A new radio occultation instrument system, the Active Temperature Ozone and Moisture Microwave Spectrometer (ATOMMS) will be built and deployed in an aircraft-to-aircraft occultation demonstration. It will remotely sense atmospheric moisture, ozone, temperature and pressure with an unprecedented combination of features and accuracy, opening a new window into understanding atmospheric behavior. The system will be built in two years using a simple and flexible design that has been developed over several years of previous research. The system consists of transmitters and receivers that will probe the 22 and 183 GHz water lines and 195 GHz ozone line actively with a series of tunable monochromatic tones in an aircraft-to-aircraft occultation geometry. There are sufficient tones near the 22 GHz water line to separate the effects of water liquid and vapor phases. With its 30 cm refractive antenna, the system will provide the very high signal-to-noise ratios needed for precise measurements while probing relatively high optical depths down to the surface through water clouds. Simultaneous with the hardware development, we will build the initial version of the retrieval software needed to derive atmospheric profiles from the ATOMMS occultation measurements. Once built and tested, during the third year the 2 sets of ATOMMS transmitters and receivers will be integrated into the existing gimbaled optical benches in the nosecones of the two NASA WB57F aircraft. With NASA support for aircraft operations, the first set of ATOMMS occultation measurements will be conducted between the aircraft. These operations will involve flying over the Atmospheric Radiation Measurement Southern Great Plains site in Oklahoma, and coordinating with overpasses by NASA's A-train satellites and the COSMIC Global Positioning System radio occultation mission. The data acquired will be analyzed, comparing the water vertical profiles derived from the ATOMMS 22 and 183 GHz water lines to profiles obtained with surface-based and orbital sounding systems. ATOMMS offers a unique and unprecedented capability for globally measuring key atmospheric parameters fundamental to climate studies. These aircraft-to-aircraft occultations offer a cost-effective way to demonstrate and assess the ATOMMS concept and its performance without the very costly step of placing at least one if not both of the instruments in orbit. Successful aircraft demonstrations will pave the way for a future orbital mission.