A technique based on an ultrasonic ranging system (URS) that yields a simple and accurate way of identifying the vibration modes of a flexible structure is introduced. Preliminary results are reported which are concentrated on some design issues of the URS and its use as a potential displacement measuring device for flexible structures. The URS accurately distinguishes four vibration frequencies of a simple clamped-mass aluminum beam, the highest frequency being near 50 Hz. A 3-D version of the system can be used as a vibration sensor in the evaluation stages of the feedback control schemes of large flexible space structures.