Orbital Drift Effect on Advanced Very High Resolution Radiometer Phenology

The U.S. Geological Survey, Earth Resources Observation and Science Center routinely produces a remote sensing phenology (RSP) dataset derived from a 1-km Advanced Very High Resolution Radiometer (AVHRR) product compiled from six National Oceanic and Atmospheric Administration (NOAA) satellites (NOAA-11, -14, -16, -17, -18, and -19). Each NOAA satellite experienced orbital drift that influences the AVHRR reflectance measurements. To understand the effect of the orbital drift on the RSP dataset, we analyzed the impact of solar zenith angle (SZA) on RSP metrics in the conterminous United States. The AVHRR weekly composites were used to calculate the growing-season median SZA from 1989 � 2014. The results showed that the SZA values in 1992, 1993, 1994, 1999, and 2000 are substantially higher than those in other years. The high SZA (44� � 60�) in those years caused negative trends in the SZA time-series that were statistically significant (α = 0.05) in 76.9% of the study area. Temporal correlation analysis showed that the phenological metrics and SZA were significantly correlated in 4.2 � 20.3% of the study area. After eliminating the five years with high SZA (>40�), the temporal SZA trend was largely reduced, significantly affecting only 1.9% of the area. The correlation coefficients between the phenological metrics and SZA were significant in 1.0 � 6.2% of the area. Our study concluded that the NOAA satellite orbital drift increased the SZA, and in turn, influenced the phenological metrics. Eliminating the years with high SZA greatly reduced the influence of orbital drift on the RSP time-series.