Effects of orbital drift on land surface temperature measured by AVHRR thermal sensors

The NOAA series of meteorological satellites that carry the Advanced Very High Resolution Radiometer (AVHRR) suffer from orbital drift so that during each satellite's duty period the overpass time occurs later in the day. Replacement satellites restore the overpass time temporarily, but then it gradually decays. The goals of this paper are to document the effects of variable observation time owing to orbital drift on brightness temperatures (BT) and land surface temperature (LST) calculated from them in the NOAA/NASA Pathfinder AVHRR Land (PAL) data set and to consider possible corrections for the resulting trends and discontinuities in the PAL BT data. The drift effects were found to be greater for bare ground than for vegetated land cover classes, however significant effects were found for most vegetated classes. The magnitude of the orbital drift effect for most global cover types was at least as large as the other errors that affect LST measurement. A simple empirical correction for observation time based on solar zenith angle (SZA) was used to correct the PAL BT time series following Gutman [Int. J. Remote Sens. 20 (1999a) 3407]. The correction from this method was compared with that predicted by a physically based model and was found to differ in the early part of each satellite's duty cycle. Finally, the impacts of correction on the effective observation time were analyzed and the simple statistical correction was found to suffer from greater variability than has hitherto been recognized. A modification to the statistical correction to adjust the effective observation time is described. (C) 2002 Elsevier Science Inc. All rights reserved.