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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
ERS - ENVISAT SYMPOSIUM
Gothenburg 16-20 October, 2000
GOMOS
Instrument Measurement Principle
60 Km
40 Km
20 Km
F(z)
F
o
z
Wavelength
Intens
ity
Wavelength
Intens
ity
F
o
Wavelength
Intens
ity
F(z
i
)
F(z
1
)
F(z
i
)
Wavelength
T
r
an
s
m
is
si
on
F
o
T(z
i
) =
!
!
!
!
Independent of gain drift
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
200
400
2
Time [msec]
De
p
o
in
tin
g
[
µ
ra
d
]
100
300
Wavelength
Intens
ity
!
!
!
!
Facilitates Depointing/Spectral Correction
Tracking Data
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
L
1b Pro
c
essor verif
i
cation
Cali
bra
t
i
o
n
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
June
Hea
l
t
h
check
Schedule - Calibration & Processing Verification
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 Processing
Level 2 Processing
Calibration Inputs
ERS - ENVISAT SYMPOSIUM
Gothenburg 16-20 October, 2000
GOMOS
Calibration Needs
Main parameters to be re-characterised during the commissioning phase:
Correction functions
Parameter (for in-orbit calibration)
Comment / required accuracy
Bad pixel processing
Dark Signal Non-Uniformity
Median filter
Cosmic ray processing
Threshold value
Median filter
Digital 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%/deg
Star 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 pixel
Dispersion [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
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 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)
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..)
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
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
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
ERS - ENVISAT SYMPOSIUM
Gothenburg 16-20 October, 2000
GOMOS
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.
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
ERS - ENVISAT SYMPOSIUM
Gothenburg 16-20 October, 2000
GOMOS
ERS - ENVISAT SYMPOSIUM
Gothenburg 16-20 October, 2000
GOMOS
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