The Photogrammetric Appendage Structural Dynamics Experiment

The Photogrammetric Appendage Structural Dynamics Experiment (PASDE) is a Hitchhiker payload scheduled to fly as part of the International Space Station (ISS) Phase-I flight program to the Russian Space Station Mir. The objective of the first flight of PASDE on STS-74 is to obtain video images of the Mir Kvant-II solar array response to various structural dynamic excitation events. This experiment will demonstrate the use of photogrammetric techniques for onorbit structural dynamics measurements. Photogrammetric measurements will provide a low cost alternative to appendage mounted accelerometers to the ISS program. The PASDE experiment hardware consists of three instruments each containing two video cameras, two video tape recorders, a modified video signal time inserter, and associated avionics boxes. The instruments were designed and built at the NASA Langley Research Center, and are integrated into standard Hitchhiker canisters at the NASA Goddard Space Flight Center. The Hitchhiker canisters are then installed into the Space Shuttle cargo bay in locations selected to achieve good video coverage and photogrammetric geometry. The measurement resolution of the instruments is expected to be on the order of 0.25 cm (0.1 in.). Introduction The Photogrammetric Appendage Structural Dynamics Experiment (PASDE) is an experiment to mitigate technical risk and cost associated with passive, on-orbit, measurement of spacecraft appendage structural response for the International Space Station (ISS) program. The experiment will demonstrate a photogrammetric method for making appendage structural measurements, provide engineering data on solar arrays designs expected to be used on the ISS, and verify that routine on-orbit spacecraft operational events provide sufficient excitation for structural response testing. On-orbit measurements of spacecraft structural response are often desired or necessary for structural verification and loads prediction validation. Typically, acceleration response time-history data is collected and processed on the ground. From this data, structural dynamic characteristics (structural mode frequencies, damping, and mode shapes) can be determined using parametric identification algorithms such as the Eigensystem Realization Algorithm (ERA) [1]. The use of photogrammetric measurements is a low cost alternative to dedicated accelerometer-based structural response measurement systems, especially when measurements are required for articulating or rotating spacecraft components such as solar arrays or thermal radiators. Elimination of accelerometers, wiring, signal conditioning and digital conversion electronics, etc., can greatly simplify the spacecraft electrical design and integration, with corresponding reduction in spacecraft cost. For the International Space Station (ISS), Figure 1, on-orbit structural response measurements are required for loads validation and verification of structural mathematical models. Currently, accelerometer-based measurements of the US. primary truss and modules are being planned, however, accelerometer measurement of the US. solar arrays are not being considered because of cost and resource impacts. Since the current ISS design calls for numerous video cameras mounted at various external points, photogrammetric measurements of solar array structural responses is a potential alternative. The PASDE experiment will verify that photogrammetric measurements can provide measurement resolution and accuracy sufficient for ISS structural verification purposes. It is manifested as part of the ISS Phase I Risk Mitigation Program. ISS Phase I involves seven flights of the U. S. Space Shuttle to the orbiting Russian Space Agency Mir space station. Current plans call for PASDE to fly twice as part of the Phase I program. NASA Langley Research Center (LaRC) is funded by NASA Headquarters Code X for development and first flight of PASDE. PASDE hardware will be flown as a Class D, NASA Goddard Space Flight Center (GSFC) Hitchhiker payload. On STS-74, the second Space Shuttle flight to Mir, PASDE will fly along with the Glo-4 experiment [2] as the Hitchhiker Glo-4/PASDE or GPP payload. On STS-86, the seventh mission of Shuttle to Mir, PASDE hardware will be used to obtain measurements as part of the Mir Structural Dynamics Experiment (MiSDE) Risk Mitigation Experiment. Funding for the STS-86 flight of PASDE is provided by the ISS Phase-I program. The remainder of this paper is organized as follows. The concept of photogrammetric structural response measurements is discussed first, followed by specific measurement requirements for the PASDE experiment. The design and fabrication of hardware to meet the measurement requirements is covered next, followed by the STS-74 mission science objectives and planned operations. Photogrammetric Structural Response Measurement Photogrammetry is defined as "the science of making reliable measurements by the use of photographs..." [3]. In the case of a single photograph or image, certain precise measurements of an object in the image, such as geometrical size, can be made under appropriate conditions. With several images of an object, again under appropriate conditions, the orientation and/or position of Figure 1 International Space Station (ISS) the object with respect to the positions of the cameras can be determined using a triangulation process [4]. The precision of the position or orientation determination is dependent on the quality and number of images. At least two independent images are required for triangulation, with more images leading to higher precision. If the object of interest is moving, determination of the position or orientation as a function of time requires a sequence or series of images from each camera location and triangulation at each time of interest. Measurement of time-varying structural response, which is of interest here, is an example of this third type of photogrammetric measurement. The measurement of structural dynamic response using photographic cameras has previously been limited by costs of film and film processing, alignment of photographic frames in time, and the significant manual processing inherent in using photographs for triangulation. The recent advent of low-cost, charge-coupled-device (CCD) video cameras, video recorder systems, and digital image processing techniques have eliminated most of these limitations. Several efforts at measuring structural response by photogrammetric methods have been made [e.g. 5,6]. In the 1984 Solar Array Flight Experiment (SAFE) for example, photogrammetric measurements of a solar array cantilevered from the cargo bay of the Space Shuttle were made from video recorded by the Shuttle's on-board camera systems. The position of the array in the cargo bay was fixed with respect to the Shuttle camera system in this experiment. As shown in Figure 2, the motion recorded in the SAFE video images was directly proportional to the structural deformation of the array as it responded to various structural excitation sources. For the case of the International Space Station solar arrays, there will be relative, rigidbody motion of the solar array with respect to the positions of the photogrammetric cameras. This motion is due to articulation and/or rotation of the array as it tracks the sun in Earth orbit. Thus the motion measured by a photogrammetric system for ISS solar arrays will consist of combined rigid-