Abstract:
A method for filtering a message transmitted between a transmitter and a receiver in a vehicular ad hoc network, the message containing at least one position of the transmitter, including: determining a position of the receiver by a global satellite navigation system in a predetermined coordinate system; determining a decision threshold for a maximum distance between the receiver and the transmitter in the predetermined coordinate system in which the position of the receiver was determined; and discarding the message if a distance between the position of the transmitter and the position of the receiver exceeds the decision threshold.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Phase Application of PCT International Application No. PCT/EP2014/067939, filed Aug. 22, 2014, which claims the priority to German Patent Application No. 10 2013 216 626.5, filed Aug. 22, 2013, the contents of such applications being incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for filtering a message transmitted between a sender and a receiver in a vehicle ad hoc network, to a filter apparatus for performing the method and to a receiver having the filter apparatus. 
       BACKGROUND OF THE INVENTION 
       [0003]    WO 2010/139 526 AI, which is incorporated by reference discloses a mobile ad hoc network called car2X whose nodes are particular road users, such as vehicles, or other objects in road traffic, such as traffic lights. These networks can be used to provide the road users involved in the car2X network with advice of road traffic states, such as accidents, congestion, hazard situations, etc. 
       SUMMARY OF THE INVENTION 
       [0004]    An aspect of the invitation aims to improve the use of such mobile ad hoc networks. 
         [0005]    According to one aspect of the invention, a method for filtering a message, transmitted between a sender and a receiver in a vehicle ad hoc network, that describes at least one position of the sender comprises the steps of:
       determination of a position of the receiver by means of a global satellite navigation system in a predetermined coordinate system,   determination of a decision threshold for a maximum distance between the receiver and the sender in the predetermined coordinate system in which the position of the receiver has been determined, and   rejection of the message if a distance between the position of the sender and the position of the receiver exceeds the decision threshold.       
 
         [0009]    The specified method is based on the consideration that useful data describing messages are interchanged between sender and receiver in a vehicle ad hoc network usually in what are known as data packets. When the sender sends useful data, it packs them into a data packet and sends it to the receiver, which then has to unpack the useful data from the data packet for the purpose of further processing. This requires a particular degree of computation power. Since different volumes of messages and hence useful data are interchanged depending on the traffic situation, the computation power that needs to be kept available for further processing of the relevant data packets is also dependent on the traffic situation. The computation power that needs to be kept available could be harmonised, however, if some of the data packets received in the receiver are filtered out as irrelevant, for example in advance, so that the receiver no longer needs to inspect the relevant message in the first place. 
         [0010]    However, such filtering operations require the provision of decision bases in order to classify the received data packets as relevant or irrelevant. One such decision basis that could be used is a decision threshold for the physical distance between the sender and the receiver of a data packet, for example, the sender in this case being intended to be understood not as a physical sender of the data packet but rather as an originator of the data packet that has produced the useful data in the data packet. The physical sender of the data packet may also be an intermediate entity that has received the data packet from the originator, for example, and hence from the actual sender and has forwarded it to the receiver. If, by way of example, a vehicle, as receiver, is too far away from a sender, such as another vehicle, a set of traffic lights and so on, then the sender can be eliminated as irrelevant solely on the basis of its excessive physical distance from the receiver. 
         [0011]    The distance between sender and receiver first of all takes into account the absolute position of the receiver, which can be determined without any problem using the global satellite navigation system. The absolute position of the sender is transmitted via the vehicle ad hoc network, so that the distance between sender and receiver is fundamentally determinable. However, the decision threshold for filtering a data packet and hence a message on the basis of the physical distance between sender and receiver is determined in a coordinate system that is defined by a specification for the vehicle ad hoc network, since on the basis of this coordinate system, the aforementioned useful data of the data packet are also processed further when the data packet is not filtered out. For technical reasons, this coordinate system is the Cartesian coordinate system. 
         [0012]    For the purpose of filtering a received data packet and hence a message on the basis of the distance between sender and receiver, the aforementioned position of the receiver, as determined using the global navigation satellite system, must therefore first of all be converted to the coordinate system defined by the specification for the vehicle ad hoc network before the decision threshold determined in this coordinate system can be taken as a basis for making the decision to filter out the received data packet. Since, as a rule, particularly in high-load situations, most data packets are filtered out anyway and therefore not used further, an unnecessarily high level of computation complexity can arise at this juncture, however. 
         [0013]    In order to reduce this computation complexity and to achieve the aforementioned harmonisation of the computation load in times of high load, it is proposed, for the purposes of the specified method, that the decision threshold be determined no longer in the coordinate system that is defined by the standard for the vehicle ad hoc network but rather in the coordinate system that is prescribed by the global satellite navigation system. In this way, reception of a data packet can immediately be followed by a decision regarding whether or not said data packet is filtered out, and the filtering process can be performed as early as possible in the reception chain. 
         [0014]    In one development of the specified method, the predetermined coordinate system is a spherical coordinate system that describes positions in geocoordinates on the earth&#39;s surface. Such a coordinate system is also called an ellipsoid coordinate system and, depending on the system, has worldwide validity. Therefore, it does not first need to be adapted to suit a local standard. 
         [0015]    In a special development of the specified method, the decision threshold comprises a corridor around the position of the receiver, so that the message is rejected if the position of the sender is outside the corridor. In this case, the corridor defines a region around the receiver. The region moves with the receiver, in principle. In this case, the receiver may be a vehicle and the corridor may be larger in the direction of travel of the vehicle than counter to the direction of travel. In this way, the corridor can be kept constant over a predetermined period and does not have to be recalculated on every change of location of the vehicle, which likewise results in a perceptible decrease in the computation power necessary for performing the specified method. Alternatively, it is conceivable for the updating of the corridor to be performed on the basis of a perceptible or significant change in the position of the receiver. 
         [0016]    According to an alternative that is simple to implement, the corridor comprises a rectangle that is oriented at right angles to the navigation coordinate system. Thus, the corridor comprises four threshold values that correspond to distances in the four directions of the compass. The decision regarding whether a message is rejected or the relevance check then comprises two conditions that both have to be satisfied:
       latitude of the sender between north and south threshold values?   longitude of the sender between west and east threshold values?       
 
         [0019]    If this rectangle is rotated in accordance with the current characteristic heading, the conditions become more complex and simple multiplications are added. 
         [0020]    In another development, the specified method comprises the step of stipulation of the corridor if the vehicle leaves a further corridor that is located in the corridor. This further corridor can thus be used in a simple manner to compute when the superordinate corridor on which the filtering is based is intended to be recomputed. 
         [0021]    In a special development of the specified method, the further corridor is dependent on a speed of the vehicle. In this way, it is possible to take account of different traffic situations, such as an unobstructed journey on the freeway and a journey in congestion, which takes account of the fact that fewer data packets can be expected to be received at higher characteristic speeds of the receiver, that is to say of the vehicle, for example. Equally, the decision threshold itself, such as the superordinate corridor, may also be dependent on the speed of the receiver, that is to say of the vehicle, for example. 
         [0022]    In another development of the specified method, the position of the sender is stored in a header of the data packet and can therefore immediately be evaluated and taken as a basis for the filtering. 
         [0023]    According to a further aspect of the invention, a filter apparatus is set up to perform a method as claimed in one of the preceding claims. 
         [0024]    In one development of the specified filter apparatus, the specified apparatus has a memory and a processor. In this case, the specified method is stored in the memory in the form of a computer program, and the processor is provided for carrying out the method when the computer program is loaded into the processor from the memory. 
         [0025]    According to a further aspect of the invention, a computer program comprises program code means in order to perform all the steps of one of the specified methods when the computer program is executed on a computer or one of the specified apparatuses. 
         [0026]    According to a further aspect of the invention, a computer program product contains a program code that is stored on a computer-readable data storage medium and that, when executed on a data processing device, performs one of the specified methods. 
         [0027]    According to a further aspect of the invention, a receiver for a vehicle for receiving a transmission signal that is set up to carry data packets containing at least position data pertaining to vehicles comprises
       an antenna for receiving the transmission signal,   one of the specified filter apparatuses for filtering at least some of the data packets from the transmission signal,   a presentation apparatus for compiling the position data from the filtered data packets, and   an output interface for outputting the position data in the vehicle.       
 
         [0032]    According to another aspect of the invention, a vehicle comprises one of the specified receivers. 
         [0033]    A further aspect of the invention is explained below. The further aspect of the invention relates to a data selection method for reducing the computation effort of a vehicle-to-X communication system. 
         [0034]    The prior art already discloses what are known as vehicle-to-X communication systems that are designed for transmitting both traffic-related data and various service data, such as entertainment applications. In this case, the vehicle-to-X communication is based both on the data interchange between vehicles themselves (vehicle-to-vehicle communication) and on the data interchange between vehicles and infrastructure devices (vehicle-to-infrastructure communication). On account of the high demands on the reliability and data integrity of information transmitted by means of vehicle-to-X communication, such information is additionally often provided with a complex security signature or data encryption. 
         [0035]    The evaluation of such a security signature and the decoding of such data encryption are associated with a relatively high level of computation effort, however. In order to keep the computation effort and hence the purchase costs for a computation module having sufficient computation power as low as possible, the prior art additionally discloses various preprocessing methods that make a selection for the vehicle-to-X messages that are to be decoded from among all received vehicle-to-X messages. Often, such preprocessing methods are based on the distance between the receiver and the sender of the vehicle-to-X message by deducing a significance of the vehicle-to-X message for the receiver from the distance. 
         [0036]    One of the most important pieces of information in a vehicle-to-X message is the absolute position of the originator or the sender of the packet. The originator and the sender are identical in the case of direct transmission without packet forwarding or without forwarding of the vehicle-to-X message via intermediate senders. The term sender usually denotes both the intermediate senders and the actual sender, whereas the actual sender of a vehicle-to-X message is additionally often referred to as the originator. The general term sender is used below. The reason is that the method according to the invention can be used both for an originator and for a sender. 
         [0037]    From the position of the sender relative to the receiver, it is possible to compute the distance between sender and receiver. This computation involves a high level of computation effort, however, because it requires coordinate transformation from (ellipsoid) geocoordinates, such as GPS coordinates, to local Cartesian coordinates or terrestrial coordinates. Details pertaining to the computation methods for such conversion are known from “GPS und GNSS: Grundlagen der Ortung und Navigation mit Satelliten” [GPS and GNSS: Principles of satellite location and navigation] by Jean-Marie Zogg, for example. 
         [0038]    Without such conversion, the prior art does not allow any distance statement and hence also any corresponding distance-based initial filtering. This thus means that, according to the prior art, each data packet of the received vehicle-to-X messages requires coordinate transformation and always requires the necessary computation power to be kept available. 
         [0039]    The object of the present aspect of the invention is therefore to reduce the decoding computation effort of a vehicle-to-X communication system and hence the computation power that needs to be kept available therein. 
         [0040]    This is the point at which the aspect of the invention takes effect. Instead of computing a distance from the sender to the receiver in Cartesian coordinates for each data packet or each vehicle-to-X message, the method according to the invention provides for determination of the distance from the sender to the receiver directly in the transmitted geocoordinates. In this case, geocoordinates are, in a general manner, the coordinates used by the different global satellite navigation systems. 
         [0041]    In addition, the invention preferably provides for a distance-based selection method for the data packets or vehicle-to-X messages likewise to use the geocoordinates directly, for example for the purpose of comparison with a prescribed distance threshold that, when exceeded, prompts the data packets or vehicle-to-X messages to be rejected as irrelevant. This threshold value check is also preferably performed directly in geocoordinates. Hence, the complex coordinate transformation is saved in the case of those packets or vehicle-to-X messages that are rejected anyway. 
         [0042]    One possible and preferred method sequence has the following appearance:
       the distance thresholds are computed in geocoordinates and, in the event of a significant change in the characteristic position, refreshed or adapted to suit the changed characteristic position,   the position data of received data packets or vehicle-to-X messages are compared directly in geocoordinates, which means that there is no further need for conversion to local Cartesian coordinates,   the adaptation frequency for the distance threshold in geocoordinates rises as the characteristic speed increases, but at higher characteristic speeds there are normally fewer received packets anyway.       
 
         [0046]    Ideally, the method described is used as early as possible in the processing chain of received data packets or vehicle-to-X messages, that is to say e.g. directly after physical reception, before the remainder of the so-called communication stack is actually handled. 
         [0047]    The invention therefore results in the advantage that the use of distance thresholds in geocoordinates allows a distinct reduction in the computation effort required for initial filtering. This in turn results in much lower hardware complexity for the handling of vehicle-to-X messages. 
         [0048]    The distance thresholds are computed particularly preferably in rectangular from rectangular geometries or areas around the current characteristic position. These can, furthermore, also be derived in a simple manner from a multiplicity of requirements for different vehicle systems based on vehicle-to-X communication and allow better adaptation thereto than the scalar value of the Euclidean distance. 
         [0049]    According to a further preferred embodiment, the relevance area, that is to say the area within the distance thresholds around the receiver, consists of a rectangle oriented at right angles to the navigation coordinate system, so that four distance thresholds are obtained that correspond to distances in the four directions of the compass. The check on the distance thresholds then consists merely of two conditions that both have to be satisfied:
       Is the sender between north and south threshold values?   Is the sender between west and east threshold values?       
 
         [0052]    If this rectangle is rotated in accordance with the current orientation of the receiver, the computation requirements become more complex and simple multiplications are added. Nevertheless, there continues to be a reduction in the need for computation power in comparison with the position conversion to local Cartesian coordinates, since angle functions are needed only once for setting up the threshold values, rather than for each received data packet or each received vehicle-to-X message. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0053]    The properties, features and advantages of this invention that are described above and also the manner in which they are achieved will become clearer and more distinctly comprehensible in connection with the description of the exemplary embodiments that follows, said exemplary embodiments being explained in more detail in connection with the drawings, in which: 
           [0054]      FIG. 1  shows a basic illustration of a vehicle travelling on a road, 
           [0055]      FIG. 2  shows a basic illustration of the vehicle from  FIG. 1 , 
           [0056]      FIG. 3  shows a basic illustration of a vehicle ad hoc network for the vehicle from  FIGS. 1 and 2 , 
           [0057]      FIG. 4  shows a basic illustration of a coordinate system for describing a position of the vehicle from  FIG. 1  on the road, 
           [0058]      FIG. 5  schematically shows the data structure of a vehicle-to-X message, and 
           [0059]      FIG. 6  shows a flowchart that presents the individual sequence steps of a possible embodiment of the method according to an aspect of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0060]    In the figures, like technical elements are provided with like reference symbols and described only once. 
         [0061]    The invention relates to a network protocol for a vehicle ad hoc network shown in  FIG. 3 , which is called car2X network  1  below for the sake of simplicity. To provide a better understanding of the technical background to this car2X network  1 , a nonrestrictive exemplary embodiment will first of all be provided for this car2X network  1  before discussing technical details pertaining thereto in more detail. 
         [0062]    Therefore, reference is made to  FIG. 1 , which shows a basic illustration of a vehicle  3  travelling on a road  2 . 
         [0063]    In the present embodiment, the road  2  is meant to have a pedestrian crossing  4  at which a set of traffic lights  5  is used to regulate whether the vehicle  4  on the road  2  is permitted to cross the pedestrian crossing  4  or a pedestrian—not shown in more detail—on the pedestrian crossing  4  is permitted to cross the road  2 . Between the pedestrian crossing  4  and the set of traffic lights  5 , there is, for the purposes of the present embodiment, an obstacle in the form of a curve  6  that conceals the pedestrian crossing  4  from the driver of the vehicle  3  and from an ambient sensor system—which is yet to be described—of the vehicle  3 . 
         [0064]    In a direction of travel  7  ahead of the vehicle  3 ,  FIG. 1  shows a further vehicle  8  that has been involved in a road accident  10  with a vehicle  9 —shown in dots—on the pedestrian crossing  4  and is blocking the lane in the direction of travel  7  of the vehicle  3 . 
         [0065]    The pedestrian crossing  4  and the road accident  10  are hazard situations on the road  2 . If the driver of the vehicle  3  overlooks the pedestrian crossing  4  and therefore illegally fails to stop before it, he could hit a pedestrian who is crossing the pedestrian crossing  4  and who, in crossing the pedestrian crossing  4 , relies on the driver of the vehicle  3  behaving in accordance with the rules. In both hazard situations, the driver of the vehicle  3  must stop the vehicle  3  in order to avoid a collision with the hazard object in the hazard situation, that is to say the pedestrian and/or the further vehicle  8 . To this end, the car2X network  1  can be used, which will be discussed in more detail at a later juncture. 
         [0066]    In the present embodiment, the vehicle  3  has a receiver  11  for a global satellite navigation system, called a GNSS receiver  11  below, which the vehicle  3  can use in a manner known per se to determine position data in the form of its absolute geographical position  12  and to use said position data for the purposes of a navigation system  13 , for example, in order to display them on a geographical map, which is not shown further. Corresponding signals  14  from the global satellite navigation system, called GNSS signals  14  below, can be received via an appropriate GNSS antenna  15 , for example, and forwarded to the GNSS receiver  11  in a manner known per se. 
         [0067]    In the present embodiment, the vehicle additionally has a transceiver  16  that the vehicle  3  can use to be involved as a node in the car2X network  1  and to interchange messages, called car2X messages  17  below, with other nodes, such as the further vehicle  8  and/or the set of traffic lights  5 . In order to distinguish it from the GNSS receiver  11 , this transceiver  16  will be called car2X transceiver  16  below. 
         [0068]    In the car2X messages  17  interchanged via the car2X network  1 , the individual nodes  3 ,  5 ,  8  can interchange data describing various information with one another, which data can be used to increase road safety on the road  2 , for example. An example of the information that can be interchanged with the data in the car2X messages  17  would be the absolute geographical position  12 , determined using the GNSS receiver  11 , of the respective node  3 ,  5 ,  8  of the car2X network  1 . Such data can also be called position data. If the node  3 ,  5 ,  8  of the car2X network  1  that receives the geographical position  12  is a vehicle, such as the vehicle  3  that is not involved in the road accident  10  and the vehicle  8  that is involved in the road accident  10 , then the geographical position  12  received via the car2X network  1  can be used to represent the traffic movement, for example, on the navigation system  13  of the receiving vehicle  3 ,  8 , for example. If, besides the absolute geographical position  12 , the road accident  10  is also described as information with the data in the car2X message  17 , then determined traffic situations, such as the road accident  10 , can be represented on the navigation system  13  more specifically. Further possible information that can be interchanged with the car2X messages  17  will be discussed in more detail later for the purposes of  FIG. 2 . 
         [0069]    In order to interchange the car2X messages  17 , the car2X transceiver  16  either modulates a car2X message  17  onto a transmission signal, called car2X signal  18  below, and sends it via an antenna, called car2X antenna  19  below, to the other nodes  3 ,  5 ,  8  in the car2X network  1 , or it uses the car2X antenna  19  to receive a car2X signal  18  and filters the relevant car2X message  17  therefrom. This will be discussed in more detail at a later juncture for the purposes of  FIG. 3 . In this case,  FIG. 1  shows that the car2X transceiver  16  outputs a car2X message  17  to the navigation system  13  on the assumption that said message contains, in the manner described above, information that can be represented on said navigation system. This is not intended to be understood as a restriction, however. In particular, it is expediently also possible for the GNSS receiver  11  to be connected to the car2X transceiver  16  directly or, as shown in  FIG. 2 , indirectly in order to send its own absolute geographical position  12  in the car2X network  1 . 
         [0070]    The structure of the car2X message  17  and of the car2X signal  18  and hence the design of the car2X network can be defined in a communication protocol. There are already such communication protocols on a country-specific basis, inter alia for the purposes of ETSI TC ITS at ETSI in Europe and for the purposes of IEEE 1609 at IEEE and also at SAE in the United States of America. Further information in this regard can be found in the cited specifications. 
         [0071]    The vehicle  3  can optionally also have the aforementioned ambient sensor system in the form of a camera  20  and a radar sensor  21 . The camera  20  can be used by the vehicle  3  to record an image of a view that is ahead of the vehicle  3 , when considered in the direction of travel  7  of the vehicle  3 , within an image angle  22 . In addition, the vehicle  3  can use the radar sensor  21  and appropriate radar beams  23  to identify objects, when considered in the direction of travel  7  of the vehicle  3 , and to determine the distance from the vehicle  3  in a manner known per se. 
         [0072]    In order to substantiate the information that can be transmitted with a car2X message  17 , the design of the vehicle  3  and of the further vehicle  5  will first of all be discussed below on the basis of the vehicle  3  by way of example. The vehicle  3  has various safety components, of which  FIG. 2  shows an electronic braking assistant  24 , called EBA  24 , and a driving dynamics control system  25 , which is known per se. While DE 10 2004 030 994 AI provides details pertaining to the EBA  24 , DE 10 2011 080 789 AI provides details pertaining to the driving dynamics control system  25 . 
         [0073]    The vehicle  3  comprises a chassis  26  and four wheels  27 . Each wheel  27  can be slowed down in comparison with the chassis  26  by means of a brake  28 , mounted at a fixed location on the chassis  26 , in order to slow down a movement by the vehicle  3  on the road  2 . 
         [0074]    In this case, in a manner that is known to a person skilled in the art, it may occur that the wheels  27  of the vehicle  3  lose their traction and the vehicle  3  even moves away from a trajectory, for example prescribed by means of a steering wheel, which is not shown further, as a result of understeer or oversteer. This is avoided by the driving dynamics control system  25 . 
         [0075]    In the present embodiment, the vehicle  4  has speed sensors  29  on the wheels  27  for this purpose, which sense a speed  30  of the wheels  27 . 
         [0076]    On the basis of the sensed speeds  30 , a controller  31  can determine, in a manner that is known to a person skilled in the art, whether the vehicle  3  slips on the carriageway or even deviates from the aforementioned prescribed trajectory, and can react thereto accordingly with a control output signal  32  that is known per se. The controller output signal  32  can then be used by an actuating device  33  in order to use actuating signals  34  to actuate actuating elements, such as the brakes  28 , which react to the slipping and the deviation from the prescribed trajectory in a manner that is known per se. 
         [0077]    The EBA  24  can evaluate image data  35 , captured using the camera  20 , and distance data  36 , captured using the radar sensor  21 , pertaining to objects such as vehicles in the direction of travel  7  ahead of the vehicle  3  and, on the basis thereof, can detect a hazard situation. This hazard situation could arise, by way of example, when an object ahead of the vehicle  3  approaches the latter at an excessive speed. In such a case, the EBA  24  could use an emergency braking signal  37  to instruct the actuating device  33  to use the actuating signals  34  to carry out emergency braking with the brakes  28 . 
         [0078]    Each time the EBA  24  or the driving dynamics control system  25  uses the actuating device  33  to take action in the vehicle  4 , the actuating device  33  can output a report signal  38 , for example, which is shown in dots in  FIG. 2 . Expediently, the report signal  38  should substantiate whether the action was required by the EBA  24  or the driving dynamics control system  25 . Such a report signal  38  can be produced by any entity in the vehicle  3 , that is to say even by the controller  31  of the driving dynamics control system  25 , for example. A message generation device  39  could then take the report signal  38 , the absolute geographical position  12  and a timestamp  41 , which is shown in  FIG. 3  and output from a timer  40 , as a basis for generating a car2X message  17  that can be used to report the action of the EBA  24  and/or of the driving dynamics control system  25  to the other nodes  5 ,  8  as information via the car2X network  1 . The car2X message  17  generated in this manner could then be sent in the car2X network  1  via the car2X antenna  19 . 
         [0079]    In the example of  FIG. 1 , it was explained that the information about the absolute geographical position  12  of the individual nodes  3 ,  5 ,  8  and/or about events such as the road accident  10  and/or such as an action by the EBA  24  and/or the driving dynamics control system  25  that is interchanged in the car2X messages  17  could be represented on the navigation system  13  for the purpose of orienting the driver. Alternatively or additionally, the information interchanged in the car2X messages  17  can also be taken as a basis for actively generating actuating signals  34 , for example using the actuating device  33 , however. If, by way of example, the action by the EBA  24  is transmitted as information in a car2X message  17 , then it would be possible, by way of example, to take the reception of this car2X message  17  as a basis for automatically triggering the EBA  24  in the receiving vehicle  3 ,  8 . 
         [0080]    The transmission of a car2X message  17  via the car2X network  1  will be explained below with reference to  FIG. 3 , said car2X network being indicated by a cloud in  FIG. 3  for the sake of clarity. The content of the car2X message  17  is intended to be assumed to be, by way of example, an action—reported by the actuating device  33  with the report signal  38 —by the EBA  24  in the accident vehicle  8  involved in the road accident  10 . 
         [0081]    As already explained, the message generation device  39  takes the report signal  38 , the absolute geographical position  12  and the timestamp  41  as a basis for generating the car2X message  17  according to the aforementioned communication protocol. In this case, the message generation device  39  may also be part of the car2X transceiver  16 , in principle. 
         [0082]    From the car2X message  17 , data packets  43  are generated in a data packet generation device  42  in the car2X transceiver  16  of the accident vehicle  8 . The generation of data packets  43  means that car2X messages  17  from various applications in the accident vehicle  8  can be combined to form a single data stream in order to produce the car2X signal  18 . The data packet generation device  42  therefore corresponds to a network and transport layer, the task of which is known to be to route the network data from various applications. The design of the data packet generation device  42  is dependent on the aforementioned specification of the communication protocol for the car2X network  1 . 
         [0083]    The generated data packets  43  are modulated onto the car2X signal  18  in a modulation device  44  and wirelessly sent in the car2X network  1 . The modulation device  44  therefore corresponds to an interface layer, the task of which is to physically connect the accident vehicle  8  to the car2X network  1 . The design of the modulation device  44  is also dependent on the aforementioned specification of the communication protocol for the car2X network  1 . 
         [0084]    In the vehicle  3  that is not involved in the road accident  10 , the car2X signal  18  sent by the accident vehicle  8  can then be received via the car2X antenna  19 . 
         [0085]    In order to extract the car2X message  17  from the car2X signal  18 , the car2X transceiver  16  of the vehicle  3  has a demodulation device  45  that reverses the sender-end modulation of the data packets  43  in a manner that is known per se. Accordingly, a message extraction device  46  can extract the car2X messages  17  from the data packets  43  and make them available to the applications in the vehicle  3 , such as the navigation system  13  or even the actuating device  33 . Ultimately, the demodulation device  45  and the message extraction device  46  are the reception-end counterparts in accordance with the aforementioned network and transport layer and the interface layer and are likewise dependent on the aforementioned specification of the communication protocol for the car2X network  1 . 
         [0086]    For details of the individual network layers, reference is therefore made to the relevant specifications. 
         [0087]    Particularly in high-load situations when there are a multiplicity of nodes  3 ,  5 ,  8  in the car2X network  1  on the road  2 , it is necessary for correspondingly high levels of computation resources to be kept free in the respective nodes  3 ,  5 ,  8  for the purpose of processing all car2X messages  17  sent in the car2X network  1 , in order to guarantee the processing of all car2X messages  17  at the receiver end within particular time limits. The provision of these high levels of computation resources is associated with a correspondingly high outlay in terms of cost, which is intended to be reduced for the purposes of the present embodiment by the introduction of initial filters  47 ,  48 . 
         [0088]    The concept behind the initial filters  47 ,  48  is for potentially irrelevant car2X messages  17  to be eliminated as early as possible in order to avoid their needing to be processed unnecessarily by an element in the reception chain because, as it is, they contain information that is irrelevant to the receiving node. One option would be to place a first initial filter  47  from the two initial filters  47 ,  48  between the car2X antenna  19  and the demodulation device  45  and to perform the initial filtering on the basis of a signal strength of the car2X signal  18 . The concept behind the first initial filter  47  is that nodes  5 ,  8  that are further away from the receiving node, that is to say from the vehicle  3  that is not involved in the accident  10 , are less relevant because, at least from the point of view of road safety, they cannot become a danger to the vehicle  3  that is not involved in the accident  10  in the immediate proximity of time. 
         [0089]    The data packets  43  can then be demodulated from the car2X signal  49  filtered in this manner. 
         [0090]    To extend the aforementioned approach of filtering out the car2X messages  17  from transmitting nodes that are further away from the receiving node without unpacking the relevant car2X messages  17  themselves, it is proposed, for the purposes of the present embodiment, that the second initial filter  47  from the two initial filters  47 ,  48  be used to filter the data packets  43  on the basis of the position data already contained in the data packets  43 , that is to say the absolute geographical position  12 . This is possible because the absolute geographical position  12  is normally carried at a predetermined location in the data packet  43 , for example in the header of the data packet  43 . Only from the data packets  50  filtered in this manner is it then possible for the message extraction device  46  to be used to extract the car2X messages  17 , the computation complexity of the message extraction device  46  having perceptibly decreased on account of the car2X messages  17  disappearing in advance as a result of the initial filters  47 ,  48 . 
         [0091]    For the purpose of implementing the second initial filter  46 , it is proposed that a decision threshold for a distance  51  between a receiving node and a transmitting node be defined, upward of which a data packet  43  is intended to be filtered out and the relevant car2X message  17  ignored. Before the definition of the decision threshold for the aforementioned distance  51  between a receiving node and a transmitting node can be discussed in more detail, it will first of all be clarified what in the car2X network  1  is a receiving node and what is a transmitting node. 
         [0092]    In this regard, reference is made to  FIG. 4 . 
         [0093]    A receiving node in the car2X network  1  will always be a node  3 ,  5 ,  8  that receives a car2X message  17 . For the purposes of  FIG. 4 , this is the vehicle  3  that is not involved in the accident  10 , for example. Therefore, the vehicle  3  that is not involved in the accident represents the receiving node below. 
         [0094]    A transmitting node in the car2X network  1  will always be a node  3 ,  5 ,  8  that produces and sends a car2X message  17  and is therefore the origin thereof. A transmitting node can therefore also be referred to as the originator of a car2X message  17 . For the purposes of  FIG. 4 , as a transmitting node, the vehicle  8  that is involved in the accident  10  will represent the transmitting node. 
         [0095]    So that car2X messages  17  in the car2X network  1  can also be transmitted over distances  51  that exceed the transmission range of a car2X signal  17 , it is possible for nodes  3 ,  5 ,  8  in the car2X network  1  that are located between the transmitting node  8  and the receiving node  3  and that receive the car2X signal  18  having the car2X message  17 , for example from the transmitting node  8 , to forward the car2X message  17  to the receiving node  3  using a car2X signal  18  of their own. Such forwarding nodes or intermediate nodes may be any subscribers in the car2X network  1 . For this purpose,  FIG. 4  indicates a singular vehicle having the reference symbol  52 . 
         [0096]    The aforementioned decision threshold is in this case intended to be defined for distances  51  between the transmitting node  8 , as the origin of a car2X message  17  rather than as the origin of a car2X signal  18 , and the receiving node  3 . 
         [0097]    For the purposes of the present embodiment, the decision threshold is defined in the form of a corridor  53  that is positioned around the receiving node  3 . A car2X message  18  is intended to be filtered out by the second initial filter  46  if the geographical position  12  of the transmitting node  8  does not fall within the corridor  53  when the car2X message  18  is sent. 
         [0098]    The form in which the geographical position  12  of a node  3 ,  5 ,  8 ,  52  in the car2X network  1  is described is dependent on the aforementioned specification. Normally, the geographical position  12  is described in a Cartesian coordinate system, because the geographical position  12  is simplest to process by computer in said coordinate system. This coordinate system defined by the specification could also be used to define the decision threshold and hence the corridor  53 . 
         [0099]    However, on principle, the GNSS receiver  11  determines the geographical position  12  in an ellipsoid coordinate system  54  having degrees of longitude  55  and degrees of latitude  56 . 
         [0100]    Provided that the decision threshold and hence the corridor  53  have been defined in the aforementioned manner in the coordinate system defined by the specification, the geographical position  12  ought first to be transformed to the relevant coordinate system in a car2X message  17  in the second initial filter  46 . This has the advantage that the transformed geographical position  12  can then immediately continue to be used and that the decision threshold and hence the corridor  53  can be determined directly in the coordinate system that is simpler to handle by computer and that is determined by the specification. 
         [0101]    This is where the present embodiment takes effect with the proposal to determine the decision threshold and hence the corridor  53 , as shown in  FIG. 4 , directly in the ellipsoid coordinate system  54 . As far as computation complexity is concerned, this is counter-productive at first glance, because not only is the determination of the decision threshold and hence of the corridor  53  in the ellipsoid coordinate system  54  computation-intensive on account of the trigonometric correlations, it is additionally necessary for the geographical position  12  to be converted to the coordinate system defined by the specification after passing through the second initial filter  46 . 
         [0102]    Taking account of a high-load situation in the car2X network  1 , in which a multiplicity of car2X messages  26  can be expected from a multiplicity of different nodes  3 ,  5 ,  8 ,  52 , the aforementioned notion becomes clearly apparent. If the car2X messages  17  are filtered directly in the ellipsoid coordinate system  54  prescribed by the GNSS receiver  11 , then the geographical positions of all data packets  43  and hence of all car2X messages  17  that do not pass through the second initial filter  46  also do not need to be transformed to the coordinate system defined by the specification. In a high-load situation, in which the complexity for converting the geographical positions  12  in the individual received data packets  43  would exceed the complexity for determining the decision threshold and hence the corridor  53  directly in the ellipsoid coordinate system  54  by a multiple, computation resources can thus be perceptibly saved. 
         [0103]    In order to save the computation resources further, the decision threshold and hence the corridor  82  should be defined such that the corridor  53  needs to be recomputed as little as possible. To this end, the corridor  53  is set up with a first corridor section  57  in front of the vehicle  3  and a second corridor section  58  behind the vehicle  3 , as seen in the direction of travel  7  of the vehicle  3  acting as receiving node. In this case, the first corridor section  57  should be longer than the second corridor section  58  in order to give the traffic in the direction of travel  7  in front of the vehicle  3  a correspondingly higher relevance. 
         [0104]    The two corridor sections  57 ,  58  are stipulated as constant for a period that should be chosen such that firstly the corridor  53  and hence the decision threshold do not have to be updated all too frequently, but secondly not all too many data packets  43  pass through the second initial filter  46 . 
         [0105]    As a basis for updating the corridor  53 , a further, inner corridor  59  can be defined in the corridor  53 , for example. So long as the receiving node and hence the vehicle  3  stay in the inner corridor  59 , the outer corridor  53  can remain unchanged. If the vehicle  3  leaves the inner corridor  59 , then the outer corridor  53  and at the same time also the inner corridor  59  can be updated. 
         [0106]    In this case, the two corridor sections  57 ,  58  of the outer corridor  53  can be stipulated on the basis of a marginal region of the inner corridor  59 , for example. The size of the inner corridor  59  and/or the size of the outer corridor  53  may be dependent on the speed of the receiving node and hence of the vehicle  3  in this case, for example in order to take account of unobstructed journeys on the freeway and journeys on congested roads in the second initial filter  46  as appropriate. The reference symbols in  FIGS. 5 and 6  below refer to different technical elements than the reference symbols in  FIGS. 1 to 4 . 
         [0107]      FIG. 5  shows the data structure of vehicle-to-X message  61 . Vehicle-to-X message  61  comprises header data component  62 , data integrity component  63  and useful data component  64 . Header data component  62 , data integrity component  63  and useful data component  64  may for their part be divided into further subcomponents, this not being the case in this exemplary embodiment, however. Header data component  62  is uncoded, data integrity component  63  is digitally encrypted and comprises a data integrity certificate, and useful data component  64  is coded in the ASN.1 format. Since header data component  62  furthermore comprises the position data of the sender in geocoordinates, it is possible for the distance from the sender to the receiver to be determined without special computation effort. 
         [0108]      FIG. 6  shows an exemplary sequence for the method according to the invention as a flowchart. In step  71 , a vehicle-to-X message is received by means of a communication unit. In step  72  that follows, the header data component of the vehicle-to-X message, and particularly the position data comprised therein for the sender, is/are read and evaluated. In step  73 , there then follows a comparison of the position data of the sender with the distance thresholds. Provided that the position data are located within the area framed by the distance thresholds, the vehicle-to-X message or the data packets comprised by the vehicle-to-X message is/are flagged as relevant in step  74 . If the position data are located outside the area framed by the distance thresholds, however, then the vehicle-to-X message or the data packets comprised by the vehicle-to-X message is/are flagged as irrelevant in step  75  and rejected. In this case, no further processing of the data packets or of the vehicle-to-X message takes place, therefore. Finally, in step  76 , the data packets or vehicle-to-X messages flagged as relevant are decoded, so that they can be processed further and evaluated by the relevant vehicle systems in step  77 . 
         [0109]    The further aspect of the invention also includes the following principles: 
         [0110]    1. A data selection method for reducing the computation effort of a vehicle-to-X communication system, wherein a communication unit is used to receive and send vehicle-to-X messages and wherein the vehicle-to-X messages comprise position data for their sender in geocoordinates, characterized in that a distance from the receiver to the sender in geocoordinates is determined from a characteristic position of a receiver and the position data of the sender. 
         [0111]    2. The method according to principle 1, characterized in that the receiver also determines its characteristic position in geocoordinates. 
         [0112]    3. The method according to at least one of principles 1 and 2, characterized in that the distance is determined by comparison with a distance threshold, wherein the distance is determined to be above the distance threshold or to be below the distance threshold. 
         [0113]    4. The method according to at least one of principles 1 to 3, characterized in that four distance thresholds around the receiver are determined. 
         [0114]    5. The method according to at least one of principles 1 to 4, characterized in that the four distance thresholds frame a rectangular area around the receiver. 
         [0115]    6. The method according to at least one of principles 1 to 5, characterized in that the receiver is in the center of the rectangular area. 
         [0116]    7. The method according to at least one of principles 1 to 6, characterized in that the four distance thresholds are oriented parallel to coordinate axes of the geocoordinate system. 
         [0117]    8. The method according to at least one of principles 1 to 7, characterized in that the four distance thresholds are rotated in accordance with an orientation of the receiver compared with the coordinate axes of the geocoordinate system. 
         [0118]    9. The method according to at least one of principles 1 to 8, characterized in that the distance thresholds are determined for a particular characteristic position of the receiver and are redetermined if the receiver alters its characteristic position, in particular alters it by more than 50 m. 
         [0119]    10. The method according to at least one of principles 1 to 9, characterized in that the four distance thresholds are determined at high speeds of the receiver such that the receiver initially moves to the center of the rectangular area before leaving it again. 
         [0120]    11. The method according to at least one of principles 1 to 10, characterized in that the rectangular area comprises an area of 100 m×50 m. 
         [0121]    12. The method according to at least one of principles 1 to 11, characterized in that received vehicle-to-X messages that are received from senders outside the distance threshold(s) are flagged as irrelevant. 
         [0122]    13. The method according to at least one of principles 1 to 12, characterized in that vehicle-to-X messages flagged as irrelevant are not decoded and not processed. 
         [0123]    14. The method according to at least one of principles 1 to 13, characterized in that received vehicle-to-X messages that are received from senders within the distance threshold(s) are flagged as relevant. 
         [0124]    15. The method according to at least one of principles 1 to 14, characterized in that vehicle-to-X messages flagged as relevant are decoded and processed. 
         [0125]    16. The method according to at least one of principles 1 to 15, characterized in that the vehicle-to-X messages each comprise a multiplicity of data packets.