Distance measurement in air without the precise knowledge of refractive index fluctuation

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.