Refinement of the Method for Using Pseudo-Invariant Sites for Long Term Calibration Trending of Landsat Reflective Bands

The long term calibration history of the Landsat 5 Thematic Mapper instrument has recently been defined using a time history of desert sites in Northern Africa. This trend is based on the assumption that the reflectance of each site is approximately constant and Lambertian over time. As a result, the top of the atmosphere reflection is assumed constant when corrected for variation in solar illumination angle and earth-sun distance. While this is true to first order and provides the current best estimate and therefore the basis for all current temporal calibration, there are multiple known sources of residual error in the data. The two we believe to be most significant and addressable are bidirectional reflectance and atmospheric variation from observation to observation. This paper presents demonstrated trends in the existing data consistent with bidirectional reflectance variation that should significantly reduce errors in the trend data.