Daytime stellar imager

• US 7,349,804 B2
• Filed: 05/31/2005
• Issued: 03/25/2008
• Est. Priority Date: 10/18/2004
• Status: Expired due to Fees

• Current Assignee: Trex Enterprises Corporation
• Original Assignee: Trex Enterprises Corporation
• Inventors: Rye, Vincent A, Brinkley, Timothy, Belenkii, Mikhail, Bruns, Donald G.
• Primary Examiner(s): Beaulieu; Yonel
• Application Number:
  US11/141,526
• Publication Number:
  US 20070038374A1
• Time in Patent Office: 1,029 Days
• Field of Search: 701/200, 701220-223, 731/78, 332/27, 332/68, 244/3.15
• US Class Current: 701/222
• CPC Class Codes: G01C 21/025 with the use of startrackers

Abstract

An automatic celestial navigation system for navigating both night and day by observation of K-band or H-band infrared light from multiple stars. In a first set of preferred embodiments three relatively large aperture telescopes are rigidly mounted on a movable platform such as a ship or airplane with each telescope being directed at a substantially different portion of sky. Embodiments in this first set tend to be relatively large and heavy, such as about one cubic meter and about 60 pounds. In a second set of preferred embodiments one or more smaller aperture telescopes are pivotably mounted on a movable platform such as a ship, airplane or missile so that the telescope or telescopes can be pivoted to point toward specific regions of the sky. Embodiments of this second set are mechanically more complicated than those of the first set, but are much smaller and lighter and are especially useful for guidance of aircraft and missiles. Telescope optics focus (on to a pixel array of a sensor) H-band or K-band light from one or more stars in the field of view of each telescope. Each system also includes an inclinometer, an accurate timing device and a computer processor having access to catalogued infrared star charts. The processor for each system is programmed with special algorithms to use image data from the infrared sensors, inclination information from the inclinometer, time information from the timing device and the catalogued star charts information to determine positions of the platform. Direction information from two stars is needed for locating the platform with respect to the celestial sphere. The computer is also preferably programmed to use this celestial position information to calculate latitude and longitude which may be displayed on a display device such as a monitor or used by a guidance control system. These embodiments are jam proof and insensitive to radio frequency interference. These systems provide efficient alternatives to GPS when GPS is unavailable and can be used for periodic augmentation of inertial navigation systems.

Claims

1. An automatic celestial navigation system for navigating both night and day by observation of K-band or H-band infrared light from multiple stars, said system comprising:

A) at least one telescope on a movable platform for viewing at least two substantially different portions of sky with each of said at least one telescope defining a field of view and comprising;

1) telescope optics for focusing H-band or K-band light from stars in said field of view onto a focal plane,

2) a sensor located at said focal plane for detecting H-band or K-band light,

B) an accurate timing device,

C) an accurate inclination sensor, and

D) a computer processor having access to infrared star chart information, programmed to compare image data from said infrared sensors, time information from said timing device, inclination information from said inclination sensor and said star chart information to determine positions of said platform.

2. The system as in claim 1 wherein said at least one telescope is two telescopes.

3. The system as in claim 1 wherein said system is a compact system suitable for use on a missile, an unmanned aerial vehicle or other aircraft and having a volume of substantially less than one cubic meter.

4. The system as in claim 3 wherein said system further comprises a rotary means for pointing the field of view of said at least one telescope with an accuracy of less than 3 arc-seconds.

5. The system as in claim 3 wherein said system further comprises a rotary means for pointing the field of view of said at least one telescope with an accuracy of about 1 arc-second or better.

6. The system as in claim 3 wherein said system further comprises two rotary stages for pointing the field of view of said at least one telescope.

7. The system as in claim 6 wherein said at least one telescope is two telescopes.

8. The system as in claim 6 wherein said at least one telescope is three telescopes.

9. The system as in claim 1 wherein said at least one telescope is three telescopes and each of said three telescopes are rigidly mounted with respect to the platform and each other.

10. The system as in claim 1 and further comprising temperature and pressure sensors wherein said processor is also programmed to utilize information from said temperature and pressure sensors to correct for variations caused by changing temperature and pressure.

11. The system as in claim 9 wherein each of said three telescopes are pointed in azimuthal directions spaced at about 120 degrees relative to each other and at about 45 degrees relative to horizontal.

12. The system as in claim 1 wherein said system is integrated with an inertial guidance system.

13. The system as in claim 9 wherein said platform is a ship or a portion of a ship.

14. The system as in claim 3 wherein said platform is a ship or a portion of an airplane.

15. The system as in claim 3 wherein said platform is a ship or a portion of a missile.

16. The system as in claim 3 wherein said platform is a ship or a portion of an unmanned aerial vehicle.

17. The system as in claim 1 wherein each of said sensors have full well capacities in excess of 5 million electrons.

18. The system as in claim 1 wherein each of said sensors have a frame readout period of less than 30 milliseconds.

19. The system as in claim 9 wherein each of said telescopes have a pixel size of about 6 arc-seconds.

20. The system as in claim 3 wherein each of said telescopes have a pixel size of about 16 to 27 microradians.

21. The system as in claim 1 wherein said processor is programmed to detect stars in the presence of strong background noise and to reduce noise, by utilizing multiple individual frames that are aligned to a star image in a single frame and summed up.

22. The system as in claim 1 wherein sizes of search windows are determined by the root mean square error of measurements of local verticals with an inclinometer.

23. The system as in claim 1 wherein stars are identified using angular distances between stars, star relative brightness and triangle patterns.

24. The system as in claim 9 wherein each of said three telescopes have an instantaneous field of view of about 0.4×

0.5 degrees.

25. The system as in claim 9 wherein said computer processor is programmed to determine position utilizing the following steps:

A) to reduces the fixed pattern camera noise, considering the data set that consists of 20 frames and first, using 20 data frames that precede the first frame in the data set, the median data frame is calculated to determine a background threshold for each pixel, then, the median frame of pixels is subtracted from each frame in the data set, pixel by pixel;

B) next, to reduce noise, five sequential data frames in the data set are blindly summed up to spreads star illumination over a few pixels;

C) then a “

super pixeled”

image is created by down sampling the image generated in step B at the rate of 1;

4 (four adjacent pixels are summed across the pixel array);

D) determine the brightest super pixel in the frame where the star can be located and create a small (9×

9 regular pixel size) window about the brightest super-pixel location (81 pixels with the brightest 4×

4 in the middle);

E) to increase the centroid accuracy, up sample the image within the window at the rate 10;

1 using cubic spline fit algorithm image at the noise ceiling (the computer produces a digital array of 90×

90 virtual pixels and fits them with the cubic spline fit algorithm into a Gaussian-like shape);

F) calculate the intensity weighted centroid to determine an expected star location in the first data frame;

G) repeat steps B)-F) for each subsequent data frames in the data set;

H) once an expected star location in all subsequent data frames are determined, shift all 20 frames to the star position in the first frame, and sum up all frames to create a final image for the entire frame, based on the shifts obtained from the 9×

9 pixel centroiding window, the extent of the shift is being based on the location of the centroid;

I) create a “

super pixel”

representation of the shifted and added frame obtained from step 8 by down sampling at the rate of 1;

4 (four adjacent pixels are summed across the pixel array), determine brightest super pixel and create a small (9×

9 regular pixel size) window about that location;

J) up sample the image within the small 9×

9 pixel window with a ratio of 10;

1 by using cubic spline fit algorithm, remove background by chopping at noise ceiling, calculate intensity weighted centroid position as well as total intensity in the image, make an estimation of the rms noise by taking the standard deviation σ

of all pixels in the entire image frame, remove the data within the small window in order to search for the next dimmest star, repeat steps I and J until all potential stellar objects within the frame are found;

K) for each potential star location, the pixel SNR is calculated;

$\mathrm{SNR}=\frac{I_S-\langle <br><br>I\rangle <br><br>}{\mathrm{\sigma <br><br>}},$

where I_s is the total signal intensity divided by number of pixels in the image, <

I>

is the mean intensity in the image, and σ

is the rms noise and if the SNR≧

10, then the star is detected and the star coordinates are determined by intensity weighted centroid calculated in step J) but if SNR<

10, then this potential star location is rejected and treated as a noise; and

L) star coordinates along with the star intensity are used further by automated star pattern recognition algorithm and coordinates of the brightest star in the field of view are used in calculations of latitude, longitude and absolute azimuth.

26. The system as in claim 3 wherein the system is integrated with an inertial navigation system using a Kalman filter.

27. The system as in claim 1 wherein said computer processor is programmed to determine a absolute azimuth reference utilizing at least one star position.

28. The system as in claim 27 wherein said absolute azimuth reference is utilized to determine attitude of said platform.

29. The system as in claim 28 wherein the platform is an artillery gun.

30. The system as in claim 1 and further comprising a visible light camera especially for use on partly cloudy days for determing directions to cloudless regions of the sky.

31. The system as in claim 30 wherein the visible light camera is a CCD camera.

32. The system as in claim 9 wherein said three rigidly mounted telescopes each comprise a temporary fold mirror for changing the fields of view of each telescope by fixed angles.