Inter-comparison of MetOp-A and MetOp-B AVHRR and on-orbit calibration update

Abstract

Meteorological Operational (METOP)-B spacecraft was launched on September 17, 2012, and the Advanced Very High Resolution Radiometer (AVHRR) visible and near-infrared channels were activated shortly after the launch. Due to the lack of on-board calibrator, the postlaunch calibration update can only be carried out vicariously. Libyan Desert reflectance with seasonal variation consideration has been used as reference since 1995. The initial updates of the calibration are derived using the seasonal oscillations of previous AVHRR measurements. In this paper, this approach is improved using the seasonal curve regression for the MetOp-B AVHRR measurement over Libyan Desert. A new method is developed using the MetOp-A and MetOp-B inter-comparison with consideration of the Bidirectional Reflectance Distribution Function (BRDF) effect, and applied to the calibration update. Since these two satellites are in the same orbit with half orbit time (~50min) apart, it is feasible to use the inter-comparison of the AVHRR measurement on Libyan Desert between these two instruments. The solar zenith angle and sensor zenith angle are not the same for the collocated measurements, and the BRDF effects on the comparison should be corrected. The zenith angle dependent measurements have been modeled and the comparison of the reflectance with the same solar zenith angle and sensor zenith angle has been derived. The calibrations from these two methods are consistent and the updates are delivered for operational L1B product. This work is also useful to investigate BRDF effect on the inter-comparison for visible and near infrared sensors.