Abstract:
An apparatus, system and method for the radiometric calibration of an optical payload consisting of a housing with an optical aperture, at one portion. The optical aperture is utilized for passing light to an imaging device. The housing also includes at least one door located at another portion of the housing. The door receives and directs light into the housing and toward the optical aperture. The door includes a plurality of holes which are disposed directly in the door. When the housing is moved through predetermined angles relative to the sun, the plurality of holes are capable of passing light into the housing at calibrated levels of radiance.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to energy calibration equipment used in spacecraft, and more particularly, to calibration equipment for remote sensing devices. 
     BACKGROUND OF THE INVENTION 
     Planetary imagers are useful for remote sensing of atmospheric compositions, crop assessments, weather prediction and other types of monitoring activities. Monochromatic and multispectral satellite-based, remote sensors are able to measure properties of the atmosphere above the earth, when their detector arrays are properly calibrated for radiometric response. 
     A method of calibrating the radiance measured by these remote sensors is to create a reference radiation using a known source of spectral irradiance, such as the sun. The radiation from the sun may be used as a reference signal which, in turn, may provide a known radiance to a remote sensor for calibrating its detector arrays. 
     The output of the detector arrays may be measured as the remote sensor receives the known energy from sunlight or a diffusive reflector. This radiance calibration method provides sufficient information to correctly measure and calculate other types of radiance incident on the remote sensor during normal operation, when using the output of the remote sensor, as the remote sensor views the earth or other target of interest. 
     In the prior art, there are at least four different methods for calibrating the radiance measured by the sensors. One of these methods is the use of Calibration Light Source Assemblies (hereinafter “CLSA”). CLSAs use the sun as a source of illumination and provide a partial aperture illumination. A second method is the use of a Full Aperture Calibration Door (hereinafter “FACD”). An FACD provides a coating on the inner surface of a calibration door. During the calibration process, the door is opened and the coated inner surface reflects the sun towards the aperture. This provides full or partial aperture illumination. A third method is called Full-Aperture Calibration Surface (hereinafter “FACS”). In the FACS method, a medium or coating is applied to a surface. The surface is then moved into position to reflect the sun as a source of illumination. As with the FACD method, this method provides full or partial aperture illumination. Finally, another method that may be employed is an On-board Calibration Source (hereinafter “OBCS”). In the OBCS method, incandescent lamps, light emitting diodes, or other portable electromagnetic sources (including, but not limited to, radiative black bodies) are used to provide the illumination. The lamps, diodes, and/or other sources are positioned in front of the aperture or otherwise placed such that they illuminate direct energy towards the sensor when calibration is required. 
     SUMMARY OF THE INVENTION 
     To meet this and other needs, and in view of its purposes, the present invention provides a solar calibration device for an optical payload. The solar calibration device includes a housing having an optical aperture, at one portion of the housing, for passing light to an imaging device, and includes at least one door in a closed position, at another portion of the housing, for receiving and directing light toward the optical aperture. The door includes a plurality of holes, disposed directly in the door, configured to pass light into the housing, when the housing is moved to a predetermined angle relative to the sun, and the door is in a closed position. 
     The door of the radiometric calibration device may be either a single panel door attached to the housing or a multi-panel door attached to the housing. Some embodiments of the present invention may include a diffuser configured to pass light towards the optical aperture. This diffuser may be a transmissive diffuser and may be attached to the door panel. The door of the radiometric calibration device may include a plurality of supports and diffuser plates may be placed between the plurality of supports, or a diffuser may be bonded to the plurality of supports. The door may be in a honeycomb-shaped support structure. In some embodiments of the invention two or more doors, may be placed adjacent to the first door. The second door may be displaceable in regards to the first door and may be capable of being slid into a position that allows a light to pass through both doors and a position where light will not pass through both doors. 
     The plurality of holes may be arranged in any number of patterns including a rectangular pattern, a radial pattern, a random pattern or in any another geometric pattern. At least one of the plurality of holes may be tapered, curved or change the angles of redirect. In some embodiments of the present invention, the radiometric calibration device&#39;s door and the plurality of holes in the door may allow light to pass into the housing, when the housing is moved through a range of predetermined angles relative to the sun. In some embodiments of the present invention, the imaging device may also be calibrated by varying light sensitivity levels to account for different applications for the imaging device. 
     Embodiments of the present invention also relate to a method of calibrating an optical payload. The steps of this method include positioning a closed door, affixed to a housing, at a first angle with respect to the sun, for preventing light from entering the housing. The closed door may then be positioned at a second angle with respect to the sun. Light from the sun may then be transmitted through a plurality of holes formed in the closed door to an optical sensor disposed within the housing when the closed door is at the second angle with respect to the sun. Finally, this light may then be used to calibrate the optical sensor. 
     Embodiments of the present invention further relate to a system deployed in orbit around the earth. This system deployed in orbit includes an optical payload and an imaging sensor having a calibration mode and an operational mode. The imaging sensor is configured to receive radiometric radiation during the calibration mode and reflected radiation from the earth during the calibration mode. The system also includes a housing having an optical aperture, at one portion, for passing light to the imaging sensor, and the housing includes at least one door, at another portion, for receiving and directing light toward the optical aperture. The door includes a plurality of holes, disposed directly in the door, configured to pass light into the housing, when the housing is moved to a predetermined angle relative to the sun. 
     It is understood that the foregoing general description and the following detailed description are exemplary, but not restrictive, of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of exemplary embodiments of the invention, may be better understood when read in conjunction with the appended drawings, which are incorporated herein and constitute part of the specification. For the purposes of illustrating the invention, exemplary embodiments of the present invention are shown. It will be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings, the same reference numerals are employed designating the same elements throughout the several figures. In the drawings: 
         FIG. 1  is a perspective-view illustration of an interaction of the rays of the sun with a calibration door. Three different orientations of the sun with respect to the optical payload telescope assembly are included; 
         FIG. 2  is a cross-sectional illustration of the rays of the sun with a calibration door for calibrating the optical payload; 
         FIG. 3A  is a perspective-view illustration of a payload optical telescope assembly with a single panel door; 
         FIG. 3B  is a perspective-view illustration of a payload optical telescope assembly with a split panel door; 
         FIG. 3C  is a perspective-view illustration of a payload optical telescope assembly with a multiple panel door; 
         FIG. 4  is a top-view illustration of a single panel door with a rectangular hole pattern; 
         FIG. 5  is a top-view illustration of a single panel door with a radial hole pattern; 
         FIGS. 6A-6I  are cross-sectional illustrations of various shapes of panel door holes that may be used by the present invention; 
         FIGS. 6J-6L  are cross-sectional illustrations of panel door holes that may be used by the present invention together with a panel insert; 
         FIG. 7A  is a cross-sectional illustration of an unsupported panel door; and 
         FIGS. 7B-7F  are cross-sectional illustrations of various supports used to reinforce a panel door, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention. 
     The present invention relates to a calibration system for calibrating a remote sensing system optical payload. As described in the background of the invention, numerous methods are known in the prior art to calibrate optical payloads. These methods, however, include disadvantages. For instance, CLSAs provide only partial aperture illumination and remain as an obstruction in the telescope aperture when not in use. Furthermore, CLSAs require sensitive optical alignments, which in turn, require additional processing power and calibration time. 
     With FACDs, the coating disposed on the calibration surface degrades over time, thereby altering the calibration obtained at different periods. The door surface must be made extremely flat to avoid shadowing at low angles of incidence. To properly calibrate the payload, the angle of the opened door with respect to its housing must be known accurately. The FACD also consumes considerable power during calibration upon moving the door into position and holding the payload in a proper orbit. 
     With FACSs, as with FACDs, the reflective coating used on the door surface degrades, thereby altering the calibration as time goes on. Furthermore, the reflective surface must be deployed into proper position when in use, many systems of which require a high precision angular encoder, and stowed when not in use. This requires additional processing. 
     Finally, with OBCSs, the incandescent lamps, LEDs, or other electromagnetic sources (hereinafter “EMS”) may degrade and burn out over time. The incandescent lamps, LEDs, or other EMS also require additional power when illuminated during a calibration process. 
     The present invention avoids these disadvantages by including a system that uses sunlight to calibrate the optical payload, without requiring the panel door to be moved. During calibration, the panel door remains in a closed position. Therefore, no additional power is required during the calibration process. In addition, the door&#39;s angle with respect to the sun may be controlled by an onboard attitude control system used for attitude control of the satellite, rather than a specially built control mechanism used for opening and closing a door. 
     As shown in  FIG. 1 , payload optical telescope assembly  110  is fitted with calibration door  120 . Calibration holes  130  are formed in calibration door  120 . These calibration holes may be formed as straight-through apertures from the input surface to the output surface, as shown in  FIG. 1 , or in a number of other configurations. 
     Calibration holes  130  are formed at an angle with respect to the horizontal plane of the door, as shown in  FIG. 1 . This advantageously permits sunlight to pass through calibration door  120  only when the sun is located at predetermined angles with respect to the door. As shown in  FIG. 1 , the calibration holes are formed such that when the sun is at position  140 , sunlight does not pass through the calibration door. At that position, photons  142  are reflected off the top of the calibration door and away from optical telescope assembly  110 . The photons do not pass through the calibration door because the angle of entry of the photons does not align with the angle of the calibration holes. 
     Similarly, when the sun is at position  160 , photons  162  do not pass through calibration door  120 . Most photons  162  reflect off the top of the calibration door. When the sun is in position  150 , however, photons  152  pass through the calibration door. When the sun is in position  150 , the angle of photons  152  matches the angle of incidence of the calibration holes and allows the solar illumination to enter the payload optical telescope assembly. Therefore, during a normal course of the payload&#39;s orbit, when a proper alignment is achieved between the angle of incidence of the calibration holes and the sunlight angle, the photons would pass through the calibration door and a full-aperture calibration may be performed. 
     Similarly, the incidence angle of calibration holes  130  may be set such that during a normal orbit of the payload, the sun never aligns at a correct angle for full-aperture calibration. In this example, the payload may be re-oriented to achieve a proper alignment with the sun in order to perform a full-aperture calibration. Re-orienting the entire payload is easier and more accurate than in conventional calibrations that move a door between an operational position and a stowed position. No additional motors are required by the present invention over and above those required for normal payload orbiting maneuvers. 
     In some exemplary embodiments of the present invention, it may be possible to allow different concentrations of photons through the calibration door  120 . In some embodiments, as the optical payload is oriented around the sun and the relative position of the sun changes from position  140  to position  160 , it may be possible to have different angles allow different concentrations of photons to enter the calibration door  120 . This possibility depends upon the size and shape of the holes as well as the desired uses for the optical payload. The sensor system in these embodiments may be programmed to received different concentrations of photons in accordance with the angle of the optical payload to the sun. In these cases, the sensors and the optical payload may be able to be calibrated such that the optical payload has different light sensitivities that may be used in different applications. 
       FIG. 2  shows an exemplary cross-sectional view of rays of sunlight  210  entering housing  200  during a calibration process. When housing  200  is moved into a predetermined orientation relative to the sun, rays  210  align with calibration holes  230  formed in calibration door panel  220 . Once rays  210  pass into the calibration holes, the rays may be transmitted downwardly towards the opposite end of the housing as shown in  FIG. 2 , or rays  210  may be directed into an optional transmissive diffuser  260 . Importantly, rays  210  are directed toward optical device  240  through the receiving aperture  270  for calibrating its sensor array  250 . Sensor array  250  may be placed opposite calibration door  220 , or may be moved into a proper position for calibration. Sensor array  250  may also be calibrated after properly orienting one or more optical elements (not shown) to transmit the sun&#39;s rays onto the sensor array. 
     The present invention provides several advantages over the prior art. Specifically, the calibration door provides a full-aperture uniform illumination scene at the entrance aperture of the telescope, out of focus of the optical instrument. The use of rear illumination to perform the calibration eliminates the requirement that the door surface be flat. This happens because rear illumination avoids the shadowing effects that may occur with front illumination calibration. The angle of the calibration holes may be chosen such that the photons cannot enter the telescope aperture without making contact with at least one surface. This can provide the user with the option to make the contacting surface a dispersion surface which may aid the calibration process. Furthermore, the angle of the calibration holes may be designed and arranged to provide uniform illumination for specific wavelength regions and/or illumination intensities. 
     Additionally, as illustrated in  FIGS. 6K ,  6 L and  7 C- 7 F, the door panel may be fitted with a semi-transparent medium that allows for back-illumination to uniformly disperse the solar radiation. A final advantage of the present invention is that the angle of the calibration holes does not change over time. Because the angle of the calibration holes does not change, the optical payload is provided with uniform solar illumination for every calibration event throughout the life of the mission. 
     Referring next to  FIGS. 3A-3C , there are shown various types of door panels that may be used by the present invention. In  FIG. 3A , payload optical telescope assembly  300  is fitted with a single door panel  310 . Single door panel  310  is pivotally attached at one location to optical telescope assembly  310 . As shown in  FIG. 3A , the single door panel may be circular to match the circular shape of an optical telescope assembly, or may be any other shape such as rectangular, square or elliptical. 
       FIG. 3B  shows a payload optical telescope assembly  300  fitted with a split panel door  320 . Split panel door  320  is divided into two panels  322  and  324 . Panels  322  and  324  are pivotally attached to optical telescope system  300 . In instances where the optical instrument needs to be accessed, panels  322  and  324  may be opened. As shown, the split panel door may be circular to match the circular shape of the optical telescope assembly, or may be any other shape, such as rectangular, square, or elliptical. 
       FIG. 3C  shows a telescope assembly  300  fitted with a multiple panel door  330 . Multiple panel door  330  is divided into multiple panels  332 ,  334 ,  336 ,  338 ,  342 ,  344 ,  346  and  348 . Multiple panel door  330  may be attached through a track system (not shown) that allows the multiple panel door  330  to be retracted across the top of the optical telescope assembly  300  such as with a garage door, or individually pivoted such as with a louvered window. As in the other configurations, the split panel door  320  may be circular, rectangular, square, or elliptical. 
     During construction of the telescope system, a radiometric analysis may be performed to determine the shape, size and pitch of the apertures included in the door panel. A possible calibration hole layout may be a rectangular hole pattern as shown in  FIG. 4 . As shown, calibration door  400  includes a pattern of calibration holes  410  formed in multiple rows. The calibration holes have the same horizontal distance within each row, and the separation between each row may have the same vertical distance. Each row may be offset horizontally from an adjacent row, as shown in  FIG. 4 , or may not be offset. 
     The diameter of each hole may be at least 1.0 centimeter. For larger-sized calibration doors, such as 1.5 meters or larger in diameter, the space between each hole may be at least 5 centimeters. For smaller-sized calibration doors, the space between each hole may be less than 5 centimeters. It will be appreciated, however, that these numbers may change based on light sensitivity analysis of the optical payload. 
       FIG. 5  shows another pattern that may be used as an embodiment of the present invention. As shown, calibration holes  510  are arranged in a radial pattern. The density of calibration holes  510  increases towards the center of calibration door  500 , whereas the spacing between the holes increases towards the outer end of calibration door  500 . This pattern may be utilized for optical payloads that require greater brightness towards the center portion of its receiving aperture, and require less brightness towards the outer portion of the receiving aperture. Conversely, an optical configuration with a large central obstruction may opt to have no hole pattern in the vignetted portion of the optical field of view. As described with respect to  FIG. 4 , the spacing, or pitch between calibration holes  510  and the size of each calibration hole may be determined by a radiometric analysis. 
     The present invention is not limited to the patterns that are disclosed in  FIGS. 4 and 5 . In addition, the distribution of calibration holes may be completely random, or may be deterministically random in appearance. 
       FIGS. 6A-6H  illustrate exemplary embodiments of various apertures that may be used for the calibration holes. The present invention is not limited to the shapes that are shown, but rather the intent is to show the flexibility of the aperture shapes, which may be based on a radiometric analysis. Essentially, any shape, size, or taper angle is possible provided that the hole allows photons to reach the radiometric device under calibration. 
       FIGS. 6I-6L  and  FIGS. 7A-7F  illustrate different calibration doors that may be utilized by the present invention, depending on the aperture size of the receiving device under calibration and the required sturdiness of the calibration door. As shown in  FIGS. 6I and 7A , for smaller doors, the calibration door may include a single panel. For larger payloads, additional support may be needed for the calibration door. In such case, a single panel may be used supplemented with supports  610  or  710 , as illustrated in  FIGS. 6J and 7B . Supports  610 ,  710  provide the door with additional structure to prevent bending, warping, or movement during various operations of the payload, and opening and closing of the door. Supports  610 ,  710  may be of any size or shape, including longitudinal or latitudinal support slats, honeycomb shaped supports  720  (illustrated in  FIG. 7F ), or other lattice shapes. 
     Similarly, for additional support, a semi-transparent material  620 , or  720  may be placed between supports  610  or  710 , respectively, as shown in  FIGS. 6K and 7C . The semi-transparent material  620 ,  720  may be used as a diffuser to diffuse the sunlight, before it reaches the receiving aperture of the radiometer under calibration. Such diffusers may be transmissive or reflective in nature. Transmissive diffusers may be made from ground or frosted glass; they may also be made from opal glass or small particulate scatterers placed in a transparent matrix. Transmissive diffusers may also be made from screens, or pinhole arrays. Still another type of diffuser may be a diffractive diffuser, such as diffractive scatterers formed from micro-lens arrays or holographic material. 
     The semi-transparent material  620 ,  720  may be placed between supports  610 , or  710 , as shown in  FIGS. 6K and 7C , or may be bonded with the supports, as shown in  FIGS. 6L and 7D . Furthermore, depending upon the amount of diffusiveness of the transparent material, the calibration hole may be drilled through the calibration door, but not into the semi-transparent material disposed between the supports and under the calibration door. This is illustrated in  FIG. 6K . 
     In an alternative embodiment, as shown in  FIG. 6L , the calibration hole is drilled through the calibration door and partially into the semi-transparent material  620 . Although not shown, it will be understood that the calibration hole may be drilled through the entire width of the calibration door and the entire width of semi-transparent material  620 . 
     As illustrated in  FIGS. 7C and 7E , the semi-transparent material inserted between the supports may be made as thin or as thick as the supports themselves. The thickness of the semi-transparent material may depend on the amount of support needed, and the diffusive qualities of the semi-transparent material. 
     In yet another embodiment of the present invention, two or more calibration doors may be stacked on top of one another. These doors may be displaced in relation to one another, so that the calibration holes on each door may be aligned to allow light to pass through each door. With this double or multiple door alignment, the optical payload may be rotated into position, and the doors may be aligned to each other to allow passage of light to the radiometer under calibration. In multiple-panel door embodiments, using a multiple door alignment may not require all panels in the doors to have calibration holes. In these embodiments, this will allow only some panels to be aligned to allow light to pass through the door. 
     Although this invention is discussed primarily in use with an orbit based optical payload system, it is understood that this invention may be used with any number of optical payloads that require calibration. Some embodiments of this invention may be used with Commercial Remote Sensing Payloads including NextView, WorldView, and AdvancedView. Furthermore, it is envisioned that embodiments of this invention may also be used with various Government Remote Sensing Payloads. Embodiments of this invention may also be used with Scientific Remote Sensing Payloads such as ABI, VIIRS, GEOS or CRIIS. Finally, embodiments of this invention may also be used with Ground-based Calibration Equipment as a cost effective replacement for very Large Integrating Spheres. 
     While preferred embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. Accordingly, it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.