Estimation of long-term throughput degradation of GOES 8 and 9 visible channels by statistical analysis of star measurements

Each Imager and Sounder on the present generation of GOES satellites has an eight-channel visible detector array. These visible arrays perform star-sensing measurements to establish inertial attitude references so that earth observations can be registered to the earth's latitude and longitude coordinates. The Imager's visible array also performs earth observations. We have used the archived signal levels of star measurements to estimate the long-term throughput loss in these channels of the Imager and Sounder on GOES 8 and GOES 9. An exponential decay rate was determined for each sensor by averaging the values derived from each of approximately 30 stars over a time interval of at least 500 days. Large degradations in image quality occur during local night, when direct sunlight enters the optical ports of the two sensor. Therefore, we have deleted observations made during the 10 hour interval around midnight, satellite time, from our analysis. Variable stars and stars with low signal-to-noise ratios were also excluded. The annual throughput losses for the four sensors, derived from measured star signal levels, range from 3.8 percent to 9.6 percent.