WO1999021062A1

WO1999021062A1 - Provision of accurate time to a timepiece

Info

Publication number: WO1999021062A1
Application number: PCT/AU1998/000866
Authority: WO
Inventors: Roger John La Salle
Original assignee: H.P.M. Technologies Pty. Ltd.
Priority date: 1997-10-20
Filing date: 1998-10-20
Publication date: 1999-04-29
Also published as: AUPO986997A0

Abstract

A system for the provision of accurate time to a timepiece (112) involves the transmission (110) of signals containing information on accurate time to remote timepieces (112). Each transmission 110 consists of at least two signals, the information contained in each of the signals being such that a timepiece (112) is capable of obtaining accurate correct time from the two signals or if the transmission (110) consists of more than two signals, from two of those signals. Ideally, in each transmission (110) there are three signals, designated 'mark' (M), 'update one' (U1) and 'update two' (U2). Accurate time may be obtained from the signal combinations M+U1, M+U2 and U1+U2. Specific data, such as that relating to the time of the introduction or cessation of Daylight Saving Time, which does not change for lengthy periods of time, is transmitted with only one of the three signals. Each timepiece (112) may have a unique identifier, such as a serial number, and each signal may contain a reference to that identifier to enable the timepiece (112) associated with the identifier to be individually addressed, for sending a specific message to the timepiece (112) or for disabling said timepiece so that it is unable to receive further time message signals.

Description

PROVISION OF ACCURATE TIME TO A TIMEPIECE

This invention relates to the provision of accurate time to a timepiece.

In this specification, the term "timepiece" refers to a horological device for keeping and/or displaying time, and to devices adapted to calculate and/or receive and/or store and/or supply time information, whether or not such time information is displayed.

Timekeeping devices have been in use for some 3,500 years. Mechanical timepieces have been in existence for 900 years, and clocks have been known since the fourteenth century.

Every location on a particular degree of longitude has the same mean solar time. However, in a town or village a few kilometres to the east or west of such a location, noon, for example, occurs a few minutes earlier or later. Life was regulated by the rising and setting of the sun. Until railways brought improved travel and communications in the 19th Century, there was no need to impose a time standard over larger areas. With the coming of the railways, and subsequently the telegraph, it became necessary to impose a single standard time.

In a country which is not large in terms of land area, such as the United Kingdom, a single standard time could be applied to an entire country. In Britain, the standard time was based on Greenwich Mean Time (GMT), the mean solar time of the 0° meridian. Every 15° of longitude represents a one hour time difference, and that led to the provision of time zones, each of which was represented a time which was not too far away from mean solar times at individual locations within the zone. From 1884, GMT was use to define a world time regime and the time zones which formed part of that regime. Examples of larger countries which span a number of time zones are the United States of America, Canada and Australia. Australia is divided into three time zones: Eastern Standard Time [EST] (applicable in Queensland, New South Wales, the Australian Capital Territory, Victoria and Tasmania: GMT+10), Central Standard Time [CST] (applicable in South Australia and the Northern Territory: GMT+9¹/₂) and Western Standard Time [WST] (applicable in Western Australia): GMT+8. Some States have adopted Daylight Saving, advancing clocks one hour ahead of the relevant Standard time for a period of some months between spring and autumn each year. Others have not, which results in there being up to five time zones in Australia during the period in which Daylight Saving operates. In 1998/1999, with Tasmania commencing Daylight Saving ahead of Victoria, New South Wales and the ACT, four time zones were created in early October 1998, with a different set of five time zones subsequently created by the commencement of Daylight Saving in New South Wales and Victoria, with Queensland remaining on Eastern Standard Time, and by the commencement of Daylight Saving in South Australia, with the Northern Territory remaining on CST. As a result, there are difficulties for persons, particularly business persons, conducting activities in relation to States and/or Territories which are nominally in the one time zone, but which are for much of the year in an effectively different time zone.

Despite the problems caused by having between three and five time zones, and the twice-yearly change of the time regime in all but three of the eight States and Territories, most timepieces in Australia operate independently of each other, and require a manual operation to correct and change the time displayed, and day and date when applicable. In fact, until relatively recent times, truly accurate time has not been required for day-to-day personal and even commercial activities, and the maintenance of accurate time has often been a "hit or miss" operation. For example, many timepieces associated with cash registers, facsimile machines and other equipment are left on the applicable standard time throughout the year, with resultant inaccuracies in time printouts during the period of Daylight Saving, without considering the fact that many of the clocks presently used in commercial and other applications drift alarmingly over relatively short periods of time. In Australia, Telstra Corporation Limited is responsible for providing reference times, producing the "pips" heard on the hour (on the half-hour in South Australia and the Northern Territory) on some radio stations, providing the dial- up "speaking clock", and providing equivalent facilities for computer users via modems. However, the reference time provided by Telstra is rarely directly and/or continuously used to provide a timepiece with accurate time.

There is clearly a need in Australia and other countries for a system which is able to remotely synchronize and reset timepieces, so that the timepieces display accurate time. For example, it is essential to be able to maintain the correct time on timepieces - most of which have digital displays - which are associated with personal computers, cash registers, fax machines, automated teller machines (ATMs), debit and credit card terminals, automated ticket machines and ticket validators for public transport systems, and so on. This is particularly so for accurate records of such things as credit card transactions, ATM transactions, faxes sent and received, ticket validation, and so on, as well as dealing with the problems caused by Daylight Saving changes. On a domestic level, having inaccurate time on a VCR can mean that a VCR set to record a television program will start too late or finish too early. In addition, there is rarely any structure in place to check and correct the time shown on such a timepiece; it is also quite common for such clocks to not be reset for and/or after Daylight Saving changes.

At present in Australia there are installations of a plurality of clocks, operating on a "master-slave" basis, where a master clock is connected by electrical cabling to a number of slave clocks, which are controlled by the master clock and as a result all show the same time. Such installations may be found in locations such as hospitals and airport terminals. They are cumbersome, and expensive to construct, and are only useful where no slave clock is very far from the master clock.

The concept of using a radio signal to carry time information was first published some forty years ago. In the Federal Republic of Germany there has operated for many years a system called the German Radio Clock, the operation of which is described in AU-A-59372/94. A powerful VLF transmitter broadcasts "time telegram" signals which are received by a large number of clocks throughout Europe (much of which operates on Central European Time), and which synchronizes those clocks, bringing each of them to the correct time.

In US-A-4,204,398 to Lemelson, published on 27 May 1980, there is disclosed a method and apparatus for automatically setting timepieces in a time zone. The method involved the setting of a timepiece - such as a watch worn by a traveller passing from one time zone to another - by the transmission of a time-related signal, which results in the timepiece displaying or being able to display the correct time for the time zone in which the timepiece is located.

In US-A-4,713,808 to Gaskill et al, published on 15 December 1987, a paging system is disclosed in which paging messages are transmitted to watch pagers. The pagers include a conventional electronic clock with an analog display time, and although time information is transmitted for short-term digital display on the watch pager along with information such as telephone numbers and icons representing common types of paging messages, there is no ability to update the analog time display from the paging signals.

EP-A-0 475 298 by NEC Corporation, published on 18 March 1992, relates to time-keeping apparatus which utilises a cellular mobile communications system as a source of transmitted signals which cause a correct time to be displayed on a timepiece which is adapted to be mounted on a vehicle or to be personally carried.

US-A-5,241 ,305 to Fascenda et al, published on 31 August 1993, describes a pager-based information system, in which the pager includes a real-time pager clock which is updated by pairs of broadcast time messages, the second time message having a time correction for the first time message. Each of the first and second time messages includes time information, and a time correction which is used to correct the time of the preceding time message to account for delays between the time stated in a particular time message and the actual time of broadcast of that time message.

In AU-A-16102/95 by H.P.M. Technologies Pty. Ltd., published on 5 October

1995, there is disclosed a method and apparatus for synchronizing a timepiece to a reference time, which includes the preparation of an encoded and/or encrypted signal for transmission from a paging network transmitter. The signal, which contains a time message and information on Daylight Saving changes and corrections to UTC (Co-ordinated Universal Time, the replacement from 1972 for GMT) time, is transmitted a number of times in a twenty-four hour period, with brief intervals between each transmission. A timepiece has a receiver and control circuitry which receive and decode the signal. The decoded time information is used to automatically set the time of the timepiece to the correct time for a predetermined time zone. A locality switch is provided on the timepiece to determine which time zone time is to be displayed.

The invention described in EP-A-0 703 514 by ETA SA Fabriques d'Ebauches, published on 27 March 1996, is a system for generating and transmitting a succession of time message signals via a paging network to a number of receivers. Each time message signal is representative of the real time of receipt by a receiver of a preceding time message signal. The receivers include clock means which are reset in response to a first time message signal, and for advancing the clock means in accordance with a subsequent time message signal.

In AU-A-54723/96 by H.P.M. Technologies Pty. Ltd., published on 19 December

1996, there is disclosed improvements in a method and apparatus for synchronizing a timepiece to a reference time, utilizing signals which are transmitted over a paging network. To correct for delays which may be encountered in the network, in queuing and transmission, a second signal may be transmitted as a result of the first signal being received by a monitor, the second signal including information about the time of receipt of the first signal by the monitor, such that the timepiece, upon receipt of the second signal, is able to correct for any delay between the transmitted time and the time of receipt of that transmitted time by the monitor, and to then display highly accurate time.

WO 98/14842 (PCT/AU97/00659) by H.P.M. Technologies Pty. Ltd., published on 9 April 1998, discloses a method and apparatus, in a system for the synchronization of a timepiece to a reference time, to correct for the possibility that data may be corrupted, updates may not take place until a third signal is transmitted. A master unit located in the paging network coverage area, which has a source of accurate time, receives the time update and determines that the updated time is correct or incorrect. If it is correct, the third signal will be a "confirm" signal. If the updated time is incorrect, the third signal will be an "ignore" signal.

It is an object of this invention to provide an improved system for the provision of accurate time to a timepiece.

The invention provides a system for the provision of accurate time to a timepiece, in which signals containing at least information on accurate time are transmitted to remote timepieces, characterized in that in each transmission there are at least two signals, the information contained in each of said signals being such that a timepiece is capable of computing correct time from said two signals or from two of said at least two signals.

The invention also provides a method for the provision of accurate time to a timepiece, in which steps signals containing at least information on accurate time are transmitted to and received by remote timepieces, characterized by the steps of:

composing at least two signals, the information contained in each of said signals being such that a timepiece is capable of computing correct time from said two signals or from any two of said at least two signals;

transmitting said at least two signals; receiving at least two of said signals at a timepiece;

decoding said two received signals; and

computing correct time from the information contained in said received signals.

The invention further provides apparatus for the provision of accurate time to a timepiece, which apparatus is adapted to compose and transmit signals containing at least information on accurate time to remote timepieces, which remote timepieces are adapted to receive said signals, characterized in that said apparatus is adapted to compose at least two signals, the information contained in each of said signals being such that a timepiece is capable of computing correct time from said two signals or from any two of said at least two signals, and said apparatus is further adapted to transmit said at least two signals, which said at least two of said signals are adapted to be received at a timepiece, said two received signals being decoded at said timepiece, accurate time being able to be computed from the information contained in said received signals.

An embodiment of the invention, which may be preferred, will be described in detail hereinafter with reference to the accompanying drawings, in which :-

Fig 1. is a block diagram of an embodiment of a system, method and apparatus for the provision of accurate time to a timepiece according to the present invention.

To assist in describing the embodiment of the present invention, the entire contents of the specification, abstract, claims and drawings of AU-A-16102/95, AU-A-54723/96 and WO 98/14842 are hereby incorporated into this specification by reference.

The block diagram of Fig. 1 of AU-A-16102/95 represents an arrangement which utilises a Radio Paging Network (RPN) such as that operated in Australia by Link Telecommunications. The RPN is adapted to send messages to paging devices carried by persons. The RPN services most of the geographic area of Australia, and a very high percentage of the population of that country. The network is currently operated by sending signals via landlines to master transmitters, which relay signals to a larger number of slave transmitters. In the future, signals may be transmitted from an Earth station to a geosynchronous satellite, which in turn will transmit the signals to the territory of Australia.

The aforementioned figure also shows signal generating apparatus 10. In the prior art a signal - preferably an encoded and/or encrypted signal - is prepared using apparatus 10, and then sent to the RPN signal distribution centre for transmission from the RPN transmitters.

Timepieces 18 and 22 include or are associated with a radio receiver and control circuitry such that a time message signal transmitted from transmitter 14 or 16 is received by the receiver and decoded by the control circuitry to synchronize a clock mechanism or movement in the timepiece to adjust the mechanism to the correct time. The clock may have an analog display (20) or a digital display (24). Alternatively, the output from the control circuitry may consist of output means such as a digital signal, which may be directly or indirectly interfaced to a personal computer, slave clocks, cash register or other electronic equipment for the purpose of conveying correct time and/or time synchronizing, which may also include year, day and date information.

The clock mechanism's circuitry may be so arranged that the time (and the day, date and year information if they are included) is able to be synchronized as a result of receiving the aforementioned signal to maintain the accuracy of the mechanism, or for updating the time for changes such as the entry into or the departure from Daylight Saving, or for the occasional corrections to UTC (Coordinated Universal Time) time. As far as the latter is concerned, there are regular agreed corrections to time, often by the addition of a leap-second on a predetermined date. While UTC time - which is equivalent to GMT - is referred to throughout the specification and claims, local time, which is in itself derived from UTC time, may well be substituted when accurate time is being sourced, and any reference to UTC time may be seen to include a reference to local time.

AU-A-54723/96 discloses an arrangement for providing a 'real time' signal to enable timepieces to be synchronized to a reference time. It has been acknowledged that true real time signalling is difficult if not impossible to achieve due to transmission delays and problems associated with "queuing" delays.

In its simplest form, the approach of AU-A-54723/96 is to transmit a first signal, which may be regarded as a start signal or 'mark'. That first signal or mark is sent to the paging network as generally described earlier in this specification, and may well wait in a queue before being transmitted. A typical queue delay may be about five minutes.

The receipt of the mark signal by a timepiece starts an internal counter within the timepiece. The signal is also noted by the monitor, which is in the transmitting coverage area of the transmitter. The monitor notes the precise time the first signal (mark) was received by it. The monitor then composes a second signal which includes information on the precise time of receipt of the first signal. That second signal is then be sent to the transmitter for transmission. A timepiece, on receiving the second signal applies that precise or accurate time sent by the second signal to the elapsed time on the counter, to provide an extremely accurate time or display on the timepiece.

WO 98/14842 discloses a system to cope with the possibility, however unlikely it may be, of data being inadvertently corrupted in the time transmitting means or encoding means within the RPN, which may result in all timepieces receiving the corrupted signal and accordingly displaying a corrupted and thus incorrect time.

One suggested approach envisages the timepieces which receive the time signal mark and update do not accept the data contained therein until a third, "confirm" signal is received. Such a third signal is sent out over the RPN after a master unit located in the RPN coverage area, and having an accurate source of time such as an accurate real time clock, receives the time update and determines that the updated time is correct. If the tested time is correct, a "confirm" signa¹ is sent to all timepieces.

Alternatively, if the master unit determines that the updated time is incorrect, an "ignore" signal is sent out over the RPN, as a result of which the timepieces do not accept the time as correct. During a predetermined time period, for example 15 minutes, after receipt of the update signal, the timepieces do not accept the time sent as correct, while the "ignore" signal is being sent out over the RPN.

By way of an introduction to the present invention, as represented by the embodiment of Fig. 1 , it is noted that a paging system coverage map is generally based on a predetermined probability of an error-free signal being received within the coverage area. One example would be a probability of 50% that such an error-free signal would be received.

As described in AU-A-54723/96, a clock adapted to be synchronized to a reference time was required to receive two different signals, a "mark" signal and an "update" signal, before it could calculate and display accurate time. Since two signals are required, the statistical probability of a timepiece receiving an error- free signal, and thus being able to calculate and display accurate time, is reduced compared with the signal required by a standard pager. Thus the coverage area for a timepiece adapted through a paging system or the like to be synchronized to a reference time, would be less than that for a normal pager.

The present embodiment of the invention is shown in the block diagram of Fig. 1. Time message signals are composed at location 100, and sent by landiine 102 to a RPN facility 104 such as that operated in Australia by Link Telecommunications, where the signals join a queue for transmission over the RPN. The queued signals are sent by line 106 or the like to a transmitter 108, which regularly generates transmissions 110.

In this embodiment of the invention, three signals M, U1 and U2 are composed at 100 for sequential broadcast over the RPN by transmitter 108. The three signals are designated as follows: M is a "mark" signal, U1 is an "update one" signal and U2 is an "update two" signal. Each of the three signals M, U1 , U2 cqntains sufficient information such that each signal may be treated by a remote timepiece 112 as either a "mark" signal or an "update" signal, and such that the remote timepiece 112 is able to compute accurate time on the receipt of only two of the three signals M, U1 and U2.

The three signals M, U1 , U2 are transmitted by transmitter 108 as part of the RPN transmission 110, and are received by receiver 114, which may form part of or be associated with timepiece 112. The signals M, U1 , U2 are decoded by a decoder 116, which may also be part of or associated with timepiece 112, and may also be adapted to make comparisons of decoded times with times kept by the timepiece 112 and carry out other software-driven activities in connection with the maintenance of accurate time by the timepiece 112. Means 116 may supply accurate time for the display 118 associated with or part of timepiece 118.

There are three possibilities of signal pairs which enable accurate time to be computed. The pairs are: M+U1 , M+U2 or U1+U2. A timepiece 118 receiving through receiver 114 and decoder 116 any one of those pairs is able to calculate correct time from the information contained in the signals comprising the pair. A statistical analysis of the system of this embodiment demonstrates that the coverage area of such a system is actually larger than for the single pager signal.

Should a monitor 120 based at the RPN transmitter, as described in AU-A- 54723/96, or located elsewhere, detect the transmission of an incorrect time, it is able to communicate with composing station 100 through line or the like 122 so that a "delete" or "ignore" signal will still be transmitted via the RPN transmitter 108.

Routinely sending a package of three signals inevitably means that more transmission time is used. Accordingly, in order to reduce such transmission time, constant data, that is, data which are unchanging over a long period of time, such as pre-alerts for Daylight Saving changes for various time zones, are preferably transmitted as part of only one of signals M, U1 or U2.

It is envisaged that remote timepieces will contain software which is sufficiently "intelligent" that should a signal be received which when decoded contains time information which is clearly wrong, for example a time which bears no relation to the times the timepiece has been routinely receiving as updates, the software will cause the timepiece to store the incorrect time and await further, similar prima facie incorrect times to be decoded before accepting the times as correct.

By way of an example only, such software may be written such that any received time which is in error by, typically, more than two seconds, will be rejected. If further time signals are received in consecutive transmissions such that the time received does not accord with the time kept or displayed by the timepiece to the degree of error referred to hereinabove, the third consecutive time signal showing that the timepiece is keeping a time different to that of the signals will be accepted by the software as accurate time.

Of course, that approach cannot be used when a timepiece is being initialized. When a timepiece is first initialized by applying power to it, it has not time reference, and thus will accept any time sent to it as correct. In such a situation, the receiver adapted to receive the time information signals will not immediately shut down, which is the preferred manner of operation in a non-initializing phase, but remains powered up to detect a "delete" signal which the RPN transmitter will transmit if the time signal monitor determines that the time signal just transmitted is in error. Thereafter, once accurate time has been acquired, the receiver shuts down after it receives a time signal. In a preferment during normal operation, the receiver will only switch, that is, power up, at a predetermined time, when a time signal is expected.

A further embodiment is predicated upon the fact that constant data, referred to earlier in this specification, such as Daylight Saving time change information for all time zones, are routinely transmitted. Remote timepieces need to receive a number of transmissions containing the same information before the data are accepted as correct.

The embodiment has as its aim the reduction of the amount of data being transmitted, and consists of said data being stored in each timepiece in a read/write memory, with the transmitted constant data simply represented by a code to confirm that the stored data is still correct. In the event that there has been a change in the data, only information sufficient to change the stored data is transmitted. Again, it is preferred that in the event that the signal information is indicating that the data is incorrect, three consistent successive signals are required before the timepiece accepts the new data of those signals to be correct.

In Fascenda et al, US-A-5,241 ,305, there is disclosed in column 13 a "time message number" which is part of an exemplary time message. Each time message has a unique time message number, the information in successively- numbered time messages being employed to correct the accuracy of time kept by a paging device. There is no indication of how such time corrections could take place if one or more time messages were not received, and there is no further discussion about time message numbers.

Unique time message numbers, or some other part of a time message signal, could be used to associate two or more signals, in the event that a timepiece receiver receives signals containing conflicting information. It is already known from AU-A-16102/95 for a timepiece decoding device to be programmed in such a manner that time information and/or Daylight Saving information is correctly applied to the time zone in which the timepiece is located or the time in which zone has been selected for display by that timepiece.

In a somewhat similar manner, each of a number of signals may include an identifier which enables a receiver/decoder, when extracting information from a plurality of signals, to only extract information from a plurality of signals which share an identifier, or have an association of identifiers. A simple number identification system would appear to be the most appropriate for such an arrangement. With such a means of identifying which signals are to be associated with each other, it would not be possible to receive, decode, extract and use in calculations inherently conflicting information contained in two or more signals which may have been received by a timepiece.

Clearly, a signal transmission system other than a paging network could be used. For example, signals could be sent on a television or radio network. However, paging networks are more suitable, as they are geared to be a telephone messaging service. Furthermore, each transmission could only comprise two signals, rather than the preferred three.

It can be seen that this invention provides an improved system, method and apparatus for the provision of accurate time to a timepiece.

The entire contents of the specification, claims and drawings of Australian Provisional Patent application No. P09869 filed on 20 October 1997 are hereby incorporated into this description. The claims form part of the description of this application.

Claims

1. , A system for the provision of accurate time to a timepiece, in which signals containing at least information on accurate time are transmitted to remote timepieces, characterized in that in each transmission there are at least two signals, the information contained in each of said signals being such that a timepiece is capable of computing correct time from said two signals or from two of said at least two signals.

2. A system according to claim 1 , characterized in that in each transmission there are three signals.

3. A system according to claim 2, characterized in that said three signals are designated "mark" (M), "update one" (U1 ) and "update two" (U2), and in that accurate time may be calculated from the signal combinations M+U1 , M+U2 and U1+U2.

4. A system according to any preceding claim, characterized in that predetermined data relating to constant information or information which does not change for lengthy periods of time, is transmitted with only one of said three signals.

5. A system according to any preceding claim, characterized in that each said timepiece has a unique identifier, and in that each said signal may contain a reference to said identifier to enable the timepiece associated with said identifier to be individually addressed, for sending a message to said timepiece or for disabling said timepiece.

6. A method for the provision of accurate time to a timepiece, in which steps signals containing at least information on accurate time are transmitted to and received by remote timepieces, characterized by the steps of:

transmitting said at least two signals;

receiving at least two of said signals at a timepiece;

decoding said two received signals; and

computing correct time from the information contained in said received signals.

7. A method according to claim 6, characterized in that three signals are composed and transmitted.

8. A method according to claim 7, characterized in that said three signals are designated "mark" (M), "update one" (U1) and "update two" (U2), and in that accurate time may be calculated from the signal combinations M+U1 , M+U2 and U1+U2.

9. A method according to any one of claims 6 to 8, characterized in that each said timepiece has a unique identifier, and in that each said signal may contain a reference to said identifier to enable the timepiece associated with said identifier to be individually addressed, for sending a message to said timepiece or for disabling said timepiece.

10. Apparatus for the provision of accurate time to a timepiece, which apparatus is adapted to compose and transmit signals containing at least information on accurate time to remote timepieces, which remote timepieces are adapted to receive said signals, characterized in that said apparatus is adapted to compose at least two signals, the information contained in each of said signals being such that a timepiece is capable of computing correct time from said two signals or from any two of said at least two signals, and said apparatus is further adapted to transmit said at least two signals, which said at least two of said signals are adapted to be received at a timepiece, said two received signals being decoded at said timepiece, accurate time being able to be computed from the information contained in said received signals.

11. Apparatus according to claim 10, characterized in that three signals are composed and transmitted.

12. Apparatus according to claim 11 , characterized in that said three signals are designated "mark" (M), "update one" (U1) and "update two" (U2), and in that accurate time may be calculated from the signal combinations M+U1 , M+U2 and U1+U2.

13. Apparatus according to any one of claims 10 to 12, characterized in that each said timepiece has a unique identifier, and in that each said signal may contain a reference to said identifier to enable the timepiece associated with said identifier to be individually addressed, for sending a message to said timepiece or for disabling said timepiece.

14. In the system of any one of claims 1 to 5, the improvement comprising means associated with said timepiece and adapted to recognize a prime facie incorrect time decoded from a time message signal, and to store said time without updating the time maintained by said timepiece and/or displaying said time, said means also being adapted to utilize said prime facie incorrect time to compute correct time if said means receives a further time message signal containing information which indicates that said prime facie incorrect time is in fact correct.

15. The improvement according to claim 14, characterized in that the criterion for determining that a time is an incorrect time is that there is a difference of about two seconds between said time and the time kept by said timepiece at the time said signal is received and/or decoded.

16. The improvement according to claim 15, wherein said means only updates the time kept by said timepiece with time information from said time message signals when three consecutive time message signals indicate that the prima facie incorrect time is in fact correct.