  We propose a direct and real-time ranging scheme using an optical frequency
combs, able to compensate optically for index of refraction variations due to
atmospheric parameters. This scheme could be useful for applications requiring
stringent precision over a long distance in air, a situation where dispersion
becomes the main limitation. The key ingredient is the use of a mode-locked
laser as a precise source for multi-wavelength interferometry in a homodyne
detection scheme. By shaping temporally the local oscillator, one can directly
access the desired parameter (distance) while being insensitive to fluctuations
induced by parameters of the environment such as pressure, temperature,
humidity and CO$_2$ content.
