Spacecraft attitude variations of NOAA-11 inferred from AVHRR imagery

A series of 1 km Advanced Very High Resolution Radiometer (AVHRR) images for the period of 1991-1992 are used to compute the attitude variations of the NOAA-11 spacecraft using ground control points (GCPs or land surface reference points) in conjunction with a recently developed precise AVHRR image navigation software package (Rosborough et al. 1994). The images were first corrected for the NOAA satellite clock drift and then roughly 2-3 images per month were used to compute roll, pitch and yaw to match the GCPs in each image. Higher frequency satellite attitude variations were monitored using a 2 to 3 day sampling interval for two month-long periods in May 1991 and February 1992. The pitch and yaw behaviour of NOAA-11 was found to oscillate about a mean value near 0·0, with an amplitude of ±0·002 radians for pitch and ±0·001 radians for yaw. The variability in the roll axis was characterized by an amplitude of ±0·0015 radians about a mean offset of −0·002 radians. The higher frequency month-long samples showed similar behaviour in terms of amplitude and phase when compared with the annual time series. The NOAA-11 onboard attitude information was found to have a much smaller amplitude with extremely regular variability and was apparently not related to the AVHRR image derived attitude variations.