Positioning system

Abstract

The invention relates to a positioning system (200) comprising a hybridized inertial unit. The hybridized inertial unit comprises an inertial measurement unit (202), referred to hereinafter as an IMU, a star tracker (203) allowing stars to be observed both during the day and at night, and a computer for calculating a position by hybridizing the measurements from the IMU and from the star tracker. The star tracker (203) comprises an optical objective associated with an active-pixel image sensor, making it possible to image a field of view having a half-field θs of between 2° and 10°; and a high-pass spectral filter with a cutoff wavelength of between 600 nm and 850 nm. The relative position of the star tracker (203) and of the IMU (202) is fixed. The star tracker and the IMU are mounted on an orientable mount (201) suitable for pointing the star tracker (203) in various directions with a pointing precision of between 0.1° and θs.

Description

Positioning system

[0001] The invention relates to a positioning system. This system can be fitted, in particular, to a mobile device also called a "carrier device" or simply a "carrier," such as an airplane, a boat, etc. This system allows the carrier to be positioned on Earth. Background

[0002] Satellite positioning systems (e.g., GPS, Galileo, Beidou, etc.) determine the position of a device equipped with a signal receiver, based on signals emitted by a constellation of satellites. These systems are accurate but vulnerable: satellite signals can be jammed (signal unavailability) or spoofed (reception of an altered signal, potentially leading the device to an incorrect position). Therefore, some devices whose navigation cannot rely solely on a satellite positioning system, which is insufficiently reliable, are equipped with an alternative positioning system such as an inertial navigation system (IMS).

[0003] An inertial measurement unit (IMU) is a positioning system capable of detecting the movements of a platform and deducing its position. The platform is equipped with inertial sensors. An IMU typically consists of an inertial measurement unit (IMU) and a computer. The IMU includes linear and angular inertial sensors. Linear inertial sensors, typically accelerometers, measure a specific force resulting from the platform's movements (accelerations) and Earth's gravity, relative to an inertial frame of reference. Angular inertial sensors, typically gyroscopes, measure the platform's angular velocity relative to the inertial frame of reference. After being initialized to a known position (e.g., the platform's starting point), the computer integrates the measurements from the inertial sensors to calculate, by propagation, the platform's position relative to this known position.Calculating a position from inertial measurements is extensively described in the state of the art and will not be detailed here.

[0004] The accuracy of the position calculated by an inertial measurement unit (IMU) is inherently limited by the performance of the inertial sensors and the integrative nature of the calculations. In practice, noise and biases in the inertial measurements cause the calculated position to drift from the actual position of the vehicle. This drift, which is unbounded, increases over time.

[0005] To limit this drift, high-performance inertial sensors can be used, but these sensors are expensive, or even unavailable depending on the desired level of accuracy and the mission duration. A more accessible solution is to correct the various errors in inertial measurements using external data (e.g., external measurements of velocity, orientation, and position) from at least one measurement system external to the IMU. This is referred to as the fusion or hybridization of external data and inertial measurements, and, by extension, a hybridized inertial measurement unit. The external measurement system could be a satellite positioning system, a star tracker, etc.

[0006] A star tracker comprises an optical lens and an image sensor positioned at the focal plane of the optical lens. The image sensor is connected to an electronic processing unit programmed to recognize celestial objects, such as stars or other celestial bodies, in the images provided by the image sensor and to deduce the orientation of the star tracker relative to these objects. Typically, the electronic processing unit is configured to detect and calculate the barycentric coordinates of the celestial objects imaged on the image sensor, to deduce the direction of incidence of these objects as they enter the viewer, and then to calculate, from these directions and an ephemeris, the orientation of the viewer relative to the celestial sphere.

[0007] French patent document FR 3082614 describes a vessel equipped with a navigation system that calculates a trajectory based on position information provided by a satellite positioning system and / or a UMI (Unified Navigation Instrument) integrated into the navigation system. A star tracker is used to periodically verify, and if necessary correct, the calculated trajectory to compensate for drift or a failure of the navigation system. The navigation system is mounted inside the vessel, while the star tracker is located outside. The star tracker can be fixedly mounted on the vessel or, conversely, be movable so that it can be oriented towards a predetermined area of ​​the sky. In the case of a movable star tracker, the orientation mechanism must be very precise because it is necessary to know the orientation of the tracker relative to the navigation system with high accuracy.Such a requirement for precision translates, in practice, into high manufacturing costs and / or reliability problems.

[0008] There is a need for a reliable and accurate positioning system that can operate day and night, and has a limited manufacturing cost. General presentation

[0009] A positioning system according to the invention comprises a hybridized inertial measurement unit (IMU). The hybridized IMU comprises: an IMU including angular and linear inertial sensors, a star tracker enabling the observation of celestial bodies day and night, and a computer enabling the calculation of a position by hybridizing the measurements from the IMU (inertial measurements) and the star tracker (stellar measurements). The star tracker comprises an optical lens associated with an active pixel image sensor. The optical lens and the image sensor allow imaging of a field of view whose half-field θs is such that 2° ≤ θs ≤ 10°. The star tracker also includes a high-pass spectral filter that blocks light transmission for wavelengths less than or equal to a cutoff wavelength λc such that 600 nm ≤ λc ≤ 850 nm. The star tracker is rigidly coupled to the UMI so that the relative orientation of the tracker and the UMI is fixed.The star tracker and the UMI are mounted on a swivel mount suitable for pointing the star tracker in various directions with a pointing accuracy between 0.1° and θs.

[0010] The gain in accuracy for position calculation lies primarily in the hybridization of stellar and angular inertial measurements. Stellar measurements, produced at low frequencies but with very little drift, filter out the noise and biases of angular inertial measurements produced at high frequencies. This allows for a precise measurement of angular orientation which, when combined with the local vertical measurement provided by the UMI, enables the accurate calculation of the carrier's position.

[0011] The slewing mount allows the star tracker to be pointed in various directions to ensure the availability of stellar measurements. The pointing direction of the star tracker (and, by extension, the pointing direction of the mount carrying the tracker) is defined as the direction associated with the center of the tracker's field of view, which typically corresponds to a direction collinear with the optical axis of the tracker's objective lens passing through the pixel at the center of the image sensor. In particular, the slewing mount is adapted to orient the pointing direction of the star tracker within an observation cone with a half-angle at its apex θu between 10° and 90°. The area of ​​the observation cone therefore corresponds to the extreme positions of the pointing direction.The availability of stellar measurements, including in cases where the level of spurious signal on the image sensor is very high (for example, when the carrier is located at sea level), allows for effective filtering of noise and bias from inertial measurements.

[0012] The pointing accuracy of the mount corresponds to the angular offset between the pointing direction commanded to the slewing mount on which the star tracker is mounted and the point in the sky at the center of the star tracker's field of view. The smaller this offset, the more precise the pointing. Since both the UMI (Unified Meteorological Instrument) and the star tracker are supported by the slewing mount, and the orientation of the star tracker relative to the UMI is fixed and defined by design, the orientation of the star tracker relative to the UMI is independent of the mount's orientation. Therefore, the pointing accuracy of the mount does not factor into the overall accuracy of the position measurement. This is a significant difference compared to prior art solutions where only the star tracker is mounted on a slewing mount (as in the aforementioned document FR 3082614), with the UMI attached to the carrier using a conventional, non-slewing "strap-down" mount.In these prior art solutions, the orientation of the stellar finder relative to the UMI is dependent on the orientation of the mount. The steerable mount of the stellar finder then requires a high pointing accuracy, strictly less than 0.1° (typically, less than a few arcseconds), because the relative orientation of the stellar finder and the UMI must be determined precisely; an error of one arcsecond in the relative orientation induces an error of approximately 30 m in the estimated position. In contrast, the steerable mount according to the invention does not require such pointing accuracy. A relatively low pointing accuracy, i.e., greater than or equal to 0.1°, is sufficient to properly point at celestial objects of interest and to ensure the correct functioning of the system. This technologically undemanding pointing accuracy helps to limit the manufacturing cost of the steerable mount and improve its reliability.

[0013] However, the pointing accuracy cannot exceed the half-field θs of the star tracker to ensure proper system operation. Typically, the pointing accuracy is chosen to be less than or equal to 2°.

[0014] The field of view of a star tracker, more precisely of the assembly formed by the optical lens and the image sensor, is defined as the solid angle Ωs within which the image sensor is exposed to the electromagnetic radiation from the external scene collected by the lens. The half-field θs is defined as Ωs = 2π(1 - cosθs). To detect stars during the day, the half-field θs must be sufficiently narrow to minimize the level of spurious signal on the image sensor and thus make the star tracker more tolerant of the higher sky luminance levels in the visible band. This tolerance allows the use of an active pixel image sensor, in particular a complementary symmetry metal-oxide-semiconductor sensor, known as a "CMOS sensor," which benefits from mature technology, lower cost, and does not require an active cooling system.This limitation of the half-field θs is compensated for by the presence of the steerable mount, which allows the star tracker to be pointed in various directions and thus cover the entire (or at least a large part) of the celestial sphere. However, as mentioned above, the pointing accuracy of the steerable mount is intentionally limited. The half-field θs therefore cannot be too limited, as it must compensate for the uncertainty associated with the mount's limited pointing accuracy. Furthermore, in a navigation system, the half-field θs must compensate for uncertainties in the carrier's position, which is initially used by the positioning system to orient the star tracker. Thus, the value of the half-field θs is a compromise, and the star tracker's half-field θs can range from 2° to 10°. To further improve performance during daylight detection, the half-field θs can range from 2° to 7°.

[0015] Furthermore, some applications require minimizing the size of the star tracker, in which case the half-field θs can be greater than or equal to 4°. Thus, the half-field θs can be between 4° and 10°, in particular between 4° and 7°.

[0016] To minimize the level of unwanted signal on the image sensor, particularly the CMOS sensor, the star tracker captures images in the spectral band "I" (wavelengths between 700 nm and 1100 nm). This capability is ensured by the high-pass spectral filter, which blocks light transmission for wavelengths less than or equal to a cutoff wavelength λc between 600 nm and 850 nm. This allows the star tracker to filter out the unwanted signal associated with the lowest wavelengths, for which the sky's luminance is highest. It should be noted that the spectral filter is not a bandpass filter with a defined bandwidth for observing a particular celestial object. It is a high-pass filter whose cutoff wavelength is chosen to minimize the level of unwanted signal on the image sensor.

[0017] The positioning system according to the invention has numerous possible applications. In one such application, the positioning system is integrated into a navigation system. Thus, the invention also relates to a navigation system comprising a positioning system according to the invention. The positioning system is connected to an electronic navigation unit programmed to calculate a trajectory. The position calculated by the hybridized inertial measurement unit (IMU), which combines inertial and stellar measurements, is provided to the electronic navigation unit. The position calculated by the hybridized IMU can be used by the electronic navigation unit either to calculate a trajectory or to correct or "recalibrate" a trajectory previously calculated from other position information.In the event of a course correction, additional position information can be provided to the electronic navigation unit by a second positioning system connected to it. This second positioning system could be a satellite positioning system, an inertial measurement unit, etc. Connections to the electronic navigation unit can be wired or wireless.

[0018] The electronic navigation unit, the second positioning system, or another system can initially provide position information to the hybrid inertial measurement unit. This position information, which does not need to be precise, is initially used by the positioning system to orient the star tracker toward a portion of the sky containing celestial objects to be imaged.

[0019] The invention also relates to a mobile device equipped with a positioning system according to the invention. The base of the steerable mount is fixed to the mobile device. The mobile device may incorporate the aforementioned electronic navigation unit. Alternatively, this electronic navigation unit is located remotely from the mobile device. The mobile device may also incorporate the second positioning system mentioned above. The mobile device may be terrestrial, aerial, or maritime. It may be piloted or unpiloted. It may be a vehicle or any other means of transporting passengers, goods, and / or other loads.

[0020] The accompanying drawings are schematic and not necessarily to scale; their primary purpose is to illustrate the principles of the invention. In these drawings, identical elements (or parts of elements) are identified by the same reference numerals from one figure (fig.) to the next. Figure 1 represents an example of a positioning system. Figure 2 represents the positioning system mounted on a carrier. Figure 3 is an exploded view of an example of a star tracker. Figure 4 is a functional representation of an example of a navigation system. Detailed description of examples

[0021] Embodiments of the invention are described in detail below. Some embodiments are described with reference to the examples shown in the accompanying drawings. These embodiments illustrate the features and advantages of the invention. However, it should be noted that the invention is not limited to the embodiments described or the examples shown.

[0022] In general, as illustrated in the figure, the positioning system 200 includes a hybridized inertial unit and a steerable mount 201. The hybridized inertial unit includes an inertial measurement unit (IMU) 202, a star tracker 203 and a computer 204.

[0023] In some embodiments, the positioning system 200 can be part of a navigation system 100 for a mobile device 10. The positioning system 200, in particular the computer 204, is connected by a data link 214 to an electronic navigation unit 110 programmed to calculate a trajectory of the mobile device 10.

[0024] In some embodiments, as illustrated in Figures 1 and 2, the steerable mount 201 comprises a base 220 adapted for mounting on a mobile carriage 10, a plate 222 pivotally mounted on the base 220 with two degrees of rotational freedom, an actuator (not shown) for rotating the plate 222 relative to the base, and an actuator control unit. The actuator control unit may be integrated into the computer 204. The computer 204 may be integrated into the UMI 202 or the star tracker 203, or be separate from the UMI and the star tracker. In the latter case, the computer 204 is not necessarily on the steerable mount 201.

[0025] The star tracker 203 and the UMI 202 are both fixed to the plate 222 such that the star tracker 203 is rigidly coupled to the UMI 202 via the plate 222. Alternatively, the star tracker 203 can be directly coupled to the UMI 202, and either the star tracker or the UMI can be fixed to the plate 222. In all cases, the position and orientation of the star tracker 203 relative to the UMI 202 are fixed (i.e., the relative position and orientation of the star tracker 203 and the UMI 202 are fixed), fixed by construction and independent of the pointing direction of the star tracker 203. The movement of the plate 222 allows the star tracker 203 to be pointed in different directions.In the example shown, the platform 222 is rotatable about two axes, X and Y, so that the star tracker 203 can be pointed in any direction within a hemisphere, approximated as an observation cone, having a semi-vertical angle θu of 90°. In other words, the steerable mount 201 allows the pointing direction of the star tracker 203 to be oriented within a solid angle of 2π steradians. However, the freedom of movement of the platform 222 could be less, and the semi-vertical angle θu could be smaller, without departing from the scope of the invention.

[0026] The UMI 202 includes angular inertial sensors, typically gyroscopes, and linear inertial sensors, typically accelerometers. The UMI 202 produces increments of linear and angular measurements, which it provides to the computer 204 via a data link 212.

[0027] In some embodiments, starting from a rough position measurement 111 initially provided to the positioning system 200 and the inertial measurement increments produced by the UMI 202, the computer 204 evaluates a first rough orientation relative to an inertial reference frame. The computer 204, which has in its memory a catalog grouping the positions of celestial bodies on the celestial sphere in an inertial reference frame, then uses this first rough orientation to point the star tracker 203 towards the celestial bodies of interest, typically chosen for their brightness.

[0028] The star tracker 203 is configured to observe celestial bodies, day and night, and to calculate an orientation between its line of sight 230 and the observed celestial bodies. The star tracker 203 provides this orientation to the computer 204 via a data link 213. In order to operate day and night, the star tracker 203 must be able to detect celestial bodies through the strong signal level emitted by the atmosphere when it is illuminated by the sun. To do this, the star tracker 203 must be designed to maximize the useful signal level from the observed celestial bodies and minimize the level of spurious signal emitted by the atmosphere, which generates detection noise.

[0029] In some embodiments, as illustrated in Figure 1, the star tracker 203 comprises an electronic processing unit 231 connected to an active pixel image sensor 234 located behind an optical lens 232. The image sensor 234 is composed of a plurality of adjacent elementary detectors called pixels (from the English "picture element") forming an array. The image sensor 234 typically has a resolution of several million pixels. Each pixel is formed by a well that accumulates electrical charges depending on the photons it receives. Celestial objects are imaged by optical elements (lenses, mirrors) of the optical lens 232 onto the pixel array of the sensor.

[0030] The electronic processing unit 231 incorporates a processor and memory to execute an image acquisition program (using the image sensor 234) and to calculate an orientation between the line of sight 230 and a visible celestial body. Determining the position of the celestial body on the pixel matrix of the image sensor 234, and thus deducing the orientation of the line of sight 230 of the star tracker 203 relative to the observed celestial body, is a known technique and is not detailed here.

[0031] The image sensor 234 can be a sensor sensitive to light in the wavelength range between 300 nm and 1000 nm, that is, a sensor operating in the visible spectrum (400 to 700 nm) and the first spectral window of the near-infrared (700 to 1000 nm). The image sensor 234 can be a sensor using silicon (Si) photodiodes, such as a complementary symmetry metal-oxide-semiconductor (CMOS) sensor. The optical lens 232 associated with the image sensor 234 provides the star tracker with a field of view such that the half-field θs is between 2° and 10°. This field of view is sufficiently small to minimize the level of spurious signal on the pixel matrix of the image sensor 234. On the other hand, this field of view is large enough to compensate, in particular, for uncertainties on the approximate position 111 of the carrier 10 used initially by the positioning system 200 to orient the star tracker 203.

[0032] The star tracker 203 also includes a high-pass spectral filter 235 that blocks light transmission for wavelengths less than or equal to a cutoff wavelength λc between 600 nm and 850 nm. The spectral filter 235 can be positioned in front of the optical lens 232, between the image sensor 234 and the optical lens 232, or inside the optical lens 232. The optical lens 232 can be a fixed-focal-length or zoom lens. The spectral filter 235 also minimizes the level of spurious signal on the pixel array of the image sensor 234.

[0033] The on-board computer 204 incorporates an algorithm which, from the orientation information provided by the gyroscopes of the UMI 202 and the star tracker 203 and the local vertical information provided by the accelerometers of the UMI 202, calculates a precise position measurement which it provides to the electronic navigation unit 110. The computer also calculates the orientation to be given to the mobile mount 201 and accordingly controls the actuator of the mount 201 via the data link 217.

[0034] The embodiments described above and the examples in the figures are given by way of illustration and not limitation. A person skilled in the art could easily, in light of this description, modify these embodiments or consider others, while remaining within the scope of the invention. In particular, a person skilled in the art could easily consider variations comprising only some of the features of the embodiments described above, if those features alone are sufficient to provide one of the advantages of the invention.

Claims

Positioning system (200) comprising a hybridized inertial measurement unit, the hybridized inertial measurement unit comprising: an inertial measurement unit (202), hereinafter referred to as IMU, comprising angular and linear inertial sensors; a star tracker (203) allowing the observation of stars by day and by night; and a computer (204) allowing the calculation of a position by hybridization of the measurements of the IMU and the star tracker, characterized in that the star tracker (203) comprises: an optical lens associated with an image sensor with active pixels, allowing the imaging of a field of view whose half-field θs is between 2° and 10°;and a high-pass spectral filter blocking light transmission for wavelengths less than or equal to a cutoff wavelength (λc) between 600nm and 850nm, wherein the star tracker (203) is rigidly coupled to the UMI (202) so that the relative orientation of the star tracker (203) and the UMI (202) is fixed, the star tracker and the UMI being mounted on a steerable mount (201) adapted to point the star tracker (203) in various directions with a pointing accuracy between 0.1° and the half-field value θs.; Positioning system according to claim 1, wherein the half-field θs is between 2° and 7°. Positioning system according to claim 1 or 2, wherein a steerable mount (201) adapted to orient the pointing direction (230) of the star tracker within an observation cone having a half-angle at the apex (θu) between 10° and 90°. Positioning system according to any one of claims 1 to 3, wherein the image sensor with active pixels is a light-sensitive sensor from 300 nm to 1000 nm. Positioning system according to claim 4, wherein the active pixel image sensor is a complementary symmetry metal-oxide semiconductor (CMOS) sensor. Positioning system according to any one of claims 1 to 5, wherein the steerable mount (201) comprises a base (220) and a platform (222) pivotally mounted on the base (220), an actuator for driving the platform (222) in rotation relative to the base (220) and an actuator control unit, and wherein the star tracker (203) and the UMI (202) are mounted on the platform (222). Positioning system according to claim 6, wherein the computer (204) of the hybridized inertial unit integrates the actuator control unit and determines the orientation of the pointing direction (230) of the star tracker. Navigation system comprising a positioning system (200) according to any one of claims 1 to 7, connected to an electronic navigation unit (110) programmed to calculate a trajectory, wherein the position calculated by the hybridized inertial measurement unit is provided to the electronic navigation unit (110). Navigation system according to claim 8, comprising at least one second positioning system, in particular a satellite positioning system, connected to the electronic navigation unit (110). Navigation system according to claim 8 or 9, wherein the electronic navigation unit (110) or the second positioning system provides position information initially used by the positioning system (200) to orient the star tracker (203). Mobile device equipped with a positioning system according to any one of claims 1 to 7.

Images