Effect of orbital drift and sensor changes on the time series of AVHRR vegetation index data

This paper assesses the effect of changes in solar zenith angle (SZA) and s ensor changes on reflectances in channel 1, channel 2, and normalized diffe rence vegetation index (NDVI) from the advanced very high resolution radiom eter (AVHRR) Pathfinder land data set for the period July 1981 through Sept ember 1994, First, the effect of changes in SZA on channel reflectances and NDVI is derived from equations of radiative transfer in vegetation media. Starting from first principles, it is rigorously shown that the NDVI of a v egetated surface is a function of the maximum positive eigenvalue of the ra diative transfer equation within the framework of the theory used and its a ssumptions. A sensitivity analysis of this relation indicates that NDVI is minimally sensitive to SZA changes, and this sensitivity decreases as leaf area increases, Second, statistical methods are used to analyze the relatio nship between SZA and channel reflectances or NDVI. It is shown that the us e of ordinary least squares can generate spurious regressions because of th e nonstationary property of time series. To avoid such a confusion, we use the notion of cointegration to analyze the relation between SZA and AVHRR d ata, Results are consistent with the conclusion of theoretical analysis fro m equations of radiative transfer, NDVI is not related to SZA in a statisti cally significant manner except for biomes with relatively low leaf area, F rom the theoretical and empirical analysis, we conclude that the NDVI data generated from the AVHRR Pathfinder land data set are not contaminated by t rends introduced from changes in solar zenith angle due to orbital decay an d changes in satellites (NOAA-7, 9, 11), As such, the NDVI data can be used to analyze interannual variability of global vegetation activity.