a registration module arranged to register the initial position and the initial heading of the inertial measurement unit. The system may further comprise: a determining module arranged to determine when the portable module has been undocked; and wherein the registration module is arranged to register the initial position and the initial heading of the inertial measurement unit in response to the determining module determining that the portable module has been undocked.

The portable module may be arranged to be carried by a person.

According to yet another aspect there is provided a portable module for use with the system in accordance with any statement herein.

According to yet another aspect there is provided a dock for use with the system in accordance with any statement herein.

The invention may comprise any combination of the features and/or limitations referred to herein, except combinations of such features as are mutually exclusive.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 schematically shows a personnel location system;

Figure 2 schematically shows a base station of the location system connected to a dock and a vehicle monitoring unit;

Figure 3 schematically shows the vehicle monitoring unit of Figure 2; and

Figure 4 schematically shows the dock and portable module of Figure 2.

Figure 1 shows a personnel location system 10 for tracking, or locating, the position of one or more people 1. In this embodiment, the location system 10 is for tracking the position of one or more firefighters. However, it should be appreciated that the system 10 could be used to track any type of person. For clarity, in Figure 1 only one firefighter 1 is shown, but it should be appreciated that the location system 10 can track the position of multiple people. The personnel location system 10 generally comprises a central base station 12, a plurality of docks 14 fixedly mounted within an emergency response vehicle (such as a fire engine) 16, and a plurality of portable monitoring modules 20 each comprising an inertial measurement unit (IMU). Each firefighter 1 has a portable monitoring unit 20 associated with or attached to them. For example, a portable module 20 may be attached to a firefighter’s foot, or attached to the harness of the firefighter’s breathing apparatus. As will be described in detail below, before use, each portable monitoring module 20 is docked within a respective dock 14 within the vehicle 16. When the vehicle 16 is stationary and the portable monitoring module 20 is docked, the output of the inertial measurement unit is calibrated. When the vehicle 16 arrives at an incident, the portable monitoring module 20 is removed from the dock 14 as the firefighter 1 leaves the vehicle 16 and the location system 10 initialises the portable monitoring module 20 so as to set the absolute initial starting position and starting heading of the portable module 20. As the firefighter 1 moves around, the location system 10 tracks the movement path 18 of the firefighter 1 using data captured by the IMU of the monitoring module 20, thereby determining the absolute position of the firefighter 1.

Referring to Figure 2, the base station 12, which may be in the form of a portable tablet computer or the like or which may be integrated into the fire engine 16, comprises an initialisation module 40 for initialising each portable module 20, a memory 42 which stores the relative position and orientation of each dock 14 relative to the position and orientation of the vehicle 16, a registration module 44 for registering the absolute initial position and initial heading of each portable module 20, a wireless receiver 46 for receiving movement data from each portable module 20, a position determining module 48 for determining the current absolute position of each firefighter 1 based on the received movement data and the initial position and heading stored in the memory 42, and a display 50 for displaying the position of each firefighter 1 on a digital map.

As shown in Figure 2, each dock 14 is also connected to the initialisation module 40 such that the initialisation module 40 can initialise the portable module 20 when it is undocked. Further, the location system 10 also comprises a vehicle monitoring unit 52 which is attached to or integrated into the vehicle 16. As shown in Figure 2, the vehicle monitoring unit 52 is connected to the initialisation module 40 such that the initialisation module 40 can receive the absolute position and heading of the vehicle. The vehicle monitoring unit 52 is also connected to each dock 14 so each dock can determine whether the vehicle 16 is stationary or moving. In other embodiments, the vehicle 16 ECU could be connected to each dock 14 so each dock can determine whether the vehicle is stationary or moving.

Referring to Figure 3, the vehicle monitoring unit 52 comprises a satellite navigation receiver 54 that is provided at the front and rear of the vehicle 16. In this embodiment the satellite navigation receiver 54 is a GPS receiver. However, it should be appreciated that any other type of satellite navigation receiver could be used, for example a global navigation satellite system (GNSS) such as GLONASS, BeiDou, Galileo, or IRNSS. The satellite navigation receiver 54 is capable of determining an absolute geographic position of the vehicle 16. The vehicle monitoring unit 52 also comprises an electronic compass 58 which is arranged to determine the geographic heading of the vehicle 12 and an inertial measurement unit (IMU) 60. In this embodiment the IMU 60 is provided with three accelerometers, three gyroscopes, and three magnetometers. The sensors are arranged such the measurement axes are orthogonal to each other (i.e. x-axis, y-axis, z-axis). The IMU 60 is arranged to track (or monitor) the movement of the vehicle 16, for example as it is driven to an emergency incident. A controller 62 is provided which receives the outputs of all of these sensors/receivers and determines the absolute position of the vehicle 16 and the absolute heading of the vehicle 16. The controller 62 outputs the initial position and heading to the initialisation module 40, and outputs a signal indicative of whether the vehicle is moving or stationary to each dock 14. For ease of reference it has been schematically shown that all of the sensors and receivers are provided within a single vehicle monitoring unit 52 which is mounted to the vehicle 16, but it should be appreciated that the sensors and receivers may be individually mounted or attached to the vehicle 16.

Referring to Figure 4, each dock 14 is fixed at a specific location within the fire engine

16. For example, if there are eight seats within the fire engine, there may be eight docks 14 which are fixed in the vicinity of each seat. The relative position and orientation of each dock 14 with respect to the position and heading of the vehicle 16 is stored within the memory 42 of the base station 12. The docks 14 are all connected to a power supply, which may either be a power supply of the vehicle 16, or it may be an external power supply to which the vehicle is plugged into when stationary. Each dock 14 is also provided with a docking determining module 64 which is arranged to determine if a portable module 20 is docked within it, and a movement determining module 66 which is arranged to receive data from the vehicle monitoring unit 52 and determine whether or not the vehicle 16, and therefore the dock 14 is stationary. The dock 14 is arranged such that it can receive and secure a single portable module 20 such that the portable module 20 is fixed with respect to the dock 14. For example, a portable module 20 may snap-fit into a dock 14 to fix it in place. The dock 14 is also provided with connection terminals 68 which engage with corresponding terminals 22 provided on the portable module 20. The engagement of the terminals 22 allow the dock 14 to supply power to the portable module 20 when docked, and also allows the docking determining module 64 to determine whether or not a portable module 20 is docked. In should be appreciated that in other embodiments there may be no terminals and instead there may be a wireless communication between the dock 14 and the portable module 20 (e.g. inductive coupling).

As shown in Figure 4, each portable module 20 comprises a battery 24, a docking determining module 26 arranged to determine whether or not the portable module 20 is docked within a dock 14, an inertial measurement unit (IMU) 28, and a wireless transmitter 30 arranged to transmit movement data generated by the IMU 28 to the base station 12. The IMU 28 comprises three accelerometers, three gyroscopes, and three magnetometers. The sensors are arranged such that the measurement axes are orthogonal to each other (i.e. x-axis, y-axis, z-axis). The IMU 28 is arranged to track (or monitor) the movement of the firefighter as they move around. The portable module 20 also comprises a calibration module 32 which is arranged to calibrate the output of the sensors, in particular the output of the gyroscopic sensors, when the portable module 20 is docked and stationary.

Before use, a portable monitoring module 20 is docked within each dock 14 such that each portable module 20 is fixed with respect to the dock 14. Since the docks 14 are at fixed known locations within the vehicle 16, the portable modules 20 are also at fixed known locations with respect to the vehicle 16. When the portable modules 20 are docked, they are powered by the docks 14 and the batteries 24 are charged. Further, if the movement determining module 66 determines that the vehicle 16 (i.e. the fire engine) is stationary based on data received from the vehicle monitoring unit 52, and if the docking determining module 26 of the portable module 20 determines that the portable module 20 is docked, the calibration module 32 performs a calibration process to calibrate the output of the three gyroscope sensors of the IMU 28. In particular, the calibration process, in the form of a plurality of sequentially performed calibration steps, is continually performed whilst the portable module 20 is docked and whilst the dock 14 is stationary. As soon as the movement determining module 66 determines that the vehicle 16 is moving, or as soon as the docking determining module 26 determines that the portable module 20 is no longer docked, the calibration process is halted. Performing the calibration process when the portable module 20 is stationary and is in a known fixed orientation improves the reliability of the output of the gyroscope sensors of the IMU 28. Further, the accuracy of the output of the gyroscope sensors is improved the longer the portable module 20 is calibrated for. Hence, continually calibrating the portable module 20 whilst it is stationary improves the accuracy of the output.

When the fire engine 16 is called to an incident the calibration module 32 halts any further calibration of the gyroscope sensors of the IMU 28 of the portable module 20 as soon as it is determined that the vehicle 16 is moving. As the fire engine 16 is driven to the incident, the vehicle monitoring unit 52 continually monitors the movement of the vehicle 16 using the sensors integrated into the monitoring unit 52. Specifically, the satellite navigation receiver 54 determines an absolute geographic position of the fire engine 16, and the compass 58 determines the geographic heading of the fire engine 16. The absolute position of the fire engine 16 and the absolute heading of the fire engine 16 are continually provided the initialisation module 40. The initialisation module 40 takes the absolute position and absolute heading of the fire engine 16, together with the relative positions and orientations of each dock 14 with respect to the fire engine 16 which are stored in the memory 42, and determines the absolute position and orientation of each dock 14, and hence the absolute position and heading of each portable module 20. When the fire engine 16 reaches the incident, the firefighters exit the vehicle 16, and the hence the firefighter’s portable module 20 (which may be attached to their harness or foot, for example) is undocked from the dock 14. As soon as the dock 14 determines that the portable module 20 has been removed, it communicates this to the initialisation module 40 which causes the initialisation module 40 to register the last determined absolute position and heading of that portable module 20 in the registration module 44.

As the firefighter 1 walks around, the IMU 28 of the portable unit 20 tracks (or monitors) the movement of the firefighter. The data generated by the IMU 28 is transmitted to the base station 12 using the wireless transmitter 30 and is received by the wireless receiver 46. The position determining module 48 uses the data generated by the IMU 28, together with the absolute initial position and heading ofthe portable unit 20, to continually determine the position ofthe firefighter 1. Specifically, in this embodiment the position ofthe firefighter 1 is determined using dead-reckoning. The determining module 48 calculates (or receives) the deltas in each axis {Δχ, Ay, Δζ} which are continually added to the last determined absolute position to calculate a new current position. The determined position of each firefighter 1 is displayed on a map on the display 22, which may be in the form of the screen of a tablet computer or the like.

This allows an operator to visually observe the locations of the firefighters (which may be inside or outside a building). The automatic initialisation of the absolute position and orientation allows the absolute position ofthe firefighter to be monitored using an IMU 28, without the need for a specific additional manual registration process in which the initial position and heading ofthe portable module 20 is set. This reduces the time taken for the firefighter 1 to enter the incident.

In the embodiment described above there is both an automatic calibration and initialisation process. However, it should be appreciated that one feature could be provided without the other. Further, in the above embodiment it has been described that the docks 14 are fixed within a vehicle 16. However, in one embodiment the portable module 20 could be calibrated by docking it in a fixed dock 14 provided elsewhere, for example mounted to a wall or other static structure.

In the foregoing description, the phrase “absolute position” is intended to mean the position on a coordinate system that is pre-defined, for example a geographic coordinate system.

The various features described may be implemented in hardware, or as software modules running on one or more processors. Further, the various modules and controllers may be physically located anywhere. For example, any of the modules could be located on-board a vehicle, on-board the firefighter, or somewhere else entirely. For example, the determining module could be located at the person and the determined position could be transmitted wirelessly to the registering module which could be part of a tablet computer. In essence, any of the data signals and sensor outputs could be processed in any suitable location. The locations of the various modules given are purely examples.

Claims

CLAIMS:

1. A method of calibrating the output of one or more sensors of an inertial measurement unit, the inertial measurement unit being at least a part of a portable module, the method comprising:

docking the portable module to a dock;

determining that the portable module has been docked; and in response to determining that the portable module has been docked, performing a calibration process to calibrate the output of at least one sensor of the inertial measurement unit.

2. A method according to claim 1, wherein docking the portable module to the dock fixes the position of the portable module relative to the dock.

3. A method according to claim 1 or 2, wherein the calibration process is performed whilst the dock is stationary.

4. A method according to claim 3, wherein the calibration process is continually performed whilst the portable module is docked and whilst the dock is stationary.

5. A method according to any preceding claim, wherein the calibration process comprises a plurality of sequentially performed calibration steps.

6. A method according to any preceding claim, wherein when the portable module is docked, the dock supplies power to the portable module.

7. A method according to any preceding claim, wherein the portable module automatically determines that it has been docked.

8. A method according to any preceding claim, wherein the dock is attached to a static structure.

9. A method according to any of claims 1-7, wherein the dock is attached to a moveable object.

10. A method according to claim 9, wherein the moveable object is a vehicle.

11. A method according to any preceding claim, further comprising determining that the dock is stationary, and wherein the calibration process is performed in response to determining that that the portable module has been docked and that the dock is stationary.

12. A method according to claim 11, further comprising halting the calibration process when it is determined that the dock is no longer stationary.

13. A method according to claim 12 when appended to claim 9 or 10, wherein determining that the dock is stationary is based on movement data relating to the movement of the moveable object.

14. A method according to claim 13, wherein the moveable object is provided with one or more sensors which generate movement data relating to the movement of the object.

15. A method according to claim 13 when appended to claim 10, wherein the movement data is provided by an engine control unit of the vehicle.

16. A method according to any preceding claim, wherein the inertial measurement unit comprises at least one gyroscope sensor, and wherein the calibration process calibrates the output of the at least one gyroscope sensor.

17. A method according to claim 16, wherein there are three mutually orthogonally arranged gyroscope sensors, and wherein the calibration process calibrates the outputs of the three gyroscope sensors.

18. A method according to any preceding claim, wherein the inertial measurement unit comprises at least one accelerometer, and wherein the calibration process calibrates the output of the at least one accelerometer.

19. A method according to claim 18, wherein there are three mutually orthogonally arranged accelerometers, and wherein the calibration process calibrates the outputs of the three accelerometers.

20. A method according to any preceding claim, wherein the portable module is arranged to form part of a personnel location system.

21. A method according to any preceding claim, wherein the portable module is arranged to be carried by a person.

22. A method according to claim 1 and substantially as described herein.

23. A system for calibrating a portable module; comprising:

a portable module comprising an inertial measurement unit comprising one or more sensors;

a dock to which the portable module can be docked; a docking determining module arranged to determine whether the portable module has been docked; and a calibration module arranged to perform a calibration process to calibrate the output of at least one sensor of the inertial measurement unit;

wherein the system is arranged such that calibration module performs the calibration process in response to determining that the portable module has been docked.

24. A system according to claim 23, wherein the portable module and dock are arranged such that the position of the portable module is fixed relative to the dock when the portable module is docked.

25. A system according to claim 23 or 24, wherein the system is arranged such that the calibration module performs the calibration process whilst the dock is stationary.

26. A system according to claim 25, wherein the system is arranged such that the calibration module continually performs the calibration process whilst the portable module is docked and whilst the dock is stationary.

27. A system according to any of claims 23-26, wherein the calibration module is arranged to perform a calibration process comprising a plurality of sequentially performed calibration steps.

28. A system according to any of claims 23-27, wherein the dock is a powered dock arranged to supply power to the portable module when it is docked.

29. A system according to any of claims 23-28, wherein the portable module comprises the docking determining module.

30. A system according to any of claims 23-29, wherein the dock is attached to a static structure.

31. A system according to any of claims 23-29, wherein the dock is attached to a moveable object.

32. A system according to claim 31, wherein the moveable object is a vehicle.

33. A system according to any of claims 23-32, further comprising a movement determining module arranged to determine whether the dock is stationary, and wherein the system is arranged such that the calibration module performs the calibration process in response to determining that that the portable module has been docked and that the dock is stationary.

34. A system according to claim 33, wherein the system is arranged such that the calibration module halts the calibration process if it is determined that the dock is no longer stationary.

35. A system according to claim 34 when appended to claim 33 or 34, wherein the movement determining module is arranged to determine whether the dock is stationary based on movement data relating to the movement of the moveable object.

36. A system according to claim 35, wherein the moveable object is provided with one or more sensors which are arranged to generate movement data relating to the movement of the object.

37. A system according to claim 35 when appended to claim 32, wherein the movement data is provided by an engine control unit of the vehicle.

38. A system according to any of claims 23-37, wherein the inertial measurement unit comprises at least one gyroscope sensor, and wherein the calibration process calibrates the output of the at least one gyroscope sensor.

39. A system according to claim 38, wherein there are three mutually orthogonally arranged gyroscope sensors, and wherein the calibration process calibrates the outputs of the three gyroscope sensors.

40. A system according to any of claims 23-39, wherein the inertial measurement unit comprises at least one accelerometer, and wherein the calibration process calibrates the output of the at least one accelerometer.

41. A system according to claim 40, wherein there are three mutually orthogonally arranged accelerometers, and wherein the calibration process calibrates the outputs of the three accelerometers.

42. A system according to any of claims 23-41, wherein the portable module is arranged to form part of a personnel location system.

43. A system according to any of claims 23-42, wherein the portable module is arranged to be carried by a person.

44. A method of initialising an inertial measurement unit which is at least a part of a portable module, the method comprising:

docking the portable module to a dock which is fixed to a vehicle;

defining the initial position of the inertial measurement unit based on at least the position of the dock; and defining the initial heading of the inertial measurement unit based on at least the orientation of the dock.

45. A method according to claim 44, wherein the dock has a fixed relative position relative to the position of the vehicle and a fixed relative orientation relative to the orientation of the vehicle.

46. A method according to claim 45, wherein the initial position of the inertial measurement unit is defined based on at least the fixed relative position of the dock and wherein the initial heading of the inertial measurement unit is based on at least the fixed relative orientation of the dock.

47. A method according to any of claims 44-46, further comprising determining the absolute position and the absolute orientation of the vehicle.

48. A method according to claim 47, wherein the initial absolute position of the inertial measurement unit is based on at least the position of the dock and the absolute position of the vehicle and wherein the initial heading of the inertial measurement unit is based on at least the orientation of the dock and the absolute orientation of the vehicle.

49. A method according to any of claims 44-48, further comprising registering the initial position and the initial heading of the inertial measurement unit.

50. A method according to claim 49, further comprising determining that the portable module has been undocked, wherein the initial position and the initial heading of the inertial measurement unit are registered in response to determining that the portable module has been undocked.

51. A method according to claim 44 and substantially as described herein.

52. A system for initialising a portable module, comprising:

a portable module comprising an inertial measurement unit; a dock fixed to a vehicle and to which the portable module can be docked; and an initialisation module arranged to:

define the initial position of the inertial measurement unit based on at least the position of the dock; and define the initial heading of the inertial measurement unit based on at least the orientation of the dock.

53. A system according to claim 52, wherein the dock has a fixed relative position relative to the position of the vehicle and a fixed relative orientation relative to the orientation of the vehicle.

54. A system according to claim 53, wherein the initial position of the inertial measurement unit is defined based on at least the fixed relative position of the dock and wherein the initial heading of the inertial measurement unit is based on at least the fixed relative orientation of the dock.

55. A system according to any of claims 52-54, further comprising a determining module arranged to determine the absolute position and the absolute orientation of the vehicle.

56. A system according to claim 55, wherein the initial absolute position of the inertial measurement unit is based on at least the position of the dock and the absolute position of the vehicle and wherein the initial heading of the inertial measurement unit is based on at least the orientation of the dock and the absolute orientation of the vehicle.

57. A system according to any of claims 52-56, further comprising a registration module arranged to register the initial position and the initial heading of the inertial measurement unit.

58. A system according to claim 57, further comprising:

a determining module arranged to determine when the portable module has been undocked; and wherein the registration module is arranged to register the initial position and the initial heading of the inertial measurement unit in response to the determining module determining that the portable module has been undocked.

59. A system according to any of claims 52-58, wherein the portable module is arranged to form part of a personnel location system.

60. A system according to any of claims 52-59, wherein the portable module is arranged to be carried by a person.

61. A system in accordance with any of claims 23-43 and in accordance with any of claims 52-60.

62. A portable module for use with the system of any of claims 23-43 or 52-61.

63. A dock for use with the system of any of claims 23-43 or 52-61.

64. A system, a portable module or a dock substantially as described herein with 5 reference to the accompanying drawings.

Intellectual

Property

Office

Application No: GB1614983.3 Examiner: Michael McKenzie