The present invention generally relates to the positioning of a vehicle in its environment.
It more particularly relates to a method for correcting a previously estimated position of a mobile vehicle. It also relates to a vehicle fitted with a RADAR system and a computer adapted to implement such a correction method.
The invention finds a particularly advantageous application in the resetting of an inertial unit installed on a ship.
Nowadays, great size ships are fitted with satellite geolocation systems of the GPS or Galileo type, enabling them to accurately determine their location on the globe.
However, these geolocation systems suffer from three well-known limits.
The first limit is that they can be intentionally jammed by the system provider, in particular in conflict zones. It is therefore no longer possible to enter such zones without risk.
The second limit is that these systems can be deluded, which is liable to lead to the ship sinking.
The third limit is that the ships may sometimes be located in interfering environments, such as fjords, which do not allow them to receive the data transmitted by the satellites.
It is hence known to fit a ship with an inertial unit, in addition to this navigation system. Due to its intrinsic principle of operation, such an inertial unit can indeed neither be jammed, nor deluded, nor hampered by the environment.
Such an inertial unit is able to determine the ship acceleration in the three dimensions of space and to deduce its speed therefrom. Consequently, knowing the starting position of the ship and the initial speed thereof, it is possible to determine, using only the inertial unit, the position of the ship at any time instant.
The major drawback of such an inertial unit is that the results it provides always exhibit a temporal drift. Hence, if the calculation of the ship position initially suffers from very limited errors, these errors accumulate over time, which finally lead to aberrant results.
To minimize this drift, it is then known to regularly reset the inertial unit using the data obtained from the geolocation system. But, here again, this resetting is effective only if these data are available and are neither jammed, nor deluded.
In order to remedy this drawback, the present invention proposes to reset the inertial unit no longer using a position provided by a geolocation system, but rather using images provided by a RADAR system installed on the vehicle.
More particularly, it is proposed according to the invention a method for correcting the position of a vehicle, comprising:
Hence, thanks to the invention, the real RADAR image (received from the RADAR system) is compared with the simulated RADAR image (that which should be received from the RADAR system if the estimated position of the vehicle was exact). The comparison of these two images makes it possible to determine if the estimated position is exact and, if not, what is the exact position of the vehicle.
Other advantageous and non-limitative features of the resetting method according to the invention are the following:
The invention also relates to a vehicle fitted with a RADAR system and a computer adapted to implement a correction method as mentioned hereinabove.
The following description in relation with the appended drawings, given by way of non-limitative examples, will allow a good understanding of what the invention consists of and of how it can be implemented.
In the appended drawings:
In
This ship 1 conventionally comprises a hull 16 topped with a bridge 17.
The ship 10 moreover comprises, above the bridge 17, a RADAR system 11.
This RADAR system 11 is more specifically designed to use the electromagnetic waves in order to detect the presence of coasts located near the ship 10 and in order to determine their positions.
It is herein a standard maritime navigation RADAR system, using the “X-band” frequency band.
As an alternative, an “S-band” RADAR system could be used, but the accuracy obtained within the framework of the method described hereinafter would be poorer. Higher security margins should then be used to pilot the ship 10.
The ship 100 moreover comprises an inertial unit 13. This inertial unit 13 is herein of the FOG, i.e. fibre-optic type. It can for example be an inertial unit such as those marketed by iXBlue company, for example in one of the following ranges: QUADRANS, OCTANS, PHINS, MARINS.
This inertial unit 13 is able to determine the acceleration undergone at each time instant by the ship 10 in the three directions of space. It moreover includes an electronic and/or computer unit adapted to deduce from this acceleration the speed of the ship 10.
Knowing the exact position, the exact direction and the exact speed of the ship 10 at a given time instant, the electronic and/or computer unit of the inertial unit 13 is also able to deduce therefrom an estimated position and an estimated direction of the ship 10 at any time instant.
The estimation of this position and this direction however tends to derive over time, due to initially very limited errors that accumulate over time.
It is herein considered that the inertial unit is of the gyrocompass type, that is to say it is able to detect the geographic North. That way, the inertial unit 13 will be able to correct the estimated direction of the ship 10.
To correct the estimated position of the ship 10 (it is herein talked about the inertial unit 13), the ship 10 comprises a computer 12 connected to the RADAR system 11 and to the inertial unit 13.
This computer 12 is designed to collect the real RADAR images obtained by the RADAR system 11 as well as the estimated position of the ship 10 obtained by the inertial unit 13.
It includes at least one processor (CPU), at least one memory and different input and output interfaces allowing it to communicate with the RADAR system 11 and with the inertial unit 13.
It will be noted that if the computer 12 is herein distinct from the inertial unit 13, as an alternative, it can be integrated to the latter.
Thanks to its memory, the computer memorizes data used within the framework of the estimated position correction method described hereinafter.
It memorizes in particular a cartographic model of the ship 10 environment. In practice, this cartographic model is a digital terrain model 14, for example of the SRTM (“Shuttle Radar Topography Mission”) type. Such a digital terrain model 14 is consisted of topographic matrix and vector files, which are provided by American agencies and which give the altitude of the relief over a major part of the globe.
The computer 12 also memorizes a computer application consisted of computer programs comprising instructions whose execution by the processor allows the implementation by the computer of the method for correcting the estimated position of the ship 10.
According to the invention, this correction method comprises five main steps, i.e.:
This method is herein implemented automatically, i.e. without express instruction from an individual, from the moment that the coast is seen on the real RADAR image 100.
During the first, reception step, the RADAR system 11 elaborates a RADAR image representing the position of the different obstacles detected around the ship 10, then it transmits this image to the computer 12. This step is performed in a conventional manner.
The so-acquired image, hereinafter called the real RADAR image 100, is hence a raw image that is not reprocessed by the computer 12.
An example of real RADAR image 100 is shown in
During the second, acquisition step, the inertial unit 13 calculates an estimation of the ship 10 position, then transmits this estimated position to the computer 12.
Herein, the inertial unit 13 also transmits to the computer 12 other pieces of information, among which:
The estimated position P2 is herein expressed by a latitude and a longitude, in degrees, minutes and seconds.
As shown in
In practice, the maximum drift of the inertial unit 13 is known. In other words, if the exact position P1 of the ship 10 is unknown, the inertial unit 13 is able to know at any time instant in which area the ship 10 is located. In
Herein, the computer 12 will then discretize this area into a plurality of possible positions P3 to which the ship 10 can be.
For that purpose, the computer 12 “meshes” the disk 101 and defines a possible position P3 at the crossing of each mesh. The mesh fineness (i.e. the distance between the possible positions) is herein predefined, as a function of the desired accuracy. It can for example be comprised between 1 and 100 metres.
As an alternative, it can be provided that this mesh fineness is variable, as a function for example of the distance separating the ship 10 from the coast.
As another alternative, the computer could define differently the possible positions, for example using a Point Mass Filter or PMF, the supply of which will be explained at the end of this disclosure.
During the third, elaboration step, the computer 12 will generate simulated RADAR images 200.
A simulated RADAR image 200 will be elaborated for each possible position P3 of the ship 10, in order to represent the RADAR image that could be received from the RADAR system 10 if the ship 10 was exactly at the considered possible position P3.
Each simulated RADAR image 200 is elaborated as a function at least of the possible position P3 that is associated thereto and as a function of the data provided by the digital terrain model 14.
Here, each simulated RADAR image 200 is elaborated also as a function of the roll angle and the pitch angle of the ship 10.
More precisely, to elaborate a simulated RADAR image 200, the computer 12 begins by finding in the digital terrain model 14 the position of the coasts located in a circle that is centred on the considered possible position P3 of the ship 10 and whose radius corresponds to the effective range of the RADAR system 11.
It can hence plot a RADAR image sketch.
It can hence correct this sketch by taking into account the roll angle and the pitch angle of the ship 10. Indeed, the roll and pitch modify the direction of the RADAR system 11 with respect to the sea, which affects the real RADAR images that this system acquires. This correction hence aims to affect in the same way the RADAR image sketch.
Once the sketch corrected, the computer 12 hence obtains a simulated RADAR image 200.
Such an image is shown in
As an alternative, this simulated RADAR image 200 could also be corrected, to take into account the state of the sea or the weather, which are also liable to affect the real RADAR images 100 that the RADAR system 11 acquires.
It will be noted herein that the exact calculations for elaborating the simulated RADAR images 200 won't be described herein, because they vary as a function of many factors such as the chosen type of RADAR system 11, the position of the RADAR system 11 on the ship 10 . . . . An algorithm for generating the simulated RADAR images 200 will hence have to be developed for each type of ship, after a test survey in real conditions making it possible to collect the required data for developing this algorithm.
These real condition tests will further make it possible to determine an error model linked to the inaccuracy of the algorithm for generating the simulated RADAR image 200. This error model will make it possible to calculate, at the generation of each simulated RADAR image 200, an index of confidence in the image correctness.
This confidence index can for example be function of the state of the sea and/or the weather and/or the quantity of coasts detected by the RADAR system 11. The usefulness of this confidence index will be described in detail hereinafter.
To sum up, during this third step, the computer 12 will calculate, for each possible position P3 of the ship 10, a simulated RADAR image 200 associated with a confidence index.
During the fourth, comparison step, the computer compares each simulated RADAR image 200 (associated with each possible position P3) with the real RADAR image 100.
This comparison step is performed by means of correlation calculations. These calculations aim to determine to what extent the simulated 200 and real 100 RADAR images overlay each other.
For that purpose, the computer firstly considers a first simulated RADAR image 200.
It then calculates the level of correlation between the real RADAR image 100 and this first simulated RADAR image 200.
Then it repeats these operations with each of the other simulated RADAR images 200.
It then selects the simulated RADAR image 200 for which the level of correlation with the real RADAR image 100 is the highest. This simulated RADAR image 200 will be called hereinafter “selected RADAR image”.
At this stage, the computer can hence estimate that the possible position P3 associated with the selected RADAR image is the possible position P3 that is the closest to the exact position P1 of the ship 10.
Preferentially, the computer 12 will refine the accuracy of the search for the exact position P1 of the ship 10. For that purpose, the computer 12 calculates several levels of correlation between the real RADAR image 100 and the selected RADAR image, by varying each time one at least of the following parameters:
This triplet of parameters indeed makes it possible to play on how the two images overlay each other before being correlated.
Selecting the triplet of parameters for which the level of correlation is the highest hence makes it possible to find the position in which the two images exhibit the best overlaying.
During the fifth, correction step, the computer 12 then corrects the position of the ship 10, by now considering that the exact position P1 of the ship 10 is the possible position P3 associated with the selected RADAR image, corrected as a function of the selected triplet of parameters.
Then, the computer 12 transmits to the inertial unit 13 a signal making it possible to reset the latter.
This signal comprises at least the corrected position of the ship and the confidence index associated with the selected possible position P3.
These two data are then stored in the inertial unit 13 so that the latter can subsequently determine new estimates of its position.
It will be noted herein that the inertial unit 13 uses, to determine at each time instant a new estimate of the ship 10 position, a calculation method based on a state observer. This state observer is herein a Kalman filter, which can then be reset by the new position that is obtained and which takes into account the received confidence index.
The present invention is not limited in any way to the embodiment that has been described and represented, but the person skilled in the art will be able to apply thereto any variant in accordance with the invention.
Hence, in the embodiment described hereinabove, only the selected possible position P3 and the associated triplet of parameters are used to correct the position of the ship that had been estimated by the inertial unit 13.
As an alternative, the computer 12 could keep in memory other possible positions P3, in particular those which are associated with high levels of correlation, so as to supply the above-mentioned PMF filter with news probabilities of possible positions P3 (the weight of these probabilities being formed by the corresponding level of correlation). Hence, this PMF filter would make it possible to provide at each time instant new possible positions P3 of the ship 10, these positions delimiting a pool of uncertainty in which is located the real position of the ship 10.
According to another alternative of the invention, the errors of the inertial unit and of the RADAR system could be taken into account in order to correct the estimated position of the vehicle. By way of example, in the case where a PMF filter is used, the size of the pool of uncertainty will vary as a function of:
Number | Date | Country | Kind |
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1859750 | Oct 2018 | FR | national |
Filing Document | Filing Date | Country | Kind |
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PCT/FR2019/052483 | 10/18/2019 | WO | 00 |