  Vibrations are among main causes of fatigue and damages leading to
destruction of rotating blades. Consequently, motions of blades have to be
carefully studied, and in particular, periodic components. Blade Tip-Timing
(BTT) methods achieve non-intrusive measurements which is a great advantage
with respect to classical methods. They can be modelled by a Periodic
Nonuniform Sampling of order L equal to the number of probes (PNSL). In this
paper, we develop the case L=2, and we prove that this model is sufficient for
finding the places of asynchronous frequency lines, with their amplitudes and
their phases. To increase L will increase the redundance and then the accuracy
of the method.
