Climate change and air quality are the most pressing environmental issues of the 21st century. Despite decades of research, the sources and sinks of key greenhouse gases remain highly uncertain [IPCC, 2007] making atmospheric composition predictions difficult. The Geostationary Remote Infrared Pollution Sounder (GRIPS) will measure carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), and nitrous oxide (N2O) with unprecedented precision to reduce substantially this uncertainty. The GRIPS instrument uses gas filter correlation radiometry (GFCR) to detect reflected and thermal IR radiation from geostationary orbit. GRIPS is designed to haves sensitivity down to the Earth's surface at ~8 km nadir resolution. GRIPS can also resolve CO2, CO, and CH4 anomalies in the planetary boundary layer and the free troposphere to quantify lofting, diurnal variations and long-range transport. With repeated measurements throughout the day GRIPS can maximize the number of cloud free measurements determining biogenic and anthropogenic sources, sinks, and fluxes. Finally, the GFCR technique is, to first order, insensitive to aerosols interference. GRIPS is highly complementary to the Orbiting Carbon Observatory, OCO-2, and other existing and planned missions.