Practical aspects of on-orbit modal identification using free-decay data

Future large space structures such as Space Station Freedom can be tested as a total structure on Earth. Size, zero g design, and the existing atmosphere conflict with traditional structural dynamic testing. On-orbit modal identification will become necessary. Practical aspects of performing such tests using time domain analysis of free decay data are discussed. The effects of environmental constraints, structural characteristics, and excitation and sensing are reviewed. A recent laboratory application and sensing are reviewed. In this test, an on-orbit experiment is simulated using free decay data and a limited number of excitation and measurement points. The test article is dynamically similar to the truss/solar arrays of Space Station. The identified modal parameters from this simulation correlate well, though not uniquely, with those obtained in a complete modal survey. Practical difficulties in performing the correlation are illustrated. Model parameters are identified with the Eigensystem Realization Algorithm.