WO1999018412A1

WO1999018412A1 - The indication of the position of objects

Info

Publication number: WO1999018412A1
Application number: PCT/US1998/020713
Authority: WO
Inventors: Andre Van Den Berg; George Francis Fyfe
Original assignee: Marine Data Systems (Proprietary) Limited; Handelman, Joseph, H.
Priority date: 1997-10-03
Filing date: 1998-10-02
Publication date: 1999-04-15
Also published as: AU9598498A

Abstract

Equipment (10) for indicating the position of an object comprises a processor module (12) having a CPU (14) and a modem (16). The modem (16) communicates via a navigational unit (20) with a navigational system. The equipment (10) further includes a transceiver unit (28) in communication with the CPU (14). A system for indicating the position of objects or stations incorporates the equipment (10), each station being equipped with the equipment (10). The equipment (10) of the stations communicate with one another and synchronise their transmission signals in a time division multiplexed manner using internal synchronisation data and obviating the need to rely on a time signal from the navigational system.

Description

THE INDICATION OF THE POSITION OF OBJECTS

THIS INVENTION relates to the indication of the position of

objects. More particularly, the invention relates to a method of, and equipment

for, indicating the position of an object in a population of such objects.

According to a first aspect of the invention, there is provided a

method of indicating the position of an object in a population of such objects,

the method including the steps of

determining a geographical position of the object;

generating a data carrying signal incorporating data relating to the

position of the object and time synchronisation data;

transmitting the data carrying signal to be received by at least one other

object;

monitoring, at said object, a data carrying signal transmitted by said at

least one other object; and

time synchronising the transmission of the data carrying signal of said

one object with the transmission of the data carrying signal of said at least one

other object. When an incoming data carrying signal is received, the method

may include sampling bits of the data carrying signal to switch at a

predetermined part of an incoming data bit of the data carrying signal, so that

a modem "trains" itself to switch either on a high or a low of an incoming data

bit.

The method may then include monitoring the incoming data

carrying signal to determine when a synchronisation characteristic sequence

is received.

When the synchronisation characteristic sequence has been

received, the method may include enabling a processing means (which includes

the modem) to monitor and extract from the incoming data carrying signal an

information carrying portion of the data carrying signal. The information

carrying portion of the data carrying signal may incorporate a predetermined

type of message, information relating to the position of the object transmitting

the received message, a time slot being employed by the object transmitting

the message, identification data of the transmitting object as well as other

information as will be described in greater detail below.

The method may include extracting from the information carrying

portion of the data carrying signal information relating to a time slot selected

by the object for transmission of its data carrying signal. Thus, the method may include generating a time slot map comprising at least one frame which

is subdivided into a plurality of discrete time slots and effecting frame

synchronisation between the objects to synchronise actual slot numbers of the

objects.

The method may further include frame synchronising to a

dominant object in the population of the objects.

Once a valid information carrying portion of the data carrying

signal has been received, the method may include re-setting a synchronisation

timer of a modem carried by the object in readiness for receipt of a further

signal from said at least one other object or another object in range of said object.

According to a second aspect of the invention, there is provided

equipment for indicating the position of an object in a population of the

objects, each object carrying the equipment, in use, and the equipment

including

a position determining means for determining a geographical position of

its associated object;

a processing means for processing data relating to the position of the

object and for generating a data carrying signal including the data relating to

the position of the object and time synchronisation data; a transmitter means for transmitting the data carrying signal to similarly

equipped other objects; and a receiver means for receiving data carrying signals from the other

objects, the signals including time synchronisation data for enabling

transmission of the data carrying signals of the objects to be time synchronised

with respect to one another in a time division multiplexed manner.

The position determining means may include a position

determining unit of a navigational system. The unit may be responsive to an

available navigational system such as a global positioning system (GPS),

GLONASS, Chayka, LORAN-C, or the like.

It will be appreciated that these navigational systems make use

of an accurate time signal or stamp. However, in the event that, for any

reason, the navigational system is unavailable, it is an object and an advantage

of the invention that use is made of an internal time synchronisation

arrangement.

The processing means may include a modem which has a clock

signal which is operable to sample continuously for an incoming data carrying

signal and to switch at a predetermined part of an incoming data bit of the

data carrying signal i.e. to switch either on a high or a low of the incoming

data bit. The modem may be operable to effect bit to bite synchronisation

and may include a shift register of a predetermined size to enable a high

accuracy bit to byte synchronisation system to be employed, for example, a

tri-sync bit to byte synchronisation system.

The equipment may include a memory means in which the data

relating to the geographical position of the object are stored.

Further, a time slot map may be stored in the memory means, the

time slot map comprising at least one frame, the, or each, frame being

subdivided into a plurality of discrete time slots so that the objects can

transmit the data carrying signals in a time division multiplexed manner using

internal synchronisation between the objects to effect the time division

multiplexing.

In one embodiment of the invention, the map may incorporate a

plurality of time frames such as, for example, twenty time frames to

accommodate a high volume of traffic, i.e. signals transmitted by a large

population of objects. Each frame may be subdivided into a plurality of

discrete time slots so that the objects can broadcast the data carrying signals

in a time division multiplexed manner using the internal synchronisation

between the objects to effect the time division multiplexing. For example, each time frame may be divided into approximately

100 slots arranged in 5 cycles of 20 slots each. In each cycle, a

predetermined group of time slots may be used for positional reports or data

slots, a further group of slots may be used as positional reports or data slots

plus dGPS (an additional GPS service for enhanced positional accuracy) data

slots, when available, with a final group of slots being used as announcements

slots relating to positional reports. The dGPS data may be employed to provide

enhanced accuracy of positional information for the receiving objects.

When the time slot map comprises a plurality of frames, the

number of frames employed may be dependent on the volume of transmission

traffic on the communications channel used, with the number of frames being

increased or decreased to optimise the transmission load on the channel.

In another embodiment of the invention, the time slot map may

incorporate a single time frame subdivided into a large number of discrete time

slots. For example, the time slot map may be subdivided into 2 000 slots to

be transmitted each minute. Then, each object may transmit in a

predetermined number of slots depending on the required update rate of the

object. Where the object is a mobile object, the required update rate of the

object may, for example, be dependent on the speed of the object and the

density of the population. Then, the data carrying signal transmitted by any one object may

include data relating to the time slot or slots selected by that object.

The data relating to the time slot may be incorporated into an

information carrying portion of the data carrying signal, the information

carrying portion of the signal further including data relating to a predetermined

message type and identification data of the transmitting object.

The data carrying signal may include, at the end of the

information carrying portion, a frame check sequence (FCS) which utilises a

cyclic redundancy check for determining if a valid message has been received.

Further, the data carrying signal may include, after the frame check sequence,

a buffer portion for accommodating delays in transmission resulting from

distance of transmission and processing time.

The message type may either be a controlled mode message type

or an autonomous mode message type.

A controlled mode message type may either be generated by a

base object, which would normally be a fixed object, or a mobile object. In the

event that a mobile object receives a data carrying signal from a base or fixed

object, the message may include an instruction to adopt a predetermined time

slot in the time slot map for data transmission purposes. Further, a dGPS signal may be incorporated in the data carrying signal from the base object for

effecting improved accuracy of the positional information of the object

receiving the signal from the base object.

In addition, a controlled mode message type may be received from

one mobile object by another mobile object. It is envisaged that this mode will

be used when the object transmitting the data carrying signal is, itself, under

the control of a controlling object, for example, the base object.

Where no base object is in the field of transmission, the message

transmitted from one mobile object to another may be an autonomous mode

message type.

In the case of an autonomous mode message type, further data

may be incorporated in the information carrying portion of the data carrying

signal. Said further data may be transmitted alternately with the data relating

to the time slot in a predetermined ratio, for example, a 1 : 1 ratio.

This said further data, hereinafter referred to as a "semaphore

value", may relate to the number of other objects in range of, or "visible" to, the transmitting object.

When an object transmitting the data is, itself, under a controlled mode, the semaphore value of a receiving object under autonomous mode may

be set to a default value being the highest possible value of the semaphore

value to indicate to other receiving objects that the receiving object is in

communication with another object which is under control of a base object.

In addition, the semaphore value may include data relating to an identification,

for example, a registration number, of the transmitting object which is

monitored and stored by the, or each, receiving object.

If a receiving object has a semaphore value higher than a received

semaphore value, the receiving object may increase its rate of transmission for

the period of time that it maintains the highest semaphore value. Should the

semaphore value of the receiving object at a later time drop below the

semaphore value of another transmitting object, the receiving object may then

lower its transmission rate to a normal transmission rate.

The invention is now described by way of example with reference

to the accompanying diagrammatic drawings.

In the drawings,

Figure 1 shows a block diagram of equipment, in accordance with the

invention, for indicating the position of an object;

Figure 2 shows a schematic representation of a data carrying signal

transmitted by the equipment; and Figures 3A to 3D show a flow chart of the operation of the equipment.

Referring firstly to Figure 1 of the drawings, reference numeral 1 0

generally designates equipment, in accordance with the invention, for

indicating the position of an object or station.

The equipment 1 0 comprises a processor module 1 2 having a

central processing unit (CPU) 1 and a modem 1 6 interconnected by a bit bus

1 8. The modem 1 6 communicates via a navigational unit in the form of a GPS

receiver module 20 via communications links 22, a connector 1 9 and a

universal asynchronous receiver transmitter (UART) 24. The navigational unit

20 is further connected via the links 22 to a navigational unit front end

processor 26.

The equipment 1 0 also includes a transceiver unit 28 comprising

a transmitter and a receiver. The transceiver unit 28 communicates with the

CPU 14 via an address and data bus 30, a transmitter synthesizer 32 and a

receiver synthesizer 34. The CPU 1 4 also communicates with a memory

module 36 via the bus 30.

Finally, the equipment 1 0 comprises a further processor 38 to

which ancillary equipment (not shown) , such as a telemetry card, another

transceiver, or the like is connectable. Additional equipment such as a display means for displaying or entering data in a graphic format for overlaying on

maps, or the like, is connectable to the connector 1 9. In addition, an alarm

generating means for generating an alarm signal, such as an audible alarm or

a visual alarm, when an emergency situation is imminent is also connectable

to the connector 1 9.

The equipment 1 0 is intended for indicating the position of a

station relative to a population of such stations. For example, the equipment

1 0 could be used in a shipping environment, for aircraft, for land based

vehicles or equipment, or the like. In addition, certain fixed stations,

depending on the application of a system using the equipment 10, would also

include the equipment 1 0. Thus, the invention is particularly suitable for

tracking those mobile stations, incorporating the equipment, in a system

relative to one another for collision avoidance purposes and to enhance traffic

flow of mobile stations incorporating the equipment 1 0.

Thus, each station, be it a mobile station or a fixed station, would

have the equipment 1 0 installed. Using the appropriate navigation system, for

example, a GPS system, the geographical position of each station is

determined and data relating to the geographical position of the station is

stored in the memory module 36 of the equipment 1 0.

Each station, via its processor module 1 2 also generates time synchronisation data. Further, an identification tag of each station is stored

in its memory module 36.

The equipment 1 0 is operable at radio frequencies and it is

envisaged that a dedicated radio channel will be allocated to enable the

equipment 10 to be utilised worldwide. However, should one global frequency

not be available, the system 1 0 will operate equally well with regional

frequencies. More than one radio channel may be provided, particularly where

a large number of stations are operable. Thus, the equipment 1 0 may operate

in the VHF or UHF range.

Each station transmits a signal 40 as illustrated in Figure 2 of the

drawings. The signal 40 comprises various components and is of a

predetermined duration. Thus, the signal 40 has a key-up phase 42 where a

power amplifier of the equipment 1 0 is energised, with no data being included

in this part of the signal 40. This is followed by a preamble phase 44 where

a clock of the modem 1 6 of the equipment is synchronised, as will be

described in greater detail below. Thereafter the signal 40 incorporates a

"sync" phase 46 which enables the modem 1 6 to effect bit to byte

synchronisation. Once a synchronisation characteristic sequence has been

detected by the modem 1 6, a start flag is set and what follows in the signal

40, as indicated by reference numeral 48, is regarded as the information

carrying portion. The portion 48 incorporates a message byte 50, a data packet 52 and a frame check sequence 54 which utilises a CCITT sixteen bit

cyclic redundancy check to validate the message byte 50 and the data packet 52. A buffer period 56 is provided to accommodate delays due to the distance of transmission and processing time. Once again, no data are included in the

buffer period 56.

The equipment of the invention makes use of internal time division

multiplexing for enabling signals 40 transmitted by the various stations in a zone to be time division multiplexed and for inhibiting interference of signals.

By "internal time division multiplexing" is meant that no reliance is placed on

time signals external of the system.

Hence, each station generates a time slot map. In a first

embodiment of the invention, the time slot map is divided into a plurality of frames. The number of frames will be dependent on the volume of traffic

being transmitted in the particular radio channel and up to twenty frames can be accommodated in each time slot map. Table 1 below shows an example

of one frame of the time slot map.

TABLE 1 Referring to the above table, it is to be noted that each frame is

subdivided into five cycles of twenty slots each. Each cycle has a duration of

600 mS at a baud rate of 9600. Further, it is to be noted that each cycle has

three different types of slots. Slot types A are used for positional reports or

data slots. Slot types B are also used for positional reports or data slots. In

addition, for transmission by fixed stations, these slots can be used for dGPS

slots for controlled mode transmission, as will be described in greater detail

below. Finally, slot types C are announcement slots which are utilised by a

station entering a particular zone for the first time, as will be described in

greater detail below.

In another embodiment of the invention, the time slot map

incorporates a single time frame subdivided into a large number of discrete

time slots, for example, 2 000 slots per minute. In other words, each slot is

of 30 mS duration or 288 bits at a baud rate of 9600. In this embodiment,

each station may transmit in a predetermined number of slots, depending on

the required update rate of the particular station. Where the station is a mobile

station, the required update rate of the station may, for example, be dependent

on the speed of the station and the density of the population of stations. As

an example, in the case of a maritime application of the system, a ship

incorporating the equipment 1 0, travelling at a speed of approximately 1 2

knots, may have a required update rate of 1 2 seconds. As the time slot map is re-transmitted every minute, the ship may use 5 slots, more or less evenly

spaced, in the time slot map for transmitting its signal 40.

Further, in this embodiment of the invention, there is no

differentiation between the time slots of the map and a station entering a

particular zone for the first time may announce itself in any one of these slots.

In order to achieve time division multiplexing, two types of

synchronisation are employed according to the method of the invention for

achieving synchronisation between stations in the population of the stations.

The first type of synchronisation is slot phase synchronisation which is a

method to align message boundaries of the participating stations with one

another. The second type of synchronisation is frame synchronisation which

is a method to align actual slot numbers assigned to a specific time slot by an

object with the exact time slot assigned to other stations in the population.

The slot phase synchronisation aspect is now described below.

Each station utilising the particular radio channel repeats its

transmission at the required update rate in the case of the second embodiment.

In the case of the first embodiment, if all frames of the time slot map are being

used, each station will be able to transmit its message at least every sixty

seconds or, if only one frame is being used, every 3 seconds. Hence, depending on the volume of traffic over the channel, the use of the radio

channel can be optimised either by an internal control mechanism or an

external control mechanism. If there is no other transmission in the channel

the station will repeat its transmission every 600 mS.

It will be appreciated that, when each station is not transmitting

its own data carrying signal 40, it will be monitoring the transmitted data

carrying signals 40 of the other stations in the zone.

Initially, when a mobile station enters the relevant zone after

having selected a predetermined radio channel, the station monitors the radio

channel via its transceiver 28. This station will listen to the selected channel

for at least 2 frames whereafter it selects an available slot or slots in the time

slot map. The station then uses one or more randomly selected slots to

announce itself to other stations in the zone and to advise which slot or slots

has/have been selected for repetitive use.

The incoming station monitors data carrying signals 40

transmitted by the other stations in the zone and the modem 1 6 of the

equipment "trains" itself to sample bits of the incoming signal 40 and to set

its clock sampling. This is achieved in the preamble phase 44 of the signal 40.

Thereafter tri-sync bit to byte synchronisation of the incoming signal 40 is

effected by the modem 1 6 and the three eight bit data strings are entered into a 24 bit register of the modem 1 6. Each time a bit is received in the register

and the byte shifted, the modem 1 6 tests to determine whether or not a

synchronisation characteristic sequence has been received. When the

synchronisation characteristic sequence is detected, the bytes received after

t1 are regarded as the information carrying portion 48. The portion 48 is

checked by the frame check sequence 54 to determine that it is valid packet.

Upon successful completion of the check, the timer, which is a downcounter,

of the modem 1 6 is reset to time tO including compensating for the remaining

part of the buffering portion 56 to be ready for the next signal.

If the incoming station should receive a message from another

station indicating that it has selected the same slot as that other station the

incoming station selects another slot and repeats the announcement procedure.

Preferably two announcements are done before the incoming station actually

changes to the selected slot. In this regard, it is to be appreciated that data

relating to a selected slot is incorporated in the data packet 52 in the signal

40. Once the incoming station transmits in its selected slot it indicates the

actual slot number or numbers in the data packet 52 in the signal 40. During

transmission, the station at intervals randomly changes to other slots to inhibit

the likelihood that two incoming stations simultaneously select the same time

slot. Before the station changes, it announces that it is to do so for two

frames also indicating to which slot or slots it is to change. After the two

frames the station switches to the new slot/s. When an incoming station detects that there is no available slot

from which it can select, the incoming station continuously announces itself

at randomly generated intervals until a slot becomes available.

Should an incoming station come into the "footprint" of a

controlling station the incoming station will continuously announce itself at the

randomly generated intervals until it receives an instruction from the controlling

station to use a specific slot or slots. The footprint refers to the area covered

by a signal transmitted from the controlling station.

Various message types can be incorporated in the message byte

50. Thus, there are two main categories of message types being a controlled

mode, with sub categories in respect of, firstly, a base or controlling station

and, secondly, another mobile station in the zone; and an autonomous mode

which is used for communication between mobile stations only.

The above message types will be described now with reference

to various possibilities. Thus, should an incoming station come into the

footprint of a controlling station, it will, as described above, continue to

announce itself at randomly generated intervals until instructed to transmit

from a designated slot in the frame. Also, the controlling station transmits a

dGPS signal, where available, to enable the incoming station more accurately

to determine its geographical position. As indicated above, when the incoming station comes under the influence of the controlling station, the controlling

station instructs it as to which time slots in the frame are to be used and

which are then used for transmission purposes. The incoming station then

synchronises its frame with that of the controlling station. This is known as

the frame synchronisation procedure as shown in Figures 3A to 3D of the

drawings.

To effect frame synchronisation between stations, the incoming

station monitors the slot of the transmitting station. The incoming station

may, for example, be receiving in its perceived time slot 25 with the

transmitting station in the transmitting station's perceived time slot 65 as

indicated in the data portion received. Thus the frames of the stations are not

in frame synchronisation. Then, referring to Table 2 below, the incoming

station will enter the transmitting station in the Network Domain in slot 25

with a Relative Slot Number of 65 and a Slot Offset Value of 40. If the

transmitting station is dominant, the incoming station then moves all the

existing entries in the Local Domain to the corresponding slot number entries

in the Network Domain, setting their Slot Offset Values to zero and clearing

the Local Domain Database. All entries in the Network Domain with a Slot

Offset Value of 40 will then be moved to the corresponding slot numbers in

the Local Domain, clearing these entries in the Network Domain. Slot Offset

Values of all entries remaining in the Network Domain will then be adjusted by

a value of 40. The Slot Map Index Offset Value is then adjusted by 40 and the Current Slot Number indicator of the incoming station is adjusted to 65. In so

doing frame synchronisation is achieved. A controlling station is by default the

dominant station.

The further type of controlled mode message is an autonomous-

like mode type and is where an incoming station, which is not in the footprint

of the controlling station, receives a transmission from another, normally

mobile, dominant station which itself is under the control of a controlling

station.

The incoming station then locks on to the dominant station and

effects the slot phase synchronisation procedure to ensure time

synchronisation of transmitted signals between the dominant station and the

incoming station.

In addition, the dominant station emits an identification signal

referred to as a "semaphore value". Initially, a semaphore ID is received by the

incoming station which is stored in the memory module 36 of the equipment

1 0 of the incoming station. Thereafter, the dominant station emits a

semaphore value signal which is a representation of the number of stations

which are visible to the dominant station and which are registered in the

database of the dominant station. A station is regarded as a dominant station

if it has the highest semaphore value, i.e. most stations visible to it. The dominant station alternates the transmission of the slot

number in the data packet 52 with the semaphore value at a 1 : 1 ratio.

If the incoming station's own semaphore value is greater than the

highest semaphore value received, the incoming station increases its rate of

transmission for the period of time it maintains the highest "visibility" i.e.

semaphore value. It will be appreciated that the semaphore value of any

station will be incremented or decremented due to stations entering or

departing from the footprint of that particular station.

Should a previously dominant station determine that there is

another station with a higher semaphore value, it resumes transmission of its

data carrying signal 40 at the normal required rate.

When an incoming station falls in the footprint of a controlled

station, i.e. one which, in turn is in the footprint of the base station, the

incoming station changes its semaphore value by default to the highest

possible value being 2047. The semaphore data string is an 1 1 bit word and,

accordingly, the highest possible value is 2047. This is done so that other

stations coming into the footprint of the incoming station know that the

incoming station is indirectly under control of the base station.

In an autonomous mode message type, the semaphore value procedure is again followed to enable communications between the stations

which would normally all be mobile stations. This message type is employed

where none of the stations is under the control of a controlling station.

In the semaphore value procedure, the data relating to the

semaphore ID and the semaphore value are only maintained for a particular

period of time governed by a time-out-timer which is periodically reset by the

CPU 1 4 of the equipment 1 0 of the relevant station.

Each time a station receives a signal 40 from another station it

compares this signal with the data stored in its database. This procedure is

shown in Figure 3D of the drawings and a database configuration of each

station is shown in Table 2 below.

TABLE 2 Where frame synchronisation has been effected, as described

above, the other station is inserted into the particular location in the slot map

of the Local Domain. Where there has been no frame synchronisation, the

data received are inserted in the Network Domain of the database. Thereafter,

frame synchronisation is effected to shift the data from the Network Domain

to the Local Domain, as described above.

It is to be noted that both the Local Domain and the Network

Domain make use of time-out-timers so that the database is periodically

updated.

The differentiation between the Local Domain and the Network

Domain is to enable stations to determine whether or not they are in frame

synchronisation with each other and, if not, to effect frame synchronisation.

In order to improve the quality of the slot phase synchronisation

procedure, it will be appreciated that the more stations to which a station can

slot phase synchronise, the better. Each time a station receives a signal 40

from another station, other than the semaphore value, which is frame

synchronised to it and the slot phase synchronisation of which is within

acceptable parameters, that station registers the specific station as a possible

candidate to which to slot phase synchronise. After sampling said other

station a number of times, for example, a minimum of three frames, an indicator is set to use that other station's messages to which to slot phase

synchronise. Each time a message does not comply with the required

parameters, the slot phase synchronisation indicator of the station is re-set and

the procedure has to be re-initiated.

As indicated above, the invention has particular application in

maritime applications or aircraft control applications but can also be used with

land-based systems such as vehicles.

It is a particular advantage of the invention that a fully automated

system is provided where the position of the stations can be monitored and controlled both for traffic management purposes and collision avoidance

purposes. Further, due to the use of an internal time synchronisation

mechanism, the system operates independently of any external systems.

Thus, the need to rely on, for example, the time signal of a GPS system is

obviated. This is particularly advantageous since it may arise that the GPS

system is rendered inoperative, for example, when selective availability of the

GPS system is activated. Also, particularly in land-based systems in city

centres, loss of the satellite signal is a very real possibility. In addition, when

the GPS system is utilised, there is total dependability on the GPS satellites for

operation of a system and use of the internal time synchronisation mechanism

obviates this dependence. Further, due to the flexibility of the time slot map, the radio channel is used in an optimal manner for a particular transmission

load.

Claims

1 . A method of indicating the position of an object in a population

of such objects, the method including the steps of

determining a geographical position of the object;

generating a data carrying signal incorporating data relating to the

position of the object and time synchronisation data;

transmitting the data carrying signal to be received by at least one other

object;

monitoring, at said object, a data carrying signal transmitted by said at

least one other object; and

time synchronising the transmission of the data carrying signal of said

other object.

2. The method as claimed in Claim 1 which includes, when an

incoming data carrying signal is received, sampling bits of the data carrying

signal to switch at a predetermined part of an incoming data bit of the data

carrying signal.

3. The method as claimed in Claim 2 which includes monitoring the

incoming data carrying signal to determine when a synchronisation

characteristic sequence is received.

4. The method as claimed in Claim 3 which includes, when the

synchronisation characteristic sequence has been received, enabling a

processing means to monitor and extract from the incoming data carrying

signal an information carrying portion of the data carrying signal.

5. The method as claimed in Claim 4 which includes extracting from

the information carrying portion of the data carrying signal information relating

to a time slot selected by the object for transmission of its data carrying signal.

6. The method as claimed in Claim 5 which includes generating a

time slot map comprising at least one frame which is subdivided into a plurality

of discrete time slots and effecting frame synchronisation between the objects

to synchronise actual slot numbers of the objects.

7. The method as claimed in Claim 6 which includes frame

synchronising to a dominant object in the population of the objects.

8. The method as claimed in any one of Claims 4 to 7 inclusive

which includes, once a valid information carrying portion of the data carrying

signal has been received, re-setting a synchronisation timer of a modem carried

by the object in readiness for receipt of a further signal from said at least one

other object or another object in range of said object.

9. Equipment for indicating the position of an object in a population

of the objects, each object carrying the equipment, in use, and the equipment

including a position determining means for determining a geographical position of

its associated object; a processing means for processing data relating to the position of the

object and for generating a data carrying signal including the data relating to

the position of the object and time synchronisation data;

a transmitter means for transmitting the data carrying signal to similarly

equipped other objects; and

a receiver means for receiving data carrying signals from the other

objects, the signals including time synchronisation data for enabling

with respect to one another in a time division multiplexed manner.

10. The equipment as claimed in Claim 9 in which the position

determining means includes a position determining unit of a navigational

system.

1 1 . The equipment as claimed in Claim 9 or Claim 1 0 in which the

processing means includes a modem which has a clock signai which is

operable to sample continuously for an incoming data carrying signal and to

switch at a predetermined part of an incoming data bit of the data carrying signal.

1 2. The equipment as claimed in Claim 1 1 in which the modem is

operable to effect bit to bite synchronisation and which includes a shift register

of a predetermined size.

1 3. The equipment as claimed in any one of Claims 9 to 1 2 inclusive

which includes a memory means in which the data relating to the geographical

position of the object are stored.

1 4. The equipment as claimed in Claim 1 3 in which a time slot map

is stored in the memory means, the time slot map comprising at least one

frame, the, or each, frame being subdivided into a plurality of discrete time

slots so that the objects can transmit the data carrying signals in a time

division multiplexed manner using internal synchronisation between the objects

to effect the time division multiplexing.

1 5. The equipment as claimed in Claim 14 in which the data carrying

signal transmitted by any one object includes data relating to the time slot or

slots selected by that object.

1 6. The equipment as claimed in Claim 1 5 in which the data relating

to the time slot is incorporated into an information carrying portion of the data carrying signal, the information carrying portion of the signal further including

data relating to a predetermined message type and identification data of the

transmitting object.

1 7. The equipment as claimed in Claim 1 6 in which the data carrying

signal also includes, at the end of the information carrying portion, a frame

check sequence for determining if a valid message has been received.

1 8. The equipment as claimed in Claim 1 7 in which the data carrying

signal includes, after the frame check sequence, a buffer portion for

accommodating delays in transmission resulting from distance of transmission

and processing time.

1 9. The equipment as claimed in any one of Claims 1 6 to 1 8 inclusive

in which the message type is either a controlled mode message type or an

autonomous mode message type.

20. The equipment as claimed in Claim 1 9 in which, in the case of an

autonomous mode message type, further data are incorporated in the

information carrying portion of the data carrying signal.

21 . The equipment as claimed in Claim 20 in which said further data

are transmitted alternately with the data relating to the time slot in a predetermined ratio.

22. The equipment as claimed in Claim 20 or Claim 21 in which said

further data relates to the number of other objects in range of the transmitting

object.