DE102013221321A1 - Method for synchronizing road users on a common timebase - Google Patents

Abstract

A method is proposed for synchronizing road users on a common time basis, in which data is transmitted periodically or event-based between a plurality of moving road users in messages over a common transmission medium. In this, a respective internal clock of the road users by means of a primary synchronization method, which processes the clock signals of a global navigation satellite system, synchronized to the time of an internal satellite navigation receiver of the road user. From a respective road user is switched to a secondary synchronization method when the primary synchronization method is not available to the respective road user, the secondary synchronization method is based on a self-organization for synchronization of internal clocks, in which a required for the synchronization of the respective internal clock identifier in the used Communication method is coded without disturbing the actual communication method.

Description

The invention relates to a method for synchronizing road users on a common time base, in which data is transmitted periodically between a plurality of moving road users in messages over a common transmission medium.

Radio-based multi-user communication networks use media access control (MAC) protocols or methods that govern access to the shared transmission medium. A special case of such networks are so-called Mobile Adhoc Networks (MANET). Here the users or network subscribers are mobile and transfer data to other subscribers in their environment, without resorting to infrastructure such as base stations.

Due to the movement of users, the network is dynamic. This means that for a single user, the communication partners constantly change their radio range. New unknown users are added while others disappear.

Such networks may be used, for example, for vehicle-to-vehicle communication systems. In this case we speak of a vehicular adhoc network (VANET). The road users represent transmitters (transmitting road users) and receivers (receiving road users) of the vehicle-to-vehicle communication system. Each road user in a vehicle-to-vehicle communication system is both transmitter and receiver. For safety-relevant applications, such as As the avoidance of collisions between vehicles, it is necessary that each vehicle regularly transmits traffic-related information to the other road users in the area. For this purpose, each user repeatedly sends short status messages as a broadcast, which contain the current position, speed, direction and other safety-relevant information. These sent information should be received reliably and as quickly as possible by the other road users and, above all, regularly inform about changes at short intervals. In order to be able to correctly process the information exchanged between the road users, with some characteristics a temporal synchronization of the road users is necessary.

For this purpose, every road user has an internal clock. The internal clocks of the road users are synchronized on a common time base. For example, this enables a distributed, time-controlled switching between different channels of a radio system used. This can e.g. can be used to periodically, e.g. every 50 ms, switching between a control channel and one of several service channels.

Even within a channel, an accurate time reference of all participating road users allows a predictable behavior within certain limits, eg in the observance of waiting times for a freely determined time slot or in the prioritization of a defined priority class, such as in IEEE 802.11e / p specified, which are implemented over different waiting times.

The internal clocks of the road users should therefore be synchronized on a common timebase. For this purpose, it is possible to synchronize to the comparatively precisely determined time of an internal or external satellite navigation receiver of a respective road user. A satellite navigation receiver is also referred to as a Global Navigation Satellite System (GNSS) receiver, e.g. GPS or Galileo, called. This time is very accurate, since the basic principle of position determination via ranging, i. the transit time determination of satellite signals in the nanosecond range works.

Unsynchronized, the quartz-controlled internal clocks drift in the range of a few tens to a few hundred milliseconds per day, which is why the re-synchronization is usually made about once per minute, so that the drift does not develop too far until the next re-synchronization. Usually protocols like NTP ( RFC 5905 . RFC 1305 ), which, among other things, ensure that drift corrections are made continuously.

A problem of this type of time synchronization arises when over a period of time no GNSS reception is possible, and the internal time update of the GNSS receiver also falls in time to the quality measure of quartz-controlled clocks. This can happen, for example, in longer tunnels or in parking garages due to the shadowing of the satellite signals. Of these, several vehicles are usually affected in the same environment, so that can build up relatively fast time divergences of more than 20ms under the affected vehicles, which is a maximum tolerated limit for IEEE 802.11p is seen. From this limit, certain messages are no longer sent, because the information contained in position etc. can no longer be assigned to an exact point in time.

It is an object of the invention to provide a method which provides improved synchronization enabled by multiple road users of a vehicle-to-vehicle communication system.

This object is achieved by a method according to the features of claim 1 and a computer program product according to the features of claim 16. Advantageous embodiments emerge from the dependent claims.

A method is proposed for synchronizing road users on a common timebase, in which data is periodically transmitted between a plurality of moving road users in messages over a common transmission medium. The news eg at IEEE 802.11p are so-called Cooperative Awareness Messages (CAMs), which are transmitted to all road users in the environment as a multi-broadcast message (each participant sends unidirectional messages to all other road users in the reception area). Each receiving road user can evaluate these messages and take information about position, speed etc. of all other road users (vehicles) and relate them with information about their own vehicle. On this basis, cooperative applications such as collision avoidance, threading assistants etc. are made possible. The frequency of the transmission of the CAMs depends, among other things, on the speed of the road user or other factors. The time of transmission at a certain absolute transmission time is not exactly predictable, since mechanisms of media access control (MAC) of the common transmission medium dominate here.

In the method, a respective internal clock of the road users is synchronized to the time of an internal satellite navigation receiver of the road user by means of a primary synchronization method, which processes the clock signals of a global navigation satellite system. The synchronization generally ensures that the information exchanged between the traffic participants can be processed correctly.

According to the invention, a respective traffic participant switches to a secondary synchronization method if the primary synchronization method can not be used by the respective traffic participant, the secondary synchronization method being based on a self-organization for the synchronization of the internal clocks, wherein the synchronization method of the respective internal clock is used in the secondary synchronization method required identifier is encoded in the communication method used without disturbing the regular data transmission.

It is thus resorted to a "backup synchronization method" when the reception of satellite navigation signals is disturbed. As a result, the synchronization of data is made possible by several road users, even if the reception of satellite navigation signals is disturbed, as may be the case for example in long tunnels or parking garages. In addition, the synchronization can be ensured even if, for some reason, deviations occur between the internal clocks of road users and the clock signals of the global navigation satellite system.

In the present specification, a road user means not only road vehicles, but also aircraft (aircraft, helicopters, and the like), rail vehicles (trains, trams, and the like) and watercraft (ships), as well as pedestrians and cyclists.

The primary synchronization method can not be used if a predetermined number of simultaneously received satellite signals are undershot. The case is e.g. if less than four satellites can be received by the road user, since then the navigation equation system in the means of transport can not be solved and the clock error can not be clearly determined. Another advantage is that the secondary synchronization method can also be used before the so-called "Time To First Fix" of the satellite navigation system has expired and the information from the satellite navigation system is not yet fully available.

It is useful if the secondary synchronization method based on the principle of a so-called. Firefly synchronization (German: firefly synchronization). In this method, male fireflies ("fireflies") in nature adapt the emission of a light impulse to their neighbors in such a timely manner that all fireflies flash at the same time, without there being a "master" who makes a temporal or otherwise objective specification , The algorithm runs distributed between the fireflies of a region. This principle, e.g. can also be used for distributed channel access in the communication, according to the invention for time synchronization of the decoupled from the primary synchronization method transport a VANETs, ie a vehicle-to-car (car-to-car) or a vehicle-to-infrastructure (car-to -infrastructure).

According to an expedient embodiment, at least some of the messages comprise an identifier which is the transmission of the message signaled at a particular time, wherein by the one or more traffic participants receiving the message, a synchronization is performed relative to the specific time by adjusting the internal clock at the time represented by the identifier. The qualifier, which in the later description will also be called a flash, corresponds to the light pulse emitted by the fireflies and enables the autonomous, self-organizing synchronization of the road users who can not perform the primary synchronization procedure.

The identifier can be inserted by a sending road user as additional information in the message.

The identifier may have a given confidence on a temporal reference of a traffic participant synchronized, in particular via the primary synchronization method. That is, the synchronized road user works with the clock signal of the global satellite navigation system and provides this as a time reference for the backup process.

Upon receipt of the message with the identifier, the internal clock of the receiving traffic participant is adapted to the internal clock of the traffic participant sending the message with the identifier by means of the secondary synchronization method. This can e.g. be realized analogously to the procedure described in [1].

The message is transmitted according to an embodiment as an OFDM frame with a preamble, wherein the identifier is represented by a predetermined preamble of the OFDM frame, and wherein the predetermined preamble can be distinguished from a conventional preamble by the receiving road user. If this predetermined, unambiguous preamble is emitted by all time-synchronized transmitting traffic participant (in general: transmitter) at the signaling time, a non-synchronized receiving traffic participant (in general: receiver) can resolve this and use it for re-synchronization. Should the signal, i. the OFDM frame, possibly due to multiple synchronized transmitters in the near multiple and slightly time-delayed arrive at a non-synchronized receiver, this represents from the point of view of the receiver a multipath reception, which can be resolved by the receiver with methods known in the art. In particular, even in the event of a collision, the identifier can still be used, even if the user data can not be utilized by a receiver because of the collision (for example as a result of the hidden-terminal problem).

In an alternative embodiment, the message is transmitted as an OFDM frame, the identifier being represented by a particular pattern of pilot symbols of the OFDM frame to identify a time base for synchronization. Pilot or training symbols are auxiliary signals of OFDM transmission, for example, to enable the receiver to estimate current parameters of the transmission channel.

In a further alternative embodiment, the message is transmitted as an OFDM frame, wherein the identifier is represented by a code word from a spreading code, which is transmitted in addition to the OFDM frame. The spreading code can be decoded at the receiver from a (even low) power density spectrum when searching for the known code word. The receiver must have a correlator in a known manner for this purpose. For a non-seeking receiver, the codeword is "hidden" in the noise. The procedure of coding and decoding a spreading code is well known to those skilled in the art, new here is the use as "flash", i. for marking a time on a relative or absolute time axis. An advantage of this variant is that the identifier can be made independent of the time of transmission to the OFDM frames.

According to a further expedient embodiment, messages with identifiers are transmitted at a predetermined time at predetermined time intervals with a predetermined probability, in particular by one or more traffic participants synchronized via the primary synchronization method. For example, sending the messages with the identifier every minute or every x minutes, where x is greater than 1, can be done by the minute. In particular, it can be controlled via the probability that randomly occurring data transmissions falling randomly on the given time will not be permanently disturbed.

According to a further expedient embodiment, the transmission of the message with the identifier by the road user sending the message is made dependent on a given confidence in the own time base. In this way it can be ensured that only such road users send out messages that have a "correct" clock information lying within predefined limits. This usually applies to those road users who are synchronized after the primary synchronization process. However, a sending road user can also synchronized according to the backup synchronization method Be a road user who, for example, has recently switched to the secondary procedure.

According to a further expedient embodiment, the message with the identifier is sent only by a road user whose internal clock is synchronized according to the primary synchronization method. This means that only unsynchronized road users at 1-hop distance to a sending road user are post-synchronized in this way. According to this embodiment, the traffic participants who receive the identifier do not themselves send out messages with the additional identifier after their re-synchronization, which would correspond to a multi-hop transmission. As a result, a high accuracy in the synchronization can be achieved. However, in another embodiment it is also possible for the road users receiving the identifier to send their own messages with the identifier after being re-synchronized, e.g. allows synchronization of road users in long tunnels.

In another embodiment, a message is transmitted as an OFDM frame with a payload, wherein the message sending the road user whose internal clock is synchronized according to the primary synchronization method, the internal time is included in the payload of the message. This embodiment does not require an additional identifier, whereby no additional load on the transmission channel or no potentially useful data transmission disturbing signal is emitted. Instead, a synchronized road user rewrites the payload of the message immediately prior to transmission (PHY layer) and sets the system time, i. the time of the internal clock, included. The receiving, unsynchronized road user synchronizes his internal clock with this data. In this embodiment, the transmission time between transmitter and receiver should be negligible. Furthermore, in transmitting and receiving-side processing between writing the internal time of the transmitter and reading out this time from the message by the receiver, at most a defined, deterministic time should pass (ideally these times should be zero) since the time offset can not be determined by the receiver is.

In another embodiment, from the data and / or assumptions received by the other road users about their own movement by the means of transport, which can not be performed by a primary synchronization method, the time profile is extrapolated via a movement model. In this embodiment, a synchronization to the time of the satellite navigation receiver takes place even if less than four, but at least one satellite signal are available and therefore a 3D position solution is no longer possible. The time synchronization then takes place on the basis of pseudoranges to one to three available satellites, making appropriate assumptions as to how the road users move according to the motion model. Due to the updated (and thus with increasing error afflicted) position information, the temporal drift can be limited to a calculable extent. As movement models, for example, "constant speed", "constant acceleration" or the known Krauss model are available, with or without reference to a map (map matching, etc.).

According to a further advantageous embodiment, the secondary synchronization method is executed in parallel to the primary synchronization method, even if the primary synchronization method can be used by a respective road user. The secondary synchronization method is then, when the first synchronization method is no longer available, already in the steady state, so that switches without delay and an accurate time base can be used.

There is also provided a computer program product that can be loaded directly into the internal memory of a digital computer and includes software code portions that perform the steps of any one of the preceding claims when the product is run on the computer. The computer is in particular a control device of a car-to-car communication.

The invention will be explained in more detail below with reference to exemplary embodiments in the drawing. Show it:

1 a known time-controlled switching between different channels of a radio system at predetermined times;

2 a schematic representation of a plurality of orthogonal carriers of an ODFM frame;

3 a schematic representation of the symbols of an OFDM frame with preamble and payload data;

4 a schematic representation of an OFDM preamble in a representation in bits;

5 a schematic representation of the symbols of a particular OFDM preamble, the acts as an identifier for signaling a time base;

6 the OFDM frame 3 , wherein in the payload an identifier for signaling a time base is integrated; and

7 a schematic representation of a spread code contained in a power density spectrum.

1 shows a distributed, timed switching between different channels of a car-to-car radio system, which is switched every 50 ms between a control channel CCH and one of several service channels SCHn. In the 1 The illustration shown follows the method of the sub-standard IEEE 1609.4 a car-to-car radio system. According to this representation, at a time t _0, after a so-called guard interval GI, the first switching to the control channel CCH takes place via which periodic messages such as CAMs or DENs are transmitted. After 50 ms, switching takes place to the service channel SCH, via which further specific user data is transmitted. The transmission of user data is again preceded by the Guard Interval GI to protect against overlapping transmissions and to compensate for minor clock errors. Together, the successively transmitted control channel CCH and the service channel SCH form a synchronization interval SI with a length of 100 ms. After ten synchronization intervals SI, the time t _{1 has been} reached and the sequence is repeated. The times t ₀ and t ₁ coincide with the start of each UTC (Coordinated Universal Time) second, which gives an image of the absolute timeline.

In order to be able to realize the distributed, time-controlled switching between the two channels synchronously between all nodes of the car-to-car radio network, the internal clocks of transmitters / receivers of car-to-car communication technology ( IEEE 802.11p / ITS-G5, recently led as IEEE 802.11-2012 OCB mode) synchronized to a common time base. A node of the car-to-car radio network is a road user or infrastructure component, each comprising a transmitter / receiver with an internal clock. In the following description is shortened only by a (sending or receiving) road user spoken, it being understood by those skilled in the fact that the transmission of a message by the transmitter and the receipt of a message by the recipient of the relevant means of transport takes place.

To synchronize the internal clocks of the road users is usually resorted to the time of an internal satellite navigation receiver. A satellite navigation receiver is also referred to as a Global Navigation Satellite System (GNSS) receiver, e.g. GPS or Galileo, called. This time is very accurate, since the basic principle of position determination via ranging, i. the transit time determination of satellite signals in the nanosecond range works. If the synchronization of the internal clocks of the road users by means of the satellite navigation system, this is referred to as primary synchronization.

If the primary synchronization procedure is not available, the system switches to a secondary synchronization procedure. The secondary synchronization method is based on a self-organization for the synchronization of the internal clocks, in which the data required for the synchronization of the respective internal clock are usually transmitted periodically between the plurality of moving road users.

The primary synchronization method is performed by a road user e.g. not usable if its satellite navigation receiver can receive less than four satellite signals simultaneously (e.g., in a tunnel or in a parking garage).

The secondary synchronization method is an algorithm that mimics the effect known in nature as "firefly synchronization". Here, male fireflies (English: "fireflies") adjust the emission of a light pulse in time to their neighbors, that as possible flash all fireflies simultaneously, without that there is a "master", which makes a temporal or otherwise kind of specification. The process is "distributed" and self-organizing between the fireflies of a region.

The secondary synchronization method preferably runs in the background (i.e., also in parallel with the execution of the first synchronization method), so that it is already in the steady state when there is a need to switch over to it.

The secondary synchronization method can also be used before the "Time To First Fix" of the satellite navigation system has expired and the satellite navigation information is not yet fully available.

The procedure for transmitting a synchronization-enabling information is as follows: Each road user periodically sends status messages, in the case of IEEE 802.11p so-called Cooperative Awareness Messages (CAMs) to all road users in the area as broadcast. Each receiving road user can evaluate these messages and take from these CAMs information on position, speed, etc. of all other vehicles and relate them to information about their own vehicle. On this basis, cooperative applications such as collision avoidance, threading assistants etc. are made possible. Technically, CAMs are sent as payloads of so-called OFDM frames. OFDM (Orthogonal Frequency Division Multiplexing) offers a special structure over frequency and time, which addresses both efficiency and robustness, as in principle in the 2 and 3 is shown.

2 Figure 12 shows a known schematic representation of a plurality of orthogonal carriers of an ODFM frame plotted against frequency f. 3 shows a likewise known, schematic representation of the symbols 10 an OFDM frame having a preamble P and a payload area ND. The symbols 10 comprise user data in a manner known to those skilled in the art 11 , Training symbols 13 and auxiliary information symbols / pilots 14 , The frequency spectrum used is subdivided into so-called subcarrier SC, on which in 3 in each case the symbols arranged in a respective column are transmitted. A so-called guard band is marked GB. This serves to protect against so-called intersymbol interferences, ie overlays of several symbols. Since the individual symbols and their function are known in the art and the exact function for the invention also plays a minor role, a detailed description of the OFDM frame is omitted here.

The frequency of the transmission of the CAMs depends i.a. from vehicle speed and other factors. In particular, the time of transmission at a certain absolute transmission time can not be very accurately predetermined, since mechanisms of media access control (MAC) dominate here. The focus here re-synchronization of the system time of a receiver (= receiving road users) in relation to an absolute time reference thus takes place via the evaluation of a special identifier.

Instead, an additional signal in the form of a message provided with an identifier is broadcast by transmitters (= sending road users) synchronized with good confidence to a temporal reference. This message containing the identifier, referred to as the flash signal, is e.g. a modified OFDM frame that does not disturb the regular OFDM symbol sequences, as far as they are emitted by other transmitters. The flash signal is used by unrelated or no longer synchronized receivers, in particular analogous to the principle of firefly synchronization, the absolute time base of the receiver (ie the internal clock) to that of the flash transmitter (= the message sending the message with the identifier road users ).

As a flash signal, various embodiments can be used:
A first embodiment is a predetermined OFDM preamble, which is constructed in the system similar to the standard preamble, but can be clearly distinguished from this receiver side. Such a standard OFDM preamble in a representation in bits is in 4 shown. For example, in the given OFDM preamble, the Reserved field may have a predetermined value that is interpreted as an identifier and synonymous with a time reference for self-organization. Likewise, one of the other fields could be modified as appropriate. In the symbol representation of the OFDM preamble P ( 5 ) makes the modification noticeable in one or more symbols (not visible).

If this unique, predetermined preamble of an OFDM frame is radiated with the identifier from all time-synchronized transmitters, a non-synchronized receiver can resolve this and use it for re-synchronization. Should the signal u.U. Due to several synchronized transmitters in the vicinity, arriving at a non-synchronized receiver several times and slightly time-delayed, this looks like a multipath reception, which the receiver can resolve with methods known to the person skilled in the art.

A second embodiment uses special pilot symbols of the OFDM frame structure at the time of transmission of the flash signal. This is in 6 schematically labeled NK, wherein the transmitted at a given time t symbols represent the identifier or flash. Disadvantage of this variant is that only such a synchronized transmitter can send a flash NK according to the second embodiment, which also sends a data message at the selected flash time.

A third embodiment is a codeword of a suitably chosen spreading code such as Direct Sequence Spread Spectrum (DSSS), which can be decoded at the receiver even from a low power density spectrum when searching for the known codeword. For a non-seeking receiver, the codeword is "hidden" in noise NL, such as 7 shows. In this figure, the power PW of the transmitted DSSS signal Sig is represented over the frequency f.

All three aforementioned forms of the flash signal according to this method at a certain time t_n, for example at the minute passage, on the absolute time reference of, in particular synchronized according to the primary synchronization method transmitter at certain time intervals, for example 1 × per minute, with a fixed probability p, eg 50%, sent. In particular, it can be controlled via the probability p that randomly occurring data transmissions incident on the time t_n are not permanently disturbed.

Furthermore, the transmission of a flash signal by a transmitter can also be made dependent on a specific confidence in its own time base. In one embodiment, e.g. flash signals are emitted only by transmitters that are synchronized with an accurate time base, i. of transmitters synchronized according to the primary synchronization method. This means that only unsynchronized receivers in 1-hop distance are post-synchronized in this way, but they do not emit flash signals even after resynchronization.

The limitation to the 1-hop distance has the disadvantage that in larger contiguous areas (greater than the 1-hop range) without satellite navigation coverage not all road users can be resynchronized via the described method. This can e.g. be in the case of long tunnels or parking garages. It may therefore be expedient for re-synchronized receivers to become transmitters of flash signals. This may be useful, in particular, if a certain amount of possibly accumulated residual drifts (for example 1 ms) is not exceeded.

references

• [1] A. Tyrrell, G.Auer, C. Bettstetter: Firefly Synchronization at Ad Hoc Networks, 3rd MiNEMA Workshop, Leuven, Belgium, February 7, 2006

LIST OF REFERENCE NUMBERS

GI

 Guard interval

SI

 synchronization interval

CCH

 Control Channel

FAST

 Service channel n (service channel)

t ₀

 time

t ₁

 time

f

 frequency

ND

 Payload area of an OFDM frame

P

 Preamble of an OFDM frame

GB

 Guard Band

SC

 Sub-Carrier

NL

 sough

PW

 power

NK

 Message with identifier (flash signal)

Sig

 DSSS signal

10

 symbol

11

 Nutzdatensymbol

13

 training symbol

14

 Auxiliary information symbol

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• IEEE 802.11e / p [0006]
• RFC 5905 [0008]
• RFC 1305 [0008]
• IEEE 802.11p [0009]
• IEEE 802.11p [0012]
• IEEE 1609.4 [0041]
• IEEE 802.11p [0042]
• IEEE 802.11-2012 [0042]
• IEEE 802.11p [0049]

Claims (16)

A method for synchronizing road users on a common time base, in which - data is transmitted periodically or event-based between a plurality of moving road users in messages over a common transmission medium, - a respective internal clock of the road users by means of a primary synchronization method, the clock signals of a global satellite navigation system, is synchronized to the time of an internal satellite navigation receiver of the road user, characterized in that - is switched from a respective road user to a secondary synchronization method, if the primary synchronization method for the particular road user is not available, the secondary synchronization method on a self-organization is based on the synchronization of the internal clocks, where a required to synchronize the respective internal clock Identifier is coded in the communication method used, without disturbing the actual communication process. A method according to claim 1, characterized in that the primary synchronization method is not usable when a predetermined number of simultaneously received satellite signals is exceeded. A method according to claim 1 or 2, characterized in that the secondary synchronization method is based on the principle of a Firefly synchronization. Method according to one of the preceding claims, characterized in that at least some of the messages include an identifier which signals the transmission of the message to a, the message receiving the road users, certain time, by the one or more road users who receive the message, a Synchronization is performed relative to the specific time. A method according to claim 4, characterized in that the road user takes into account a given confidence on a temporal reference of a, in particular via the primary synchronization method, synchronized road user and advantageously transmits with the identifier. A method according to claim 5, characterized in that upon receipt of the message with the identifier, the internal clock of the receiving road user is adapted to the internal clock of the message with the identifier sending road user. Method according to one of claims 4 to 6, characterized in that the message is transmitted as an OFDM frame with a special preamble (P), wherein the identifier is represented by a predetermined preamble of the OFDM frame, and wherein the predetermined preamble of a conventional preamble can be distinguished by the receiving road user. Method according to one of claims 4 to 6, characterized in that the message is transmitted as an OFDM frame, wherein the identifier is represented by a predetermined symbol pattern in the OFDM frame. Method according to one of claims 4 to 6, characterized in that the message is transmitted as an OFDM frame, wherein the identifier is represented by a code word (Sig) from a spreading code which is transmitted in addition to the OFDM frame. Method according to one of claims 4 to 9, characterized in that the messages at a predetermined time at predetermined time intervals with a predetermined probability, in particular of one or more synchronized via the primary synchronization process road users, sent out. Method according to one of Claims 4 to 10, characterized in that the transmission of the message with the identifier by the traffic participant sending the message is made dependent on a given confidence in its own time base. Method according to one of claims 4 to 11, characterized in that the message is sent with the identifier only by a road user whose internal clock is synchronized according to the first synchronization method. Method according to one of Claims 1 to 3, characterized in that the message is transmitted as an OFDM frame with a payload (ND), the internal time being transmitted by the traffic participant transmitting the message whose internal clock is synchronized according to the first synchronization method the payload of the message is included. Method according to one of claims 1 to 13, characterized in that from the received from the other road users Data and / or assumptions about the own movement, a movement model is determined, in particular to extrapolate the position of the moving road users, then, when the predetermined number of satellite signals is exceeded, but at least one satellite signal is received, assuming the estimated position one To be able to determine the time correction factor. Method according to one of the preceding claims, characterized in that the secondary synchronization method is executed in parallel to the primary synchronization method, even if the primary synchronization method can be used by a respective road user. A computer program product that can be loaded directly into the internal memory of a digital computer and includes software code portions that perform the steps of any one of the preceding claims when the product is run on the computer.

Images