gomos long-t -t calibration pemits.sso.esa.int/emits-doc/esrin/gomoscalibrationv3.pdf · 2010. 1....

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GOMOS_Calibration_v3.doc

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GOMOS LGOMOS LGOMOS LGOMOS LONGONGONGONG----TTTTERM ERM ERM ERM CCCCALIBRATION ALIBRATION ALIBRATION ALIBRATION PPPPLANLANLANLAN

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A P P R O V A LA P P R O V A LA P P R O V A LA P P R O V A L

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TTTT A B L E A B L E A B L E A B L E OOOO F F F F CCCC O N T E N T SO N T E N T SO N T E N T SO N T E N T S

1 INTRODUCTION ......................................................................................................................5 1.1 Purpose...............................................................................................................................................5 1.2 Scope..................................................................................................................................................5 1.3 References ..........................................................................................................................................6

2 GOMOS INSTRUMENT............................................................................................................8 2.1 Science ...............................................................................................................................................8 2.2 Instrument Description.......................................................................................................................9

2.2.1 Observation Principle.................................................................................................................9 2.2.2 Instrument Dessign ..................................................................................................................10

2.3 Instrument Operations......................................................................................................................12 2.4 Data Products Summary...................................................................................................................14

2.4.1 Level 0......................................................................................................................................15 2.4.2 Level 1......................................................................................................................................15 2.4.3 Level 2......................................................................................................................................15 2.4.4 Auxiliary Databases .................................................................................................................16

3 CALIBRATION/VERIFICATION REQUIREMENTS ...............................................................16 3.1 Acquisition and tracking performance .............................................................................................17 3.2 Radiometric Performance.................................................................................................................18 3.3 Spectral Performance .......................................................................................................................19 3.4 Imaging Performance .......................................................................................................................20 3.5 In-Flight Calibration ........................................................................................................................20 3.6 CCD Performance ............................................................................................................................22

4 CALIBRATION/VERIFICATION APPROACH........................................................................22 4.1 Acquisition, detection and pointing performance ............................................................................23 4.2 Radiometric Calibration ...................................................................................................................25

4.2.1 Spectral Response Linearity.....................................................................................................25 4.2.2 Spectral Response Stability......................................................................................................26 4.2.3 Spectral Response Uniformity .................................................................................................26

4.3 Spectral Calibration..........................................................................................................................27 4.4 Imaging calibration ..........................................................................................................................28 4.5 In-Flight Calibration ........................................................................................................................29 4.6 Thermal Performance.......................................................................................................................29 4.7 CCD Performance ............................................................................................................................30 4.8 Processing Approach........................................................................................................................33

5 CALIBRATION MEASUREMENTS........................................................................................33


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s 5.1 Calibration Measurements Analysis ................................................................................................33


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1 INTRODUCTION

1.1 Purpose This document describes the rationale, methods and implementation of the long-term calibration and characterisation of the GOMOS Instrument (Global Ozone Monitoring by Occultation of Stars) flown on Envisat. Envisat is a European Space Agency (ESA) mission for Earth Observations launched on 1 March 2002. The document contains a brief summary including detailed references of calibration and characterisation activities and results from the Commissioning Phase, and defines the calibration plan for the Envisat Exploitation Phase. In high-level discussions of the combination of calibration, characterisation and verification in this document, this combined set of activities will be referred to as calibration for brevity.

1.2 Scope After the introductory first chapter, the second chapter of this document reproduces the CEOS definitions of fundamental terms relevant to Calibration, Characterisation and Verification, and clarifies their use in this document. Chapter 3 outlines the GOMOS science objectives and introduces instrument and data processing properties relevant for the remainder of the document. Chapters 4 to 6 describe the GOMOS calibration approach, measurements and analysis respectively.

1.3 Acronyms and abbreviations ACVT Atmospheric Chemistry Validation Team ADF Auxiliary Data File ADS(R) Annotation Data Set (Record) ANX (time) (time of) Ascending Node crossing (intersection of Envisat orbit with x-y plane in Earth fixed coordinate system) BB Blackbody (calibration etc.) CBB Calibration Blackbody CEOS Committee on Earth-Observation Satellites CFI Customer-Furnished Items CTI Configurable Transfer Item DPM/PDL Detailed Processing Model/Parameter Data List Document DS Deep Space (calibration etc.) DSD Data Set Descriptor DSR Data Set Record FOS Flight Operations Segment FOV Instrument Field of View GADS Global Annotation Data Set IECF Instrument Engineering and Calibration Facility IODD Input / Output Data Definition Document IPF (GOMOS) Instrument Processing Facility


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s LOS Instrument Line-of-Sight LRAC Low Rate Archiving Centre MCMD Macro command MDS(R) Measurement Data Set (Record) MJD Modified Julian Day MPS Mission Planning System MPH Main Product Header (non-)LTE (non-) Local Thermodynamic Equilibrium NESR Noise Equivalent Spectral Radiance NRT Near Real Time p Atmospheric pressure PCD Product Confidence Data PDHS (-K /-E) Payload Data Handling Station (-Kiruna / - ESRIN) PDS ENVISAT Payload Data Segment PSM Parameter Setting Macro command RGT ENVISAT Reference operations plan Generation Tool ROP ENVISAT Reference Operations Plan SBT Satellite Binary Time SNR Signal-to-Noise Ratio SODAP Switch On and Data Acquisition Plan SPH Specific Product Header SVD Singular Value decomposition T Atmospheric kinetic Temperature TBC To Be Confirmed TBD To Be Defined (/Detailed) TBC To Be Confirmed TEP Test Entry Point USF User Service Facility UTC Universal Time Correlated VMR Atmospheric volume mixing ratio MW Microwindow z Line-of-sight tangent altitude

1.4 Reference Documents [1] �ENVISAT-1 SATELLITE FLIGHT ACCEPTANCE Review, Data Package, Part 2b,

Payload Instruments�, Ref: EN-DP-AST-SY-0001, issue 1, Jan 2001. [2] �On ground performance Characteristics and Calibration of the ENVISAT GOMOS

Instrument�, T. Paulsen, Ref: PO-RP-ESA-GM-0856, issue 1, 3 February 1999 [3] �GOMOS star selection w.r.t double-stars�, Ref: PO-TN-ESA-GM-930, T. Paulsen, issue 1,

revision 1, 29 September 1999 [4] �GOMOS CCD performance evaluation� GOMOS CAL/VAL Team Technical note No 5,

G. Barrot, J.L. Bertaux, C. Cot, O. F. d� Andon, R. Fraisse, A. Hauchecorne, A. Mangin, A. Mohammadzadeh, P. Snoeij, July 15, 2002.


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s [5] �ENVISAT Phase E CAL/VAL Acquisition Plan�, M. de Laurentis, Ref. ENVI-SPPA-

EOPG-TN-03-0008, draft, 18 March 25, 2003 [6] IODD Ref: PO-RS-ACR-GS-003, issue 5, rev 1, November 1999 [7] �GOMOS level1b/2 Algorithm Description document for Enviview pre-release�, T.

Paulsen, Ref: PO-TN-ESA-GM-1019, issue 1, 29 February 2000 [8] �GOMOS FM Instrument Operations Manual�, Ref: PO.MA.MAT.GM.17, issue 2, rev 5,

28 April 2000 [9] �GOMOS, an ENVISAT Instrument for Atmospheric Chemistry and Climate Research�,

Science Report, J.L. Bertaux, F. Dalaudier, A. Hauchecorne, M. Chipperfield, D. Fussen, E. Kyrola, G. Leppelmeier, H. roscoe, version 0.4, April 2000

[10] �ENVISAT 1 Mission and System Summary�, ESA, Daimler-Benz aerospace, Matra

Marconi space, Thomson-CSF Services and Sytems SOL Spatiaux [11] �GOMOS Calibration on ENVISAT � Status on December 2002�, G. Barrot, J.L. Bertaux,

R. Fraisse, A. Mangin, A. Hauchecorne, O.F. d�Andon, F. Dalaudier, C. cot, E. Kyrola, J. Tamminen, B. Theodore, D. fussen, R. koopman, L. Saavedra, P. Snoeij

[12] �GOMOS Procedure for routine Monitoring and Calibration�, GOMOS CA/VAL Team

Technical note No 15, R. fraises, G. Barrot, issue 1.0, 20 December, 2002

2 DEFINITIONS In agreement with the CEOS guidelines [e.g. ADx], the following definitions apply. Accuracy Accuracy is the absolute uncertainty or error of a measurement result, assuming that the �true� value of a measurable parameter is known exactly. In general, the accuracy of a measurement is affected by both, random and systematic error sources. In many cases the assignment of accuracies is difficult or even impossible as independent, accurate measurements taken under identical conditions are not available. Often, the accuracy of a measured parameter is estimated through comparisons with other measurements acquired under variable conditions or indirectly, by means of analyses. Calibration Calibration is the process of quantitatively defining the system response to known controlled signal inputs. It involves transformation of signals in engineering units into estimates of a physical quantity, expressed in physical units. Characterisation Characterisation represents a set of measurements of a specific quantity under well-defined, variable conditions. The purpose of a characterisation is to allow the assignment of an expected result valid at a later instant, given the exact conditions valid at that instant and the results of the


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s characterisation measurements. An example is the characterisation of non-linear detector response curves. Here, a detector output is recorded while the irradiance of a target is varied in a controlled way over a pre-defined range of values. The result is used later to correct a measured radiance level (e.g., an unknown scene) for the effect of the non-linear detector response. Precision Precision is the uncertainty within which a specific measurement can be reproduced. Assuming that fluctuations in the result of a repeated measurement are of pure random origin, precision is given as the estimated standard deviation (1 s) of the difference (relative or absolute) between individual samples and the average over all measurements. Verification Verification is the process of testing a measurement result, an assumption or a functional component through comparison with an expected result or by testing against the performance of a reference component, respectively, making use of a set of pre-defined error margins. The result of verification is always binary, i.e., �passed� or �failed�. In high-level discussions of the combination of calibration, characterisation and verification in this document, this combined set of activities will be referred to as calibration for brevity.

3 THE GOMOS INSTRUMENT

3.1 Science objectives Stratospheric ozone is the main absorber of solar ultraviolet (UV) radiation. Without ozone life as we know it on the Earth surface would not exist. Any significant decline in stratospheric ozone is therefore of serious environmental concern. The ozone concentration and distribution are also responsible for the temperature structure in the stratosphere. Any change in stratospheric ozone can therefore have an effect on the heat budget in the atmosphere and could impact the climate. Stratospheric ozone production and destruction plus stratospheric transport and troposphere-stratosphere exchange controls the ozone levels. The only source of ozone is the combination of molecular and atomic oxygen. Atomic oxygen is produced through dissociation of molecular oxygen by solar UV radiation. Ozone is destroyed by the absorption of solar UV radiation, which constitutes the UV filter effect f the ozone layer. Fragments of the destroyed ozone molecule, molecular and atomic oxygen, can recombine again to ozone molecules. Besides this natural ozone destruction, ozone is depleted by a variety of catalytic cycles involving the oxides of hydrogen, nitrogen, bromine and chlorine. Man-made emissions of ozone depleting substances play a key-role in the drastically reduced ozone amounts observed in Arctic and Antarctic regions. Reductions in ozone levels are also observed at mid-latitudes but the mechanisms responsible are not clear. So, despite the significant improvements in our understanding of stratospheric chemistry and dynamics, the science is still far from being completely understood.


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s Stratospheric ozone content has been declining worldwide since the 1970s. The decline is in the order of 4 to 8% per decade. After international agreements regarding the reduction of ozone destroying substances, there is hope that the destruction of ozone will slow down and eventually recovery would occur. However, this is a process that will take many decades to occur and the careful monitoring of ozone and substances related to ozone destruction, is necessary. Therefore, stratospheric ozone research these days needs further observation of ozone and other parameters relevant to the problem: firstly, in order to understand long-term changes of the stratospheric composition, accurate measurements are needed over a respective long period of time, i.e. decades; secondly, many relevant parameters cannot be measured accurately enough, which calls for new or improved measurement techniques. GOMOS responds to both requirements, namely ozone monitoring with high accuracy and vertical resolution. GOMOS has the large advantage of a very simple observation technique and comparably easy algorithms for retrieval of the atmospheric parameters. It has the potential to become a relatively inexpensive standard ozone-monitoring instrument. As GOMOS measures the attenuation of stellar light through the atmosphere, the atmospheric transmittance can be directly retrieved from the ratio of the stellar light outside the atmosphere and passing through the atmosphere. The stellar occultation techniques enable GOMOS to measure with high vertical resolution and with a global coverage. Since the instrument is in principle self-calibrating, linear ozone trends should be observable with accuracy in the order of 0.2 to 0.3 % per year.

3.2 Instrument Description

3.2.1 OBSERVATION PRINCIPLE GOMOS is implemented on Envisat with its overall field of view oriented opposite to the velocity vector, allowing the instrument to observe the successive occultations of various stars while the platform is moving along its orbit. The principle of the stellar occultation is quite simple. When the star is high above the horizon, the light spectrum of the star is recorded by GOMOS without any atmospheric absorption. A few seconds later, the light spectrum of the same star observed while setting through the atmosphere, is recorded. The spectrum is modified by the absorption of all atmospheric constituents integrated over the line-of-sight from the satellite to the star (fig. 2.2-1) according to the Beer-Lambert law.


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s During one single occultation, a series of line densities are obtained at various altitudes. The vertical distribution of the local ozone (or other trace gas) density (molecules/cm3) can be retrieved from this series, assuming that the atmosphere is locally spherically symmetric. This vertical retrieval is straightforward with the so-called �onion-peeling� technique. The good performance of the GOMOS instrument stems from:

• The self-calibrating measuring scheme by detecting a star spectrum outside and through the atmosphere

• The drift and background compensating measurement algorithms introduced by the use of two-dimensional array detectors, which allow stellar and background spectra to be recorded simultaneously

As a result, the spectra are therefore corrected for background or straylight and detector dark current contributions. Successive recordings of stellar spectra outside and through the atmosphere allow any long-term changes in spectral emission characteristics as well as drifts in sensor spectral sensitivity to be compensated. This protection against long-term drift is of course ideal for the study of trends of ozone and other constituents. Additional measurements provided by two fast photometers allow correcting the spectral data from the high frequency component introduced by the atmospheric scintillations. The 930 nm band of the near-infrared spectrometer allows deriving vertical profiles of water vapour, which is a major contributor to the ozone depletion process. From the 760 nm band of this spectrometer, the vertical temperature profile can be retrieved which adds useful data for the extraction of the ozone concentration profile and for its long term trend monitoring.

3.2.2 INSTRUMENT DESIGN The GOMOS instrument has been designed to measure trace gas concentrations and other atmospheric parameters in the altitude range between 20 and 100 km.


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s The instrument optical design is based on a single telescope concept: the telescope simultaneously feeds through an optical beam dispatcher placed at its focal plane, a UV-visible medium resolution CCD spectrometer, for signal measurements in the Huggins and Chappuis bands (250-675nm), and a near IR high resolution CCD spectrometer, for O2 (around 760nm) and H2O (around 930nm) observations (see fig. 2.2-2). A redundant CCD based star tracker, which may operate either in dark limb or in bright limb conditions, shares the same focal plane. It provides the pointing and tracking accuracy required to maintain the star image at the centre of the spectrometers entrance slit during the observation. Two fast photometers with two different spectral channels, receives part of the telescope signal for a high frequency monitoring (1kHz sampling rate) of the input signal scintillations. All the spectrometers, fast photometers and star tracker optics and detection modules are implemented on a thermally controlled optical bench. This optical bench and the telescope are mounted together and fixed on the instrument interface plate via 3-point attachment (fig. 2.2-3). The Steering Front Assembly (SFA) is made of one flat mirror (3000mm x 4000mm size) mounted on a two-stage mechanism. The first stage is the orientator, used to move the instrument line of sight towards the target star. The second stage is the tracking device operated during the occultation phase. It provides the fine centering of the star image within the spectrometers entrance slit. Major electronics subassemblies are the Detection Modules (DM), the Science Data Electronics (SDE) which processes the detector signals and outputs the data to the measurement data interface, the Instrument Control Unit (ICU) which performs pointing loop control (using data from the star tracker) and overall instrument management and finally, the Mechanics Drive Electronics (MDE) which regulates the mirror steering mechanism under the control of the ICU. These electronic boxes are directly mounted on a Payload Equipment Bay (PEB) panel.

UVIS 1 UVIS 2

SATU 1/2

FP 1

250 500 750 1000375 625 875 λ [nm]405 675

FP 2

IR1 IR2

Figure 3.2-2 Spectral ranges for GOMOS spectrometers (blue & red), photometers (green) and SATU (grey)


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s

3.3 Instrument Operations

Light from Stars Opto-Mechanical Assembly PEB

IR Spectrometer

(DMSB)

Fast Photometer

(DMFP)

Star Tracker(SATU)

UV/VISSpectrometer

(DMSA)

Science Data

Electronic (SDE)

Instrument Control

Unit (ICU)

MechanismDrive

Electronic (MDE)

Measurement Data

Command & Control IF

Power I/F

Telescope

Steering Front Mechanism

Figure 3.2-3: Functional block diagram of GOMOS


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s The GOMOS instrument modes are divided into two categories, the support modes (used to achieve or maintain full instrument operational conditions):

• Off • Reset/Wait • Standby • Standby/Refuse • Heater and Heater/Refuse

and the operations mode (in which the instrument performs its nominal operation to fulfil the measurement objectives):

• Auxiliary modes o Pause: the objectives in the pause mode are: the preparation of an observation

sequence by updating operational parameters; loading of the observation program; ability to switch immediately to any of the measurement modes with some limitations. This mode is automatically reached at the end of a Measurement mode, however, pause can be used to interrupt a current measurement phase

• Measurement modes o Occultation: perform measurements from a multi-spectral observation of star

occultation. The star is acquired and tracked during the occultation. This is the nominal mode operation for GOMOS and the instrument autonomously executes the sequence: maintain current position, coarse rallying, fine rallying, detection, centering and tracking submodes.

o Fictive star occultation: the occultation mode can perform the observation of a fictitious star, i.e. the orbital motion is compensated so that the instruments points in the direction of a fictitious star.

o Linearity monitoring: performs periodical monitoring of the instrument radiometric response linearity by observation of stars with variable integration times. The integration time does not vary continuously during a single observation but the measurement can be repeated with various integration times and with the same target.

o Spatial Spread monitoring: performs periodical monitoring of the star spectrum and star image location by the observation of the instrument pixel to pixel response

o Uniformity monitoring: perform a periodical monitoring of the instrument pixel-to-pixel response uniformity by observation of an extended uniform target (i.e. earth limb)

The occultation mode can implement two types of observations:

• Asynchronous: the star is processed only once • Orbit synchronous: the star list is processed over several orbits (number < 25) with an on-

board interpolation of the star parameters In measurement mode, the ICU uses the star apparent magnitude (mv) and the limb flag (dark or bright) in order to configure the SATU/DM video gain and the integration times (tables 2.3-1 and 2.3-2).


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s Table 3.3-1: DM parameters vs. star characteristics

DMSA gain

DMSB gain

Integration time (sec)

Bright and any mv

1 1 0.25

Dark and any mv

4 1 0.5

Dark and mv > -1

8 1 0.5

Table 3.3-2: SATU parameters vs. star characteristics

SATU gain (detection)

SATU gain (centering /tracking)

SATU integr. time in msec (detection)

SATU integr. time (centering /tracking)

mv = -2 1 1 5 5 mv = -1 1 1 10 5 -1 < mv < 6 dark

1 2

15 (mv = 0) 20 (mv = 1) 25 (mv = 2) 35 (mv = 3) 50 (mv = 4) 50 (mv = 5)

10

-1 < mv < 6 bright

1 1 15 (mv = 0) 20 (mv = 1) 25 (mv = 2) 35 (mv = 3) 50 (mv = 4) 50 (mv = 5)

10

mv = 6 2 2 50 10

3.4 Data Products Summary The ground processing of GOMOS data is divided into the near-real time processing and the online processing and the levels are 0, 1 and 2. An overview of the GOMOS data products generated by the PDS is provided in table 2.4-1.

Table 3.4-1: Summary of GOMOS products generated by the PDS

Processing level Product ID Description Size GOM_NL__0P Nominal mode level 0

(occultation) Typical: 50 Mb/orbit Level 0: Re-formatted

satellite data, computer compatible format GOM_MM__0P

Monitoring modes level 0: linearity, uniformity, or spatial spread data

Typical: < 1Mb/orbit

GOM_TRA_1P Geolocated Calibrated Transmission Spectra and Fast photometer fluxes

Typical: 4Mb/occultation Level 1b: Geo-located engineering calibrated product GOM_LIM_1P Geolocated Calibrated

background/limb Spectra Typical: 3Mb/occultation

GOM_NL__2P Atmospheric constituents & temperature profiles

Typical: <0.5 Mb/occultation

GOM_EXT_2P Residual extinction data Typical: 2Mb/occultation Level 2: Geo-located geophysical product

GOM_RR__2P Extracted atmospheric profiles for NRT dissemination to Meteo users

Typical: 30 Kb/occultation


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s 3.4.1 LEVEL 0 There are two GOMOS Level 0 products. One for when the sensor is in nominal occultation measurement mode, and one for when the sensor is in calibration monitoring modes. Both products contain data corresponding to a full orbit. The GOMOS Nominal Level 0 Product is a file containing time ordered AISPs, which record the occultation measurements of the GOMOS instrument. It is archived and is the basis for all-further GOMOS processing. The GOMOS Nominal Level 0 is formed systematically when the instrument is in occultation mode and the NRT version is available from the PDHS after 3 hours from data acquisition. The OFL (fully consolidated) version is available 2 weeks after acquisition from the LRAC. The GOMOS Monitoring Modes Level 0 Product contains time ordered AISPs that hold data acquired while the instrument is in spatial, uniformity or spatial spread monitoring modes. In each of these three modes, GOMOS is not acquiring stellar occultation data, but it is acquiring data which is used to establish operating parameters, and to set up look-up tables which are used in subsequent GOMOS data processing.

3.4.2 LEVEL 1 Level 1 data are divided into two categories: level 1a and level 1b. Level 1a are the level 0 data after they have been sorted and filtered by low-level quality checks. The main components of the level 1b data are the transmission spectra; these are the lowest level data likely to be of interest to the general user. The main goal of the GOMOS level 1 processing is to estimate a set of horizontal transmission functions using data measured by the GOMOS spectrometers. This so-called full transmission function will serve as basic data for the level 2 processing where profiles of atmospheric constituents are to be determined. The fast photometer data is also corrected for instrument dependent factors. Even if the GOMOS measurement method, stellar occultation, is a self-calibrating measurement method, several calibration-type corrections are needed. The self-calibration is taking care of general slow changing radiometric drifts but many pixel-level details are not included. Outliers connected to bad pixels and cosmic rays, for example, are also to be corrected. A possible non-linearity cannot be corrected by occultation self-calibration.

3.4.3 LEVEL 2 Level 2 processing of spectrometer data consists in essence of a spectral inversion of the transmission spectra in order to retrieve the constituent line densities, followed by a vertical inversion to give the local density profile for each constituent. All products are accompanied by the associated errors, which can be as low as 0.5% for Ozone, 1% for air and 5% for NO2 line densities.


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s 3.4.4 AUXILIARY DATABASES The auxiliary product files are used by the GOMOS geophysical processing facility and the calibration processing environment (IECF). The calibration and characterization database files contains all information needed by the geophysical processor and mainly by the level0, level 1b and level 2 processing. In table 2.4-2, a summary of the auxiliary files used by GOMOS processors is provided.

Product ID Description Size (Kb)

GOM_INS_AX Characteristics of the instrument which should not vary frequently during its lifetime: CCD size, static spatial and static spectral PSF, ...

10

GOM_CAL_AX Look-up tables derived from on-ground characterization and updated by in-flight measurements

2806

GOM_PR1_AX Configuration parameters of the different algorithms of the level 1b processing including thresholds

2

GOM_CAT_AX Star catalogue used by the processing chain; it contains the characteristics of the stars that might be observed by GOMOS

426

GOM_STS_AX Stellar spectra used for quality control and also as back-up of the reference spectra

6482

GOM_PR2_AX Configuration parameters of the different algorithms of the level 2 processing including thresholds

23

GOM_CRS_AX Cross-sections and transmissions required by the level 2 processing

11906

AUX_ECF_AX ECMWF analysis/forecast atmosphere files 14000

4 CALIBRATION/VERIFICATION REQUIREMENTS The scientific objective of GOMOS is, as it has been discussed before, to perform an accurate mapping of the ozone layer around the Earth with a fine and accurate observation of long-term trends. GOMOS is aimed at providing, on a daily basis, a global coverage of ozone vertical distribution with high precision and reliability. It will support monitoring of seasonal, latitudinal and long term trends and will provide also simultaneous measurements of NO2, NO3, H2O, aerosols and temperature vertical distribution - trace gases and parameters of primary importance in ozone chemistry. The major requirements derived from these scientific objectives are (ref. [1]):

• Measurements shall be performed in the following minimum spectral bands - UV-visible (UV/VIS) spectral band * 250 nm to 675 nm - Near infrared (IR1) spectral band 1 756 nm to 773 nm - Near infrared (IR2) spectral band 2 926 nm to 952 nm - Visible photometric (PHOT1) channel 650 nm to 700 nm - Visible photometric (PHOT2) channel 470 nm to 520 nm - Gaps are allowed at 375 to 405 nm and 525 to 555 nm

• The instrument shall record three spectra simultaneously


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s - Spectrum of the target star - "Upper spectrum", i.e. the background radiance above the star - "Lower spectrum", i.e. the background radiance below the star

• From the nominal ENVISAT-1 orbit, the instrument shall be able to track stars over the altitude range from 300 km to 5km and shall be able to acquire stars at any altitude within this range

• An acquisition probability of 85 % is specified, along with background brightness constraints under which this shall apply

• The instrument shall have a field of view (FOV) of: Azimuth -10° to +90° Elevation +62° to +68°

which gives the required altitude coverage in fine pointing (tracking) and gives coverage of the heavens from behind the satellite, parallel to the plane of the orbit, around to 100° from the plane of the orbit

• The instrument shall produce measurement data in the following operational modes: - Occultation Mode: synchronous and asynchronous observation - Linearity Monitoring Mode - Uniformity Monitoring Mode - Spatial Spread Monitoring Mode

• The instrument shall perform its "nominal mission" (comprising both UV and VIS spectrometers plus a photometer) with a reliability of 0.93 and shall perform an "extended mission" (comprising both UV and VIS spectrometers, both IR spectrometers and both photometers) with a reliability of 0.75.

• Over the duration of the mission the instrument shall be able to observe an average more than 45 stars per orbit.

The major requirements coming from the satellite are: - Mass ≤ 175 kg - Power consumption ≤ 207 W - Data rate ≤ 226 kbps

4.1 Acquisition and tracking performance Further to its clear fulfillment of the acquisition and tracking requirements, it can be stated that GOMOS:

• Possesses an azimuth field of view of -10° to +90° • Has an elevation field of view well in excess of the required 5 km to 100 km altitude • Can rally the line of sight to the next target star at 2 °/sec • Can acquire the target star within the allocated 10 sec • Can track the target star for up to 250 sec, occultation duration permitting


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s 4.2 Radiometric Performance A summary of the radiometric requirements and expectations (performance measured on ground) are given in table 3.2-1 (from ref. [1]).

Table 4.2-1: Summary of Radiometric Performance requirements and measured on ground

Radiometric Performance parameter Expected Performance Requirement Spectral Response Linearity

UVIS IR 1 IR 2

≤ 0.5 % 0.33 % 0.34 %

less than 1 %

Dark limb UVIS

IR 1

IR 2

Typical / Worst ≤ 1.25 % / ≤ 1.0 % ≤ 1.8 % / ≤ 2.0 % 0.55 % / 0.6 % 0.7 % / 0.7 % 2.4 % / 2.6 % 3.0 % / 3.2 %

Stability per samples≤ 1.5 % for 95 % ≤ 2.5 % for rest ≤ 1.8 % for 95 % ≤ 5.0 % for rest ≤ 4.0 % for 95 % ≤ 6.5 % for rest Spectral Response Stability

Bright limbUVIS

IR 1

IR 2

Typical / Worst ≤ 1.3 % / ≤ 1.5 % ≤ 2.1 % / ≤ 2.4 % 0.6 % / 0.8 % 0.8 % / 0.9 % 2.9 % / 3.7 % 3.6 % / 4.3 %

Stability per samples≤ 2.5 % for 95 % ≤ 8.0 % for rest ≤ 3.0 % for 95 % ≤ 9.0 % for rest ≤ 4.2 % for 95 % ≤ 10.0 % for rest

Spectral Response Uniformity UVIS IR 1 IR 2

Mean / Maximum 1.7 % / 4.3 % 1.9 % / 6.6 % 12 % / 40 %

Mean / Maximum ≤ 1 % / 4 % ≤ 3 % / 15 % ≤ 9 % / 50 %

The non-compliances for the mean value of radiometric uniformity in UVIS are due to the slit straightness. This effect can however be corrected because it is stable. The provided characterized uniformity maps can be used for this correction. The other non-compliance in VIS (max value) is due to the bad pixel located in line 92 and column 428. Except this pixel, the maximum radiometric uniformity value over the FIX band is 2.7 %. The non-compliance in IR2 is due to the choice of the CCD. In order to increase the radiometric stability, the CCD has been chosen with a large inter-fringe distance. The stability is much better but the uniformity (more sensitive to large non-uniformities) is affected and becomes non-compliant. All these deviations from the specified spectral response uniformity are covered by PO-NC-MAT-GM-0744, which was accepted by ESTEC (ref. [1]). The ultimate detection-chain is a perfect linear one, were the physical quantity in question is equal to the measurement value in digital units multiplied by a constant value called gain (eq. 1). In most cases (real cases), as GOMOS one, this is not true and the gain is a function of the input signal (eq. 2). In these cases the deviation from a perfect linear behaviour is found in a fraction of the gain, and this input dependant-fraction is called the non-linearity of the gain. The purpose of linearity


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s calibration is to find the inverse (eq. 3), enabling to remove, for each possible measurement value, the non-linear fraction of the relevant gain.

Ne = g. D (eq. 1) Ne = g. Knl(D) . D (eq. 2) D = A . Ne + B (eq. 3)

where Ne is the number of electrons in the CCD shift register, D is the resulting signal (in ADU), g is the gain in e-/ADU, Knl is the non-linearity factor (per definition is a function of D), A is ≅ 1/g and B is the electronic gain offset (B > 0). The on ground characterization of the gain (taken from ref. [2]) is reported in table 3.2-2.

Table 4.2-2: Total chain gain (e-/LSB)

Gain DMSA1 DMSA2 DMSB1 DMSB2 1 281.4 272.8 28.7 28.6 2 141.1 136.8 14.4 14.3 4 70.3 68.2 7.2 7.1 8 35.15 34.1 3.58 3.57

4.3 Spectral Performance A summary of the spectral requirements and expectations (performance measured on ground) are given in table 3.3-1 (from ref. [1]).

Table 4.3-1: Spectral Performance summary (spectral bands taken from ref. [2])

Parameter Expected Performance Requirement Spectral bands UV / visible near infrared, IR 1 near infrared, IR 2 visible photometric, PHOT 1 visible photometric, PHOT 2

248 nm to 693 nm 750 nm to 776 nm 915 nm to 956 nm 466 nm to 528 nm 644 nm to 705 nm

250 nm to 675 nm 756 nm to 773 nm 926 nm to 952 nm 470 nm to 520 nm 650 nm to 700 nm

Spectral Gaps between UV and visible

382 nm to 385 nm

within 375 to 405 nm

Spectral Resolution UVIS IR 1 IR 2

Typical / Worst case < 0.89 nm / < 1.02 nm 0.12 nm / 0.15 nm 0.14 nm / 0.17 nm

1.2 nm 0.2 nm 0.2 nm

Spectral Sampling UVIS IR 1 IR 2

≤ 0.314 nm 0.0465 nm 0.0571 nm

< 0.32 nm < 0.05 nm < 0.06 nm

Spectral Rejection UVIS IR 1 IR 2

Typical / Worst case ≤ 4.7 pixels / ≤ 4.95 pixels 3.85 pixels / 4.25 pixels 3.15 pixels / 3.5 pixels

85 % of signal over: < 4.5 pixels < 4 pixels < 4 pixels


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s The small deviations from the specified spectral rejection are covered by PO-NC-MAT-GM-0743, which was accepted by ESTEC (ref. [1]).

4.4 Imaging Performance A summary of the imaging requirements and expectations (performance measured on ground) are given in table 3.4-1 (ref. [1]). The rejection is defined as the width of the symmetric integral of the line spread function, either along the spatial or the spectral direction containing the 85 % of the LSF area (or energy of the spectral line observed). The spatial rejection value is particularly important for setting the band binning width (or number of rows per binned line)

Table 4.4-1: Imaging Performance parameter summary

Parameter Expected Performance Requirement

Spatial extent of the spectra

UVIS Target Background Separation IR 1 & IR 2Target Background separation

11 to 74 arcsec 16 to 100 arcsec 5.3 to 21 arcsec 11 to 74 arcsec 16 to 100 arcsec 5.3 to 21 arcsec

≥ 18 to 70 arcsec ≥ 24 to 96 arcsec ≥ 8 to 20 arcsec ≥ 13 to 52 arcsec ≥ 18 to 70 arcsec ≥ 8 to 20 arcsec

Spatial rejection

UVIS 254 nm 375 nm VIS 405 nm 675 nm IR 1 IR 2

Typical / worst case 25.4 arc" / 27.7 arc" 22.3 arc" / 22.9 arc" 22.7 arc" / 23.7 arc" 21.0 arc" / 26.4 arc" 20.2 arc" / 24.1 arc" 21.2 arc" / 25.0 arc"

95 % of energy contained within 26 arcsec

The deviations from the specified spatial rejection, only under worst-case conditions, are covered by PO-NC-MAT-GM-0745, which was accepted by ESTEC (ref. [1]).

4.5 In-Flight Calibration Having in mind the definition of �calibration� in the context of the ENVISAT program (those instrument parameters are subject to �in-flight� calibration which enable an upgrade of the current performance to the required performance level) there are two areas that require at least an initial calibration.


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s Two parameters were identified for in-flight calibration:

• Location of the spectrometer target area on the spectrometer detection module CCD�s • Location of the photometers instantaneous field of view (IFOV) on the photometer

detection module CCD�s In order to perform a calibration of these two parameters dedicated algorithms have been implemented in the ground segment:

• Spectrometer Target Area Calibration • Photometer IFOV Calibration

For the spectrometers, the instrument SNR performance is dependent on the programmed size of the CCD target area. The number of lines of the central band can vary this target area. To improve the SNR the central band size shall be as small as possible (but big enough to avoid the lost of star signal). Therefore it becomes necessary to determine the position of the star spectrum on the spectrometer CCD with a good accuracy. A worst-case budget of 7 lines (overall) can be expected under the assumption that the spectrum is monthly reprogrammed to +/- 0.5 pixel (in the spatial direction). The long-term stability budget, taking into account micro setting, moisture, etc., amounts to about 4 pixels along the spatial direction. However, most of the effects are due to launch, thus the first spectral measurements will lead to an initial positioning of the target area suffering afterwards only from moisture and thermal effects (up to 2 pixels). Therefore the objective of the in-flight calibration of the spectrometer target area is to locate the star spectra to +/- 0.5 pixels on the spectrometer CCD along the spatial direction. In order to perform a calibration of the spectrometer target area, the instrument will be operated in the Spatial Spread Monitoring Mode. In this case stars shall be selected which are located outside the earth limb in order to avoid background disturbances. Then, a bary-centering algorithm applied to 5 to 10 spectral samples (regularly positioned within the spectral bands) will permit to identify the star spectrum as imaged onto the 33 transmitted lines of the spectrometer detector. A monthly execution of the spectrometer target area calibration is recommended. As for the spectrometers, the instrument photometers SNR performance is dependent on the size of the area on which the target is imaged. A size of 14 lines x 5 columns is consistent with both radiometric and thermo elastic drifts. The worst-case long-term stability budget (overall) is currently more than 2 pixels. As for the spectrometers, a large part of this drift is due to launch effects, thus the first acquisitions onto photometers will take them into account. The objective of the photometer IFOV calibration is to locate the star images to +/- 0.5 pixels on to the photometer CCD along the spectral direction. In order to perform a calibration of the photometer IFOV the instrument will be operated in the Spatial Spread Monitoring Mode. In this case stars shall be selected which are located outside the earth limb in order to avoid background disturbances. Then, a bary-centering algorithm over the


page 22 of 34

s whole 14x14 pixel CCD area will permit to trace-out the star on the photometer detector. The bary-centering algorithm shall be applied to the first images taken by the photometer and confirmed on the following ones. A monthly execution of the photometer IFOV calibration is recommended.

4.6 CCD Performance The static error contributors and the total theoretical static variance have been characterized (table 3.6-1). The spectrometers electronic chain noise (n0) and quantization noise are extracted from PO-TR-MAT-GM-038, and reflect the pre-launch characterization of CCD EEV 2610. The spectrometers electronic chain noise (n0) was computed as the square root of the quadratic sum of several contributors: DM noise, SDE noise and CCD output stage noise. The static DC noise is an estimate derived from the static DC measured in flight. The DC pre-launch characterization is given in table 3.6-2. These data derived from PO-TR-MAT-GM-041 and have been measured pre-launch.

Table 4.6-1: Pre-launch characterisation of static noise contributors

Noise contributors SPA1 SPA2 SPB1 SPB2 Spectrometers electronic chain noise (n0) 29 28.4 12.2 12.1 Quantisation noise 10.2 9.8 8.3 8.3 Static DC noise (e) 7.7 8.8 8.6 7.9 Total static noise (e) 31.7 31.3 17.1 16.7 Total theoretical static variance 1004.3 980.0 291.7 277.7

Table 4.6-2: Pre-launch DC characterization

Temperature Parameter SPA1 SPA2 SPB1 SPB2 Dark charge (e) 1520 1661 1840 1739 Error (e) 55.2 49.8 44.5 45.5

16.8° (SPA) 18.15° (SPB)

Variance (e2) 3051 2479 1979 2068 Dark charge (e) 562 610 683 632 Error (e) 48.5 36.5 32.4 30

9.9° (SPA) 11.23° (SPB)

Variance (e2) 2353 1331 1052 902 Dark charge (e) 98.4 109.1 114.8 97.2 Error (e) 43.9 31 20.7 20.3

-1.65° (SPA) -0.28° (SPB)

Variance (e2) 1930 963 427 412

5 CALIBRATION/VERIFICATION APPROACH The verification of the instrument health and performance activities have started on March 22, 2002 (commissioning phase), just after the SODAP phase which had validated the correct behaviour of the Service Module, Payload Equipment Bay and Payload instruments, checked the GOMOS level 0 products format and roughly verified that the instrument can be commanded as expected. Due to the launch and in-orbit environments, several instrument characteristics may have


page 23 of 34

s been modified with respect to the ones measured on ground and this calls for calibration and monitoring of GOMOS parameters behaviour and performance: electronic gain chain, read-out noise and offset, non linearity, dark charge, pixel response no-uniformity, radiometric sensitivity, spectral line spread function, wavelength assignment, vignetting and straylight. During the commissioning phase the performances have been measured, analyzed and compared to the requirements and expectations. The main output of this study is that the GOMOS instrument in-flight performances are globally in line with expected budget, except for the CCD behaviour. During the whole mission, the performances should be kept as good as they are now or, if possible, even better. For that reason a continuous monitoring of all the performance parameters and a calibration strategy are needed.

5.1 Acquisition, detection and pointing performance The most important performance parameter of GOMOS is the star detection probability and tracking loss probability. Without a very high performance tracking system, the instrument would never acquire the stars and maintain the tracking of the star through the turbulent atmosphere. Once the star has been acquired and the instrument is tracking, the stability of the LOS is the performance driver. The pointing stability is reflected in the spectral stability monitoring accuracy in the SATU data. The azimuth range is specified to �10o /+90o with allowed degraded SNR performance on the IR pupil (IR spectrometers and photometers) between -5o /-10o. The measured ranges are consistent with the expected (measured on ground) values and the specifications (table 4.1-1). The rallying time is defined as the time to orientate the instrument LOS from its last position to the new rendezvous direction and to get ready for the acquisition phase. The specifications refer only to the angular rate, which shall be greater than 2 o/s (at LOS level) after a maximum of 5 s of acceleration phase. Table 5.1-1: Summary of parameters related to the acquisition

Expected Measured Specification/comment SFM range: azimuth elevation

-11 � 91° 61.7 � 69°

> -10.8 � 90.8° > 62 � 68.8°

-10 � 90° 62 � 68°

Rallying angular rate 4°/s > 2°/s > 2°/s Acquisition uncertainty cone < 0.16° 0.3° x 0.1° (during SODAP)

< 0.01° x 0.017° (actual) Worst case : 0.2°

Acquisition duration = (1) + (2) + (3)

8.1 s = 0.14 + 7 + 1

< 7 s 10 s (allocated) = 0.5 + 8.5 + 1

Tracing FOV 7.4°; 6.5° > 7.1°; 5.3° 7°; 5° The acquisition mispointing is the difference between the expected and the observed position of the star in the SATU FOV after rallying. The expected values are less than 0.16o whereas the observed value during SODAP was 0.3 o in elevation and 0.1° in azimuth. This mispointing is


page 24 of 34

s linked to the acquisition uncertainty cone (the cone in which the star may be detected with a probability greater than 99 %). The analysis of this unexpected mispointing in elevation has shown:

• A contribution due to a forgetting of GOMOS optical frame / GOMOS reference cube alignment in the configuration matrix (0.1° in elevation and 0.06° in azimuth)

• A possible impact of the gravity compensation of the mechanism. A shift of a few tens of microns could easily induce a bias of 0.1° in elevation

• A possible contribution due to the calibration curves of SFA angles The mispointing has been corrected and the stars are now detected at the centre of the SATU, with a typical acquisition cone (calculated as the difference between the actual star direction and the theoretical direction defined by the centre of the SATU CCD at the expected time of acquisition, ref. [3]) less than 0.01° x 0.017° (5 x 8 pixels). The acquisition duration is defined as the maximum time between the rendezvous time and the beginning of the useful tracking phase. The following durations are used for the acquisition duration budget calculation:

• Star first detection [(1) in table 4.1-1], which determines the detection algorithm • Star centering and transition to tracking mode [(2) in table 4.1-1], which determines the

centering profile • Stabilization to tracking mode [(3) in table 4.1-1]

For the performance in detection, a limited number of stars have been tested and the detection threshold has been kept to a safe value. Indeed, a too small value can lead to a class C anomaly which makes the instrument get into Refuse mode. Even if there is no danger for the instrument it has been decided not to test the limits of the detection. Anyway, the number of tested stars is already much larger than the scientific need and the measured detection probability is better than the specifications (table 4.1-2)

Table 5.1-2: GOMOS detection probability

Expected Measured Specification Hot star, dark limb 4.6 > 3.7 2.4 Cold star, dark limb 6.8 > 4.5 4 Hot star, bright limb 2.2 > 3.2 1.6 Cold star, bright limb 4.4 > 4 3.2

Also the tracking FOV, defined as the maximum angular range of the azimuth and elevation angles over a complete occultation sequence, is well within the specifications (table 4.1-1). Tracking noise and robustness: the instrument provides the ecartometry data of the SATU at 100 Hz. With these data, the noise equivalent angle can be calculated, which consist in the statistical angular variation of the SATU data above the atmosphere. The SATU Noise Equivalent Angle (NEA) expected in dark limb is 13 µrad for a specification of 18 µrad. The measured value is better


page 25 of 34

s than 3 µrad in dark limb and 10 µrad in bright limb (at 35 km altitude). Noise is less than 1 µrad (3σ).

Expected (µrad) Measured (µrad) Specification (µrad) SATU NEA (dark limb) 13 < 3 18 (3σ) SATU NEA (bright limb) 20.6 <10 (at 35 km altitude) 23 (3σ)

The average altitude at which the star is lost is 15 km in dark limb and 22 km in bright limb, better than the expected. It is nearly not dependent on magnitude since the tracking loss is mainly due to the refraction and the scintillation that depend on the atmosphere conditions. The minimum altitude reached in tracking is below 5 km for the brightest stars. Te refraction induces a displacement on SATU up to 20 µrad at 10 km and the effect on the spectrum (bending in UV) appears when flux is already nearly completely absorbed by atmosphere. The values of the ranges (in acquisition/tracking) and the durations (in rallying/centering) that have been confirmed have been implemented in the RGT Operational Parameter file. Margins have been taken with respect to the GOMOS expected capability but should be kept for the lifetime since it is already larger than the scientific need. These performances are not expected to change during the life and only an analysis of the possible anomalies is required. For detection, the number of stars which can be acquired is larger than 500 (already enough for the mission). The detection threshold (15 LSB) should be kept like this and should anyway not be decreased below 5 LSB since there could be a risk of a class C anomaly. The mispointing performance and the SATU NEA will be monitored, at least, over one complete orbit per week. The mean tangent altitude point, at which the star is lost in dark and bright conditions, will be monitored in order to detect any trend in the pointing performance.

5.2 Radiometric Calibration

5.2.1 SPECTRAL RESPONSE LINEARITY The electronic chain linearity has been verified using GOMOS measurements in Monitoring Linearity mode. Same stars have been observed with different integration times, from 0.25 seconds up to 10 seconds. The non-linearity specification is 1 % per decade and the in-flight calibration has shown an average non-linearity better than 0.4 % on full dynamic range. Also the gain and the electronic chain offset have been characterized being the offset very stable. The offsets since the beginning of the mission and the actual gain settings are reported in the table 4.2-1


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s Table 5.2-1: Gain setting and offsets (from ref. [4])

CCD Offset (orbit < 800)

Offset (800 < orbit < 1021)

Offset (orbit > 1085)

Gain (e-/ADU)

SPA1 123.4 122.7 122.6 35.15 SPA2 118.6 117.6 117.5 34.1 SPB1 107.7 104.5 104.3 28.7 SPB2 119.5 117.2 117.1 28.6

The electronic gain will be calibrated every three months. linearity calibration to be written offset calibration to be written

5.2.2 SPECTRAL RESPONSE STABILITY The radiometric calibration is performed by comparing the stellar spectra with external reference sources. A good consistency between measurements and on-ground characterisation is observed. The variation level between orbits is compatible with the theoretical noise. Radiometry is very stable in all the spectral range coverage of GOMOS. It should be noted that the absolute radiometric calibration has no significant impact on the level 2 products (species density profiles). The radiometric sensitivity monitoring, that consist in monitor the absolute sensitivity of each CCD, will be performed in order to detect any need of new radiometric calibration (which typically should be performed once a month).

5.2.3 SPECTRAL RESPONSE UNIFORMITY The two main contributors to this performance are the CCD Pixel Response Non Uniformity PRNU (pixel defects and IR-interference) and the optical path non-uniformity, which are related mainly to the slit rectitude, and the filter non-uniformity. The PRNU has been measured being the performance consistent with the expectations (table 4.2-1). The interferences present the same shape and amplitude than on ground. An important aspect of the uniformity characterisation in the two infrared bands is the temperature stability. The location of the fringes (fig. 4.2-1) could change over the orbit due to a detector temperature drift, and this would endanger the uniformity correction and the background removal. As a consequence, the thermal stability needs to be very good. There is no degradation of the pixel response even for the pixels showing a degraded dark current. One bad pixel in DMSA2 already seen on ground has not moved nor degraded. The slit profile is consistent with the slit profile measured on ground.


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s

Table 5.2-2: Radiometric Spectral response uniformity results

Radiometric uniformity Expected PerformanceMean / Maximum

Measured Mean / Maximum

Specification Mean / Maximum

UV 1.7 % / 4.3 % 1.4 % / < 4 % ≤ 1 % / 4 % VIS 1.7 % / 4.3 % 1.6 % / 4.3 % ≤ 1 % / 4 % IR 1 1.9 % / 6.6 % 2.4 % / 12 % ≤ 3 % / 15 % IR 2 12 % / 40 % 12 % / 40 % ≤ 9 % / 50 %

The PRNU will be monitored once a month, especially in IR, where the temperature influences the interference position.

5.3 Spectral Calibration For the GOMOS spectral bands, there are no specific requirements concerning the sharpness or shape of the band edges, other than that the SNR and radiometric requirements are to be met within these bands. This means that the spectral band calibration is not considered as a critical parameter with respect to the accuracy. The start and stop values of the spectral band edges are reported in table 4.3-1 where the values stated as measured are the ones included in the calibration database and are used for the nominal wavelength assignment. It should be remembered that the difference measured on ground between the TOBA (Telescope and Optical Bench Assembly) and the thermal vacuum measurements, in particular for the UV start spectrum, is due to the SMU (Steering Front Unit) enhanced aluminium coating that degrades the transmission below 250 nm on the transmission.

Figure 5.2-1: Interference pattern measured on IR 2 spectrometer


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s Table 5.3-1: Spectral band characterization summary.

Spectral bands Measured on ground (nm) Measured (nm) Requirement (nm) UV / visible

TOBA 242 - 693

Thermal Vacuum248 > 690

248 - 690

250 nm to 675 nm

near infrared, IR 1 749 - 777 750 - 776 755 - 774 756 nm to 773 nm near infrared, IR 2 916 - 952 916 - 956 926 - 954 926 nm to 952 nm visible photometric, PHOT 1 466 - 528 472 - 526 470 nm to 520 nm visible photometric, PHOT 2 644 - 705 646 - 698 650 nm to 700 nm

Due to the very stable radiometry (stated in section 4.2) the complete spectral range of GOMOS is now considered as valid and used in the ground processing. The spectral calibration (assigning a wavelength to every column) is performed by comparing the spectral assignment of several columns with external well-known reference sources. It should be noted that a vacuum wavelength scale is adopted for GOMOS. While one column per CCD becomes the reference column, the other calibrated columns are used to update the spectral dispersion law of the spectrometer. Lastly, the spectral assignment of any CCD column is done using the reference column and the spectral dispersion law. The accuracy of the wavelength calibration is of the order of 1/30 pixel. Due to the high sensitivity of the retrieval performance to the correctness of the wavelength assignment, the spectral calibration will be carefully monitored in a monthly basis. other spectrometer CCD�s: to be written calibration of spectral resolution and rejection to be written

5.4 Imaging calibration The spatial extend of the spectrum that is the spatial with (angle in the vertical direction) of the CCD binned lines is confirmed to be within the expectations (table 4.4-1). The 95 % of the star energy in the spatial direction is on average contained in 4 lines. This knowledge is important in order to set the band binning width (number of rows per binned line).

Table 5.4-1: Imaging Performance summary

Parameter Expected Performance

Measured

Requirement


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s

Spatial extent of the spectra

UVIS Target Background Separation IR 1 & IR 2Target Background separation

11 to 74 arcsec 16 to 100 arcsec 5.3 to 21 arcsec 11 to 74 arcsec 16 to 100 arcsec 5.3 to 21 arcsec

37.5 arcsec 16 arcsec 16 arcsec 37.5 arcsec 16 arcsec 16 arcsec

≥ 18 to 70 arcsec ≥ 24 to 96 arcsec ≥ 8 to 20 arcsec ≥ 13 to 52 arcsec ≥ 18 to 70 arcsec ≥ 8 to 20 arcsec

Spatial rejection

UVIS 254 nm 375 nm VIS 405 nm 675 nm IR 1 IR 2

Typical / worst case25.4 arc" / 27.7 arc" 22.3 arc" / 22.9 arc" 22.7 arc" / 23.7 arc" 21.0 arc" / 26.4 arc" 20.2 arc" / 24.1 arc" 21.2 arc" / 25.0 arc"

(mean) 22.2 arcsec

95 % of energy contained within 26 arcsec

5.5 In-Flight Calibration The spatial displacement of the spectrum location is checked analysing the spatial spread monitoring mode data of GOMOS. The mean value of the star location of the maximum of the star signal during the occultation is calculated. If the values (for all the spectrometers) are different by more than 0.5 pixels with respect to the calibration database values, the GOMOS instrument band setting (programmation) will be changed. The Optomechanical displacements due to the launch are less than 0.3 pixels for all CCD�s and the displacements produced by a temperature variation are inside the expected budget (0.5 pixels/10o C for DMSA and 0.1 pixel for DMSB). The variation during one orbit is lower than 0.03 pixels.

5.6 Thermal Performance The thermal performance has an important impact on the band setting, PRNU and spectral performance and has been studied during the three different cooling phases (from +10oC down to around -4oC). At +10oC the thermal behaviour of GOMOS is as expected, similar to the �Beginning of Life hot� thermal case. The temperature variations along the orbit are very consistent with the model. The temperature variation of DMSA and DMSB along one orbit is around 0.35o (see fig. 4.6-1). Knowing the thermal sensitivity of the spectrometer pixels, this variation leads to an inaccuracy of 5 to 6 % on the dark charge level. As the accuracy of the thermistor measurements (coding accuracy) is also 0.40 °C, the final inaccuracy may be of the order of 10 %.


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s

Temperature variation of OB units over orbit 04602

-12

-10

-8

-6

-4

-2

0

2

13:42:28 13:59:48 14:21:56 14:37:56 15:03:48 15:19:48

16-JAN-2003 time

Tem

p (d

eg C

)

DM SB HK TEM P

OSB HK TEM P

SATU2 HK TEM P

SATU1 HK TEM P

DM FP HK TEM P

DM SA HK TEM P

OSA HK TEM P

OBD HK TEM P

OBSTRUCTUREHK T

Figure 5.6-1: Typical temperature variation of OB units during one orbit (after third cooling)

The thermistor temperature of the spectrometer and photometer CCD�s is monitored every day using the data inside the level 0 products while the variation over one complete orbit is monitored once a week using the housekeeping data.

5.7 CCD Performance During the commissioning phase it was observed that several phenomena degrade the CCD performance:

• Cosmic hits: it generates, on one reading only, an extra charge on one or several contiguous pixels

• Hot pixels: the clean dark charge is the average DC signal over a spectrometer, characterised by its temperature, assumed to be uniform. All the pixels exhibiting a non-normal behaviour (>3σ) are excluded from the computation of the average and classified as �hot�. Apparently, this phenomenon is due to the effect of radiation belt protons on the pixel, creating a dislocation in the volume of the Si crystal, generating DC electrons. In fig. 4.7-1 there are plotted two Dark Signal Non Uniformity (DSNU) spectra, normalised with respect to their mean clean dark charge value


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s • RTS (Radio Telegraphic Signal) phenomena: This is the name given by ESA to the abrupt

change (positive of negative) of the CCD pixel signal. It seems that it affects only the DC part of the signal, and not the photon-generated signal

• Modulation signal: an additional parasitic signal was found to be systematically present, added to the useful signal, at least for spectrometers A1 and A2

Th Irwh

0.5

1.5

2.5

3.5

4.5

5.5

6.5

7.5

8.5

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 23000

1

2

3

4

5

6

7

8normalised DSNU on upper band

ground

flight orbit 792

same scale but with a shift

Figure 5.7-1: Dark Signal Non Uniformity before launch (blue and left axis) and two months after launch (red and right axis)

he cosmic rays hits is in line with respect to what was expected but the hot pixel and the RTS are igher with respect to the expectations. The modulation signal was not expected.

n September 2002, 60% of the pixels have a mean dark charge higher than 100 electrons, which esults in the values displayed in table 4.7-1. The values of the clean dark charge are consistent ith the pre-launch values, but taking into account all pixels the values of dark charge are sensibly igher.


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s Table 5.7-1: Mean dark charge in electrons per CCD

SPA1 SPA2 SPB1 SPB2 Clean dark charge (SPA: -2.9 o) (SPB: -1.4 o)

79 80 93 92

All pixels (SPA: -2.9 o) (SPB: -1.4 o)

215 218 254 223

Pre-launch (SPA: -1.65 o) (SPB: -0.28 o)

98.4 109.1 114.8 97.2

The observations indicate that the RTS has a high variability in duration and stability, characteristics that makes this phenomenon difficult to characterize. A preliminary study of the impact on the level 2 products has been performed and shows a degradation of several percents on ozone and other species. The cooling of the CCD (three times) to decrease this effect by decreasing the mean dark charge level has been performed. Anyway, the minimum temperature reached lead to a state where the RTS has still an important impact on the quality of the retrieval of several species (especially NO2, NO3). A dark charge calibration at orbit level (one DC map measured every orbit) has also been programmed and implemented in the level 1b processing chain. Using this dark charge map brings a clear improvement, of the order of a recovery of half the degradation due to RTS (conclusion reached from the analysis of a set of end-to-end simulations, using the GOMOS simulator, performed during the CAL/VAL activities). When analyzing dark sky observations, an oscillating regular parasitic signal has been detected in addition to the dark charge:

- A modulation signal of +/-1.4 ADU has been found for spectrometer A1 - A similar modulation signal of +/-0.76 ADU has been found for spectrometer A2 - No modulation has been detected for spectrometers B1 and B2

An algorithm has been developed to remove the modulation. Considering the improvement in the variance obtained with this algorithm, its implementation in the nominal GOMOS processing chain has been done. The total static variance (i.e. excluding photon noise and including electronic chain noise and quantization noise) is significantly different from the on ground values for the UVIS spectrometers when the correction for signal modulation is applied. The resulting variance for each spectrometer is given in the table 4.7-2. The pre-launch static variance derivation is calculated by applying the equation varT = varS + DC (varT is the total variance, varS is the static variance, DC is the dark charge) to the values given in table 3.6-2.

Table 5.7-2: Total static variance (from ref. [4])

SPA1 SPA2 SPB1 SPB2 Total measured static variance (e2) (excluding photon noise)

602 525 297 339

Pre-launch static variance derivation(SPA: -1.65 o) (SPB: -0.28 o)

1832 854 312 315


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s The dark charge will be monitored carefully every day in order to detect and characterize the trend. The modulation signal will be monitored once a week as the pattern could change mainly due to a temperature variation.

5.8 Processing Approach

6 CALIBRATION MEASUREMENTS As already stated, GOMOS is able to work in different measurement modes. All of them are used to characterize and calibrate the instrument, even the nominal occultation mode data:

• Occultation mode (OCC): a star is tracked as it sets behind the atmosphere, with

integration time measurements typically of 0.25 seconds (bright limb) and 0.5 seconds (dark limb). This is the nominal mode operation for GOMOS and the instrument autonomously executes the sequence: maintain current position, coarse rallying, fine rallying, detection, centering and tracking submodes

• Fictive star occultation: in this mode, the instrument tracks no star and the inertial trajectory of a hypothetical star is followed. The objectives are: to take measurements of a Dark Sky Area (DSA, containing only dark current signal to be used afterwards by the processing), and performing spectral measurements of bright limb scattered light.

• Linearity monitoring (LIN): the objective is to perform a periodical monitoring of the instrument radiometric response linearity by observation of stable targets (so above the atmosphere) with a variable integration time. This mode is only possible by exploiting the binned operation and will thus generate the same spectrometer data as in occultation mode, but with the signal level varying over a range of times 40

• Spatial Spread monitoring (SSM): the objective is to perform a periodical monitoring of the instrument line spread function by observation of a stable star (no fluctuations) above the atmosphere. In this way the instrument function (spatial and spectral directions), including the effects of satellite jitter, can be observed.

• Uniformity monitoring (UNI): the objective of this mode is to perform a periodical monitoring of the instrument pixel-to-pixel response uniformity by observation of an extended uniform target (limb). A pixel map is obtained and will be used for the flat field correction, for checking bad pixels and for confirming binning operations.

6.1 Calibration Measurements Analysis During the commissioning phase all the monitoring modes of GOMOS (and also data in occultation mode) have been fully exploited for calibration and characterization purposes. In table 5.5-1 there is a summary of the GOMOS monitoring mode measurement strategy that is currently the baseline for the routine phase (from ref. [5]).


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s Table 6.1-1: Calibration measurement strategy for GOMOS (from ref. [5])

Type Characteristics Mode Parameter

DSA only orbit

- One asynchronous orbit per week (during week-end) - All available DSAs - Altitude range: 5-130 km

OCC mode • Dark Charge • Straylight

- One asynchronous orbit - Dark, eclipse and bright limb stars - Altitude range: 0-250 km - Integration time = 0.5 sec - H1, H2, H3 = 7, 3, 7 DMSA gains: 00

for bright stars, 11 for dark stars, 10 for Sirius; DMSB gains: always 00

SSM mode • Band setting • Straylight

- Two asynchronous orbits - Altitude: 10 km first orbit, 30 km

second orbit - Integration time = 0.5 sec (except for

last acquisition 5 sec) - Azimuth = 30 deg - Start ANX time and duration variables - DMSA gains: always 11; DMSB gains:

always 00

UNI mode • PRNU characterization

- Three asynchronous orbits - Dark, eclipse and bright limb stars (at

least 3 stars per limb) - Altitude range: 100-300 km - Integration time = 0.25 sec first orbit,

0.5 sec second orbit, 0.25 sec third orbit - H1, H2, H3 = 7, 3, 7 DMSA gains: 00

for bright stars, 11 for dark stars, 10 for Sirius; DMSB gains: always 00

LIN mode • Electronic chain gain • Non linearity calibration • Offset

Full calibration sequence. One

sequence per month (first days of the

month)

- Two asynchronous orbits - All available DSAs Altitude range: 0-

300 km first orbit, 200-300 km second orbit

OCC mode • Dark Charge and straylight

Nominal occultation observations

- Observation of stars in bright and dark limb OCC mode

• Spectral calibration • Radiometric sensitivity