Abstract- ATOMMS represents a new class of active, airborne, limb-viewing spectrometer that is a cross between Global Positioning System (GPS) occultations and NASA's Microwave Limb Sounder. ATOMMS will characterize atmospheric water vapour and ozone by actively probing the absorption lines at 22.2 GHz, 183.3 GHz and 195 GHz, respectively. Two instrument packages are being constructed for NASA's WB-57F high altitude research aircraft, now equipped with precise WAVES gimballed pointing systems. One aircraft will generate multiple tones near the 22 GHz water line and 183 GHz to 204 GHz absorption lines and transmit them across the Earth's limb through the atmosphere to receivers on a second aircraft. Flight paths of the two aircraft begin over the horizon, with the two aircraft flying at 65 kft altitude. This creates a rising occultation geometry as the aircrafts fly towards each other. ATOMMS provides the sensitivity, vertical spatial resolution and accuracy needed to satisfy key monitoring needs for temperature, pressure, moisture and ozone. The 100 to 200 m ATOMMS vertical resolution will far surpass the 1 to 4 km vertical resolution of present state-of-the-art satellite radiometers opening a window into atmospheric scales previously inaccessible from space. Predicted precisions of individual ATOMMS temperature, pressure and moisture profiles are unprecedented at ~0.4 K, 0.1% and 1-3% respectively, extending from near the surface to the flight altitude of ~20 km. ATOMMS ozone profiles precise to 1-3% will extend from the upper troposphere well into the mesosphere. Other trace constituents such as water isotopes can be measured with performance similar to that of ozone. The ATOMMS experiment is a pathfinder experiment for eventual implementation on a constellation of satellites. Space observations from multiple satellites in precessing orbits will allow for global spatial coverage and increased altitude coverage. Our long term goal is a constellation of approximately a dozen small spacecraft making ATOMMS measurements that will provide dense, global coverage and complete cloudpenetration and diurnal sampling every orbit. The ATOMMS instruments have been completed and are now undergoing extensive laboratory and ground testing. We report on the laboratory testing results including the differential amplitude and phase stability of the instrument and systems integration testing. We will also report on ground testing experiments, where the ATOMMS instruments, located on two building tops, were used to measure atmospheric water vapour content. Comparison measurements were made using in-situ hygrometers. Further ground-based tests are planned to exercise the full ATOMMS system, including the GPS-based positioning and time correction system, accelerometer system and dual-one-way phase correction system. We will also discuss planned instrument upgrades to be implemented in preparation for air-to-ground and air-to-air flights on the WB-57F aircraft.