Abstract:
An improved system and method for obtaining photogrammetric measurements which eliminates optical distortions created by thermal gradients on windows that are used to protect the photogrammetry camera. A gate valve is introduced between the window and the wall that opens for a limited time to allow the camera to take measurements of a test article contained within a thermal testing chamber. This limits or eliminates any thermal gradients on the window and improves photogrammetric measurements. With a gate valve the window can be removed entirely, as the gate valve can prevent a thermal gradient from being introduced to the lens of the photogrammetry camera. The improved system is suited for making close-in photogrammetric measures of test articles on earth.

Description:
TECHNICAL FIELD 
     The present invention relates generally to photogrammetry, and more particularly, to improved low temperature photogrammetry for satellite systems. 
     BACKGROUND 
     Historically, photogrammetry of test objects has been performed in a thermal test chamber that incorporates a transparent window into an outer test chamber wall. The transparent window acts as a barrier between the measurement equipment and the test environment, while providing visual access for the photogrammetric equipment through the window. At low temperatures it has been necessary to heat the outer surface of the window to prevent the formation of condensation and consequent loss of visibility. As a result of this heating, a thermal gradient exists across the thickness of the window. Historical data suggests that the thermal gradient causes both a physical and optical distortion in the window, which negatively affects the accuracy of photogrammetry measurements (data) acquired through it. The disadvantage of the current system is that measurement accuracy is degraded at temperatures approaching or equaling those experienced by satellite components in space. 
     In order to improve low temperature (i.e. below condensing or frosting temperature) photogrammetry, the optical distortions should be limited or eliminated. This can be accomplished by limiting or eliminating the thermal gradient that exists across the visual access window or by removing the transparent window entirely. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to limit or eliminate thermal gradients induced upon photogrammetry system windows by moving the window from the outer thermal wall to a position inside the housing containing the photogrammetry camera. A gate valve is added between the outer thermal wall and the window to act as a barrier to the outside environment. The gate valve opens for a limited time to allow individual photogrammetry exposures, and closes before a significant thermal gradient can be induced upon the transparent photogrammetry window. To assist in the thermal isolation of the transparent photogrammetry window within the housing, pressurized dry gas (typically nitrogen) at ambient temperature and compatible with the housing environment is introduced into the housing. The gas maintains the transparent photogrammetry window near ambient temperatures when the gate valve is closed and acts as a thermal barrier between the transparent photogrammetry window and the thermal test chamber when the gate valve is open. 
     It is another object of the present invention in an alternative embodiment to eliminate the use of the transparent photogrammetry window entirely by replacing it with a gate valve. The gate valve is opened for a limited time to allow for photogrammetry exposures. The gate valve remains closed at all other times to maintain the photogrammetry camera near ambient temperatures. To assist in the thermal isolation of the camera, pressurized dry gas is introduced in the housing, thus preventing the formation of condensation on the camera body or lens. In addition, while the gate valve is open, additional pressurized dry gas can be introduced near the camera and lens to act as an additional thermal barrier. 
     Because of these features the present invention is particularly suitable for making photogrammetric measurements of satellite components on earth in simulated extreme temperature conditions. In this way, satellite components may be evaluated for such things as thermal stability prior to being placed in space. 
     Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simple view photogrammetry chamber containing the thermal test chamber and housing containing the photogrammetry camera as seen in the prior art; 
     FIG. 2 is a cross-sectional view of one preferred embodiment of the present invention containing a transparent window member; 
     FIG. 3 is a cross-sectional view of another preferred embodiment of the present invention without the transparent window member; 
     FIG. 4 is a cross-sectional view of another preferred embodiment of the present invention containing a transparent window member; and 
     FIG. 5 is a more detailed view of FIG. 4 containing a satellite component test article. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is described with respect to a photogrammetry camera for use in satellite applications. However, those skilled in the, art would recognize that the embodiments of the present invention might have applications beyond satellite applications. 
     FIG. 1 shows a simple view of the thermal test chamber  118  and the housing  102  of a photogrammetry system  10  according to the prior art. The housing  102  contains a photogrammetry camera  100 . An object (test article)  122  is placed within the thermal test chamber  118 , and a photogrammetric measurement is made by the photogrammetry camera  100  through a window  108  formed integrally within a wall  112  between the thermal test chamber  118  and the housing  102 . The window  108  is typically composed of fused silica and serves to protect the object  122  from the outside environment. To prevent condensation or frost on the window  108 , pressurized gas warms the outer surface  115  of the window  108 . The pressurized gas may be heated if necessary. However, by warming the window  108 , a thermal gradient is induced which can cause an optical distortion at the window  108 . The optical distortion in turn can adversely affect photogrammetric measurements of the test object  122 . 
     FIG. 2 shows one preferred embodiment of the present invention. In FIG. 2, a photogrammetry system  10  has a photogrammetry camera  100  contained within a housing  102  that is mounted on a turntable bearing  104 . The turntable bearing  104  allows the photogrammetry camera  100  to rotate 360 degrees around its optical axis. The system  10  further contains a window  108 , a gas purge plenum  110 , a gate valve  106 , a wall  112 , and a thermal test chamber  118  which contains an object  122  that can be photogrammetrically measured. The wall  112  has an opening  105  that allows for photogrammetric measurements to be taken whenever the gate valve  106  is opened. 
     In the preferred embodiment of FIG. 2, the window  108  is not formed integrally with the wall  112 , instead the window  108  is mounted within the housing  102  in a position located adjacent to the lens  120  of the camera  100 . In addition, the window  108  rotatably cooperates with the camera  100  such that the distortion contribution of the window  108  will not need to be accounted for by the software used to calibrate the lens  120  in the photogrammetric measurements. A gate valve  106  is added between the window  108  and the wall  112 . A chamber  113  formed within the housing  102  is filled with pressurized gas (not shown) from the gas purge plenum  110  when the gate valve  106  is closed. A thermal gradient is thus created across the gate valve  106 , not the window  108 . 
     To operate the photogrammetry camera  100 , the gate valve  106  is opened to expose the window  108  to the thermal test chamber  118  containing a test article  122 . The photogrammetry camera  100  then takes a measurement of the test article  122 . The gate valve  106  is then closed. The entire process typically takes under five seconds. The pressurized gas within the housing  102  maintains the window  108  at approximately ambient temperatures during the entire photogrammetry process, and therefore eliminates a significant thermal gradient from being induced on the window  108 , which in turn eliminates any optical distortion which may adversely affect the accuracy of the photogrammetry data obtained. The pressurized gas may be heated prior to introduction to the chamber  113  to maintain the chamber  113  at acceptable temperatures for the camera  100  equipment. Preferably, the pressurized gas is nitrogen. 
     FIG. 3 shows another preferred embodiment of the present invention. In FIG. 3, the window  108  of FIG. 2 is eliminated entirely. Pressurized gas from the gas purge plenum  110  is introduced into the chamber  113  of the housing  102  and acts to keep the camera  100  at nearly constant temperature while the gate valve  106  is open. Without a window  108 , there is no chance of optical distortion. Thus, the precision and accuracy of the photogrammetric measurements are optimized. 
     To operate the photogrammetry camera  100  of FIG. 3, the gate valve  106  is opened to expose the photogrammetry camera  100 , and specifically the lens  120  of the photogrammetry camera  100 , to the thermal test chamber  118  containing the test article  122 . After the photogrammetry camera  100  takes a measurement of a test article  122 , the gate valve  106  is closed. The entire process typically takes under five seconds. Pressurized gas (not shown) within the chamber  113  keeps the camera  100  at approximately ambient temperatures while the gate valve  106  is opened, thus preventing a thermal gradient from forming on the lens  120 . If desired, one or more nozzles  117  may be added from the gas purge plenum  110  to direct the gas output towards the photogrammetric camera  100  to form a flowing barrier across the camera  100  whenever the gate valve  106  is opened, thus providing for additional thermal isolation of the camera  100  and the lens  120 . Again, the pressurized gas may be heated pressurized gas if necessary to maintain the camera  100  equipment at acceptable temperatures. 
     FIGS. 4 and 5 show another preferred embodiment of the present invention. The system has a window  208 , a photogrammetry camera  200 , a gas purge plenum  210 , and a gate valve  206 . In this embodiment, the turntable bearing  204  cooperates with the thermal chamber wall  212  and the housing  202 . The entire housing  202  can rotate 360 degrees around its optical axis. In addition, the thermal test chamber  218  is much larger in size than the housing  202  and is able to hold larger test objects, such as satellite components  240  as shown in FIG.  5 . This system is particularly suited for making photogrammetric measurements of larger components, such as satellite components  240 , as shown in FIG.  5 . It is also contemplated that the window  208  may be removed and an array of nozzles  117  may be coupled to the gas purge plenum  210  to provide additional thermal isolation of the photogrammetry camera  200  when the gate valve  206  is opened. 
     FIG. 4 illustrates the system when the gate valve  206  is in the closed position. The gate valve  206  creates a barrier between the cold environmental nitrogen in the thermal test chamber  218  and the pressurized nitrogen that surrounds the transparent window  208  within the chamber  213 . 
     FIG. 5 illustrates the system when the gate valve  206  is in the open position to allow the photogrammetry camera  200  to acquire the satellite component  240 . In the open position, a relatively small amount of positive pressure nitrogen enters the thermal test chamber  218 . 
     The embodiment as shown in FIGS. 4 and 5, both with and without a window  208 , have particular use for making close-in photogrammetric measurements of satellite components  240  while on the earth. 
     It is thus the object of the present invention to provide an apparatus and method for obtaining photogrammetric measurements of a test article  122  in a low temperature environment without optical distortion with or without the use of a window  108 ,  208 . The method comprises the steps of introducing a quantity of pressurized gas from a gas purge plenum  110 ,  210  into the chamber  113 ,  213  of the housing  102 ,  202  such that thermal gradients are lessened or eliminated across either the window  108 ,  208  or the lens  120 ; opening a gate valve  106 ,  206  for a few seconds so as to expose the photogrammetry camera  100 ,  200  to a test object  122  contained within the thermal test chamber  118 ,  218 ; obtaining photogrammetric measurements from the photogrammetry camera  100 ,  200  on the test object  122 ; and closing the gate valve  106 ,  206 . When the window  108 ,  208  is not provided, the present invention may also include nozzles  117  located on the gas purge plenum  110 ,  210  that are directed at the lens  120  that provide additional pressurized gas to the lens  120  to provide additional thermal stability. 
     While particular embodiments of the invention have been shown, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. It is, therefore, contemplated by the appended claims to cover any such modifications as incorporate those features that constitute the essential features of these improvements within the true spirit and the scope of the invention.