method of verifying timestamps provided by a road control radar

Abstract

A computer-implemented method comprising the steps of: obtaining (600) an image timestamp indicating a time of acquisition, by a traffic control radar, of an image showing a license plate of a vehicle, the image timestamp having been indicated by a clock of the traffic control radar; obtaining (602) time synchronization data, said time synchronization data having been received by the traffic control radar during a predefined time interval including the image timestamp for time synchronizing the clock of the traffic control radar with a time server; and testing (604) the image timestamp for consistency with the time synchronization data, the consistency test producing a test result indicating that the image timestamp is valid or invalid. Figure for abstract: Fig. 4

Description

Title of the invention: method for verifying timestamps provided by a road control radar FIELD OF THE INVENTION

[0001] The present disclosure relates to a method for verifying timestamps provided by a road control radar. STATE OF THE ART

[0002] To check that the speed of vehicles traveling on a road does not exceed a maximum authorized speed, it is known to use a pair of road control radars, arranged at a distance from each other along the road.

[0003] Each of the two road control radars acquires an image showing the registration plate of a vehicle traveling on the road. Two images are thus successively acquired.

[0004] One way to estimate the speed of the vehicle represented in the two images is to divide the distance between the two road control radars, which is known in advance, by the time separating the times when the two images were acquired.

[0005] To identify these two instants, the road control radars rely on their respective internal clocks, which measure the time that passes.

[0006] However, such internal clocks are not always reliable. As a result, the acquisition times of the two images may be misjudged by road control radars, and this may lead to an inaccurate estimation of the speed of the vehicle represented in the images.

[0007] To compensate for the unreliability of internal clocks, one solution could be to synchronize these internal clocks with a remote time server, providing a time reference deemed reliable. A road control radar could thus regularly send synchronization requests to the time server, and the time server would return synchronization data to the road control radar, to allow the radar to synchronize with the time server.

[0008] However, this solution remains imperfect. When an image is acquired by the road control radar relatively long after the last synchronization performed (for example very shortly before the next synchronization), it is possible that the radar's internal clock has drifted sufficiently significantly to negatively affect the accuracy of a speed to be estimated subsequently. Statement of the invention

[0009] An aim of the present disclosure is to detect a situation likely to lead to a poor estimation of a speed using a control radar. road.

[0010] This object is achieved by a computer-implemented method comprising the following steps: obtaining an image timestamp indicating a time of acquisition, by a road control radar, of an image showing a license plate of a vehicle, the image timestamp having been indicated by a clock of the road control radar; obtaining time synchronization data, said time synchronization data having been received by the road control radar during a predefined time interval including the image timestamp for temporally synchronizing the clock of the road control radar with a time server; and testing the consistency of the image timestamp with the time synchronization data, the consistency test producing a test result indicating that the image timestamp is valid or invalid.

[0011] This method constitutes a first object of the present disclosure and can also include the following optional features, taken alone or in combination whenever it makes technical sense.

[0012] Optionally, the time synchronization data comprises a first value relating to a first synchronization parameter, and the consistency test comprises a comparison between a value to be tested and a first predefined threshold, and the test result indicates that the timestamp is invalid when the value to be tested is greater than the first predefined threshold, the value to be tested being the first value or a corrected value resulting from a correction of the first value implemented using the time synchronization data.

[0013] Optionally, the time synchronization data comprises a sequence of first values ​​relating to the first synchronization parameter, the first values ​​having been sequentially received by the road control radar during the predefined time interval, and the method comprises a selection of an extreme value in the sequence of first values, and in which the first predefined threshold is selectively compared with the extreme value or with a corrected extreme value resulting from a correction of the extreme value implemented using the time synchronization data.

[0014] Optionally, the first values ​​comprise at least one value received by the road control radar before the acquisition of the image, and at least one other value received by the road control radar after the acquisition of the image.

[0015] Optionally, the time synchronization data comprises a sequence of data sets, the data sets having been sequentially received by the road control radar during the predefined time interval, wherein the data sets comprise respective first values ​​relating to the first synchronization parameter and respective second values ​​relating to the first synchronization parameter. relating to a second synchronization parameter different from the first synchronization parameter, and the method comprises steps of: preselecting, in the sequence of data sets, data sets whose respective second values ​​are not greater in absolute value than a second predefined threshold; selecting an extreme value from among the respective first values ​​of the preselected data sets, wherein the first predefined threshold is selectively compared with the extreme value or with a corrected extreme value resulting from a correction of the extreme value implemented using the time synchronization data.

[0016] Optionally, the time synchronization data comprises a second value relating to a second synchronization parameter different from the first synchronization parameter, the consistency test comprises a comparison between another value to be tested and a second predefined threshold, the other value to be tested being the second value or another corrected value resulting from a correction of the second value using the time synchronization data. The test result indicates that the timestamp is invalid when at least one of the following two conditions is met: the value to be tested is greater than the first predefined threshold, and the other value to be tested is greater than the second predefined threshold.

[0017] Optionally, the second synchronization parameter is a time jitter representative of a variation in network latency between the clock of the road control radar and the time server.

[0018] Optionally, the first synchronization parameter is a time offset between the clock of the road control radar and the time server.

[0019] Optionally, correcting a value relating to a synchronization parameter using the time synchronization data comprises subtracting from said value an average of time offsets between the clock of the road control radar and the time server, or even subtracting from said value a minimum jitter during the time interval.

[0020] Optionally, the predefined time interval has a duration greater than a time period used by the road control radar to periodically request time synchronization data from the time server.

[0021] Optionally, the method comprises an estimation of a speed of the vehicle from a predefined distance between the road control radar and another road control radar, and a duration between the image timestamp and another image timestamp, the other image timestamp indicating an instant of acquisition, by the other road control radar, of another image showing the registration plate of the vehicle.

[0022] A second object of the present disclosure is a computer program product comprising program code instructions for executing the steps of the method constituting the first subject of the disclosure, when this program is executed by a computer.

[0023] This program may use any programming language (for example, an object language or the like), and be in the form of interpretable source code, partially compiled code, or fully compiled code.

[0024] A third object of the present disclosure also relates to a non-transitory recording medium, readable by a computer, on which is recorded a computer program product comprising code instructions for implementing the steps of the method constituting the first object of the disclosure when this program is executed by a computer.

[0025] A fourth subject of the present disclosure is a system comprising: a road control radar and a processor. The road control radar comprises: a camera configured to acquire an image showing a license plate of a vehicle, a clock configured to produce an image timestamp indicating an instant of acquisition, by the road control radar, of the image, and a communication interface for receiving time synchronization data during a predefined time interval including the image timestamp, the time synchronization data being provided by a time server, and adapted to temporally synchronize the clock of the road control radar with the time server.The processor is configured to perform a consistency test of the image timestamp with the time synchronization data, the consistency test producing a test result indicating whether the image timestamp is valid or invalid. DESCRIPTION OF FIGURES

[0026] Other characteristics, aims and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and which must be read in conjunction with the appended drawings in which:

[0027] [Fig.l] schematically illustrates a system according to one embodiment.

[0028] [Fig.2] represents the internal components of a road control radar and a control server, according to one embodiment.

[0029] [Fig. 3] is a flowchart of steps of a method implemented by a road control radar, according to one embodiment.

[0030] [Fig.4] is a flowchart of steps of a method implemented by a control server, according to one embodiment.

[0031] [Fig.5a], [Fig.5b] and [Fig.5c] are flowcharts constituting three different embodiments of a consistency test step implemented by a server of control.

[0032] Throughout the figures, similar elements bear identical references. DETAILED DESCRIPTION OF THE INVENTION

[0033] [Fig.l] shows a system comprising a first road control radar 1, a second road control radar 2, a time server 4 and a control server 6.

[0034] The first road control radar 1 and the second road control radar 2 are positioned along a road, so that a vehicle traveling on the road passes close to the first road control radar 1, then close to the second road control radar 2. The distance separating the two road control radars is known.

[0035] With reference to [Fig.2], the first road control radar 1 comprises a camera 10, a network communication interface 12, a clock 14, a processor 16 and a memory 18.

[0036] The camera 10 is configured to acquire images showing the license plate of vehicles traveling on the road.

[0037] The network communication interface 12 is configured to communicate with the time server 4 (not shown in [Fig.2]) and the control server 6. The network communication interface 12 is of any type, either wired (for example Ethernet) or wireless radio (cellular, Wi-Fi, etc.).

[0038] The clock 14 is adapted to provide timestamps indicating the times at which certain events detected by the road control radar 1 occurred. These events will be described later.

[0039] The processor 16 is configured to control the transmission, via the network communication interface 12, of synchronization requests to the time server 4, and this periodically.

[0040] The processor 16 is also configured to synchronize the clock 14 using a synchronization data set transmitted by the time server 4 and then received by the network communication interface 12, in response to a synchronization request. The synchronization, known from the prior art, aims to recalibrate the clock 14 so that the time measured by the clock corresponds to a time measured by the time server. Thus, a synchronization can advance or reverse the time measured by the clock 14, if by chance the clock 14 had drifted relative to the time server since a previous synchronization.

[0041] A conventional synchronization data set includes the parameters listed in the table below. Parameter Definition Delay The average time between sending a synchronization request and receiving a set of synchronization data in response to the synchronization request. Time offset The average time difference between the clock and the time server. Jitter Represents a variation in network latency between the clock and the time server.

[0042] The memory 18 is configured to store synchronization data received and images acquired by the camera 10. The memory 18 is in particular configured to store these data and images by classifying them in a temporal manner, by assigning them timestamps provided by the clock 14. The memory 18 is of any type, for example Flash, EEPROM (for “Electrically-erasable programmable read-only memory” in English), HDD (for “Hard Disk Drive” in English), SSD (for “Solid-State Drive” in English), etc. The memory constitutes or comprises a non-transitory recording medium readable by computer.

[0043] The second road control radar 2 comprises the same components as those described above for the first road control radar 1.

[0044] The time server 4 constitutes a time reference. Its operation is known from the state of the art. For example, the time server 4 is an NTP server, which means that the communications between each road control radar 1, 2 and the time server 4, as well as the synchronization calculations implemented by the road control radars 1, 2, comply with the NTP protocol (Network Time Protocol).

[0045] Still with reference to [Fig.2], the control server 6 comprises a network communication interface 60, a memory 62 and a processor 64.

[0046] The network communication interface 60 is configured to communicate with each road control radar 1, 2. It is of any type, for example any of the types mentioned above for the network communication interface 12.

[0047] The memory 62 is configured to store data received via the network communication interface 60 or produced by the processor 64.

[0048] The processor 64 is configured to process data received via the network communication interface 60 or stored by the memory 62. These processes will be described later.

[0049] Method implemented by road control radars

[0050] With reference to [Fig.3], a method implemented by the road control radar 1 comprises the following steps.

[0051] In a step 100, the processor 16 controls the transmission, via the network communication interface 12, of a synchronization request to the recipient of the time server 4.

[0052] In a step 102, the processor 16 detects the reception, by the network communication interface 12, of a set of synchronization data originating from the time server 4, and responding to the synchronization request.

[0053] In a step 104, the processor 16 synchronizes the clock 14 of the road control radar 1 using the received synchronization data set.

[0054] In a step 106, the processor 16 controls the transmission to the control server 6, via the network communication interface 12, of the synchronization data set, in association with a timestamp provided by the clock 14, the timestamp indicating the time of reception of the synchronization data set (therefore when step 102 occurred) or the time of synchronization of the clock 14 using the synchronization data. By convention, such a timestamp is hereinafter called a “synchronization timestamp”.

[0055] The preceding steps are repeated over time. In particular, the sending step 100 is triggered periodically.

[0056] The method implemented by the road control radar 1 also comprises the following steps.

[0057] In a step 110, the road control radar 1 detects the passage of a vehicle in the field of vision of its camera 10, using appropriate detection means (known to those skilled in the art).

[0058] In a step 112, the camera 10 acquires an image showing a license plate of the detected vehicle.

[0059] In a step 114, the processor 16 commands the transmission to the control server 6, via the network communication interface 12, of a timestamp indicating the instant of acquisition of the image by the camera 10, this timestamp having been indicated by the clock 14. By convention, such a timestamp is called in the following "image timestamp", to differentiate this timestamp from synchronization timestamps. The processor 16 may also send the acquired image to the control server 6 in association with the image timestamp relating thereto.

[0060] The preceding steps are also repeated over time, for several vehicles traveling on the road.

[0061] Repeated implementation of the transmission steps 106 and 114 causes the road control radar 1 to transmit to the control server 6 timestamps Tl... TN. The N timestamps form an ordered sequence. By convention, Tl is the oldest timestamp, and TN the most recent timestamp. As indicated above, each timestamp Ti is either an image timestamp, which means that this timestamp indicates the instant of acquisition of an image by the camera 10 of the road control radar 1, or a synchronization timestamp, which means that this timestamp is associated with a synchronization data set Si, also provided to the control server 6.

[0062] The table below contains an example of 7 successive timestamps T1 to T7, forming a sequence. T1 and T4 are in this example image timestamps. The other Ti are synchronization timestamps, they are therefore associated with respective synchronization data sets Si. Tl (frame timestamp) T2 S2 T3 S3 T4 (frame timestamp) T5 S4 T6 S5 T7 S6

[0063] Of course, the transmissions of these data to the control server 6 can be carried out synchronously, or in a deferred manner, so as to group the transmissions. In this second case, the data transmitted to the control server 6 can be temporarily stored in the memory 18 of the road control radar 1.

[0064] The above method is also implemented by the second road control radar 2. Process implemented by the control server

[0065] We will now describe a method implemented by the control server 6 to verify the data provided by the first road control radar 1, with reference to [Fig.4],

[0066] It is assumed at this stage that the timestamps T1 to TN provided by the first road control radar 1 have been received by the network communication interface 60 of the control server 6, as has each synchronization data set Si associated with a synchronization timestamp.

[0067] In a step 600, the processor 64 obtains an image timestamp Ti indicating the time of acquisition, by the road control radar 1, of the image. This obtaining is typically done by reading in its memory 62.

[0068] In a step 602, the processor 64 obtains time synchronization data having been received by the road control radar 1 during a predefined time interval [Ta, Tb] including the image timestamp Ti. We therefore have Ta < Ti < Tb.

[0069] In one embodiment, the predefined time interval has a predefined duration AT, such that AT = Tb - Ta. Furthermore, the temporal position of the terminals Ta, Tb of the interval relative to the image timestamp Ti is also predefined. For example, the time interval is centered on the image timestamp. In this case, the predefined time interval is of the form [Ta, Tb] = [Ti - AT / 2, Ti + AT / 2].

[0070] To obtain the time synchronization data received by the road control radar 1 during the predefined time interval [Ta, Tb], the processor 64 compares each synchronization timestamp present in the memory 62 and originating from the road control radar 1, with the predefined time interval. If a synchronization timestamp has a value included in [Ta, Tb], then this timestamp is retained.

[0071] In a step 604, the processor 64 applies a consistency test of the image timestamp with the synchronization data received by the road control radar 1 during the predefined time interval. The consistency test produces a result which indicates either that the image timestamp is valid (in the case where it is considered during the test as consistent with the obtained synchronization data), or that the image timestamp is invalid (in the opposite case where it is considered during the test as inconsistent with the obtained synchronization data).

[0072] When the result indicates that the image timestamp is valid, the processor 64 may implement a step 606 of correcting this image timestamp based on the synchronization data. This step is not implemented when the result indicates that the image timestamp is invalid.

[0073] The correction performed in step 606 may use only an average of the time shifts observed in the range [Ta, Tb], as follows:

[0074] Timestamp_corrected = timestamp + average (offsets)

[0075] Alternatively, a similar reasoning can be applied to the temporal jitter. A possible correction would then consist of searching for the minimum observed value (MIN(abs(JITTER)) in the range, then adding it to the image timestamp considered. Indeed, if in the range we observe a "network latency" of x microseconds, we can then estimate that the image timestamp is late by this latency. We thus have:

[0076] Timestamp_corrected = timestamp + Average(offset) + MIN(abs(jitter))

[0077] The preceding steps are repeated by the control server 6 for different image timestamps, or even all the image timestamps that the control server 6 receives from the road control radar 1.

[0078] The control server 6 repeats the same steps on the data provided by the second road control radar 2. In other words, the control server 6 tests the consistency of the image timestamp provided by the second road control radar 2.

[0079] We will now describe in more detail different embodiments of the consistency test applied by the processor 64.

[0080] Consistency test 1: taking into account a first synchronization parameter

[0081] In a first embodiment, the steps of which are represented in [Fig.5a], the consistency test uses only a first synchronization parameter.

[0082] Each synchronization data set Si, associated with a synchronization timestamp Ti, comprises (or even consists of) a value relating to this first synchronization parameter.

[0083] Thus, the synchronization data that the control server 6 obtained in step 602 (which were previously received by the road control radar 1 during the predefined time interval [Ta, Tb] including the image timestamp that is the subject of the consistency test) comprise a sequence of first values ​​relating to the first synchronization parameter (offset), the first time values ​​having been sequentially received by the road control radar 1 during this time interval.

[0084] Preferably, the first values ​​comprise at least one value received by the road control radar 1 before the acquisition of the image, and at least one other value received by the road control radar 1 after the acquisition of the image.

[0085] In a step 700, the processor 64 selects an extreme value from the sequence of first values. The extreme value is a maximum value in absolute value. This means that when the first values ​​are signed, then the extreme value is the maximum of the respective absolute values ​​of the first values.

[0086] In a step 702, the processor 64 compares the extreme value with a first predefined threshold.

[0087] Preferably, the first predefined threshold is selectively compared with the extreme value. This means that all other first values ​​of the sequence are not compared with the first threshold. Only one comparison is performed in step 702. This saves computational resources.

[0088] In a step 704, the processor 64 generates the test result based on the comparison 702, observing the following logic: • The test result indicates that the image timestamp is invalid when the extreme value is greater than the first predefined threshold. • The test result indicates that the image timestamp is valid when the extreme value is not greater than the first predefined threshold.

[0089] For example, the first synchronization parameter is the time offset (offset in English) described above. In this case, the sequence of first values ​​is a sequence of time offsets, and the first predefined threshold is a time offset threshold selectively compared to a maximum time offset in the time interval [Ta, Tb].

[0090] The logic used by the processor 64 to invalidate an image timestamp in this first embodiment can be summarized by the following formula:

[0091] max(abs(OFFSET)) > offset threshold

[0092] As an illustration of this first embodiment, let us assume that: • The time offset threshold is set to 500 milliseconds (we do not want the clock of road control radar 1 to be more than 500 ms ahead or behind the clock of time server 4). • The duration of the time interval [Ta, Tb] is equal to 10 min, and the image timestamp Ti to be tested is centered on this interval. The interval is then [Ti - 5 min, Ti + 5 min].

[0093] The minimum value MIN and the maximum value MAX of the time shifts whose image timestamps fall within this interval are determined. The processor checks that abs(MAX) < 500 ms and that abs(MIN) < 500 ms. If abs(MAX) > 500 ms or abs(MIN) > 500 ms, then the test result is negative (inconsistency of the image timestamp). Indeed, this situation means that there is a probability that the image timestamp has a shift of more than 500 ms compared to the instant at which the image was actually acquired by the camera 10.

[0094] Indeed, we can approximate the probability that the offset X at time T (radar timestamp) is greater than our threshold 500 ms according to a Gaussian distribution:

[0095] P(X > 500ms) = 1 - F ((500 - u) / s)

[0096] Where: • u = average of the offsets measured in the range • s = standard deviation of the offsets measured in the range • F = distribution function of the standard normal distribution

[0097] In the case where at least one first value (time shift) is greater than the threshold of 500 ms, we can consider that the average is less than or equal to this max, and therefore in a worst-case scenario, the average is greater than 500, the standard deviation tends towards zero, and therefore the probability tends towards 1. We can therefore legitimately consider that the use of the extreme value makes it possible to identify the case where there is a non-zero probability that the offset of the timestamp T exceeds the first predefined threshold.

[0098] If none of the first values ​​is greater than 500 ms, then the MAX is less than 500 ms, and the probability that the acquisition of the image occurred at a time offset by more than 500 ms relative to the uploaded image timestamp becomes negligible.

[0099] Consistency test 2: taking into account a first synchronization parameter and a second synchronization parameter

[0100] In a second embodiment illustrated in [Fig.5b], the consistency test uses not only the first synchronization parameter discussed previously, but also a second synchronization parameter different from the first synchronization parameter.

[0101] Each synchronization data set Si, associated with the synchronization timestamp Ti, thus comprises: • A value relating to the first synchronization parameter, and • A value relating to the second synchronization parameter.

[0102] Thus, the synchronization data that the control server 6 obtained in step 602 (which were previously received by the road control radar 1 during the predefined time interval [Ta, Tb] including the image timestamp that is the subject of the consistency test) comprise a sequence of data sets Si, in which the data sets comprise: • The first respective values ​​relating to the first synchronization parameter, • Second respective values ​​relating to the second synchronization parameter.

[0103] Preferably, the data sets Si comprise at least one set received by the road control radar 1 before the acquisition of the image, and at least one other set received by the road control radar 1 after the acquisition of the image.

[0104] Steps 700 and 702 of the first embodiment are also implemented in the second embodiment, in connection with the first synchronization parameter.

[0105] Furthermore, in a step 701, the processor selects a second extreme value in the sequence of second values. The extreme value is a maximum value in absolute value. This means that when the second values ​​are signed, then the extreme value is the maximum of the respective absolute values ​​of the second values.

[0106] In a step 703, the processor compares the second extreme value with a second predefined threshold.

[0107] Preferably, the second predefined threshold is selectively compared with the second extreme value. This means that all other second values ​​in the sequence are not compared with the second threshold. This saves computational resources.

[0108] Ultimately, steps 701 and 703 are steps similar to steps 700 and 702, except that they concern the second synchronization parameter.

[0109] In a step 705, the processor generates the test result based on the comparisons made in steps 702 and 703, as follows: • The test result indicates that the image timestamp is invalid when at least one of the following two conditions is met: • The first value is greater than the first predefined threshold, or • The second value is greater than the second predefined threshold. • The test result indicates that the image timestamp is valid when both of the following conditions are met: • The first value is not greater than the first predefined threshold, and • The second value is not greater than the second predefined threshold.

[0110] For example, the first synchronization parameter is the time offset described above, and the second synchronization parameter is the jitter also described above. In this case: • The sequence of first values ​​is a sequence of time shifts, and the first predefined threshold is a time shift threshold selectively compared to a maximum time shift in the time interval [Ta, Tb], and • The sequence of second values ​​is a sequence of jitters, and the second predefined threshold is a jitter threshold selectively compared to a maximum jitter in the time interval [Ta, Tb].

[0111] The logic used to conclude that the image timestamp is invalid in this second embodiment can be expressed as follows:

[0112] max(abs(OFFSET)) > offset threshold or max(abs(JITTER)) > jitter threshold

[0113] Consistency test 3: taking into account a first synchronization parameter and using a second different synchronization parameter for upstream filtering

[0114] In a third embodiment illustrated in [Fig.5c], the consistency test uses a first synchronization parameter and a different second synchronization parameter.

[0115] As in the second embodiment, the synchronization data that the control server 6 obtained in step 602 (which were previously received by the road control radar 1 during the predefined time interval [Ta, Tb] including the image timestamp subject to the consistency test) comprise a sequence of data sets Si, in which the data sets comprise: • The first respective values ​​relating to the first synchronization parameter, • Second respective values ​​relating to the second synchronization parameter.

[0116] However, we will see that the second synchronization parameter plays a role which is not symmetrical to that of the first synchronization parameter, as was the case in the second embodiment.

[0117] In a step 800, the processor 64 preselects, in the sequence of data sets, data sets whose respective second values ​​are not greater in absolute value than a second predefined threshold. The result of this preselection is therefore a subset of the input sequence (certain data sets have been eliminated).

[0118] In a step 802, the processor 64 selects an extreme value from among the respective first values ​​of the preselected data sets. This selection step 802 is similar to step 700, except that this selection 802 takes as input the result of the preselection 800.

[0119] In a step 804, the processor 64 compares the extreme value with a first predefined threshold.

[0120] In a step 806, the processor generates the test result based on this comparison: • The test result indicates that the image timestamp is invalid when the extreme value is greater than the first predefined threshold. • The test result indicates that the timestamp is valid when the extreme value is not greater than the first predefined threshold.

[0121] Preferably, the first predefined threshold is selectively compared with the extreme value. This means that all other first values ​​of the sequence are not compared with the first threshold. This saves computational resources.

[0122] For example, the first synchronization parameter is the time offset described above, and the second synchronization parameter is the jitter also described above. In this case: • The sequence of first values ​​is a sequence of time shifts, and the first predefined threshold is a time shift threshold selectively compared to a maximum time shift in the time interval [Ta, Tb], and • The sequence of second values ​​is a sequence of jitters, and the second predefined threshold is a jitter threshold selectively compared to a maximum jitter in the time interval [Ta, Tb].

[0123] To illustrate this third embodiment, let us take an example in which: • The time offset threshold is set to 500 milliseconds (we do not want the radar clock to be more than 500 ms ahead or behind the time server clock). • The duration of the time interval [Ta, Tb] is equal to 10 min, and the image timestamp Ti to be tested is centered on this interval. The interval is then [Ti - 5 min, Ti + 5 min]. • The jitter threshold is 1 millisecond.

[0124] The algorithm implemented then consists of filtering in this interval the measurements with a "jitter" in absolute value greater than 1 millisecond. For the remaining values, that is to say those having been preselected, the processor checks whether the following condition is respected:

[0125] max(abs(OFFSET)) > offset threshold

[0126] Ultimately, the third embodiment can be seen as an extension of the first embodiment, incorporating an additional preselection step for filtering first (time shift) values ​​associated with aberrant second (jitter) values.

[0127] Exploiting image timestamps to estimate vehicle speeds

[0128] Returning to [Fig.4], the control server 6 can also implement the following steps, after having obtained synchronization data from the first road control radar 1 and the second road control radar 2.

[0129] In a step 608, the processor 64 detects that the following conditions are met: • An image timestamp provided by the first road control radar 1 shows a vehicle registration plate, • Another image timestamp, provided by the second road control radar 2 shows the license plate of the same vehicle.

[0130] In a step 610, the processor 64 estimates a speed of the vehicle from the following data: • The distance between the first road control radar 1 and the second road control radar 2, which is known in advance, • The duration between the image timestamp (provided by the first road control radar 1) and the other image timestamp (provided by the second road control radar 2).

[0131] Step 610 may be implemented whenever the conditions of step 608 are met. Alternatively, step 610 may be implemented only if the image timestamp and the other image timestamp have been declared valid during respective implementations of the consistency test 604, or if these timestamps have been corrected during step 606 (if this is implemented by the control server 6). In other words, the processor 64 does not estimate any speed from an image timestamp declared as invalid during the consistency test 604. Other variants of implementation

[0132] In the embodiments discussed previously, the consistency test applied to image timestamps emanating from the road control radar 1 is carried out by a control server 6 constituting an entity remote from the first road control radar 1. As a variant, this consistency test is implemented by the processor 16 of the first road control radar 1.

[0133] In the embodiments discussed above, the control server 6 may have the function of estimating vehicle speeds. This is not mandatory. The consistency test of the timestamps and the subsequent steps of estimating vehicle speeds may be implemented by different entities, for example two separate servers.

[0134] In the embodiments discussed previously, it has been assumed that the predefined time interval is defined by its duration. This is not mandatory. Alternatively, the predefined time interval could be defined by its size, expressed as a number of timestamps. For example, the time interval may be chosen to be an interval that allows for searching K synchronization timestamps prior to the image timestamp to be tested, and K synchronization timestamps subsequent to the image timestamp to be tested. Thus, the total number of synchronization timestamps taken into account is equal to 2K. When an extremum is selected, this extremum is thus selected from a set of 2K values ​​(possibly a subset in the third embodiment incorporating a preselection).

[0135] In the embodiments discussed previously, it has been assumed that the image timestamps emanating from the two road control radars 1, 2 are tested, or even corrected. However, it is sufficient to carry out these steps on one of the two road control radars to improve the situation described in the introductory part.

[0136] In the embodiments discussed previously, the thresholds used (first threshold or second threshold) are compared with values ​​that are part of the time synchronization data provided by one of the road radars. It has also been envisaged to correct the timestamps of images that successfully pass the consistency test, therefore after this consistency test, by adding a corrective term to them. However, another strategy may consist of correcting the values ​​intended to be compared with one or other of the thresholds using the time synchronization data, by subtracting this corrective term from them.

[0137] For example, in a variant of the consistency test 1, the extreme value in the sequence of first values ​​is replaced by a corrected version of this extreme value. This correction may consist of subtracting from the extreme value the average of the time offsets between the clock of the road control radar and the time server in the time interval considered. Thus, the value to be compared to the threshold changes from the form “MAX(abs(OFFSET))” to the form “MAX(abs(OFFSET - average(offsets)))”. Another more advanced correction can also subtract the minimum jitter over the time interval. The compared value is then of the form “MAX(abs(OFFSET - average(offset) - MIN(abs(JITTER))).

[0138] Of course, consistency test 2 and consistency test 3 can be subject to similar variants with correction before comparison with a threshold.

Claims

Claims

1. A computer-implemented method comprising steps of: • Obtaining (600) an image timestamp indicating a time of acquisition, by a road control radar, of an image showing a license plate of a vehicle, the image timestamp having been indicated by a clock of the road control radar, • Obtaining (602) time synchronization data, said time synchronization data having been received by the road control radar during a predefined time interval including the image timestamp for temporally synchronizing the clock of the road control radar with a time server, • Testing (604) the consistency of the image timestamp with the time synchronization data, the consistency test producing a test result indicating that the image timestamp is valid or invalid.

2. Method according to the preceding claim, in which: • The time synchronization data comprises a first value relating to a first synchronization parameter, • The consistency test comprises a comparison between a value to be tested and a first predefined threshold, and the test result indicates that the timestamp is invalid when the value to be tested is greater than the first predefined threshold, the value to be tested being the first value or a corrected value resulting from a correction of the first value implemented using the time synchronization data.

3. Method according to the preceding claim, in which: • The time synchronization data comprises a sequence of first values ​​relating to the first synchronization parameter, the first values ​​having been sequentially received by the road control radar during the predefined time interval, • The method comprises selecting an extreme value from the sequence of first values, and wherein the first predefined threshold is selectively compared with the extreme value or with a corrected extreme value resulting from a correction of the extreme value implemented using the time synchronization data.

4. Method according to the preceding claim, in which the first values ​​comprise at least one value received by the road control radar before the acquisition of the image, and at least one other value received by the road control radar after the acquisition of the image.

5. A method according to any one of claims 2 to 4, wherein: • The time synchronization data comprises a sequence of data sets, the data sets having been sequentially received by the road control radar during the predefined time interval, wherein the data sets comprise: • Respective first values ​​relating to the first synchronization parameter, • Respective second values ​​relating to a second synchronization parameter different from the first synchronization parameter, • The method comprises steps of: • Preselecting, from the sequence of data sets, data sets whose respective second values ​​are not greater in absolute value than a second predefined threshold, • Selecting an extreme value from among the respective first values ​​of the preselected data sets,wherein the first predefined threshold is selectively compared with the extreme value or with a corrected extreme value resulting from an extreme value correction implemented using the time synchronization data.,

6. A method according to any one of claims 2 to 4, wherein: • The time synchronization data includes a second value relating to a second synchronization parameter different from the first synchronization parameter, • The consistency test includes a comparison between another value to be tested and a second predefined threshold, the other value to be tested being the second value or another corrected value resulting from a correction of the second value using the time synchronization data, • The test result indicates that the timestamp is invalid when at least one of the following two conditions is met: • The value to be tested is greater than the first predefined threshold, and • The other value to be tested is greater than the second predefined threshold.

7. Method according to any one of claims 5 and 6, in which the second synchronization parameter is a time jitter representative of a variation in network latency between the clock of the road control radar and the time server.

8. Method according to any one of claims 2 to 7, in which the first synchronization parameter is a time offset between the clock of the road control radar and the time server.

9. A method according to any one of claims 2 to 8, wherein correcting a value relating to a synchronization parameter using the time synchronization data comprises subtracting from said value an average of time offsets between the clock of the road control radar and the time server.

10. A method according to the preceding claim, wherein correcting the value relating to a synchronization parameter using the time synchronization data comprises subtracting from said value a minimum jitter during the time interval.

11. A method according to any preceding claim, wherein the predefined time interval has a duration greater than a time period used by the road control radar to periodically request time synchronization data from the time server.

12. Method according to any one of the preceding claims, comprising a step of: • Estimating (610) a speed of the vehicle from a predefined distance between the road control radar and another road control radar, and a duration between the image timestamp and another image timestamp, the other image timestamp indicating an instant of acquisition, by the other road control radar, of another image showing the registration plate of the vehicle.

13. A computer program product comprising program code instructions for executing the steps of the method according to one of the preceding claims, when this program is executed by a computer.

14. Non-transitory, computer-readable recording medium on which is recorded a computer program product comprising code instructions for implementing the method according to one of claims 1 to 12 when this program is executed by a computer.

15. System comprising: • A road control radar (1) comprising: • A camera (10) configured to acquire an image showing a license plate of a vehicle, • A clock (14) configured to produce an image timestamp indicating an instant of acquisition, by the road control radar, of the image, • A communication interface (12) for receiving time synchronization data during a predefined time interval including the image timestamp, the time synchronization data being provided by a time server, and adapted to temporally synchronize the clock of the road control radar with the time server, • A processor (64) configured to implement a consistency test of the image timestamp with the time synchronization data, the consistency test producing a test result indicating whether the image timestamp is valid or invalid.

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