  The accuracy of long distance measurement in air is limited by the
fluctuation of refractive index. In this paper, we propose a technique which
allows us to measure an absolute distance in air without the knowledge of air
turbulence. The technique is based on a femtosecond frequency comb. The
fluctuation of the environmental conditions is monitored by two independently
operating reference interferometers. The deviations of optical path lengths,
caused by the fluctuation of air refractive index, is compensated by feedbacks
from the reference interferometers. The measured optical path length is then
locked to certain environmental conditions, determined at an optimized moment
before the measurement process.
