The global ozone monitoring by occultation of stars instrument on envisat: GOMOS Commissioning: Instrument calibration and processor verification

Abstract

The GOMOS Commissioning phase can be summarised in three main objectives: a) verification of instrument health and operational capability, b) instrument calibration as per level-1b processor needs and c) end-to-end verification of level 1b (engineering) and level 2 (geophysical) processor. The GOMOS commissioning phase is carried out in accordance with the 'GOMOS Commissioning Verification Scenario' document. This has been established as a joint effort of ESA, the GOMOS science institutes and industry (ESA Support Laboratory, ESL). Following the initial instrument checkout by the flight operation segment (FOS), the instrument will start to collect occultation and calibration data using targets defined in the calibration mission plan and optimised to verify the on-ground performance and to update the calibration database such as to reflect the in-orbit performance. The GOMOS Instrument Engineering Calibration Facility is based on a stand-alone calibration processor, CALEX. The functionality of this processor reflects the content of the calibration database and satisfies all the needs of the level-1b processor. In addition, CALEX includes additional functionality useful for the instrument performance evaluation during and after the commissioning phase. A palette of tools necessary to process and visualise the CALEX computational files will facilitate the analysis of the data and will provide a comprehensible man-machine interface. Once the instrument calibration database has been updated and verified, the level-2 validation will commence and will particularly focus on the internal consistency of the geophysical algorithms, products and error assessment thereof. The calibration versus processor validation is executed in an iterative manner and will result in a tuned level-1b/2 processor, updated calibration and configuration database and error budget assessment. The GOMOS commissioning planning is well under way with calibration already performed for all major performance drivers. The paper presents and demonstrates the appropriateness the GOMOS specific calibration and validation plan, keeping a focus on the calibration and level-1b processing chain verification.