A sampling technique in the star-based monitoring of GOES imager visible-channel responsivities

Monitoring the responsivities of the visible channel of the Imagers on operational GOES satellites is a continuing effort at NOAA. To estimate the rate of degradation of the responsivity, we have been analyzing the time series of star signals measured by the Imagers for attitude and orbit determination. In this report, we begin by showing our latest results of monitoring of the responsivities of the visible channels of GOES-8, GOES-9, GOES-10, GOES-11, GOES-12, and GOES-13. One complicating factor in the analysis has been the presence in the time series of an annual cycle that modulates the gradual long-term degradation whose rate we are trying to infer. We describe a method we are developing to reduce the influence of the annual cycle on the analysis. The method enables us to include in the analysis the star observations near local midnight, which had been excluded in the past to prevent loss of accuracy in the derived long-term degradation rate. With a fuller set of data, we can subdivide the data within each year into 48 bins and estimate the degradation separately in each bin, thereby reducing the influence of the annual cycle on the derived degradation rates. One indication that the method is valid is that the degradation rates estimated in all the bins are consistent.