EP1183573B1 - Method and device for synchronisation of distant clocks to a central clock via satellite - Google Patents

Description

In addition to terrestrially transmitted time signals, eg DCF-77, satellite-based time signals have recently been increasingly emitted ( D. Kirchner: "Two-Way Time Transfer Via Communication Satellites", Proceedings of the IEEE, Vol. 7, July 1991, pages 983-990 ; US 4,494,211 A ; EL Gurevich et al .: "Synchronization of Remote Time Scales Via Satellite Communication Channels", Measurement Techniques, US, Consultants Bureau, New York, Vol. 35, No. 7, 1 July 1992, pages 825-828 ). The best known methods are the GPS and GLONASS systems.

A serious disadvantage is the need for a highly accurate satellite positioning, as well as the exact knowledge of the transmission path, in particular the ionosphere and Troposhäre to see, which is essential for a user of the highest accuracy. In addition, the satellite signals for civilian users are deliberately falsified ('Selective Availability') in order to prevent non-military use with the utmost accuracy. Methods have been developed that allow partial compensation of these uncertainties (e.g., Differential GPS). The difficulties of using the GPS signal for high-precision time applications are still not satisfactorily solved.

The methods mentioned are widespread because of the cost-effective availability of suitable receiving devices. An operational disadvantage is seen precisely in the military nature of the systems which hinder utilization under industrial responsibility. Satellite-based time signals require extensive monitoring and verification infrastructure. A further disadvantage is that high-precision data from the systems mentioned are only available with time delays of hours or more.

Particularly suitable for metrological purposes is the two-way method (TWSTFT, two-way satellite time and frequency transfer) for time transmission. It is a method used by national calibration authorities (eg PTB Braunschweig) to compare existing atomic clock-based time-scales.

The advantage of this method is the inherent independence of the satellite position and errors through the transmission path. It can be derived directly from the symmetry of the method. Since both partners of a connection need both a transmitting and receiving device, the application of the method remained limited to a few national authorities, in particular because of the relatively high cost (D, UK, F, OE, USA, JA, IT, ES, NL). ,

The increasing availability of small, low-cost satellite earth stations with transmitting device makes the system-related disadvantages move more and more into the background today. It makes sense to make the long-proven 2-way method as an alternative to one-way methods (GPS, GLONASS) widely available.

So far, the two-way method, also called TWSTFT ( T wo- W ay S tellite T ime and F requency T ransfer), has been limited to the comparison of existing clocks external to the devices described herein the results of the measurements are published only after a delay of up to several days according to calculations by the BIPM (Bureau International des Poids et Mesures, Paris).

The invention has for its object to provide a method and apparatus for synchronization of remote clocks via satellite with a central clock, said disadvantages do not occur.

The object is achieved by providing the subject matters of the independent claims. Advantageous embodiments emerge from the subclaims.

1. a) Verbinden der Zentralen Uhr mit mindestens einer an einer Satelliten-Bodenstation angeordneten Entfernten Uhr über eine bidirektionale Satellitenverbindung;
2. b) Bidirektionales Senden und Empfangen von Zeitsignalen zwischen der Zentralen Uhr und der Entfernten Uhr über die Satellitenverbindung in Echtzeit;
3. c) sowohl die Zentrale Uhr als auch die Entfernte Uhr bestimmen jeweils die Zeitdifferenz zwischen dem Empfangszeitpunkt des von der Gegenstation gesandten Signales gegenüber der lokalen Uhr;
4. d) Austausch der beidseitig ermittelten Zeitdifferenzen der Entfernten Uhr und der Zentralen Uhr über die Satellitenverbindung, nämlich über die Funkstrecke, über die auch die Zeitsignale der Zentralen Uhr und der Entfernten Uhr ausgetauscht werden, in Echtzeit und
5. e) Synchronisation der Entfernten Uhr nach Stand und Gang auf die Zentrale Uhr in Abhängigkeit der über die Satellitenverbindung ausgetauschten Zeitsignale und ermittelten Zeitdifferenzen.

The object is achieved in particular by providing a method for synchronization of remote clocks via satellite with a central clock at a central ground station, which comprises the following step:

1. a) connecting the central clock to at least one remote clock located at a satellite ground station via a bidirectional satellite link;
2. b) Bidirectional transmission and reception of time signals between the central clock and the remote clock via the satellite link in real time;
3. c) both the central clock and the remote clock each determine the time difference between the reception time of the signal sent by the other station to the local clock;
4. d) Exchange of the mutually determined time differences of the remote clock and the central clock via the satellite connection, namely via the radio link, via which the time signals of the central clock and the remote clock are exchanged, in real time and
5. e) synchronization of the remote clock by level and gear to the central clock in dependence of the satellite signals exchanged over the time signals and determined time differences.

• eine an einer zentralen Bodenstation angeordnete Zentrale Uhr mit einer ersten Sende- und Empfangseinrichtung für Satellitensignale,
• eine an einer Satelliten-Bodenstation angeordnete Entfernte Uhr mit einer zweiten Sende- und Empfangseinrichtung für Satellitensignale, wobei die ersten und zweiten Sende- und Empfangseinrichtungen über eine bidirektionale Satellitenverbindung miteinander in Verbindung stehen,
• eine Einrichtung zur Bestimmung von Messdaten, umfassend die sowohl von der Zentralen Uhr als auch von der Entfernten Uhr jeweils zwischen dem Empfangszeitpunkt des von der Gegenstation gesandten Signales gegenüber der lokalen Uhr bestimmte Zeitdifferenz, sowie
• einen an der Entfernten Uhr vorgesehenen Regelkreis zur Synchronisation der Entfernten Uhr nach Stand und Gang auf die Zentrale Uhr in Abhängigkeit der von der Zentralen Uhr an die Entfernte Uhr in Echtzeit übermittelten ersten Zeitdifferenz und der zweiten Zeitdifferenz, wobei
• die Vorrichtung ausgebildet ist, um die Zeitdifferenzen in Echtzeit direkt über die Funkstrecke auszutauschen, über die auch die Zeitsignale der Entfernten Uhr und der Zentralen Uhr ausgetauscht werden.

The object is also achieved in particular by providing a device for synchronization of remote clocks via satellite with a central clock, in particular for carrying out an embodiment of the method described above, the device comprising:

• a central clock arranged at a central earth station with a first transmitting and receiving device for satellite signals,
• a remote clock located at a satellite ground station having a second satellite signal transceiver, the first and second transceivers communicating via a bi-directional satellite link;
• a device for determining measurement data, comprising the time difference determined by both the central clock and the remote clock between the time of reception of the signal sent by the remote station and the local clock, and
• a provided on the remote clock loop for synchronization of the remote clock by level and gear to the central clock in response to the transmitted from the central clock to the remote clock in real time first time difference and the second time difference, wherein
• the device is designed to exchange the time differences in real time directly over the radio link, via which the time signals of the remote clock and the central clock are also exchanged.

1. 1. In der Entfernten Station befindet sich eine physikalische Uhr mit zusätzlicher Gangreserve. Es ist also nicht wie bisher beim 2-Weg Zeit-Transfer eine hochgenaue externe Uhr erforderlich, sondern es wird die direkt im Gerät eingebaute Uhr verwendet.
2. 2. Die der Zeitübertragung dienenden Signale werden gleichzeitig für den bidirektionalen Austausch der 2-Weg Meßdaten genutzt.
3. 3. Aufgrund der ständig erneuerten Meßdaten synchronisiert sich die Entfernte Uhr über einen Regelkreis auf die Zentrale Uhr unter Anbringung der sytembedingten Korrekturen, die ebenfalls zwischen den Stationen ausgetauscht werden.
4. 4. Die an der Entfernten Uhr vorhandene Zeit- und Frequenzinformation steht in Form extern zugänglicher elektrischer Signale dem Nutzer zur Verfügung.
5. 5. Die Qualität der Synchronisation ist aufgrund der ständigen Aufdatierung der Meßdaten mit minimalem Zeitverzug überprüfbar.

The aforementioned disadvantages are remedied by five major innovations:

1. 1. In the remote station is a physical clock with additional power reserve. So it is not necessary as before with the 2-way time transfer a highly accurate external clock, but it is used directly in the device built clock.
2. 2. The time transmission signals are used simultaneously for the bidirectional exchange of the 2-way measurement data.
3. 3. Due to the constantly renewed measurement data, the remote clock synchronized via a control loop on the central clock under attachment of the system-related corrections, which are also exchanged between the stations.
4. 4. The time and frequency information available at the remote clock is available to the user in the form of externally accessible electrical signals.
5. 5. The quality of the synchronization is verifiable due to the constant updating of the measured data with minimal delay.

1. 1. Unabhängigkeit von Infrastrukturen mit militärischem- und/oder multinationalem Charakter.
2. 2. Es besitzt keinerlei aus militärischen Gründen bewußt eingeführte Verschlechterung der Datenqualität ('Selective Availability').
3. 3. Das System gewährleistet unter Ausnutzung des eingeführten Meßverfahrens nach dem 2-Wegeprinzip eine hohe Unabhängigkeit von der Satellitenposition. Es arbeitet ohne Kenntnis der Ausbreitungszeit längs des Übertragungsweges.
4. 4. Die Qualität der in der Entfernten Station eingebauten Uhr kann im Vergleich zu Atom-Uhren deutlich geringer und kostengünstiger sein, da diese Uhr durch einen ständigen Regelkreis an die Zentrale Uhr angeglichen wird.
5. 5. Das Verfahren ist geeignet, gerade auch Langzeitfehler (Drift) des Systemes so zuverlässig zu verhindern, wie es im praktischen Betrieb selbst kommerzielle Atomuhren höchster Qualität aus prinzipiellen Gründen nicht vermögen,
6. 6. Das Verfahren arbeitet in Echt-Zeit ohne aufwendige Nachprozessierung der Daten.
7. 7. Dem Nutzer stehen direkt verwendbare Zeitsignale zur Verfügung.
8. 8. Das Verfahren besitzt durch direkte Relation zu einer anerkannten Zeitskala Eich-Qualität.
9. 9. Das Meßverfahren ist einer Kalibration direkt zugänglich.

For the user, the following advantages result from the method:

1. 1. Independence of infrastructures with military and / or multinational Character.
2. 2. It does not have any intentionally degraded data quality ('Selective Availability') for military reasons.
3. 3. The system ensures a high degree of independence from the satellite position by utilizing the introduced measuring method according to the 2-way principle. It works without knowing the propagation time along the transmission path.
4. 4. The quality of the clock installed in the remote station can be significantly lower and less expensive compared to atomic clocks, since this clock is adjusted to the central clock by means of a permanent control loop.
5. 5. The method is suitable, especially to prevent long-term error (drift) of the system as reliable as in practical operation even commercial atomic clocks of the highest quality for reasons of principle, not able,
6. 6. The method works in real time without expensive post-processing of the data.
7. 7. The user has directly usable time signals available.
8. 8. The method has calibration quality by direct relation to a recognized time scale.
9. 9. The measuring procedure is directly accessible to a calibration.

Preferably, the remote clock is integral to a satellite ground station. The central clock at a central ground station communicates either uninterrupted or intermittently with one or more remote clocks via bi-directional satellite communication links called two-way links. Both sides of the communication link are equipped with both a transmitting and receiving device for satellite signals. Both the central clock and the remote clock each determine the time difference between the reception time of the sent from the other station Signals against the local clock. These differences are called "measured data". The central and remote clocks intermittently exchange these "measured data" obtained on both sides together with system-related correction data. The remote clock is synchronized to the central clock via a control loop based on the "measured data" by status and gear. For the data exchange no additional data channels need to be used apart from the satellite signals carrying the time information. The time and frequency information thus obtained in the ground station are physically available to the user in the form of suitable pulsed and / or sinusoidal signals, called "time signals", including any digital correction values.

The remote, synchronized timepiece preferably has a built-in power reserve that allows communication breaks to be bridged with reduced accuracy. In order to increase the accuracy of the information contained in the time signals, the user may be provided with additional digital correction data. The unique time and date is available at a data output. The overall system does not require any special facilities on board the satellite, although these are not excluded. The entire system works without information about the current satellite position. It is a real-time method with constant, up-to-date availability of date, time and frequency information.

The remote ground station is preferably connected via a frequency division multiplexing (FDMA) by means of the central clock.

The remote ground station preferably communicates with the central clock via a code division multiple access (CDMA) method.

The remote ground station is preferably in connection with the central clock via a time-division multiplexing (TDMA) method.

The remote ground station is preferably connected to the central clock via one or more satellites.

The remote ground station preferably communicates with a system of redundant central clocks via a multiplexing process.

Preferably, any number of remote ground stations communicate with the central clock via a multiplexing process.

Preferably, any number of remote ground stations communicate via a multiplexing process with a redundant system of centralized clocks.

Preferably, there is a transparent transponder aboard the satellite.

Preferably, there is a regenerative transponder aboard the satellite.

Preferably, the user is informed in digital form of the current state of the remote clock with respect to the central clock.

Preferably, a warning signal is sent to the user if the deviation of the remote clock with respect to the central clock exceeds a limit value.

The respective state of the remote clock in the form of telemetry data is preferably available at the central clock.

The invention will be described in more detail with reference to FIG. 1 shows, using the example of a simple combination consisting of a central clock (1) at a satellite ground station (5) and a remote clock (2) in another satellite ground station (11), with a suitable Meassapparatur consisting of a transmitting (7 ) and receiving unit (8) at the central station and the corresponding transmitting (12) and receiving unit (13) at the remote station, a control signal (17) is obtained so that the remote clock (2) by level and gear with the central Clock (1) is synchronous. For this purpose, both stations communicate with a bidirectional radio link (9.1) and (9.2) via a satellite (10) and exchange in real time the results (15, 16) from time difference measurements (6, 14) in both stations directly over the radio link (9.1, 9.2), via which also the time signals of the stations are exchanged. The manipulated variable of the control loop (17) is formed from the difference between the two time difference measurements in the remote ground station. It affects the frequency of the remote clock (2). The reference time (3) of the central clock is made available to the user at the remote clock in the form of time signals (18). The symmetry of the overall structure and the radio link are decisive for the elimination of the unknown time delays of the transmission path and by the satellite.

Claims (21)

1. A method for the synchronisation of remote clocks with a central clock at a central ground station via satellite, comprising the following steps:

   a) connecting the central clock to at least one remote clock arranged at a satellite ground station via a bidirectional satellite connection;

   b) bidirectionally transmitting and receiving time signals between the central clock and the remote clock via the satellite connection in real-time;

   c) both the central clock and the remote clock determine the time difference between the point when they receive the signal transmitted by their counter station as compared with the local clock;

   d) exchanging in real-time the mutually determined time differences of the remote clock and the central clock via the satellite connection, i.e., via the transmission path that is also used to exchange the time signals of the central clock and the remote clock; and

   e) synchronising the remote clock regarding deviation of indicated time to actual time and clock drift to the central clock depending on the time signals exchanged via the satellite connection and the determined time differences.
2. The method according to claim 1, characterised in that the determined time differences are exchanged continuously or intermittently.
3. The method according to claim 1 or 2, characterised in that the synchronisation of the remote clock with the central clock is achieved by means of a control loop which generates an actuating variable from the difference of the two time differences.
4. The method according to claim 3, characterised in that the frequency of the remote clock is effected by means of the actuating variable.
5. The method according to any one of the preceding claims, characterised in that the reference time of the central clock is available at the remote clock in the form of time signals.
6. The method according to any one of the preceding claims, characterised in that the method is a real-time method with continuous up to date availability of date, time and frequency information.
7. The method according to any one of the preceding claims, characterised in that the time and frequency information available at the remote clock is provided to a user in the form of pulsed and/or sinusoidal signals.
8. The method according to any one of the preceding claims, characterised in that the unambiguous time and date specification is provided at a data output of the remote clock.
9. The method according to any one of the preceding claims, characterised in that the remote clock is in communication with the central clock using a frequency division multiple access (FDMA) method.
10. The method according to any one of the preceding claims, characterised in that the remote clock is in communication with the central clock using a code division multiple access (CDMA) method.
11. The method according to any one of the preceding claims, characterised in that the remote clock is in communication with the central clock using a time division multiple access (TDMA) method.
12. The method according to any one of the preceding claims, characterised in that the remote clock is in communication with the central clock using one or several satellites.
13. The method according to any one of the preceding claims, characterised in that the remote clock is in communication with a system consisting of redundant central clocks using a multiplex method.
14. The method according to any one of the preceding claims, characterised in that any number of remote clocks is in communication with the central clock using a multiplex method.
15. The method according to any one of the preceding claims, characterised in that any number of remote clocks is in communication with a redundant system of central clocks using a multiplex method.
16. The method according to any one of the preceding claims, characterised in that a transparent transponder is disposed on board the satellite.
17. The method according to any one of the preceding claims, characterised in that a regenerative transponder is disposed on board the satellite.
18. The method according to any one of the preceding claims, characterised in that a user is informed about the current deviation of indicated time to actual time of the remote clock relative to the central clock in digital form.
19. The method according to any one of the preceding claims, characterised in that the user is supplied with a warning signal if the difference of the remote clock from the central clock is in excess of a limit value.
20. The method according to any one of the preceding claims, characterised in that the respective deviation of indicated time to actual time of the remote clocks is available at the central clock in the form of telemetry data.
21. A device for the synchronisation of remote clocks with a central clock via satellite, more particularly for the application of the method according to any one of claims 1 to 20, comprising:

    - a central clock that is arranged at a central ground station and comprises a first transmitting and receiving device for satellite signals;

    - a remote clock that is arranged at a satellite ground station and comprises a second transmitting and receiving device for satellite signals, wherein the first and second transmitting and receiving devices are in communication with each other via a bidirectional satellite connection;

    - a device for the determination of measurement data, comprising the respective time difference determined by the central clock as well as by the remote clock between the point when they receive the signal transmitted by their counter station as compared with the local clock; and

    - a control loop that is provided at the remote clock and is intended for the synchronisation of the remote clock to the central clock regarding deviation of indicated time to actual time and clock drift depending on the first time difference and the second time difference transmitted by the central clock to the remote clock in real-time, wherein

    - the device is formed to exchange the time differences in real-time directly via the transmission path that is also used to exchange the time signals of the remote clock and the central clock.

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