1. ERS - ENVISAT SYMPOSIUM Gothenburg 16-20 October, 2000 GOMOS GOMOS Calibration and Processor Verification Instrument Principle Calibration Instrument and Engineering Data Checkout Instrument Calibration Processor Verification Level 1b/2 processing verification & tuning Mission Planning Verification of Mission Plan and Calibration Plan

2. ERS - ENVISAT SYMPOSIUM Gothenburg 16-20 October, 2000 GOMOS Instrument Measurement Principle 60 Km 40 Km 20 Km F(z) FoFo z Wavelength Intensity Wavelength Intensity FoFo Wavelength Intensity F(z i ) F(z 1 ) F(z i ) Wavelength Transmission FoFo T(z i ) = Independent of gain drift

3. ERS - ENVISAT SYMPOSIUM Gothenburg 16-20 October, 2000 GOMOS Instrument Measurement Principle As far as the star spectrum is concerned, GOMOS is a slit-less spectrometer, meaning that a movement of the point-like star target results in a spectral and spatial shift on the focal plane. To characterise such movements, the instrument star-tracker continuously samples the star image position. This information is then used in the ground processing to accurately align the individual spectra. Slit/Baffle Star Tracker Image x y 200400 2 Time [msec] Depointing [ rad] 100300 Wavelength Intensity Facilitates Depointing/Spectral Correction Tracking Data

4. ERS - ENVISAT SYMPOSIUM Gothenburg 16-20 October, 2000 GOMOS SODAP Signal level and band setting Tracking capability Performance evaluation CALEX Verification First in-flight calibration Second in-flight calibration and Trend Analyis Definition of routine calibration opeations Schedule and obs plan for in-flight calibration Launch L+ 3.5 month = Instrument Calibrated Level 1b Processing chain verification (prototype) Level 1b Processing Tuning (prototype) Level 1b Processing Tuning (PDS) Level 1b chain Update (PDS) Level 1b product coherency Val of geoloc & atm prods L1b Processor verification Calibration OctNovDecJanFebMarAprMayJune Health check Schedule - Calibration & Processing Verification

5. ERS - ENVISAT SYMPOSIUM Gothenburg 16-20 October, 2000 GOMOS On-Ground Processing Datation Geolocation Wavelength Assignment Covariance Processing Sample corrections Transmission Processing Background Processing Background Removal Transmission spectra Spectral Inversion Smoothing Vertical Inversion Cross Section Computation Level 2 Products Level 1b ProcessingLevel 2 Processing Calibration Inputs

6. ERS - ENVISAT SYMPOSIUM Gothenburg 16-20 October, 2000 GOMOS Calibration Needs Main parameters to be re-characterised during the commissioning phase: Correction functionsParameter (for in-orbit calibration)Comment / required accuracy Bad pixel processingDark Signal Non-Uniformity Median filter Cosmic ray processingThreshold valueMedian filter Digital to electron conversionSensitivity [LSB / radiometric unit]Monitoring of drift Non-linearity < 0.1 % Electronic chain offset< 1 % Dark Charge RemovalDark Signal Non-Uniformity < 1% Mirror reflectivity correctionSFM ReflectivityCalibrated on-ground; 0.05%/deg Star background estimationPixel-Response Non-Uniformity< 1 % IR Vignetting< 5% Slit transmission Star signal estimationPixel-Response Non-Uniformity< 1 % IR Vignetting< 5 % Wavelength AssignmentWavelength Assignement1/3 pixel Dispersion [nm/column] Level 2 processingLine Spread Function5 % Only a very limited number of parameters are expected to change (red) and thus requires recalibration

7. ERS - ENVISAT SYMPOSIUM Gothenburg 16-20 October, 2000 GOMOS Instrument Calibration Support Modes Besides the normal occultation mode, GOMOS has four other operation modes useful for calibration purposes. All modes will be fully exploited for calibration. SATU= Star Acquisition and Tracking Unit The linearity mode is the same as the occultation mode except that the integration time can be programmed from 0.25 to 10 second in steps of 0.25 second. The uniformity mode produces pixel images of both spectrometer, photometer and SATU 1 sensors. In this mode the instrument operates in open loop (no active tracking of stars, only orbit compensation). The Spatial Spread Mode produces, as in uniformity mode, pixel images of spectrometers, photometers and SATUs. The difference is that the mode operates in closed loop with the tracking chain and hence needs a star target to track.. The Fictive Star Mode is similar to the occultation mode (same data format) except that there is no active tracking (the star is specified with a specific position and speed)

8. ERS - ENVISAT SYMPOSIUM Gothenburg 16-20 October, 2000 GOMOS Calibration Preparation The following resources and tools are prepared, to support the GOMOS commissioning activity: PDS = Payload Data Segment (geophysical processor in ESRIN) CALEX - GOMOS Calibration Processor. This software is installed and operated within the Instrument Engineering Calibration Processor. The main product is the GOMOS calibration database; used as input to the GOMOS geophysical product processor. GOSS - GOMOS system simulator. Includes atmospheric and instrument model to generate real GOMOS source packet products (measurement data). GOPR - GOMOS level 1b (calibration and transmission spectra processing) and level-2 (geophysical processing) prototype. This processor generates exactly the same products as the PDS one and is used as reference processor. STARSEL - GOMOS star observation mission tool. Various other analysis tools (specific tools, Excel and IDL routines etc..)

9. ERS - ENVISAT SYMPOSIUM Gothenburg 16-20 October, 2000 GOMOS Level 1b verification and validation Level 1b processing chain verification (step by step) : implementation of calibration and characterisation parameters. Verification of data coherency (range) : Check the range of each level 1b product items. Analyse the self coherency of GOMOS products (e.g. level 2 products from same geolocation but observed with two different stars) Validation of stellar derived products : Analyse the stellar spectra products and create a stellar spectra database Data flagging monitoring and statistics : Monitor the data flagging of level 1b and limb products (quality flags) Recommendations for routine observation plan : Provide inputs to the observation plan derived from level 1b product analysis Update of level 1b processor

10. ERS - ENVISAT SYMPOSIUM Gothenburg 16-20 October, 2000 GOMOS Level 2 processing chain verification (step by step) : Check functionality of the level 2 processing chain using real measurements Coherency with model - climatology verification : Verification of the coherency using forward models (real star spectra, climatology models) Tuning of Level 2 processing parameters : analyse and validate the level 2 products tuning of the parameters of the level 2 processing chain to identify the new initial operational configuration (iterative tuning). Tuning of the Level 2 processor : modify, adjust the algorithms of the level 2 processing chain as appropriate. Upgrade of the level 2 operational processor : implementation of the level 2 processing chain updates into the PDS operational chain. Level 2 Processing Verification

11. ERS - ENVISAT SYMPOSIUM Gothenburg 16-20 October, 2000 GOMOS IECF Instrument Engineering Calibration Facility GOMOS Commissioning Centre FOS Flight Operation Segment PDS Processing Data Segment RGT Reference Operation Plan Generation Tool NILU Norwegian Institute for Air Research AO Groups External Scientific Studies Processor update Data / Processing Config Observation planning Products Observation commanding SODAP Switch-On & Data Acquisition Phase Group Supporting Facilities and Interfaces

12. ERS - ENVISAT SYMPOSIUM Gothenburg 16-20 October, 2000 GOMOS

13. ERS - ENVISAT SYMPOSIUM Gothenburg 16-20 October, 2000 GOMOS Spectral Line Spread Function (LSF) The GOMOS Level-2 algorithm basically consist of correlating a measured spectrum by a synthesised one; generated from the cross sections. A condition for this correlation is that the two spectra has the same resolution. As the instrument sets the limit, the synthesized one must first be convoluted with the instrument spectral LSF. The instrument LSF will be assessed in-orbit by observing narrow stellar lines (preferrably emission lines). The on-ground measurement is not expected to have changed (major contributing effects are defocus) and will be used.

14. ERS - ENVISAT SYMPOSIUM Gothenburg 16-20 October, 2000 GOMOS SDE Star Light Pointing IR1/2 Photo 1/2 UV VIS Steering Front Mechanism TEL MDE ICU Satellite Data-bus GOMOS Functional Layout

15. ERS - ENVISAT SYMPOSIUM Gothenburg 16-20 October, 2000 GOMOS

16. ERS - ENVISAT SYMPOSIUM Gothenburg 16-20 October, 2000 GOMOS