Spectral assessment of deployment platform perturbations in above-water radiometry.

Abstract

Deployment platforms such as ships, towers or buoys may affect the accuracy of nearby radiometric measurements. Aiming at expanding the know-how on platform perturbations in above-water radiometric measurements, this study investigated the spectral impact of the Acqua Alta Oceanographic Tower (AAOT) on the remote-sensing reflectance RRS as a function of the distance d between the tower and the sensor footprint at the sea surface. This was accomplished by exploiting measurements performed with radiometers operated on deployment rigs extending beyond the AAOT superstructure with sensor viewing angle θ = 40° and relative azimuth ϕ = 90° between sensor and sun. AAOT perturbations were also investigated by increasing the reflectance of the tower through white sheets covering part of its superstructure. Results indicate a spectral dependence of perturbations in RRS more pronounced in the near infrared, significantly increasing with the tower reflectance and decreasing with the inverse square of the distance d. In particular, for distances approaching the platform height, AAOT perturbations are found to be generally well below 1% for measurements performed in the visible spectral region and exceed 2% beyond 800 nm. However, with identical measurement geometry, but increasing the AAOT reflectance through the white cover, perturbations approach 1% in the blue-green spectral region and exceed 2% beyond approximately 600 nm. These findings, yet derived from a distinct tower and for specific measurement conditions, raise awareness on spectral perturbations of deployment platforms in above-water radiometry and additionally provide practical elements for the implementation of measurement protocol allowing to constrain these perturbations below required thresholds.