Mesa Photonics proposes development of a passive optical sensor for simultaneous high-precision measurement of oxygen and carbon dioxide profiles within the full atmospheric column. The approach, which is based on near-infrared heterodyne spectroscopy using solar occultation (i.e., direct solar viewing), is called Precision Heterodyne Oxygen-Calibrated Spectrometer, or PHOCS. Oxygen measurements will provide dry gas corrections and more importantly will determine accurate temperature profiles that, in turn, improve the precision of the carbon dioxide column retrievals to better than 1%. Planned instruments will complement results anticipated from the Orbiting Carbon Observatory (OCO-2), Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS), and ground-based Fourier transform spectrometers. PHOCS instruments will be small (not much bigger than a pair of binoculars), light weight, and low power. In keeping with one of the goals of this SBIR topic, planned instruments will be initially configured for operation on the ground, and have size, weight, and power (SWAP) characteristics suitable for easy ground mobility and well as airborne or space-borne deployment. The Phase I project will test an all-fiber-optic heterodyne receiver that will simplify optical design and ensure long-term optical alignment. Oxygen measurements will use the near-infrared band the 1.27 micron wavelength region instead of the more commonly used band at 0.76 microns. The longer wavelength band is weaker; precise lineshapes of many individual rotational lines will be measureable without complications due to highly saturated absorbances or instrument line shape functions (ILS). Carbon dioxide measurements will use the well-characterized band at 1.57 microns.