The Phase II project will lead to the design, construction, and field-testing of a prototype PHOCS instrument for atmospheric column retrievals of oxygen and carbon dioxide. The carbon dioxide wavelength range may also be extended to add methane and carbon monoxide measurements depending on laser performance. The prototype will include the improvement to heterodyne spectroscopy invented in Phase I. The project will move Mesa Photonics' technology toward commercializing compact, portable instruments that combine high spectral resolution with high sensitivity and rapid data acquisition. The requirements for implementing a successful Phase II prototype are straightforward responses to the lessons learned in Phase I. The Phase II work plan will implement the following hardware improvements: • Mount the light collection optics on a commercial solar tracker that has a built-in GPS to simplify setup at different locations. • Replace the current external cavity laser used for the oxygen spectral range with a laser having two orders of magnitude better wavelength repeatability and an order of magnitude faster wavelength switching speed. • Eliminate the electronic noise canceller. Phase I results showed that fluctuations in collected sunlight intensity is the dominant PHOCS noise source. Laser excess noise is small in comparison. The Phase I invention does a much better job of improving sensitivity than does the noise canceller. • Switch from a narrowly tunable laser to a widely tunable one for carbon dioxide measurements. As with oxygen, the improved laser will allow fast measurements of selected absorption lines. • Consolidate the signal processing electronics onto a custom-built, compact, low-power circuit board and replace the commercial lock-in amplifier with new dual, single-board lock-in amplifier that Mesa Photonics has developed.