DAY AND NIGHT ASTRAL ELECTRONIC SEXTANT WITH INERTIAL PLATFORM

Abstract

The present invention relates to an electronic astral sextant, which can use stars, the sun, the moon, and planets to operate. The sextant is a device designed to measure the height of a star above the horizon. With the help of astronomical tables, the latitude and longitude of the observation location can be deduced.

Description

Title of the invention: DAY AND NIGHT ASTRAL ELECTRONIC SEXTANT WITH INERTIAL PLATFORM Field of invention

[0001] The present invention relates to an electronic astral sextant, which can use stars, the sun, the moon, and planets to operate. The sextant is a device intended to measure the height of a star above the horizon. With the help of astronomical tables, the latitude and longitude of the observation location can be deduced.

[0002] It was designed for marine navigation and can also be used to find one's way on land or in the air. Measuring the height of the sun in the sky at noon indicates, for example, the latitude of the location, provided that the date is known. Thus, on the day of the equinox, at noon, if the sun is on the horizon, one is at the north pole (latitude 90°); if it is at the zenith, one is on the equator (0°); if it is at 30°, one is at (90-30) = 60° north latitude.

[0003] At night, the sextant can be used to measure the angular height in the sky of recognizable stars and then look at astronomical tables to find, there too, the latitude of the place.

[0004] A mechanical sextant is composed of a small telescope, to aim at the horizon, two mirrors (which project the image of the object targeted), possible filters (for the Sun), a movable arm and a graduated arc of a circle. The opening angle is 60°, or one-sixth of a circle, hence the name of the instrument. The sextant is an improvement of the older octant, which opened to 45°, or one-eighth of a circle.

[0005] The development of electronics has led to the replacement of mechanical components with optoelectronic solutions. State of the art

[0006] Patent FR3007128 describes a prior solution of an electronic sextant with a three-axis inertial system and a method for determining the position which makes it possible to calculate its position from a three-dimensional inertial platform and a camera which is pointed at the vault.

[0007] Also known in the state of the art is European patent application EP2472224 disclosing a system and method for determining the position using an electronic device comprising a camera for acquiring a celestial image with at least one celestial object. The device comprises a celestial object indicator for selecting a celestial object. The position of the electronic device is determined by comparing the location of the detected celestial object on the image and angle information calculated at the time of shooting with the contents of a database.

[0008] Patent EP3447447 describes another position detection solution, comprising at least the steps: • capture at least one sunlight parameter with a detection means, • determine the position of the detection means by matching the sunlight parameter with a position table • of the sun, the sun position table including data concerning the position of the sun according to the date and time.

[0009] The detection means comprise a plurality of photoreceiving units each referring to a different cardinal direction, and a main photoelectric barrier unit arranged between the light receiving units to reduce the amount of light reaching another receiving unit depending on an angle of incidence of the sunlight, in particular an angle between the sunlight and the horizontal.

[0010] Also known is the Chinese utility model CN202748024U proposing a semi-automatic sextant allowing the observation angle of a star to be measured in real time.

[0011] US patent application US2006282217 describes a method for determining a terrestrial location of an apparatus that is deployed in a generally known geographic region. An optoelectronic apparatus captures an image of the sky from a terrestrial location at a determined time. The position of the apparatus is then determined by matching the celestial image captured by the apparatus to a usual mapping of the sky from the Earth's surface.

[0012] Japanese patent application JP2006153473 proposes another portable terminal for determining the current position based on a date-time group, and on the azimuth and elevation angle of a celestial body. When this portable terminal is pointed towards the sun, a button for starting the measurement of time, and terrestrial magnetism as well as the date-time group is pressed. A processor calculates the longitude of the measurement location as well as the latitude by using a table showing the relationship between the present time stored in the memory, the altitude of the sun and the latitude of the present point. Disadvantage of prior art solutions

[0013] The solutions of the prior art such as that which is the subject of the present invention aim to calculate the position of a mobile from the observation of stars and the measurement of the local vertical.

[0014] However, with the solutions of the prior art, the calculation is carried out either with approximations which are not compatible with a search for performance in precision, either with portable or steerable systems that require the user to point at a particular direction in the sky.

[0015] The quality of the measurement is therefore highly dependent on the precision of the orientation of the sighting device: in the solutions of the prior art, the user of the sextant will point the instrument towards a star in the celestial vault. Precise knowledge of the site and approximate knowledge of the azimuth makes it possible to recognize the star.

[0016] The recognized star is characterized by a star vector expressed in the Earth's reference frame.

[0017] During image acquisition, the electronic system integrated into the sextant will measure the local vertical of the optical axis. It will thus determine the site of the observed star expressed in the local geometric trihedron (TGL). Solution provided by the invention

[0018] The present invention aims to remedy this drawback by proposing a solution installed at a fixed position on a mobile carrier, and making it possible to carry out measurements day and night, without the need for pointing.

[0019] To this end, the invention relates, in its most general sense, to an electronic sextant comprising a camera dedicated to the acquisition of images of low-luminosity celestial bodies as well as a means delivering a signal representative of the angle formed by the optical axis of a camera relative to the local vertical, in the local terrestrial reference frame, a clock and a camera associated with a calculator controlled by celestial body recognition software to determine the coordinates of the bodies in the inertial reference frame of the J2000 universe, said sextant comprises a memory in which the data are recorded for the determination of a transformation matrix making it possible to recalculate the coordinates of the celestial bodies in the local terrestrial reference frame, characterized in that said sextant further comprises a brightness reduction filter for the daytime acquisition of the position of the sun.

[0020] According to a first variant, said brightness reduction filter is retractable relative to a single camera, for the daytime acquisition of the position of the sun when said filter is placed in the field of said camera, and for the acquisition of low-luminosity celestial bodies when said filter is retracted.

[0021] According to a second variant, the sextant comprises a first camera for acquiring night images and a second camera equipped with said brightness reduction filter for daytime acquisition of the position of the sun.

[0022] According to a first variant, said means delivering a signal representative of the angle formed by the optical axis relative to the local vertical in the local terrestrial reference frame is a three-axis inertial system integrated into said sextant.

[0023] According to a second variant, said means delivering a signal representative of the angle formed by the optical axis relative to the local vertical in the reference frame local terrestrial in the local terrestrial reference frame is constituted by an input of a signal coming from an inertial system of a support vehicle mechanically coupled with said sextant.

[0024] Preferably, said camera(s) have an optical field greater than 90°.

[0025] Advantageously, said software controls the determination of the attitude and position of the user from the same star identified on a plurality of successive images.

[0026] According to a variant, said software controls the determination of the attitude and position of the user from a plurality of different stars identified on an image.

[0027] The invention also relates to a method for calculating the attitude of a sextant mentioned above, characterized in that it comprises a step of comparing the star vectors measured by a camera and said reference star vectors identified in an astronomical catalog or by ephemerides.

[0028] The invention also relates to a method for calculating the position of a sextant having the above-mentioned characteristics, characterized in that it comprises a step of merging the attitude with the local vertical data obtained by the internal or external 3D inertial platform.

[0029] The invention also relates to a computer program comprising program portions for executing the steps of the above-mentioned method.

[0030] Detailed description of a non-limiting example of embodiment

[0031] The present invention will be better understood on reading the following description, concerning a non-limiting example of embodiment illustrated by the appended drawings where:

[0032] [Fig-1] [Fig.l] is a perspective view of a sextant according to the invention. General principle of the invention described

[0033] The invention relates to a sextant whose operating principles are those described in French patent FR3007128, in particular with regard to the astronomical bases, the determination of the star vector expressed in the terrestrial reference frame and the use of this information for determining the position and / or altitude of the sextant.

[0034] The sextant which is the subject of the invention is distinguished from that described in the prior art by the fact that it comprises two cameras (10, 20) with a wide field of view, called "fisheye", with a field angle greater than 90°.

[0035] The first camera (10) is a daytime camera protected by a solar filter (11). Its function is to acquire the position of the sun and optionally of the celestial bodies visible during the day.

[0036] The second camera (20) is a night camera, with greater sensitivity, protected by a night porthole (21) which can be made opaque to avoid exposure to an intensity excessive light. This second camera (20) has the function of acquiring a plurality of celestial bodies at night.

[0037] Optionally, the device may include two motorized shutters to protect the two cameras (10, 20) when they are not in use.

[0038] The optical assembly is fixed on a rigid frame enclosed in a housing (1). The system also comprises a three-axis inertial unit (30).

[0039] This inertial unit (30) can be integrated into the housing (1). It can also be replaced by an input of the signal provided by an inertial unit equipping a support such as a land or sea vehicle on which the housing (1) is rigidly fixed.

[0040] This internal or external inertial unit (30) delivers a signal representative of the angle formed by the optical axes of the two cameras (10, 20) relative to the local vertical, in the local terrestrial reference frame. These signals are transmitted by an electronic circuit (40) to a computer (50) controlled by software, and a clock.

[0041] The software detects the stars, performs the recognition of celestial stars by pattern comparison for the stars or by pre-designation or characterization for the other stars. The system includes a memory in which the position data of the stars, the algorithms and the time information (date - time group) are recorded allowing the software to perform the calculations of passage from an inertial reference frame (J2000 or other) in which the positions of the stars towards the Earth frame are determined.

[0042] Advantageously, the camera is configured to capture a plurality of stars (stars, sun, moon, planets) in the wide optical field of the camera.

[0043] Given that the attitude is defined as the angular position of a moving object in a reference frame, the invention also relates to a method for calculating the attitude of the sextant in the reference frame which may be J2000, the terrestrial frame or the local geographic trihedron, by comparing the measured star vectors and the reference star vectors. The invention also relates to a method for calculating the position of a sextant from the attitude of the camera and the determination of the local vertical by a three-axis inertial system.

[0044] The invention also relates to a computer program for controlling equipment comprising two wide-field cameras, a computer and a three-axis inertial unit (or inclinometer), characterized in that it determines its attitude in a reference frame (J2000, Terrestrial Reference Frame or TGL) and its position by merging the attitude and the local vertical data obtained by the inertial unit.

[0045] The sextant makes it possible to determine the user's attitude and position from:

[0046] - A star seen at several different times (example: sun, moon, star, planet)

[0047] - Multiple stars (depending on whether initial information is available or not) viewed in 1 time by the wide-field optics.

[0048] Alternative embodiment

[0049] The general principle of the sextants according to the present invention is to allow the use of two channels: one during the day and the other at night. The preceding description concerns an embodiment using two cameras, one for the acquisition of night images, the other for the acquisition of day images or even the use of the "night" channel combined with the "day" acquisition to detect the Moon or certain planets in broad daylight in order to speed up the measurement and optimize precision.

[0050] It is however also possible to implement the invention with a single camera associated with a retractable filter system placed in front of the lens. This filter is positioned in front of the lens during the acquisition of daytime images, and erased during the acquisition of nighttime images.

[0051] This variant is of particular interest in the aeronautical field because it allows a reduction in weight, a reduction in volume, and only requires a single orifice on the skin of the aircraft, drone or missile.

Claims

Claims

1. - Electronic sextant comprising at least one camera dedicated to the acquisition of images of low-luminosity celestial bodies as well as a means delivering a signal representative of the angle formed by the optical axis of a camera with respect to the local vertical, in the local terrestrial reference frame, a clock and a calculator executing celestial body recognition software to determine the coordinates of the bodies in the inertial reference frame of the J2000 universe, said sextant comprises a memory in which are recorded the data and the algorithms for the determination of a transformation matrix making it possible to recalculate the coordinates of the celestial bodies in the local terrestrial reference frame, characterized in that said sextant is installed in a fixed position on a mobile carrier, said sextant further comprises a brightness reduction filter for the daytime acquisition of the position of the sun.

2. - Electronic sextant according to claim 1 characterized in that said brightness reduction filter is retractable relative to a single camera, for the daytime acquisition of the position of the sun when said filter is placed in the field of said camera, and for the acquisition of low-luminosity celestial bodies when said filter is retracted.

3. - Electronic sextant according to claim 1 characterized in that it comprises a first camera for the acquisition of night images and a second camera equipped with said brightness reduction filter for the daytime acquisition of the position of the sun.

4. - Electronic sextant according to claim 1 characterized in that said means delivering a signal representative of the angle formed by the optical axis of said at least one camera, relative to the local vertical in the local terrestrial reference frame, is a three-axis inertial system integrated into said sextant

5. - Sextant according to claim 1 characterized in that said camera(s) have an optical field greater than 90°.

6. - Sextant according to any one of the preceding claims, characterized in that said software controls the determination of the attitude and position of said sextant from the same star identified on a plurality of successive images.

7. - Sextant according to claim 1 characterized in that said software controls the determination of the attitude and position of said sextant from a plurality of different stars identified on an image.

8. - Method for calculating the attitude of a sextant having the characteristics of claim 1, characterized in that it comprises a step of comparing the star vectors measured by a camera and said reference star vectors identified in an astronomical catalog or by ephemerides.

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