Remote sensing of the atmosphere and surface of the earth is performed by the Imager and Sounder instruments onboard the GOES (Geostationary Operational Environmental Satellite) Satellites. The current versions of these instruments have two and four detectors per band, respectively, that are scanned across the earth. Large photovoltaic, Hg1-xCdx Te Focal Plane Arrays (FPAs) will permit faster coverage, improved resolution, and improved sensitivity for future designs like the Advanced Baseline Imager (ABI) and the Advanced Baseline Sounder (ABS). However, the transition away from the current small number of detectors requires a technology demonstration of the same or better radiometric precision and uniformity across available FPAs. These measurements, using appropriate flux levels, f-numbers, and readout rates for GOES, are underway at MIT Lincoln Laboratory. Both corrected response from pixel to pixel (residual spatial non-uniformity) and temporal stability of each pixel during the calibration period are required to better than 0. 1 K NEdT. The test set-up and the measurements of dark current and signal performance will be discussed for two arrays.