<title>Evaluating the GOES-8 and -9 earth IR horizon sensor error response using the GOES-9 sounder IR channel 1 through 4 data</title>

Abstract

Fourteen days of sounder observations were scheduled during the GOES-9 post launch test to perform an inflight measurement of the effects of IR radiance variations in the stratosphere on altitude control using the GOES Type II IR Earth Sensor. Two 16.33 degree by 1.65 degree scenes corresponding to the Earth sensor north and south chords were scheduled every four hours for the 14 days from July 19, 1995 until August 2, 1995. The data from IR channels 1 through 4 are integrated over a 1.65 degree square FOV and weighted to synthesize the GOES ES medium passband RI filter and a narrow IR passband filter. The integrated scenes were converted to time dependent Earth sensor radiance input pulses for each passband, north and south chord, for six times per day over the 14-day interval. The qualitative features of these synthesized Earth sensor IR input pulses are compared graphically and correlated with image navigation registration (INR) star residuals for the operational GOES-8 spacecraft. Comparisons of the altitude of the IR horizon on the raw sounder scans in channel 1 with channel 7 are proposed to detect variations the 14.7 micron horizon altitude. Schemes for removing the effects of attitude motion from the sounder scenes are discussed and the Earth sensor pulses are processed through a linear model of the Earth sensor electronics to evaluate the relative error response to the synthesized IR input pulse shapes. These results appear to be influenced by thermal mechanical distortion of the spacecraft. Correlation analysis shows that the GOES Earth sensors are responding to nonrepeatable and therefore unpredictable variations in stratospheric radiance and that the time evolution of these variations is consistent with the INR star residual data from GOES-8. Geometric and time adjustments are made for the stratospheric rotation rate and the 15 degree eastward displacement of GOES-8 relative to GOES-89. The performance of the narrow and wide passband filters shows no difference for the pitch and roll errors computed at this level of accuracy. Finally, based on this work methods of implementing sounder data from the Earth IR limbs scenes into the daily INR process to improve image registration accuracy may be developed.