WO2013132409A1

WO2013132409A1 - Local positioning system and method for determining a position

Info

Publication number: WO2013132409A1
Application number: PCT/IB2013/051701
Authority: WO
Inventors: Stephen Michael Pitchers; Paul Richard Simons
Original assignee: Koninklijke Philips N.V.
Priority date: 2012-03-07
Filing date: 2013-03-04
Publication date: 2013-09-12

Abstract

A local positioning system is provided comprising a mobile unit (10) and at least one stationary unit (20). The mobile unit (10) comprises a sound generating unit (11), an acoustic-to-electric transducer (12) and a light detector (13), all electrically coupled to a control unit (14). The at least one stationary unit (20) comprises a sound generating unit (21), an acoustic-to-electric transducer (22) and a lamp (23), all electrically coupled to a control unit (24). The control unit (14) of the mobile unit (10) is operative to control the sound generating unit (11) of the mobile unit (10) to generate an initiation sound at an initiation time T₀ and to determine a distance between the mobile unit (10) and the stationary unit (20) based on the initiation time T₀, a time R₁₀ of detecting an acoustic response by the acoustic-to-electric transducer (12) of the mobile unit (10) and a light coded data portion detected by the light detector (13). The control unit (24) of the stationary unit (20) is operative to schedule a reply message at a scheduled time T₁, the reply message comprising the light coded data portion and the acoustic response, the light coded data portion comprising a time interval between a time R₀₁of detecting the initiation sound by the acoustic-to-electric transducer (22) of the stationary unit (20) and the scheduled time T₁. The control unit (24) of the stationary unit (20) is further operative to control the lamp (23) to send out the light coded data portion and to control the sound generating unit (21) of the stationary unit (20) to send out the acoustic response at the scheduled time T₁.

Description

LOCAL POSITIONING SYSTEM AND METHOD FOR DETERMINING A

POSITION

Field of the invention

This invention relates to a local positioning system comprising a mobile unit and at least one stationary unit. The mobile unit comprises a sound generating unit. The at least one stationary unit comprises an acoustic-to-electric transducer. A control unit of the mobile unit controls the sound generating unit to generate an initiation sound at an initiation time To. The acoustic-to-electric transducer of the stationary unit detects the initiation sound at a time Roi . From the time required for the initiation sound to travel from the mobile unit to the stationary unit, a distance between the mobile unit and the stationary unit can be calculated.

This invention further relates to a method of determining a position of a mobile unit relative to at least one stationary device in the above described system, a mobile unit and also a stationary unit.

The invention also relates to a computer program product for causing a processor to perform such a method.

Background of the invention

With a system as described above, it is possible to calculate the distance between the mobile unit and the stationary unit after measuring how much time it takes for the initiation sound to travel from the one unit to the other. The mobile unit can use its own clock for registering the initiation time To. The moment Roi of reception of the initiation sound by the acoustic-to-electric transducer of the stationary unit is determined using a clock of the stationary unit. In order to make it possible to accurately measure the travel time of the initiation sound, the clocks in the different units must be synchronized or should have a known time difference. Alternatively, the stationary unit replies to the initiation sound with an acoustic response which is generated at a known time interval after receiving the initiation sound. Such a two-way ranging system is described in IEEE Std 8022.15.4a-2007, pp 124-125. An acoustic-to-electric transducer of the mobile unit will then detect the acoustic response at a time Rio. The total travel time of the initiation sound and the acoustic response can then be determined using only the clock of the mobile unit and equals the difference between R₁₀ and T₀, which is denoted T_A , minus the known time interval TB between the reception of the initiation sound and the generating of the acoustic response at the stationary unit. The distance between the mobile unit and the stationary unit then is half of the distance sound can travel in the total travel time.

A disadvantage of the above described ways of determining the distance between the mobile unit and the stationary unit is that it requires either synchronized clocks or some way of informing the mobile unit of the time interval between the reception of the initiation sound and the generating of the acoustic response at the stationary unit. If this time interval has a varying length, a communication network is needed for communicating the length of the time interval to the mobile unit. If the time interval has a fixed value, acoustic responses of different stationary units may

coincidentally arrive at the mobile unit at the same moment, which may lead to collision of signals and communication problems.

Object of the invention

It is an object of the invention to provide a local positioning system without the above mentioned disadvantages.

Summary of the invention

According to a first aspect of the invention, this object is achieved by providing a local positioning system comprising a mobile unit and at least one stationary unit. The mobile unit comprises a sound generating unit, an acoustic-to-electric transducer and a light detector, all electrically coupled to a control unit. The at least one stationary unit comprises a sound generating unit, an acoustic-to-electric transducer and a lamp, all electrically coupled to a control unit. The control unit of the mobile unit is operative to control the sound generating unit of the mobile unit to generate an initiation sound at an initiation time To and to determine a distance between the mobile unit and the stationary unit based on the initiation time To, a time Rio of detecting an acoustic response by the acoustic-to-electric transducer of the mobile unit and a light coded data portion detected by the light detector. The control unit of the stationary unit is operative to schedule a reply message at a scheduled time T_{l s} the reply message comprising the light coded data portion and the acoustic response, the light coded data portion comprising a time interval between a time Roi of detecting the initiation sound by the acoustic-to- electric transducer of the stationary unit and the scheduled time Ti . The control unit of the stationary unit is further operative to control the lamp to send out the light coded data portion and to control the sound generating unit of the stationary unit to send out the acoustic response at the scheduled time Ti .

With the system according to the invention, the clocks of the stationary unit and the mobile unit do not have to be synchronized. The mobile unit determines the initiation time To and the time of receipt of the acoustic response Rio using its own clock. The duration of the time interval between the reception of the initiation sound (Roi) and the generating of the acoustic response (Ti) at the stationary unit can be determined using the clock of the stationary unit. Because this time interval represents a time difference and is independent of what happens at the mobile unit, the clock of the stationary unit does not need to be synchronized with the clock of the mobile unit. It is to be noted that the time interval may be provided as a single value indicating the length of the interval or as a start and end time of the time interval as determined by the stationary unit.

When knowing how long it takes for an audio signal to travel from the mobile unit to the stationary unit and vice versa, the distance between the mobile unit and the stationary unit can be calculated using the known value of the speed of sound. As such, the distance of the mobile unit to one stationary unit already is a description of its position. When more stationary units are added to the system, the position can be determined more accurately. With at least three stationary units, it is possible to precisely determine the position without ambiguity of the mobile unit in two dimensions. Position can be resolved without ambiguity in three dimensions from at least four stationary units. Adding more stationary units to the system improves accuracy and reliability of the position calculations.

The time interval is sent from the stationary unit to the mobile unit by use of coded light. As a result, the stationary devices do not need to be connected to a wired network. If multiple stationary units are used, no interconnections between the separate stationary units are needed. Stationary units can be added to or removed from the system without needing to reconfigure a complete communication network.

In a preferred embodiment, the control unit of the stationary unit is operative to schedule the reply message in accordance with a predetermined

communication protocol, such as ALOHA. Using, e.g., the ALOHA protocol reduces the probability of multiple stationary devices transmitting their reply messages at the same time, thereby interfering with each other and preventing the safe reception of the light coded data portions and the acoustic responses. The reply message preferably further comprises an identification code, identifying the stationary unit. The identification code may be used by the mobile unit to 'recognize' the stationary unit sending the reply message. When multiple stationary units are used this is an important part of the light coded data portion for enabling the mobile unit for finding out which stationary unit is situated at which determined distance.

Alternatively the identity of the stationary unit may be derivable from the acoustic response, e.g. by varying the frequency, amplitude and/or length of the acoustic response. Such variations may be applied within and/or between acoustic responses of different stationary units.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

Brief description of the drawings

In the drawings:

Figure 1 schematically shows a way of determining a distance between two units,

Figure 2 shows a system according to the invention,

Figure 3 shows a diagram illustrating operation of a system according to the invention, and

Figure 4 shows a flow diagram of a method according to the invention.

Detailed description of the invention

Figure 1 schematically shows a way of determining a distance between two units 100, 200. Both units 100, 200 comprise a sound generating unit, such as a speaker 11, 21, and an acoustic-to-electric transducer, such as a microphone 12, 22. Both units 100, 200 also comprise a control unit 14, 24 which is coupled to the speaker 11, 21 and the microphone 12, 22 for processing incoming sounds and controlling the generating of sound by the speakers 11, 21. In this exemplary embodiment, we assume that one of the units is a mobile unit 100 and the other on is a stationary unit 200. The determining of the distance is however performed in exactly the same way when the first unit 100 would be stationary and the other unit 200 mobile or when both units 100, 200 would be mobile/stationary. Both control units 14, 24 comprise a clock for timing operations. The clocks (not shown) of the different control units 14, 24 do not have to be synchronized. At an initiation moment To, the speaker 11 of the mobile unit generates an initiation sound. The sound may be audible or ultrasonic. The generated sound may be a single tone of a fixed length, amplitude and frequency a special, more complex, melody or pseudo-random noise. The microphone 22 of the stationary unit 200 receives the initiation sound at moment Roi . The time it takes for the initiation sound to reach the stationary unit 200 depends on the speed of sound and the distance between the mobile unit 100 and the stationary unit 200. If the control unit 24 of the stationary unit 200 recognizes the incoming sound as an initiation sound, it instructs the speaker 21 to send a reply sound at moment Ti . When the reply sound is received by the microphone 12 of the mobile unit at time Rio, the control unit 14 of the mobile unit 100 can calculate the time interval T_A = Rio - T₀ between sending the initiation sound and receiving the reply sound. If also the time interval TB = Ti - Roi between receiving the initiation sound and sending the response sound by the stationary unit 200 is known, it can be calculated how much time the two audio signals needed for traveling from one unit to the other ((TA - TB)/2).

The time interval TB may be known by the control unit 14 of the mobile unit 100 if it is a fixed and predetermined interval or if both units 100, 200 are part of a communication network that allows the stationary unit 200 to communicate the time interval TB to the mobile unit 100. According to the invention, the control unit 14 of the mobile unit 100 is informed about the time interval TB via coded light.

Figure 2 shows a system according to the invention. The mobile unit 10 in this system is similar to the mobile unit 100 in figure 1 , but additionally comprises a coded light receiver 13 which is also coupled to the control unit 14 of the mobile unit 10. For example, the stationary units may be embodied in lighting devices 20 attached to a ceiling 30. The six lighting devices 20 of this system are similar to the stationary unit 200 in figure 1 , but comprise an additional coded light emitter 23.

Optical free space communications, i.e. visible light (VL) and infra-red (IR) communications, for the selection and advanced control of light sources has previously been proposed, and is referred to as coded light (CL). The light emitted from a lighting device or luminaire, employing coded light comprises a modulated part (which for the human eye is invisible) associated with coded light comprising information messages. The emitted light also comprises an un-modulated part associated with an illumination contribution.

Also the coded light emitter 23 is coupled to the control unit 24 (not shown in this figure) of the stationary units 20. The coded light emitter is a lamp 23, e.g. a LED lamp, which can be turned on/off under control of the control unit 24. The lamp 23 may emit light in the visible part of the spectrum and/or in invisible parts of the spectrum. At moment Ti (see figure 1), the control unit 24 controls the speaker 21 to generate an acoustic response and the lamp 23 to emit a coded light response.

The coded light response at least comprises information about the time interval T_B between receiving the initial sound (at moment Roi) and generating the response sound (at moment Ti). This information may, e.g., be provided as a time interval duration which is independent of any clock offset of the clock of the control unit 24. Alternatively, the information comprises the moments Rm and Ti which are dependent on the clock offset. However, when the control unit 14 of the mobile unit calculates the time interval T_B = Ti - Rm the dependence on the clock offset disappears.

In addition to the to the time interval information, the light coded data portion of the reply message may include an identification of the stationary unit 20. In a system as shown in figure 2, with multiple different stationary units 20 at different positions, it is important to know which reply is coming from which stationary unit 20. . If the distance between the stationary units and the mobile unit is not too large, the audio part will arrive soon after the light coded data portion of the reply message, which provides an easy way to associate the identity of the stationary devices 20 with the acoustic response received at the mobile unit 10. If the positions of the different stationary units 20 are already known, the mobile unit can use well known tri- lateration/multi-lateration techniques for calculating its current position based on the determined distances to the different known stationary unit positions. For this purpose, the mobile unit may comprise a database with all positions and identities of the installed stationary devices. With at least four stationary units 20, the position of the mobile unit can be determined in 3D. However, additional stationary units 20 will contribute to the accuracy and reliability of the position calculation.

In a similar way, when the positions of the stationary units 20 are already known, but not the exact arrangement, repeated measurements of the distances to the mobile station at several different locations can be used for determining which stationary unit is located at which position. This can be very useful for setting up the local positioning system, without having to know beforehand which stationary unit is placed at which position.

Figure 3 shows a diagram illustrating operation of a system according to the invention. This system comprises a mobile unit M and three coded light lamps A, B and C. The mobile unit M is a mobile unit 10 and the coded light lamps are stationary units 20 as described above with reference to figure 2. In the horizontal direction, the diagram indicates a position. The lamp A is nearest to the mobile unit M and the lamp C is the furthest away from it. In the vertical direction, the progress of time is indicated. Time progresses from top to bottom. At time To, the mobile unit M sends out the initiation sound. At a somewhat later moment Roi, the initiation sound reaches the nearest lamp, lamp A. Lamp B, which is located further away from the mobile unit M receives the initiation sound at a later moment Ro₂ and lamp C on moment R03. When the lamps receive the initiation sound, a reply message is scheduled. The reply message comprises an acoustic response and a light coded data portion. The light coded data portion comprises information about the moment Roi, Ro₂, R03 that the initiation sound was received and about the moment T_{l s} T₂, T₃ on which the reply message is scheduled to be sent. The information may, e.g., be provided in the form of the measured moments or as the calculated time intervals in between those moments. For lamp C this time interval (T₃ - R03) is indicated in the figure. As described above, the lamps can use their own clocks with an unknown clock offset.

When scheduling the reply messages, it should be avoided that multiple lamps send their replies at the same moment because that might lead to collision of the acoustic responses and/or the light coded data portions and would made it difficult to determine which acoustic response is coming from which lamp. For example, the well- known ALOHA algorithm may be used to ensure that each lamp sends its reply message at different times. According to the ALOHA algorithm, each lamp sends out its transmission periodically, at randomized intervals, with a low duty cycle. Even without synchronization between lamps, this procedure ensures that collisions are statistically rare. The same ALOHA transmission time is utilized to send both the coded light data portion and to the acoustic response at the same moment. Both these signals can be expected to arrive intact at the mobile unit as collisions will rarely occur. These signals will arrive at approximately the same moment, which provides an easy way to associate the identity of the stationary units with the acoustic response received at the mobile unit.

When the acoustic responses of the lamps A, B and C are received by the microphone of the mobile unit M at the moments Rio, R₂₀, R30 all information needed for calculating the distances to the different lamps is available. For example, the distance to lamp B can be calculated using the formula

S_B = c_sound * ½ * ((R₂₀ - To) - (T₂ - Ro ), or SB = ½ * Csound * (R20 + R02 - o - T2), wherein ¾ is the distance to lamp B and c_SOund is the speed of sound in the relevant medium (probably air) and at the relevant temperature (probably room temperature).

Figure 4 shows a flow diagram of a method according to the invention. It starts with an initiation step 41 at time To, wherein the sound generating unit 11 of the mobile unit 10 generates the initiation sound. In a receiving step 42 at time Rm , a first stationary unit 20 receives the initiation sound at its acoustic-to-electric transducer 22. If a second, third, ... n^th stationary unit 20 are present, the additional stationary units 20 will receive the initial sounds at their respective acoustic-to-electric transducers 22 at moment R01 , R02, · · · Row- In a reply preparation step 43, the control unit 24 of the stationary unit 20 schedules a reply message at a scheduled time T_{l s} T₂, ... T„. In two responding steps 44, 45 at the same scheduled time T_{l s} T₂, ... T„, the reply message is sent out. In acoustic responding step 44, the sound generating unit 21 of the stationary unit generates the acoustic response. In optical responding step 45, the light coded data portion of the reply message is sent out by the light source 23. The light coded data portion comprises the information about the time interval between the moment R01 , Ro₂, . . . Ro« of receiving the initial sound and the moment T_ls T₂, ... T„ of sending the acoustic response.

In response receiving steps 46, 47 both parts of the reply message are received by the mobile unit 10. In acoustic response receiving step 47 at time Rio, R20, · · · R„o the acoustic-to-electric transducer 11 of the mobile unit 10 detects the acoustic response. In coded light receiving step 46, the light detector 13 of the mobile unit 10 detects the light coded data portion of the reply message. Because light travels faster than sound, the coded light will be received somewhat earlier than the sound. However, with the stationary units 20 not too far away from the mobile unit 10, the time difference will be relatively small and it will be easy to associate the identity of the stationary units 20 with the proper acoustic response.

In a position determining step 48, the control unit 14 of the mobile unit 10 determines the distance(s) between the mobile unit 10 and the one or more stationary units 20. The position may then, e.g., be provided to the user as a set of coordinates or as one or more distances to fixed locations. The position may also be indicated on a map of the location and may be used as input for navigation software. Further applications of the position information are, of course, possible.

It will be appreciated that the invention also extends to computer programs, particularly computer programs on or in a carrier, adapted for putting the invention into practice. The program may be in the form of source code, object code, a code intermediate source and object code such as partially compiled form, or in any other form suitable for use in the implementation of the method according to the invention. It will also be appreciated that such a program may have many different architectural designs. For example, a program code implementing the functionality of the method or system according to the invention may be subdivided into one or more subroutines. Many different ways to distribute the functionality among these subroutines will be apparent to the skilled person. The subroutines may be stored together in one executable file to form a self-contained program. Such an executable file may comprise computer executable instructions, for example processor instructions and/or interpreter instructions (e.g. Java interpreter instructions). Alternatively, one or more or all of the subroutines may be stored in at least one external library file and linked with a main program either statically or dynamically, e.g. at run-time. The main program contains at least one call to at least one of the subroutines. Also, the subroutines may comprise function calls to each other. An embodiment relating to a computer program product comprises computer executable instructions corresponding to each of the processing steps of at least one of the methods set forth. These instructions may be subdivided into subroutines and/or be stored in one or more files that may be linked statically or dynamically. Another embodiment relating to a computer program product comprises computer executable instructions corresponding to each of the means of at least one of the systems and/or products set forth. These instructions may be subdivided into subroutines and/or be stored in one or more files that may be linked statically or dynamically.

The carrier of a computer program may be any entity or device capable of carrying the program. For example, the carrier may include a storage medium, such as a ROM, for example a CD ROM or a semiconductor ROM, or a magnetic recording medium, for example a floppy disc or hard disk. Further the carrier may be a

transmissible carrier such as an electrical or optical signal, which may be conveyed via electrical or optical cable or by radio or other means. When the program is embodied in such a signal, the carrier may be constituted by such cable or other device or means. Alternatively, the carrier may be an integrated circuit in which the program is embedded, the integrated circuit being adapted for performing, or for use in the performance of, the relevant method.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A local positioning system comprising a mobile unit (10) and at least one stationary unit (20),

the mobile unit (10) comprising a sound generating unit (11), an acoustic- to-electric transducer (12) and a light detector (13), all electrically coupled to a control unit (14),

the at least one stationary unit (20) comprising a sound generating unit (21), an acoustic-to-electric transducer (22) and a lamp (23), all electrically coupled to a control unit (24),

the control unit (14) of the mobile unit (10) being operative to control the sound generating unit (11) of the mobile unit to generate an initiation sound at an initiation time To and to determine a distance between the mobile unit (10) and the stationary unit (20) based on the initiation time To, a time Rio of detecting an acoustic response by the acoustic-to-electric transducer (12) of the mobile unit (10) and a light coded data portion detected by the light detector (13),

the control unit (24) of the stationary unit (20) being operative to schedule a reply message at a scheduled time Ti, the reply message comprising the light coded data portion and the acoustic response, the light coded data portion comprising a time interval between a time Rm of detecting the initiation sound by the acoustic-to-electric transducer (22) of the stationary unit (20) and the scheduled time T_{l s}

the control unit (24) of the stationary (20) unit further being operative to control the lamp (23) to send out the light coded data portion and to control the sound generating unit (21) of the stationary unit (20) to send out the acoustic response at the scheduled time Ti.

2. A local positioning system according to claim 1, wherein the control unit (24) of the stationary unit (20) is operative to schedule the reply message in accordance with a predetermined communication protocol, such as ALOHA.

3. A local positioning system according to claim 1 comprising n stationary units (20) with n respective sound generating units (21), acoustic-to-electric transducers (22) and lamps (23), all electrically coupled to n respective control units (24),

the control unit (14) of the mobile unit (10) being operative to determine the respective distances between the mobile unit (10) and the respective stationary units (20) based on the initiation time T₀, n respective times R„o of detecting acoustic responses by the acoustic-to-electric transducer (12) of the mobile unit (10) and n respective light coded data portions detected by the light detector (13),

the control units (24) of the n respective stationary units (20) being operative to schedule n respective reply messages at respective scheduled times T„, the reply messages comprising a light coded data portion and the acoustic response, the light coded data portion comprising an identification code identifying the respective stationary unit (20) and a time interval between a time Ro„ of detecting the initiation sound by the acoustic-to-electric transducer (22) of the stationary unit (20) and the scheduled time T„, the control units (24) of the n respective stationary units (20) further being operative to control the respective lamps (23) to send out the respective light coded data portions and to control the sound generating units (21) of the stationary units (20) to send out the respective acoustic responses at the respective scheduled times T_n.

4. A local positioning system according to claim 3, wherein the control unit

(14) of the mobile unit (10) is further operative to determine the position of the mobile unit (10) based on the respective distances.

5. A method of determining a position of a mobile unit (10) relative to at least one stationary unit (20), the method comprising:

a sound generating unit (1 1) of the mobile unit (10) generating an initiation sound at initiation time To,

an acoustic-to-electric transducer (22) of the at least one stationary unit (20) detecting the initiation sound at a time Rm,

a control unit (24) of the at least one stationary unit (20) scheduling a reply message at a scheduled time Ti, the reply message comprising a light coded data portion and an acoustic response, the light coded data portion comprising a time interval between the time Rm and the scheduled time T_{l s}

the control unit (24) of the at least one stationary unit (20) controlling a lamp (23) of the stationary unit (20) to send out the light coded data portion and controlling a sound generating unit (21) of the stationary unit to send out the acoustic response at the scheduled time T_{l s}

an acoustic-to-electric transducer (12) of the mobile unit (10) detecting the acoustic response at time Rio,

a light detector (13) of the mobile unit (10) detecting the light coded data portion of the reply message, and

a control unit (14) of the mobile unit (10) determining a distance between the mobile unit (10) and the stationary unit based on the initiation time To, the time interval and time Rio.

6. A method of determining a position of a mobile unit (10) according to claim 4, wherein

the initiation sound is received and detected by n acoustic-to-electric transducers (22) of n respective stationary units (20) at respective times Ro„,

n reply messages are scheduled by n control units (24) of the n respective stationary units (20) at respective scheduled times T„, each reply message comprising a light coded data portion and an acoustic response, the light coded data portion comprising time intervals between the times Ro„ and the respective scheduled times T„ and an identity of the respective stationary unit (20),

n lamps (23) of the n respective stationary units (20) are controlled by the n control units (24) to send out the n respective light coded data portions,

n sound generating units (21) of the n respective stationary units (20) are controlled to send out the n respective acoustic responses at the respective scheduled times T_n,

the acoustic-to-electric transducer (12) of the mobile unit (10) receives and detects the n respective acoustic responses at respective times R„o,

the light detector (13) of the mobile unit (10) receives and detects the n respective light coded data portions of the n reply messages, and

the control unit (14) of the mobile unit (10) determines the respective distances between the mobile unit (10) and the respective stationary units (20) based on the initiation time To, the time intervals and times R„o-

7. A method of determining a position of a mobile unit (10) according to claim 6, wherein the control unit (14) of the mobile unit (10) further determines the position of the mobile unit (10) based on the respective distances.

8. A computer program product for determining a position of a mobile unit

(10), which program is operative to cause a processor to perform the method as claimed in claim 5.

9. A mobile unit (10) for use in a local positioning system comprising a at least one stationary unit (20),

the mobile unit (10) comprising a sound generating unit (1 1), an acoustic- to-electric transducer (12) and a light detector (13), all electrically coupled to a control unit (14),

the control unit (14) of the mobile unit (10) being operative to control the sound generating unit (11) of the mobile unit to generate an initiation sound at an initiation time To and to determine a distance between the mobile unit (10) and the stationary unit (20) based on the initiation time To, a time Rio of detecting an acoustic response from the stationary unit by the acoustic-to-electric transducer (12) of the mobile unit (10) and a light coded data portion detected by the light detector (13), the light coded data portion comprising a time interval between a time Roi of detecting the initiation sound by the acoustic-to-electric transducer (22) of the stationary unit (20) and a scheduled time Ti of transmission of the acoustic response at the scheduled time Ti .

10. A stationary unit (20) for use in a local positioning system comprising at least one mobile unit (10),

the control unit (14) of the stationary unit (20) being operative to detect an initiation sound and to schedule a reply message at a scheduled time Ti, the reply message comprising a light coded data portion and an acoustic response, the light coded data portion comprising a time interval between a time Roi of detecting the initiation sound by the acoustic-to-electric transducer (22) of the stationary unit (20) and the scheduled time T_{l s} the control unit (24) of the stationary (20) unit further being operative to control the lamp (23) to send out the light coded data portion and to control the sound generating unit (21) of the stationary unit (20) to send out the acoustic response at the scheduled time Ti .