WO2016151574A1

WO2016151574A1 - Celestial compass and methods of use and calibration

Info

Publication number: WO2016151574A1
Application number: PCT/IL2016/050272
Authority: WO
Inventors: Yifrach AHARON
Original assignee: Israel Aerospace Industries Ltd.
Priority date: 2015-03-26
Filing date: 2016-03-13
Publication date: 2016-09-29
Also published as: EP3274657A4; IL248033A0; EP3274657A1; IL237963A0; SG11201707739RA

Abstract

A celestial compass that includes one or more imagers configured to image corresponding positions of stars in the sky. A storage device storing data representative of a star almanac. A position measurement device for determining the position of the celestial compass. A clock for determining the time of measurement. A processor connected to the storage device, the position measurement device and the clock and being configured to compare a distribution of one or more stars imaged by the imagers to their listed positions in the almanac, and thereby determine the azimuth and elevation of the celestial compass with respect to the horizontal plane and true north respectively.

Description

Celestial Compass and Methods of Use and Calibration

FIELD OF THE INVENTION

The present invention relates to azimuth direction finding in the field of navigation in surveying and military applications and more specifically to the implementation of a compass based on celestial observation.

BACKGROUND OF THE INVENTION

Both in surveying as well as in military application accurate determination of azimuth is growing in importance with the increased navigation accuracy capability. The classical method to determine azimuth with a magnetic compass is limited in accuracy even after the application of the correction of magnetic pole deviation information. More modern approaches take advantage of multiple global positioning system (GPS) receivers located at a known fixed separation to determine azimuth accurately.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an azimuth measuring device that is more accurate that a magnetic compass and is independent of supporting systems such as GPS reception.

According to embodiments of one aspect of the present invention there is provided a celestial compass that uses and/or includes: (a) one or more imagers configured to image star positions in the sky, (b) a memory storing an "almanac" (an almanac, e.g. "Yale Catalogue") listing the true position of stars and/or celestial bodies and/or sun and/or moon (herein after "stars") in the sky at any desired times (i.e. date, hour, minute and second) and global location; (c) means for obtaining the celestial compass global location and time; and (d) a processor coupled to the memory and programmed to process the star almanac data, the celestial compass global location, and the camera images of star positions to calculate the direction of the true north (geodetic North) or any other basis/datum direction by comparing the data on star location in the almanac to their location as imaged by the one or more imagers, and determining the orientation of the celestial compass relative to the global coordinate system (azimuth and elevation angles).

In some embodiments, the celestial compass can be leveled by a "common leveling apparatus". In other embodiments the tilt offset from the horizontal plane of the celestial compass can be determined by a tilt measuring device.

According to embodiments of one aspect of the present invention there is provided a method of determining the direction of the true north. In some embodiments, the method can also be used to determine the elevation angle of a given target object with respect to the local horizon.

The one or more imagers image the sky and determine the actual position of stars in the sky. The meaning of 'position' as used herein refers to two-axis angular coordinates in a spherical coordinate system relative to the point of view of the one of more imagers, also referred to as azimuth and elevation. Note that each imager may include appropriate imaging technology so that stars can be imaged during the day as well as at night. The position of the celestial compass and measurement time may be determined via a Global Positioning System (GPS). Alternatively other navigation/positioning and timing methods and/or devices can be used. Using appropriate algorithms, the position of imaged stars is compared to that in a star almanac to determine the horizontal and elevation angle of one or more celestial bodies. Based on these data the processor calculates the orientation of the celestial compass relative to the global coordinate system (azimuth and elevation angles).

For most intended uses, the required accuracy of the true north determination by the celestial compass is achievable even in cases where the input parameters of position and time are not highly accurate. For example, a 1 kilometer error in the position of the celestial compass will affect the accuracy of the true north direction by less than a third of a milliradian.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of the local horizontal plane, the celestial compass at the center and the celestial sky above;

Fig. 2 is a side view of the local horizontal plane, the celestial compass at the center and the field-of-view (FOV) of the celestial sky imaged by the celestial compass;

Fig. 3 is a top-down view (from the zenith onto horizontal plane) of the local horizontal plane, the celestial compass at the center and the field-of-view (FOV) of the horizon as it is imaged by the celestial compass; and

Fig. 4 is a diagram showing pictorially the process implemented by the processor of the celestial compass, the image it produces, comparison and adjustment using data from the star almanac and the final determination of the true north direction.

DETAILED DESCRIPTION OF EMBODIMENTS

The figures are intended to aid in understanding the invention and components illustrated therein are not necessarily drawn to scale.

In many instances, the same reference numbers may be used for similar components, despite modifications thereto, in the various embodiments described below. For the sake of brevity, description details of certain components which are known in the art are not necessarily included.

The figures show a celestial compass 1 in accordance with embodiments of the invention. The celestial compass includes one or more imagers 21 to 23, for example CCD or C-MOS cameras, configured to produce an image 15 of the sky 3 or at least a suitably significant portion of the sky, with a view of several stars 4. Image 15 is compared to a star almanac, schematically depicted as a memory device 16 storing star position data for each star that can potentially be imaged by the celestial compass.

Optionally a target object 10 within the field of view (FOV) of the imagers of the celestial compass can also be imaged by one of the imagers 21 to 23. Optionally the celestial compass 1 may rest on a platform that is adjusted to accurately orient the celestial compass in the local horizontal plane 2 with the aid of a leveling device 20.

Such a device may be a spirit level. Alternatively the angular offset of the celestial compass from the horizontal plane may be measured, and the offset used to correct the viewed image of the stars. This offset may be measured with a device that is capable of accurate angular measurements of offset from the horizontal plane. In principle, provided several stars are viewed in the imagers 21 to 23, the celestial compass can derive its orientation in both azimuth and elevation. Nevertheless, in cases where only a small number of stars are visible, or indeed, when only one star can be imaged, for example the Sun in the daytime, knowledge that the celestial compass is accurately referenced to the horizontal plane significantly enhances the accuracy of determining the true north.

The imagers 21 to 23 capture the angular position of the stars within their FOV. In principle the position of a star within the FOV can be related directly to its angular position (azimuth and elevation) with respect to the reference point of the imager. The current invention also contemplates a set of imagers 21 to 23 jointly covering or exceeding the entire FOV of a hemisphere, thereby covering the entire sky. In the drawings three imagers are depicted, where each imager has a FOV of 127° in the horizontal and a 92° FOV in the vertical. Therefore three imagers proved full coverage for the hemisphere of the sky. Alternatively a larger number of imagers with smaller individual FOV values are also possible, provided their combined FOV covers the hemisphere of the sky.

A practical imager, especially if it offers a large FOV, is likely to suffer optical aberrations that might lead to erroneous angular position values for imaged stars. Furthermore, the mechanical alignment of several imagers may introduce some relative alignment errors between imagers. The invention reduces these errors by carefully calibrating the images generated by the imagers against a calibration image with visible calibration objects accurately arranged on a calibration surface in the form of a complete hemispheric target under which the imager set of the celestial compass is placed for calibration. Alternatively the calibration surface may extend to a portion of a hemisphere, and relative angular motion between the celestial compass and the calibrating target incorporated to scan the calibration target over the entire combined FOV of the imager array. Additionally and/or alternatively the set of imagers may calibrated by viewing stars distributed across the entire FOV and comparing their position to those in the almanac.

Using the comparison of the image 15 of the stars 4 and the star almanac 16, the location of pixels on the images obtained from the imagers 21 to 23 can be associated with the precise direction of the true north 7, and from this association any other pixels can be used to calculate a precise azimuth direction.

Capturing an image of target 10 in the imaged dataset enables calculation of the target's precise azimuth 6, and optionally its elevation 8. As noted above, this requires knowledge of the relative angular offset of the reference plane of the celestial compass from the horizontal plane. Such knowledge can be obtained directly from the image of stars in the sky. Alternatively, using level 20, knowledge of the local horizontal plane is obtained, and processing of that datum with the azimuth 6 of target object 10 can determine the elevation angle 8 of the target object with respect to the local horizon. If, for whatever reason, the celestial compass 1 is not leveled on a local horizontal plane, a compensation angle can be calculated utilizing a leveling offset meter to be used in place of level 20.

In summary, a sky imager images the stars and compares it with an almanac, whereby with an accurate knowledge of the day and time, and a GPS (i.e. accurate knowledge of position/location and time), one can accurately compute true north 7. The position/location used in the almanac is the position of compass 1 and imager/cameras 21 to 23. A processor (not shown) of the celestial compass extracts data from a star almanac data storage 16, such as the Yale Catalogue. The processor also extracts star positions from an image produced by the imagers 21 to 23. The processor compares the two (image matches) to compute the differential orientation between the almanac and image, to enable a computation of the true North.

It should be understood that the above description is merely exemplary and that there are various embodiments of the present invention that may be devised, mutatis mutandis, and that the features described in the above-described embodiments may be used separately or in any suitable combination.

It will also be understood that the processor according to the invention may be a suitably programmed computer. Likewise, the invention contemplates a computer program being readable by a computer for executing the method of the invention. The invention further contemplates a machine-readable memory tangibly embodying a program of instructions executable by the machine for executing the method of the invention.

Claims

CLAIMS:

1. A celestial compass, comprising:

one or more imagers configured to image corresponding positions of at least one star in the sky;

a storage device storing data representative of a star almanac;

a position measurement device for determining the position of the celestial compass;

a clock for determining the time of measurement; and

a processor coupled to the storage device, the position measurement device and the clock and being configured to compare a distribution of one or more stars imaged by the imagers to their listed positions in the almanac, and thereby determine the azimuth and elevation of the celestial compass with respect to the horizontal plane and true north respectively.

2. The celestial compass according to claim 1 , further comprising a leveling device that can be used to accurately level the celestial compass in the horizontal plane.

3. The celestial compass according to claim 1 , further comprising a leveling meter capable measuring the angular offset of the celestial compass from the horizontal plane.

4. The celestial compass according to any one of the preceding claims, wherein the position measurement device is a Global Positioning System (GPS) for input of Global position and time.

5. The celestial compass according to any one of the preceding claims, wherein the processor is further responsive to data representative of a target on the one or more imagers for determining the azimuth and elevation of the target.

6. The celestial compass according to any one of the preceding claims, wherein the at least one star is the sun.

7. A method for finding true north comprising the following operations:

recording an image of stars in the sky by a set of imagers;

determining local position and time;

retrieving the position of the stars for the location and time from a star almanac; and

comparing the position of stars in the star image with their corresponding positions in the almanac to determine the orientation of the imagers in space, azimuth and elevation.

8. The method of claim 7 further including measuring the offset of the imagers from the horizontal plane so as to improve the accuracy of the measurement when a small number of stars is imaged.

9. The method of claims 7 or 8, further including determining the azimuth and elevation of a target by imaging the target with the imagers.

10. A method for calibrating the imagers in the celestial compass according to any one of claims 1 to 6, including:

directing the imagers at a calibration surface with objects at known locations distributed thereon and spanning a portion of a sphere; and

scanning the calibration surface across the complete field of view of the imagers to determine and correct for any optical distortions;

whereby direct relationship between the position of a calibration object on the calibration surface and its position in the image is obtained.

11. A method for calibrating the imagers in the celestial compass according to any one of claims 1 to 6, including:

directing the imagers at a calibration surface covering a full hemisphere with objects at known locations distributed thereon and spanning a portion of a sphere; and determining and correcting for any optical distortions;

12. The method of claim 11 where the calibration hemisphere is the night sky and calibration objects on it are stars whose position is determined from a star almanac.

13. A computer readable memory storing data representative of program code, which when run on a suitable computer causes the computer to carry out the method of any one of claims 7 to 9.