QuinStar Technology proposes to develop an efficient, GaN based solid-state power amplifier (SSPA), operating over the 106 to 114 GHz with 4-W output power and power-added efficiency (PAE) > 25%, for the Scanning Microwave Limb Sounder on the Global Atmospheric Composition Mission and the SOFIA (Stratospheric Observatory for Infrared Astronomy) airborne observatory. The proposed amplifier will drive the LO multiplier chain for mixers in the submillimeter-wave detector. QuinStar plans to meet the power and efficiency goals of this program by employing a combination of state-of-the-art (SOA) GaN device technology, innovative circuit design techniques and power combining techniques. We plan to design the MMIC amplifier in a quasi-switching mode with high-Q matching networks implemented on chip. Second, for improved efficiencies, we are proposing to limit the MMIC power level to about 1 watt and use high-efficiency waveguide circuit combining techniques to achieve higher power levels. For this, we propose to use a high-efficiency, 4-way H-tee combiner. Computer simulations show that this approach is capable of realizing combining efficiencies of greater than 95%. QuinStar is the leader in power combining techniques at millimeter-wave frequencies. We have developed both radial and E/H-plane waveguide combiners with SOA power levels and efficiencies in the past decade. Potential NASA Applications (Limit 1500 characters, approximately 150 words): NASA Earth Science missions require submillimeter-wave remote sensing instruments to monitor air quality, climate variability and change, ozone layer stability, weather, and the global hydrological cycle. Due to its shorter wavelength, submillimeter-wave sensors can provide enhanced resolution over lower frequency sensors. A key enabler for this technology is an F-band (106-114 GHz) solid-state power amplifier (SSPA) capable of providing 4-W output power and power-added-efficiency as high as 25% such as the one described in this proposal. Potential Non-NASA Applications (Limit 1500 characters, approximately 150 words): Applications of the W/F-band high-efficiency amplifier technology abound at agencies ranging from helicopter landing and obstacle detection/avoidance radars to cloud radar, UAV, and DoDs V/W-band (Hotspots) communications systems. Space-based applications include broadband RF cross-links communications constellations and long duration reconnaissance UAV missions in airborne. Duration: 6