We realized from the outset that airborne measurements are critical for providing information about radiative fluxes, water vapor profiles, and cloud top properties in the atmosphere.  An ideal measurement platform is an aircraft that can hover continuously at high altitudes above a cloud and radiation testbed site.  The DOE asked Sandia to lead a multilaboratory program to develop the necessary instrumentation and measurement techniques to use unmanned aerospace vehicles for climate measurements. We used the General Atomics Altus unmanned aerospace vehicle on a record-setting scientific flight of twenty-six continuous hours over the Oklahoma cloud and radiation testbed site.  Subsequently, the Altus set a new altitude record of 43,500 feet for a single-stage turbocharged engine and was used as part of a large test at the atmospheric radiation measurement site.  The radiation flux profiles obtained there, in conjunction with the simultaneous in situ aerosol and cloud microphysical sampling conducted by manned aircraft, constitute a unique data set for testing radiation models.  We are currently on a joint mission with the National Aeronautics and Space Administration, using a two-stage turbocharged Altus to extend these measurements to altitudes above 50,000 feet to study tropical cirrus clouds and their effects on the radiation budget.