Satellite camera attitude determination and image navigation by means of earth edge and landmark measurement

• US 6,023,291 A
• Filed: 09/29/1997
• Issued: 02/08/2000
• Est. Priority Date: 10/16/1996
• Status: Expired due to Term

• Current Assignee: Space Systems/Loral Incorporated (Loral Space and Communications Incorporated)
• Original Assignee: Space Systems/Loral Incorporated (Loral Space and Communications Incorporated)
• Inventors: Kamel, Ahmed A, Ong, Kwok M
• Primary Examiner(s): BRITTON, HOWARD W
• Application Number:
  US08/939,616
• Time in Patent Office: 862 Days
• Field of Search: 348/147, 348/116, 348/117, 701/4, 701/13
• US Class Current: 348/147
• CPC Class Codes: G01C 11/025 by scanning the object
• CPC Class Codes: G01S 3/7868 using horizon sensors
• CPC Class Codes: H04N 7/002 Special television systems not provided for by H04N7/007 - H04N7/18 still pictures via a television channel H04N1/00098
• CPC Class Codes: H04N 7/18 Closed-circuit television [CCTV] systems, i.e. systems in which the video signal is not broadcast

Abstract

A method and system is disclosed for imaging a celestial object, typically the Earth, with a spacecraft orbiting the celestial object. The method includes steps of (a) operating an imager instrument aboard the spacecraft to generate data representing an image of the celestial object; (b) processing the image data to derive the location of at least one predetermined landmark in the image and a location of edges of the celestial object in the image; and (c) further processing the detected locations to obtain the attitude of the imager instrument. The method includes a further step of outputting the image and the imager instrument attitude to at least one end-user of the image, and/or using the imager instrument attitude to revise the image before outputting the image to the at least one end-user of the image. The generated data preferably represents a one half frame image, and the steps of processing and further processing thus occur at a one half frame rate. The step of processing includes a step of applying the a priori knowledge of the attitude coefficients in processing new observations to determine the imager current attitude.

Claims

1. A method for imaging a celestial object with a spacecraft orbiting the celestial object, comprising steps of:

operating an imager instrument aboard the spacecraft to generate data representing an image of the celestial object;

processing the image data to derive a location of at least one predetermined landmark in the image and a location of edges of the celestial object in the image; and

further processing the detected locations to obtain an attitude of the imager instrument.

2. A method as in claim 1, and further comprising the step of outputting the image and the imager instrument attitude to at least one end-user of the image.

3. A method as in claim 1, and further comprising the step of using the imager instrument attitude to revise the image before outputting the image to at least one end-user of the image.

4. A method as in claim 1, wherein the generated data represents a one half frame image, and wherein the steps of processing and further processing occur at a one half frame rate.

5. A method as in claim 1, wherein the step of processing includes a step of inputting imager instrument scan angles.

6. A method as in claim 1, wherein the step of processing the image data to derive a location of edges of the celestial object in the image operates in accordance with a spherical Earth model, and further comprising a step of applying correction terms to accommodate a non-spherical Earth model.

7. A method as in claim 1, wherein the step of processing the image data to derive a location of edges of the celestial object in the image includes an initial step of obtaining Earth edge observables by measuring imager instrument line-of-sight (LOS) scan angles during an East-West scan when the LOS enters and exits the Earth'"'"'s horizon.

8. A method as in claim 7, wherein the Earth edge observables are combined to form half-sum and half-difference pseudo observables, wherein the half-sum pseudo-observable (H_s) can be interpreted as an E/W coordinate of a mid-point of a line joining two measured Earth edges corresponding to a given North-South imager instrument scan line, and wherein the half-difference pseudo-observable (H_d) can be interpreted as a deviation of a half-chord length of the line joining the two measured Earth edges from a nominal value.

9. A method as in claim 1, wherein correlation between the parameters specifying a misalignment of an image optical axis and spacecraft axes are reduced by combining them in a predetermined manner.

10. A method as in claim 1, wherein landmark and celestial body edge measurements are fitted to an observational model to determine attitude coefficients using a sequential batch least squares procedure.

11. A method as in claim 10, wherein the step of fitting includes steps of, for each half-frame of image data:

correcting observed East-West and North-South scan angles in accordance with a non-spherical Earth model, using current orbit and attitude knowledge, to provide modelled observations; and

fitting the modified observations to a spherical Earth-based mathematical model using a sequential batch filter;

wherein

parameters that are solved are Fourier and polynomial coefficients of five attitude angles defined as three angles representing the attitude state of the spacecraft characterized by Euler angles roll (φ

), pitch (θ

), and yaw (ψ

), and the attitude state of the imager instrument, relative to the spacecraft body, described by a misalignment of the imager instrument optical axis with respect to the spacecraft body axes, expressed as two angles representing roll misalignment (φ

_ma) and pitch misalignment (θ

_ma)).

12. A method as in claim 11, wherein a priori knowledge of the coefficients are used to determine, for the case where no special event occurs immediately before the image, the current attitude solution and its full covariance matrix;

and wherein a priori knowledge of the Fourier coefficients are used for the case where a special event does occur before the image.

13. A method for imaging a celestial object with a spacecraft orbiting the celestial object, comprising steps of:

operating an imager instrument aboard the spacecraft to generate scan-line data representing an image of the celestial object;

processing the scan-line data to derive a location of at least one predetermined landmark in the image and a location, along at least two scan-lines, of edges of the celestial object in the image; and

further processing the detected locations to obtain an attitude of the imager instrument.

14. A method as in claim 13, wherein the at least two scan-lines correspond to two lines of latitude of the celestial object.

15. A method as in claim 14, wherein the two lines of latitude are about 45 degrees North latitude and about 45 degrees South latitude.

16. A method as in claim 13, and further comprising the step of outputting at least the image and the imager instrument attitude to at least one end-user of the image.

17. A method as in claim 16, and further comprising the step of using the imager instrument attitude to revise the image before outputting the image to the least one end-user of the image.

18. A method as in claim 13, wherein the scan-line data represents a one half frame image, and wherein the steps of processing and further processing occur at a one half frame rate.

19. A method as in claim 13, wherein the step of processing includes a step of inputting imager instrument scan angles.

20. A method as in claim 13, wherein the step of processing the image data to derive a location of edges of the celestial object in the image operates in accordance with a spherical Earth model, and further comprising a step of applying correction terms to accommodate a non-spherical Earth model.

21. A method as in claim 13, wherein the step of processing the image data to derive a location of edges of the celestial object in the image includes an initial step of obtaining Earth edge observables by measuring imager instrument line-of-sight (LOS) scan angles during an East-West scan when the LOS enters and exits the Earth'"'"'s horizon.

22. A method as in claim 21, wherein the Earth edge observables are combined to form half-sum and half-difference pseudo observables, wherein the half-sum pseudo-observable (H_s) are interpreted as an E/W coordinate of a mid-point of a line joining two measured Earth edges corresponding to a given North-South imager instrument scan line, and wherein the half-difference pseudo-observable (H_d) can be interpreted as a deviation from a nominal value of a half-chord length of the line joining the two measured Earth edges.

23. A method as in claim 13, wherein correlation between the parameters specifying a misalignment of the image optical axis and spacecraft roll and pitch axes are reduced by combining them in a predetermined manner.

24. A method as in claim 13, wherein landmark and cellestial body edge measurements are fitted to an observational model to determine attitude coefficients using a sequential batch least squares procedure.

25. A method as in claim 24, wherein the step of fitting includes steps of, for each half-frame of image data:

correcting observed East-West and North-South scan angles in accordance with a non-spherical Earth model, using current orbit and attitude knowledge, to provide modelled observations; and

fitting the modified observations to a spherical Earth-based mathematical model using a sequential batch filter;

wherein

parameters that are solved are Fourier and polynomial coefficients of five attitude angles defined as three angles representing the attitude state of the spacecraft characterized by Euler angles roll (φ

), pitch (θ

), and yaw (ψ

), and the attitude state of the imager instrument, relative to the spacecraft body, described by a misalignment of the imager instrument optical axis with respect to the spacecraft body axes, expressed as two angles representing roll misalignment (φ

_ma) and pitch misalignment (θ

_ma).

26. A method as in claim 25, wherein a priori knowledge of the coefficients are used to determine, for the case where no special event occurs immediately before the image, the current attitude solution and its full covariance matrix;

and wherein a priori knowledge of the Fourier coefficients are used for the case where a special event does occur before the image.

27. An observation system for obtaining an image of the Earth, comprising:

a spacecraft orbiting the Earth, said spacecraft comprising an imager instrument having a camera for obtaining an image of the Earth; and

a terrestrial data processor system linked to said spacecraft through a first communication link for receiving said obtained image and for processing said obtained image to derive a location of at least one predetermined landmark in the image and a location, along at least two image scan-lines, of edges of the Earth in the image, and further processing the detected locations to obtain an attitude of at least said imager instrument, said observation system further comprising means for providing at least the image to at least one end-user of the image.

28. A system as in claim 27, wherein said providing means further provides the imager instrument attitude to the at least one end-user.

29. A system as in claim 27, wherein said data processor system employs said obtained imager instrument attitude to revise said image before outputting said image to the least one end-user.

30. A system as in claim 27, wherein said providing means is coupled to said at least one end-user through a second communication link, said second communication link comprising said spacecraft.