Abstract:
An earth sensor for a communication satellite is provided which utilizes a camera for viewing the earth from the satellite and an optical correlator which optically processes the video data from the camera to provide attitude information to a satellite attitude control computer.

Description:
This is a continuation of application Ser. No. 08/154,326, filed Nov. 18, 1993 now abandoned. 
    
    
     TECHNICAL FIELD 
     This invention relates to earth sensors for providing spacecraft attitude information and more particularly to an earth sensor for a satellite which utilizes a camera for viewing the earth from the satellite and an optical correlator which optically processes the data from the camera to provide accurate attitude information to the satellite attitude control system. 
     BACKGROUND ART 
     An earth sensor is a critical component of a communication or remote sensing satellite&#39;s attitude control subsystem. It is generally desirable to maintain a communications or remote sensing satellite in a geosynchronous orbit about the earth so as to enable a communication beam or sensor field of view from the satellite to accurately cover a desirable area, such as a particular country, on the surface of the earth. Any deviations from this orbit will alter the coverage of the beam or view. Accordingly, satellites are provided with sensors for sensing changes in orientation of the satellite relative to the earth. 
     One prior art earth sensor relies on infrared radiation focused on thermally sensitive detectors. Another relies on the intensity of visible and ultraviolet radiation from the earth disk. The earth as seen from space does not always have the same shape in the visible light spectrum. It is therefore difficult to design a video earth sensor which accommodates this characteristic without providing prohibitively large and complex data processing capabilities. 
     SUMMARY OF THE INVENTION 
     According to the present invention a video earth sensor for a geosynchronous satellite is provided which utilizes a camera for viewing the earth from the satellite and an optical correlator which optically processes the video data from the camera to provide accurate attitude information to the satellite attitude control system. The optical correlator uses two Magneto-Optic Spatial Light Modulators (MOSLMs). MOSLMs are well known in the art and have been used in target and pattern recognition applications. In contrast to the prior art pattern recognition systems, the pattern of interest in the present invention, namely the earth as seen from orbit, is of a known and slowly changing shape. It is the shape and orientation of the earth as seen by the camera which gives the satellite attitude control system the attitude information it requires. The sensor of the present invention relies on the fact that although the earth does change shape over the course of a day, as well as over the course of a year, it does so relatively slowly. The rate at which MOSLMs can be readdressed is very fast (120 frames per second) and thus real-time or near real-time comparison from one frame to the next is possible. Comparison of the current picture of earth with those of the very recent past can give data on the location of the earth within the scene as well as orientation. The changing shape of the earth is essentially filtered out or eliminated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more thorough understanding of the present invention may be had from the following detailed description that should be read with the drawings in which: 
     FIG. 1 shows a satellite with two earth sensing camera mounted thereon; 
     FIG. 2 is a functional schematic diagram of a preferred embodiment of the invention; 
     FIGS. 3 a  and  3   b  show previous and current views of earth showing an attitude change; 
     FIGS. 4 a - 4   f  show the light incident on various correlator elements; 
     FIGS. 5 a - 5   c  show yaw data as collected with a single camera; 
     FIGS. 6 a - 6   c  show video data useful in calibrating the earth sensor and in determining pitch and roll information from the data. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring now to the drawings and initially to FIG. 1, a satellite generally designated  10  is provided with the usual solar panels  12  and  14  which provide a source of power for operating the various electrical loads on the satellite. The video earth sensor of the present invention is generally designated  16 , in FIG. 2, and comprises a primary camera  18  and a secondary camera  20  appropriately mounted on the satellite  10  as shown in FIG.  1 . Referring to FIG. 2, the output of the cameras  18  and  20  is provided to an image processor generally designated  22  which includes a MOSLM translator generally designated  24  and an optical correlator generally designated  26 . The MOSLM translator  24  converts or translates video data from the cameras  18  and  20  into MOSLM pixel addresses. The translator  24  multiplexes data from the two cameras to the correlator using a multiplexer  28  and stores frame data in a storage buffer  30 . 
     The correlator  26  includes MOSLMs  32  and  34 , a vertical polarizer  36 , horizontal polarizers  38  and  40  and a lens system generally designated  42 . The light source for the correlator  26  may include one or more solid state lasers generally designated  44 . The optical output of the correlator  26  is presented to a third camera  46  which converts the optical data to electrical signals for input to a satellite control processor (SCP)  48 . The SCP  48  derives attitude control information from the signals supplied from the cameras  18  and/or  20  and the camera  46  representing pitch, roll or yaw movement of the satellite from a desired pointing direction toward the earth. The beam of light from the laser  44  is adjusted by the lens system  42 , which may include a plurality of lenses, in order to supply parallel light over the surface area of the MOSLMs. The SCP  48  may also receive earth view data directly from the camera  18  as indicated at  50 , without being filtered through the correlator  26 , for use in calibration of the sensor. 
     The operation of the sensor is as follows considering first the pixel data presented to the correlator  26  as a result of the view of the earth from camera  18  only. The data from camera  18  is input to the MOSLM translator  24 , via the multiplexer  28  where it is translated from the pixel addresses of the camera picture to the appropriate pixel addresses of the MOSLM so that the pixel data of the MOSLM corresponds to the view of the earth as seen by the camera  18 . After translation, the data from the camera  18  is stored in the buffer  30 . The current or present output of the translator  24  is used to program the MOSLM  32 , i.e. magnetize the individual pixels of the MOSLM  32  to the appropriate state to correspond to the pixel data of the current earth scene, as viewed by the camera  18 . Similarly, the delayed pixel data from the buffer  30  is used to programs the MOSLM  34  with the previous scene&#39;s pixel data. Programming the MOSLMs  32  and  34  means setting those pixels of the MOSLM, representing the earth in the scene, to a state which causes a 90 degrees rotation of the polarization state of any light transmitted through those pixels. Also, those pixels of the MOSLM, representing the earth&#39;s background in the scene, are set to a state which produces no polarization shift of light passing through those MOSLM pixels. How often the previous and current pixel data is compared, and whether succeeding frames are compared or a number of frames are skipped between comparisons is a tradeoff between performance and MOSLM translator complexity and capability. 
     Referring now to FIGS. 3 a - 3   b , a previous view ( 3   a ) and current view ( 3   b ) of the earth, as seen by camera  18 , is shown. The difference in the two views represents an attitude shift about the pitch axis. The correlator  26  provides a video output to the camera  46  representing this difference. 
     FIGS. 4 a - 4   f  show the effect of each of the optical elements of the correlator  26  on the light from laser  44  as it passes through the correlator  26  and enters the camera  46 . The beam of the laser  44 , is adjusted with any required lenses to supply parallel light over the surface area of the two MOSLMs  32  and  34 . The light incident on polarizer  36  is depicted in FIG. 4 a  and is vertically polarized by the polarizer  36 . The vertically polarized light incident on MOSLM  32  is depicted in FIG. 4 b . This light is acted upon by the MOSLM  32  in accordance with the manner in which it is programmed i.e. the light passing through the MOSLM pixel locations corresponding to the earth, is rotated by 90 degrees, while the light passing through the MOSLM pixel locations corresponding to the background is unchanged. The light exiting the MOSLM  32  is incident on the polarizer  38  as depicted in FIG. 4 c  where it is horizontally polarized, thereby rejecting all of the background light. The horizontally polarized light corresponding to the earth passes through the polarizer  38  and is incident on the MOSLM  34  as shown in FIG. 4 d . The light falling on MOSLM  34  carries the data of the current earth view encoded via polarization. The MOSLM  34  does not affect the light incident upon those pixel locations corresponding to the background of a previous scene. The MOSLM  34  rotates the light, by 90 degrees, which passes through those pixel locations which have been magnetized to represent the earth in the previous scene. As shown in FIG. 4 e , if the previous scene does not match the current scene, some light of both vertical and horizontal polarization will be incident on the polarizer  40 . If the scenes are identical, then all of the light incident on the polarizer  40  will be vertically polarized, because it will have undergone two 90 degree rotations. If the two scenes are identical no light will fall on camera  46 , since all the vertically polarized light will be rejected by the horizontal polarizer  40 . If the scenes are different there will be some horizontally polarized light which passes through the polarizer to fall on the camera  46  as shown in FIG. 4 f . This light represents the difference between the position of the earth in the two scenes, the background light having been rejected by the polarizer  38 . 
     The image viewed by the camera  46  is converted to pixel data and input to the SCP  48 . If the data input to the SCP  48  by the camera  46  is indicative of a completely blank scene, the data is interpreted by the SCP as indicating that the spacecraft has not undergone any attitude changes. If an image is present, four things could have happened, which can be determined by the SCP  48 . 
     1) If the earth moves within the scene of camera  18 , the data from the camera  46  identifies the direction and amount of motion in roll and/or pitch. 
     2) If the earth changes orientation within the scene of camera  18 , the data from the camera  46  identifies the direction and amount of yaw motion of the spacecraft. This only applies if the earth&#39;s visible view is oblong as depicted in FIGS. 5 a - 5   c . FIGS. 5 a  and  5   b  show the first and second video frames respectively, and FIG. 5 c  depicts the image seen by the camera  46 . While FIGS. 5 a - 5   c  depict earth as viewed by a visible light camera, other types of cameras responsive to another part of the spectrum can be used as long as the camera provides pixel data output. It will be appreciated that if a visible light camera is used and the earth in the scene is full, or eclipsed, conditions which exist less than 2% of the time, yaw information is not available. Such conditions occur at noon and midnight near the time of the vernal equinoxes. If it is essential to detect Yaw at all times with the earth sensor, the visible light camera  18  may be replaced by an infra-red (IR) or other type camera which has a non-circular view of the earth. An IR camera or detector provides a non-circular view by distinguishing thermal features of the earth. Some possible thermal features to be viewed are the warm belt of the equator, or the cold nodes at the poles. With an IR camera, yaw can be detected from views presented to the camera  46  similar to those shown in FIGS. 5 a - 5   c  for visible light camera. 
     3) A combination of 1) and 2). 
     4) If the earth changes shape i.e. grows larger, as seen by camera  18 , the change will not be detected by camera  18  since the position of the earth is unchanged, and the current earth scene programmed in MOSLM  32  is larger and eclipses the smaller previous earth scene programmed in MOSLM  34 . As the earth&#39;s shape grows smaller there will be a distinct signature which distinguishes it from a movement, that is, pixels on both sides of the earth scene will be viewed by the camera  46 . They will also be recognizable by their periodicity. 
     As indicated above if the camera  18  is a visible light camera and acting alone, it cannot detect yaw rotation 100% of the time because it relies on the oblong visual shape of the earth for this rotational movement. This drawback can be eliminated by using a camera which portrays the earth in a non-circular manner. An IR camera, for example, would permit unambiguous detection of yaw rotation by relying on the pixel data portraying the thermal features of the earth, i.e. the poles or the equator. Alternatively, instead of this substitution, the secondary camera  20 , which detects a different portion of the spectrum, such as infra-red or ultraviolet radiation, may be added. By adding the second camera other benefits are derived such as greater bias capability and different resolution modes. Bias capability could be implemented by adjusting the lenses on the second camera  20  to view the earth as a smaller image than the first camera  18 . This will allow the image to be moved around in the camera view during on-orbit calibration. By using different lenses on the two camera, slight motions of the spacecraft would not cause loss of the view of earth. For example, if the spacecraft antenna is not pointing in the proper direction following launch, it is desirable to manipulate the spacecraft to achieve the desired pointing. With two cameras such spacecraft movements are not likely to lose sight of the earth. 
     If two cameras are used the translator  24 , and correlator  26  are time shared by the two cameras so that the description above with reference to the programming of the MOSLM&#39;s and processing of the laser beams in connection with the scene viewed by the camera  18  is also applicable to camera  20 . Consequently the image presented to the camera  46  is alternately the difference between the present and previous scene viewed by camera  18  and the difference between the present and previous scene viewed by camera  20 . 
     There are several ways of processing the data from the camera  46  to determine satellite attitude information. One method chosen for its simplicity could be used effectively with an IR camera viewing the full disk of the earth. There would be no yaw motion detection however, using this method. During the initial in-orbit checkout, while most of the spacecraft calibrations are being performed, a calibration of the earth sensor can be done. This task would be to find which pixel of the correlator camera represents the centroid of the earth view. This pixel represents the reference from which motion is detected. It is the desired centroid for the spacecraft mission and is identified by the numeral  52  in FIG. 6 a.    
     As was discussed earlier, when the spacecraft is in operation and there is no movement, there is no sensor output, there are no lit pixels. When motion occurs pixels are lit in the correlator camera  46  which represent the edge of the earth in the direction of the motion. FIG. 6 b  shows pictorially what the camera  46  views and produces as pixel data to the SCP  48 . FIG. 6 b  shows a pitch motion of the satellite in which the field of view of the sensor moves to the west when viewed from the earth&#39;s coordinate system. 
     The SCP  48  takes a minimum of three points from the curve in FIG. 6 b  and calculates a current centroid of the earth. The current centroid identified by the numeral  54  in FIG. 6 c , is compared with the desired centroid  52  to unambiguously provide the magnitude and direction of the rotation or can be used with a nulling routine until the current centroid  54  is moved back to the location of the desired centroid  52 . 
     While the forms of the invention herein disclosed are presently preferred embodiments, many others are possible. It is not intended herein to mention all of the possible equivalent forms or ramifications of the invention. It is understood that the terms used herein are merely descriptive rather than limiting, and that various changes may be made without departing from the spirit or scope of the invention.