The Air Force Phillips Laboratory is conducting a series of measurements at the Air Force Maui Optical Station (AMOS) facility on Maui, Hawaii, to determine system requirements for an airborne long path CO2 DIAL system. The lidar incorporates a cavity-matched 3-J laser with the 60 cm diameter AMOS laser beam director telescope. The beam propagation path is approximately 21 km, originating at the AMOS facility on Haleakala at an altitude of 3 km ASL, and terminating at a target site near sea level. Both heterodyne and direct detection techniques are being compared with respect to radiometric performance and signal statistics. Radiometric models including system optical characteristics, beam propagation considerations, target reflectivity characteristics, and atmospheric effects have been developed and validated experimentally. Predictions and results are presented, compared, and discussed.