Space Vehicle Experimental Modal Definition Using Transfer Function Techniques

In early 1975, a Hughes-built communications satellite was subjected to a ground vibration survey test to determine resonant frequencies, modes, and damping characteristics. Two different techniques were independently utilized to define the space vehicle vibration modal parameters. One consisted of the classical technique where multiple shakers were used to tune in a single resonance to define its frequency and normal mode shape. The second approach utilized recent technology and a data processing technique which provides for more complete data acquisition in a shorter actual on-the-floor test time. The latter method uses a single point random excitation to provide all the data required, which then undergoes a considerable amount of data processing. Transfer functions are computed for each accelerometer location from which the modal properties are assembled. The subject paper describes the two test techniques, data reduction, processing required, and a comparison of results.