method for verifying timestamps provided by a road traffic control radar

Abstract

A computer-implemented method comprising the following steps: obtaining (600) an image timestamp indicating the time of acquisition, by a traffic control radar, of an image showing a vehicle's license plate, the image timestamp having been provided by a clock on 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 to temporally synchronize the clock of the traffic control radar with a time server; and consistency testing (604) 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. Figure for the abstract: Fig. 4

Description

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

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

[0002] To control that the speed of vehicles travelling 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 traffic control radars acquires an image showing the license plate of a vehicle travelling on the road. Two images are thus acquired successively.

[0004] One way to estimate the speed of the vehicle shown in the two images is to divide the distance between the two road traffic radars, which is known in advance, by the time that separates the moments when the two images were acquired.

[0005] To identify these two moments, road traffic control radars rely on their respective internal clocks, which measure the passage of time.

[0006] However, such internal clocks are not always reliable. Consequently, the acquisition times of the two images may be misjudged by traffic control radars, and this can lead to an inaccurate estimation of the speed of the vehicle depicted 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 considered reliable. A traffic control radar could thus regularly send synchronization requests to the time server, and the time server would return synchronization data to the traffic control radar, allowing the radar to synchronize with the time server.

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

[0009] One purpose of this disclosure is to detect a situation that could lead to an incorrect estimation of speed using a control radar truck driver.

[0010] This goal is achieved by a computer-implemented method comprising the following steps: obtaining an image timestamp indicating an acquisition time, by a road traffic radar, of an image showing a vehicle registration plate, the image timestamp having been indicated by a clock of the road traffic radar; obtaining time synchronization data, said time synchronization data having been received by the road traffic radar during a predefined time interval including the image timestamp to synchronize the clock of the road traffic radar in time with a time server; and consistency testing 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 process constitutes a first subject of this disclosure and may also understand the following optional features, taken alone or in combination whenever it makes technical sense.

[0012] Optionally, the time synchronization data includes a first value relating to a first synchronization parameter, and the consistency test includes 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 includes 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 includes a selection of an extremum value in the sequence of first values, and wherein the first predefined threshold is selectively compared with the extremum value or with a corrected extremum value resulting from a correction of the extremum value implemented using the time synchronization data.

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

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

[0016] Optionally, 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.

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

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

[0019] Optionally, correcting a value relating to a synchronization parameter using time synchronization data includes subtracting from said value an average of time offsets between the road control radar clock 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 period of time used by the road traffic control radar to periodically request time synchronization data from the time server.

[0021] Optionally, the method includes an estimation of a vehicle speed from a predefined distance between the traffic control radar and another traffic control radar, and of a time between the image timestamp and another image timestamp, the other image timestamp indicating a time of acquisition, by the other traffic control radar, of another image showing the vehicle's license plate.

[0022] A second object of this disclosure is a computer program product including program code instructions for executing the steps of the process that constitute the first subject of the disclosure, when that program is executed by a computer.

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

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

[0025] A fourth object of this disclosure is a system comprising: a traffic control radar and a processor. The traffic control radar includes: a camera configured to acquire an image showing a vehicle license plate, a clock configured to produce an image timestamp indicating the time of acquisition of the image by the traffic control radar, and a communication interface for receiving time synchronization data over 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 traffic control radar with the time server.The processor is configured to implement a consistency test of the image timestamp with the time synchronization data; the consistency test produces a test result indicating whether the image timestamp is valid or invalid. DESCRIPTION OF THE FIGURES

[0026] Other features, objectives and advantages of the invention will become apparent from the following description, which is purely illustrative and not limiting, and which should be read in conjunction with the accompanying drawings on which:

[0027] Fig. 1 schematically illustrates a system according to one embodiment.

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

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

[0030] The [Fig.4] is a flowchart of steps of a process implemented by a control server, according to an embodiment.

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

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

[0033] Figure [1] 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 between 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 travelling 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 (e.g. 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 sending, via the network communication interface 12, of synchronization requests to the time server 4, and this on a periodic basis.

[0040] The processor 16 is also configured to synchronize the clock 14 using a synchronization dataset issued by the time server 4 and then received by the network communication interface 12, in response to a synchronization request. Synchronization, known from the prior art, aims to realign 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 dataset includes the parameters listed in the table below. Parameter Definition Delay: The average time between sending a synchronization request and receiving a synchronization data set in response to the request. 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 received synchronization data and images acquired by the camera 10. The memory 18 is specifically configured to store this data and these images by arranging them temporally, 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"), HDD (for "Hard Disk Drive"), SSD (for "Solid-State Drive"), etc. The memory constitutes or includes a non-transient recording medium readable by a 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 prior 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, conform to the NTP (Network Time Protocol).

[0045] Still referring to [Fig.2], the control server 6 includes 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 aforementioned types for the network communication interface 12.

[0047] 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 traffic 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 commands the transmission, via the network communication interface 12, of a synchronization request to the time server 4.

[0052] In a step 102, the processor 16 detects the reception, by the network communication interface 12, of a synchronization data set emanating 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 commands 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 receipt of the synchronization data set (i.e., when step 102 occurred) or the time of synchronization of the clock 14 using the synchronization data. By convention, such a timestamp is referred to hereafter as 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 includes 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 a person 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 time of image acquisition by the camera 10, this timestamp having been indicated by the clock 14. By convention, such a timestamp is referred to in the following "Image timestamp" to differentiate this timestamp from synchronization timestamps. Processor 16 can also send the acquired image to the control server 6 in association with the corresponding image timestamp.

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

[0061] Repeated implementation of transmission steps 106 and 114 leads the traffic control radar 1 to transmit timestamps Tl...TN to the control server 6. The N timestamps form an ordered sequence. By convention, Tl is the oldest timestamp, and TN the most recent timestamp. As mentioned above, each timestamp Ti is either an image timestamp, meaning that this timestamp indicates the time of acquisition of an image by the camera 10 of the road control radar 1, or a synchronization timestamp, meaning that this timestamp is associated with a synchronization dataset 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 image timestamps in this example. The other Ti are synchronization timestamps, and are therefore associated with respective synchronization datasets Si. Tl (frame timestamp) T2 S2 T3 S3 T4 (frame timestamp) T5 S4 T6 S5 T7 S6

[0063] Of course, the transmissions of this 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 preceding method is also implemented by the second road control radar 2. Process implemented by the control server

[0065] We will now describe a process 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 Tl 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 well as 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 of the image by the road traffic control radar 1. This is typically obtained by reading from 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 endpoints 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 traffic control radar 1 during the predefined time interval [Ta, Tb], the processor 64 compares each synchronization timestamp present in memory 62 and originating from the traffic control radar 1 with the predefined time interval. If a synchronization timestamp has a value within [Ta, Tb], then that 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 traffic control radar 1 during the predefined time interval. The consistency test produces a result that indicates either that the image timestamp is valid (in the case where it is considered during the test to be consistent with the synchronization data obtained), or that the image timestamp is invalid (in the case where it is considered during the test to be inconsistent with the synchronization data obtained).

[0072] When the result indicates that the image timestamp is valid, processor 64 can implement a step 606 to correct 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 can 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 line of reasoning can be applied to temporal jitter. A possible correction would then consist of finding the minimum observed value (MIN(abs(JITTER)) within the range and adding it to the image timestamp in question. Indeed, if a "network latency" of x microseconds is observed within the range, the image timestamp can then be estimated to be lagging by this latency. Thus, we 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 traffic 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 modes of implementation 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 whose steps are represented in [Fig.5a], the consistency test uses only a first synchronization parameter.

[0082] Each synchronization dataset Si, associated with a synchronization timestamp Ti, includes (or is made up of) a value relating to that 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 which 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 ​​include at least one value received by the road control radar 1 before image acquisition, and at least one other value received by the road control radar 1 after image acquisition.

[0085] In step 700, processor 64 selects an extreme value from the sequence of initial values. The extreme value is a maximum absolute value. This means that when the initial values ​​are signed, the extreme value is the maximum of the respective absolute values ​​of the initial 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 ​​in the sequence are not compared with the first threshold. Only one comparison is performed at step 702. This saves computational resources.

[0088] In step 704, processor 64 generates the test result based on 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 shifts, and the first predefined threshold is a time shift threshold selectively compared to a maximum time shift in the time interval [Ta, Tb].

[0090] The logic used by 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] By way of illustration of this first embodiment, let us suppose that: • The time offset threshold is set at 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 offsets whose image timestamps fall within this interval are determined. The processor verifies 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 (image timestamp inconsistency). Indeed, this situation means that there is a probability that the image timestamp has a shift of more than 500 ms relative to the time 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 of 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 = cumulative distribution function of the standard normal distribution

[0097] In the case where at least one first value (time offset) 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 extremal value makes it possible to identify the case where there is a non-zero probability that the timestamp offset 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 image acquisition occurred at a time more than 500 ms away from the image timestamp retrieved 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 dataset 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 was previously received by the road traffic control radar 1 during the predefined time interval [Ta, Tb] including the image timestamp being checked for consistency) comprises a sequence of datasets Si, in which the datasets include: • The first respective values ​​relating to the first synchronization parameter, • The respective second values ​​relating to the second synchronization parameter.

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

[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 step 701, the processor selects a second extremal value from the sequence of second values. The extremal value is a maximum absolute value. This means that when the second values ​​are signed, the extremal 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] In conclusion, steps 701 and 703 are similar to steps 700 and 702, except that they relate to the second synchronization parameter.

[0109] In a step 705, the processor generates the test result based on the comparisons performed 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 jigs, and the second predefined threshold is a jig threshold selectively compared to a maximum jig 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 second synchronization parameter that are different.

[0115] As in the second embodiment, the synchronization data that the control server 6 obtained in step 602 (which was previously received by the road traffic control radar 1 during the predefined time interval [Ta, Tb] including the image timestamp being checked for consistency) comprises a sequence of datasets Si, in which the datasets include: • The first respective values ​​relating to the first synchronization parameter, • The respective second values ​​relating to the second synchronization parameter.

[0116] However, we will see that the second synchronization parameter plays a role that 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 pre-selects, from the sequence of datasets, datasets whose respective second values ​​do not exceed, in absolute value, a second predefined threshold. The result of this pre-selection is therefore a subset of the input sequence (some datasets have been eliminated).

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

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

[0120] In 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 extremum value. This means that all other first values ​​in 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 jigs, and the second predefined threshold is a jig threshold selectively compared to a maximum jig 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 at 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 out measurements with an absolute jitter value greater than 1 millisecond within this interval. For the remaining values, i.e., those that have been pre-selected, the processor checks if the following condition is met:

[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 allowing filtering of first (time offset) values ​​associated with aberrant second (jitter) values.

[0127] Exploitation of image timestamps to estimate vehicle speeds

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

[0129] In step 608, processor 64 detects that the following conditions are met: • An image timestamp provided by the first traffic control radar 1 shows a vehicle's license 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 vehicle speed from the The following data: • The distance between the first speed camera 1 and the second speed camera 2, which is known in advance, • The time 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 can be implemented whenever the conditions of step 608 are met. Alternatively, step 610 can 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 step is implemented by the control server 6). In other words, the processor 64 does not estimate any speed from an image timestamp declared invalid during the consistency test 604. Other implementation variations

[0132] In the embodiments discussed above, the consistency test applied to image timestamps from the road control radar 1 is performed by a control server 6 constituting a remote entity from the first road control radar 1. Alternatively, 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 timestamp consistency test and the subsequent vehicle speed estimation steps may be implemented by different entities, for example, two separate servers.

[0134] In the embodiments discussed above, it was 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 could be chosen to be one that allows searching for 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 above, it was assumed that the image timestamps from both traffic control radars 1 and 2 are tested and possibly corrected. However, it is sufficient to perform these steps on just one of the two traffic control radars to improve the situation described in the introductory section.

[0136] In the embodiments discussed above, 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 considered to correct the timestamps of images that successfully pass the consistency test, i.e., after this consistency test, by adding a correction term. However, another strategy may consist of correcting the values ​​intended to be compared with either threshold using the time synchronization data, by subtracting this correction term.

[0137] For example, in a variant of consistency test 1, the extreme value in the sequence of first values ​​is replaced by a corrected version of that extreme value. This correction may consist of subtracting from the extreme value the average of the time offsets between the clock of the traffic 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

Demands

1. 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 vehicle license plate, 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 to time synchronize the clock of the traffic control radar with a time server, • Consistency testing (604) 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. A method according to the preceding claim, wherein: • The time synchronization data includes a first value relating to a first synchronization parameter, • The consistency test includes 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. A method according to the preceding claim, wherein: • 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 traffic control radar during the predefined time interval, • The method includes selecting an extremal value from the sequence of first values, and wherein the first predefined threshold is selectively compared with the extremal value or with a corrected extremal value resulting from a correction of the extremal value implemented using time synchronization data.

4. A method according to the preceding claim, wherein the first values ​​comprise at least one value received by the road control radar before image acquisition, and at least one other value received by the road control radar after image acquisition.

5. A method according to any one of claims 2 to 4, wherein: • The time synchronization data comprises a sequence of datasets, the datasets having been sequentially received by the traffic control radar during the predefined time interval, wherein the datasets comprise: • First respective values ​​relating to the first synchronization parameter, • Second respective values ​​relating to a second synchronization parameter different from the first synchronization parameter, • The method comprises the steps of: • Preselection, in the sequence of datasets, of the datasets whose second respective values ​​do not exceed in absolute value a second predefined threshold, • Selection of an extremity value from among the first respective values ​​of the preselected datasets,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 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. A method according to any one of claims 5 and 6, wherein the second synchronization parameter is a time jitter representative of a network latency variation between the road control radar clock and the time server.

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

9. A method according to any one of claims 2 to 8, wherein the correction of a value relating to a synchronization parameter using 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 the correction of the value relating to a synchronization parameter using time synchronization data comprises subtracting from said value a minimum jitter during the time interval.

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

12. A method according to any one of the preceding claims, comprising a step of: • Estimating (610) a vehicle speed from a predefined distance between the traffic control radar and another traffic control radar, and a time between the image timestamp and another image timestamp, the other image timestamp indicating a time of acquisition, by the other traffic control radar, of another image showing the vehicle's registration plate.

13. Product computer program comprising program code instructions for carrying out the steps of the process according to any one of the preceding claims, when such program is executed by a computer.

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

15. System comprising: • A traffic control radar (1) comprising: • A camera (10) configured to acquire an image showing a vehicle license plate, • A clock (14) configured to produce an image timestamp indicating the time of acquisition of the image by the traffic control radar, • A communication interface (12) for receiving time synchronization data over 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 traffic 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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