The Geostationary Lightning Mapper (GLM) instrument selected to fly on the National Oceanic and Atmospheric Administration (NOAA) GOES-R Series environmental satellites has very unique requirements as compared to an imaging array. GLM's requirements to monitor lightning on a continental scale will provide new insight into the formation, distribution, morphology and evolution of storms. A 500 frame per second backside illuminated frame transfer CCD imager (STA3900A) with variable pixel size has been developed to meet these requirements. A variable pixel architecture provides a near uniform mapping of the curved surface of the earth, while 56 outputs running at 20 MHz yield greater than a 1.1 Gigapixel per second data rate with low RMS noise and high MTF. This paper will provide detailed information on design trades required. We will report CCD read noise, dark current, full well capacity, and quantum efficiency (QE).