Improvement of the PARASOL Radiometric In-Flight Calibration Based on Synergy Between Various Methods Using Natural Targets

Abstract

The Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar (PARASOL) mission was launched in December 2004 and was a member of the A-train up to the end of 2009. The mission was definitively stopped in December 2013 after nine years of operation. A temporal decrease of the PARASOL radiometric sensitivity occurred with a complex signature within the field of view. Using several calibration techniques, all based on the use of natural targets, it was possible to derive an efficient set of new calibration parameters which includes the absolute calibration, as well as the temporal evolution characterized by a mean decrease plus a variation within the field of view. The physical model used to fit parameters is a classical exponential decrease with a time constant of 0.018 month-1 and an amplitude from 16% in 490 nm down to 1%–2% in the near infrared. The mean drift can be considered as estimated with an accuracy better than 0.5% per decade. The variation within the field of view, which is estimated with accuracy between 0.2% and 0.5% rms, follows a similar spectral shape as the mean instrumental drift. Some typical features were found which illustrate the ageing of the optical part of the instrument (lenses). The updated calibration, based on a synergy between various calibration methods, has significantly improved the radiometric consistency for all spectral bands of the PARASOL data archive, which represents nearly nine years of acquisitions. Finally, the PARASOL absolute calibration was confirmed with accuracy close to 2%. The last reprocessing of the full PARASOL level-1 archive has implemented these new radiometric corrections.