Method and Apparatus for Rotational Rate Determination Using a Stellar Reference

1. Field of the Invention

This invention relates generally to satellite positioning control systems, and more particularly to a satellite-based angular rate determination system and method.

2. Description of the Related Art

In the field of satellite positioning controls, continual, precise rate sensing is very important for precision satellite attitude control.

Currently, devices such as gyroscopes have been used in attitude control. However, mechanical gyroscopes have a limited life dictated by the life of the wheel suspension assembly and its sensitivity to influences such as temperature and vibration. Likewise, optical gyroscopes are limited by the life and stability of the light source, the resistance of the fiber or mirrors to ambient radiation, and other complex problems related to light and quantum limits.

In addition to gyroscopes, various forms of star sensors have been used to detect and control satellite attitude. For example, a static star sensor compares the stars seen over a relatively large area of space to a map of the universe, and calculates the direction of the sensor line of sight from the comparison of the image to the map. However, there are several limitations on this technology. First, the sensor field of view must be very large or the sensitivity of the sensor very high to obtain adequate information. Second, the brightness and position of the stars in the field of view must be compared to a very large catalog of information in order to decide which region of the heavens is in view. An alternative system, known as a tracker star sensor, follows the motion of a single star relative to the sensor base by rotating the sensor line of sight. The angle between the line of sight and the sensor base determines the base attitude. Such a system is limited by the fact that the device must be able to select a particular star.

It would therefore be advantageous to provide for a satellite attitude control system that is reliable, inexpensive and robust. It should be capable of determining angular rates without requiring the complicated hardware associated with a gyroscope. Likewise, any use of stellar references should not be dependent upon an extensive map of the universe or the ability to identify and track a single star.

It is therefore an object of the present invention to provide for a method and apparatus for determining the rate of rotation of a rotating satellite. The apparatus includes an imaging device, located on the satellite, for viewing a celestial sphere. At a first point in time, a first celestial sphere image is taken by the imaging device and stored temporarily. At a second point in time, a second celestial sphere image is taken by the imaging device, and this second image is compared with the stored first image. The difference between the first and second images, differentiated over the time between the taking the first and the second images, is used to determine the rate of rotation of the satellite.

These and other objects and features of the present invention can be better understood by referencing the foregoing description of the presently preferred embodiment as well as the drawings in which:

FIG. 1 is a pictorial representation of a satellite and its imaging device viewing a celestial sphere;

FIG. 2 is a functional block diagram illustrating the logic and computational methods used in this invention; and

FIGS. 3A and 3B are symbolic representations of the celestial sphere view as captured by the imaging device at two different points in time.

As can be seen in FIG. 1, the presently preferred embodiment is directed toward a method and apparatus for determining the rate of rotation of a rotating satellite orbiting the earth. Typically, as a satellite orbits around the earth, different portions of the heavens come into view. For example, persons in the northern hemisphere have a different view of the heavens than do persons in the southern hemisphere. That portion of the heavens associated with a particular orientation of the satellite with respect to the earth is commonly referred to as a celestial sphere. In the presently preferred embodiment, the satellite has an apparatus for determining the performance of the satellite by viewing the celestial sphere. More particularly, the satellite 10 has an imaging device, generally indicated at 12, for viewing a portion 14 of the celestial sphere 16 as the satellite 10 travels through its orbit. The imaging device 12 includes a lens assembly 17 for focusing the view of the celestial sphere 16 onto a detector array 18. The detector array is electronically linked to logic circuitry 19, where the logic circuitry 19 controls the taking of images and the computation of rotational rates. By comparing celestial views taken at different points in time, the method and apparatus of the present invention calculates the rotational rate of the satellite 10 about the X, Y, and Z axes.

More particularly, as can be seen by referencing the functional block diagram of FIG. 2, the imaging device 20 is designed to focus on a portion of the celestial sphere 14. At a first point in time, denoted as t.sub.1, a first celestial sphere image 22 is taken by the imaging device 20 and temporarily stored 24. At a second point in time, denoted as t.sub.1 +T, a second image 26 is taken by the imaging device 20. The first 22 and second 26 images are compared 28 by the electronics of the imaging device and related circuitry, from which the relative movement along the X, Y and Z axis 30 can be determined. From these determined changes in relative positions, the rates of rotation about the X, Y and Z axis can be determined 32. Once the rates of rotation about the various axes are determined, an overall rate of rotation 34 can be determined, if desired.

For example, as illustrated in FIGS. 3A and 3B, the first celestial view, generally indicated at 40, can be represented as a plurality of relative light levels 42 across a detector array 44. In this example, a low level of light is represented by a 3 while a high level of light is represented by a 1. It should be appreciated by those of ordinary skill in the art that such a detection array 44 could be made up of a CCD array or other detection means. This first celestial view image 40 taken at time t=t.sub.1 has relative positional coordinates X.sub.1 and Y.sub.1 and an orientation in the X-Y plane here depicted as Z.sub.1. This first image 40 is stored in the circuitry of the rate determination apparatus. At a second point in time, t=t.sub.1 +T, a second image 50 is taken. As can be seen in FIG. 3B, the light incident upon the detection array 44 has similar intensity levels 42 as before, but the pattern created by the light has moved to position X.sub.2, Y.sub.2 and orientation Z.sub.2. By comparing these relative coordinate movements, the relative X, Y and Z axis travel can be determined as .DELTA.X 52, .DELTA.Y 54 and .DELTA.Z 56. Given that the time, T, between the first 40 and second 50 images is known, the apparent rates can be determined as:

Comparing these apparent rates with the known orientation of the imaging device relative to the satellite axes, the satellite rotation rates about the X, Y, and Z axes can be determined. It should be appreciated by one of ordinary skill in the art that the time between successive views (T) can be equal to or relatively large in proportion to the scan rate of the detector array 44. For example, if a CCD array is employed having a 1,024.times.1,024 pixel detection array, being scanned at a rate of 1 KHz and having a lens assembly providing a one degree field of view for the detection array, taking sequential views of the celestial sphere provides a level of accuracy whereby rates from just under .+-.0.001 degrees per second to approximately .+-.1,000 degrees per second can be measured.

The preceding description of the presently preferred embodiment has been provided for the purposes of illustration only. It should therefore be appreciated by one of ordinary skill in the art that modifications could be made to this embodiment without departing from the spirit or scope of the invention disclosed herein.