Modal Identification from Dynamic Environmental Testing of the PPF/ENVISAT-1 Satellite

Following the mechanical verification logic of spacecraft structures, a comprehensive programme of system identification and qualification tests was performed on the PPF/ENVISAT-1 Flight Model, the largest European satellite up to now. In the course of this test programme it was required to obtain a modal identification of the satellite from the Sine Vibration Test data, because a dedicated Modal Survey test as for the Structural Model was not foreseen for the Flight Model. The Sine Vibration Test was performed on HYDRA hydraulic multi-axis shaker at ESTEC. The Sine Vibration Test data were to be utilized to identify the dynamic properties used to derive the flight loads and to validate the structural dynamic mathematical model of the spacecraft in terms of eigenfrequencies, mode shapes, and damping values. It was anticipated, that this baseline approach of modal identification could possibly be complicated by the cross-couping of the excitation axes of the HYDRA facility. Therefore an alternative approach of modal identification was requested. From the experiences with the Structural Model, DLR proposed to perform a simplified Modal Survey Test to complementary identify a number of modes. This "Mini Modal Survey Test" was to be performed in a simplified manner due to test and schedule constraints. The present paper describes the test performance and the modal identification of the Flight Model of PPF/ENVISAT-1 following both approaches. The benefits and limitations of both methods are outlined. Summarizing recommendations are given to improve the approach for future applications