  Long-distance measurements (10 m - 1000 m) with an accuracy of 10-7 is a
challenge for many applications. We show that it is achievable with Frequency
Shifted Feedback (FSF) laser interferometry technique, provided that the
determination of the radio frequency beat order be made without ambiguity and
on a time scale compatible with atmospheric applications. Using the pulsed-FSF
laser that we developed for laser guide star application, we propose and test,
up to 240 m, a simple method for measuring the beat order in real time. The
moving-comb and Yatsenko models are also discussed. The first of these models
fails to interpret our long-distance interferometry results. We show that the
accuracy of long-distance measurements depends primarily on the stabilization
of the acoustic frequency of the modulator.

Optics