Assessment of the impact of the orbital drift of SPOT-VGT1 by comparison with SPOT-VGT2 data

The Systeme Pour l'Observation de la Terre 5 - VEGETATION 2 (SPOT 5 - VGT2) instrument started to drift in orbit from early 2011, but currently the overpass time is still within mission requirements (less than 20min deviation from 10:30 local equator crossing time). To determine the operational lifetime of the VGT2 instrument, it was necessary to investigate the impact of orbital drift beyond these mission requirements such that the VGT time series remains consistent over time. To this purpose, the impact of orbital drift on reflectance values and on the normalized difference vegetation index (NDVI) from the VGT1 instrument (onboard SPOT 4) in the period May 2009 to April 2012 was investigated using paired observations from VGT1 and VGT2. The comparison is thus a relative one, which has the advantage that land-cover change or trends in land-cover change are not interfering with the analysis. VGT2 acted as the reference against which the VGT1 data were compared. First, the magnitude of the solar zenith angle (SZA) change in the overlapping time period was investigated. This SZA change is largest near the equator and decreases considerably with increasing latitude. The overpass difference of VGT1 and VGT2 of 37min resulted in maximum SZA changes of 9 degrees, which is still well within the SZA variability of standard VGT products (i.e. 20 degrees). Second, for a number of selected sites of 1 degrees x1 degrees size with one dominant land cover, we investigated in detail the time evolution of some measures of agreement and disagreement. In the third analysis, systematic subsamples of the global images were used. From these images, the relative difference between VGT1 and VGT2 - with VGT2 as reference - was assessed as a function of the SZA of VGT2 and for different ranges of SZA difference. An increase of 10-20% in the relative difference between VGT1 and VGT2 was observed for the four spectral bands. The impact on the NDVI was negligible. Based on the obtained results, an alternative compositing approach is formulated in order to maintain the quality of the VGT standard products with a similar orbital drift as investigated in this study.