Calibration of ocean color scanners: how much error is acceptable in the near infrared?

Abstract

We simulate vicarious calibration (VC) of a Sea-viewing Wide Field-of-view Sensor (SeaWiFS)-like ocean color sensor relative to its longer near infrared (NIR) spectral band (865 nm) to understand the influence of calibration error at 865 nm, which is difficult to assess in orbit. We show that as long as the calibration error at 865 nm less than ∼10% in magnitude, the post-vicarious-calibration-corrected radiances are sufficiently accurate to retrieve useful water-leaving reflectances at moderate aerosol optical depths. This is completely independent of the initial calibration error in the shorter-wave bands, but assumes an atmospheric correction approach similar to that currently used with SeaWiFS. Retrievals are only slightly improved by reducing the magnitude of the error at 865 nm below ∼5%. The simulations immediately suggest that pre-launch calibration is necessary only to the extent required to set the sensitivity of the instrument in the desired range. Rather than trying to achieve a highly accurate pre-launch calibration, e.g., uncertainty <5%, we assert that resources would be better expended on improved radiometric stability (and its monitoring) and complete characterization of the instrument, e.g., polarization sensitivity, out-of-band response, etc. However, these assertions assume the existence of a permanent vicarious calibration facility, e.g., the Marine Optical Buoy (MOBY).