ers - envisat symposium gothenburg 16-20 october, 2000 gomos gomos calibration and processor...

ERS - ENVISAT SYMPOSIUMGothenburg 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


GOMOS

Instrument Measurement Principle

60 Km

40 Km

20 Km

F(z)

Fo

z

Wavelength

Inte

nsity

Wavelength

Inte

nsity

Fo

Wavelength

Inte

nsity

F(zi)F(z1)F(zi)

Wavelength

Tra

nsm

iss

ion

FoT(zi) =

Independent of gain drift


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

200 400

2

Time [msec]

Dep

oint

ing

[ra

d]

100 300

Wavelength

Inte

nsity

Facilitates Depointing/Spectral Correction

Tracking Data


GOMOS

SODAP

Signal level and band setting

Tracking capability

Performance evaluationCALEX Verification

First in-flight calibrationSecond 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

Pro

cess

or v

erif

icat

ion C

alib

rati

on

Oct Nov Dec Jan Feb Mar Apr May June

Hea

lth

chec

k

Schedule - Calibration & Processing Verification


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 SectionComputation

Level 2 Products

Level 1b Processing Level 2 Processing

Cal

ibra

tion

Inp

uts


GOMOS

Calibration NeedsMain parameters to be re-characterised during the commissioning phase:

Correction functions Parameter (for in-orbit calibration) Comment / required accuracyBad pixel processing Dark Signal Non-Uniformity Median filterCosmic ray processing Threshold value Median filterDigital to electron conversion Sensitivity [LSB / radiometric unit] Monitoring of drift

Non-linearity < 0.1 %Electronic chain offset < 1 %

Dark Charge Removal Dark Signal Non-Uniformity < 1%Mirror reflectivity correction SFM Reflectivity Calibrated on-ground; 0.05%/degStar background estimation Pixel-Response Non-Uniformity < 1 %

IR Vignetting < 5%Slit transmission

Star signal estimation Pixel-Response Non-Uniformity < 1 %IR Vignetting < 5 %

Wavelength Assignment Wavelength Assignement 1/3 pixelDispersion [nm/column]

Level 2 processing Line Spread Function 5 %

Only a very limited number of parameters are expected to change (red) and thus requires recalibration


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 SATU1 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 SATU’s. 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)


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..)


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


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


GOMOS

IECFInstrument Engineering

Calibration Facility

GOMOS Commissioning

Centre

FOSFlight Operation

Segment

PDSProcessing

Data Segment

RGTReference Operation Plan Generation Tool

NILUNorwegian Institute

for Air Research

AO GroupsExternal Scientific

Studies

Processor update

Data / Processing Config

Observation planningProducts

Observation commanding

SODAPSwitch-On & Data

Acquisition Phase Group

Supporting Facilities and Interfaces


GOMOS


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.


GOMOS

SDE


GOMOS

ers - envisat symposium gothenburg 16-20 october, 2000 gomos gomos calibration and processor...

Documents

gomos gomos calibration

b calibration

gomos calibration database

gomos calibration preparation

processing calibration

calibration purposes

calibration plan slide

gopr gomos level