NOVELTY - The system (10) has a receiver (40) for receiving a returned online signal and two returned offline signals from a target (70) i.e. Earth, where the signals are transmitted from a transmitter (20) toward the target. A processor determines a concentration path length (CPL) value based on the online signal and one of the offline signals and another CPL value based on the online signal and the other offline signal. A filter correlates the former and the latter values with respective CPL mean and CPL standard deviations for determining whether the values are useable.    USE - Remote sensing system i.e. airborne platform-based three-line tunable differential absorption lidar (DIAL) laser optical sensor, for detecting chemical species i.e. natural gas such as methane, above natural gas distribution pipelines and explorations areas, in a remote sensing platform i.e. Cessna Caravan (RTM: single turboprop engine, fixed-gear short-haul regional airliner and utility aircraft) aircraft, utilized for security, conservation and monitoring by government and private industry. Can also be used for detecting ethane, carbon dioxide, and propane on a moving platform such as fixed-wing aircraft, helicopter, unmanned aerial vehicle (UAV), balloon, automobile, lorry, and a fixed platform such as a building and tower.    ADVANTAGE - The system ensures that the average CPL value allows better relative measurement of the CPL above the background CPL if two measurements of the CPLs for a single column of air are correlated, so that multiple samples of the same column of air are not required to obtain a good measurement. The system provides an indication that an issue such as laser alignment, occurs within the DIAL sensor, if the two measurements show no correlation, so that area survey work can be accomplished in a rapid manner without missing a leak indication or reporting a false positive leak indication.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of a remote sensing system including an online and offline laser sources.    Remote sensing system (10)    DIAL sensor (15)    Transmitter (20)    Receiver (40)    Target (70)