Mesa Photonics proposes to develop an Airborne Multi-Gas Sensor (AMUGS) based upon two-tone, frequency modulation spectroscopy (TT-FMS). Mesa Photonics has developed a fiber-coupled implementation of TT-FMS that leverages telecommunications lasers as sources and WiFi electronics for signal processing. With this approach, sensitive trace gas detection in a robust, compact and low-power package has been achieved. In work to-date, we have demonstrated the capability of this TT-FMS concept in a single channel. In the proposed project, we aim to demonstrate multiplexing of TT-FMS to provide simultaneous, real-time measurement of carbon dioxide (CO2) and methane (CH4). In addition to demonstrating the extensibility of TT-FMS, the Phase I project would address key performance limitations encountered in early feasibility work. The objectives of Phase I are to: - Implement and test polarization management during phase modulation to reduce baseline variation induced by observed in previous work, - Build a two-channel TT-FMS with breadboard components, - Establish baseline (single-channel) performance of TT-FMS when polarization maintaining phase modulation is included, - Calibrate and evaluate the full AMGUS (two-channel, TT-FMS) apparatus. Accomplishment of Phase I objectives would yield a benchtop technology ready for transition to a UAV-compatible AMUGS prototype in Phase II.