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GOME Diffuser and Dark Signal Trends
Angelika Dehn adehn@esrin.esa.itClaus Zehner czehner@esrin.esa.it
ESA-ESRIN, Via G. Galilei, 00044 Frascati, Italy
Introduction
The Global Ozone Monitoring Experiment (GOME) was launched on ERS-2 in April 1995 and has been continuously operational since then. It is a scanning nadir-viewing spectrometer, with its primary scientific objective being to retrieve total column ozone globally. A more detailed description of the instrument can be found in [1] . In common with previous instruments to measure total column ozone from space such as TOMS and SBUV, it measures the back-scattered radiance from the Earth's atmosphere and surface, and the solar irradiance which is viewed via a diffuser plate to provide a reference spectrum at comparable intensity. These diffuser plates have been found to be subject to degradation (see [2] for example) particularly when subject to shorter wavelength ultra-violet light, and efforts have been made to characterize this degradation for instruments such as SBUV/2, where the diffuser plate was exposed for a total of around 750 hours between 1979 and 1986. GOME has been designed with a cover for its diffuser plate in an attempt to minimise this degradation, with exposure usually being for a short time for one orbit each day to obtain a reference solar spectrum, and characterisation of any degradation is possible by means of the on-board Pt/Cr/Ne calibration lamp. In order to investigate the GOME diffuser degradation, and to see if the measures taken have reduced the effect, a first analysis was done in January 1997. The results and detailed description are presented in technical note [9]. Updates on the analysis were done for the time periods June 1995 to December 1997, and June 1995 to December 1998 (see technical notes [10], [11]). This document provides the update on the degradation analysis, using the monthly calibration data from June 1995 until September 1999 following the same algorithms as in [9].Algorithm Descriptions
The detailed description of the algorithms used for the calculation of diffuser reflectivity and dark signal components can be found in [9].Dark Current Analysis
The dark signal for GOME is defined as being comprised of two parts - a constant value of between 140 and 150 binary units (BU) which is the fixed pattern readout noise (FPRN) and a time dependent component of around 2 to 3 binary units per second which is the leakage current (LC).Trends were calculated for both the FPRN and the LC, and for the noise on these measurements. The results are shown in Table 1 below and Figures 1 to 2.
Figure 1: Leakage Current
Figure 2: Fixed PatternReadout Noise
Diffuser Reflectivity
The diffuser reflectivity is calculated as the ratio of calibration lamp measurements and the lamp measurements via the diffuser. The result of the analysis for the diffuser reflectivity can be seen in Figure 3. The data are dark signal corrected.
Figure 3: Diffuser Reflectivity
(to be multiplied by 10 -4)
Results
Over a period of about 4.5 years, the following conclusions regarding trends in the GOME dark signal and diffuser reflectivity have been reached.- No significant change is seen in the fixed pattern readout noise
- There is an increase of ~13-14% per year in leakage current for all detectors
- No significant change is seen in the diffuser reflectivity in any channel
References
ESA Publications Division September 1995.[2] : Report of the International
Ozone Trends Panel
WMO Report No. 18 Vol. 1, §2.3.6 1988.
[3] : ERGO Software User's
Manual
DOR-GO-QA-SUM Issue 1.0 12/12/1995
[4] : ERGO Design Document
DOR-GO-QA-DD Issue 2.0 12/12/1995
[5] : GOME Data QA -
Specification of Instrument Parameters - S. Slijkhuis
SRON-GOME-QA-TN01 Issue 2/A 30/06/1995.
[6] : ERGO Test Report
of Final S/W Delivery Dec-1995 - S. Slijkhuis
SRON-GOME-QA-TN04 Issue 1 09/02/1996.
[7] : Remote Sounding
of Atmospheres by J.T. Houghton, F.W. Taylor and C.D. Rodgers
Cambridge University Press 1st Edition 1986.
[8] : Functional / Performance
Test on GOME BBM - Olij, C. & Zoutman, A. E.
TPD-ERS-GO-MIR-11 Issue 2 1993
[9]: GOME Diffuser Reflectivity
and Dark Signal Analysis - D. Pemberton
ERS2-GO-DDS-TN-001 Issue 1.0 1997
[10]: Long Term Monitoring
of GOME Diffuser Reflectivity and Dark Signal Analysis- A. Dehn
ERS2-GO-DDS-TN-002 Issue 1.0 1998
[11]: Long Term Monitoring
of GOME Diffuser Reflectivity and Dark Signal Analysis- A. Dehn
ERS2-GO-DDS-TN-003 Issue 1.0 22/03/1999
Appendix A
Monthly Calibration Data Sets
Appendix B
Lamp Lines Used For Diffuser Calibration
Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry