5.1.3 Content of the products
5.1.3.1 General common fields to all products
SPH descriptor

ASCII string describing the product.

Start time

start time of the measurements included in the MDSR. Expressed
in Julian Date 2000.

Attachment flag

flag indicating if MDSRs are attached to the current ADSR
1: no corresponding MDSRs
0: there are corresponding MSDRs
In the case of GOMOS, the attachment flag is always set to
0, indicating that a MDSR is attached to the ADSR.

Data Quality indicator

indicator of the quality of the data included in the MDSR
1: blank MDSR
0: data are available and valid
In the case of GOMOS, the attachment flag is always set to
0, indicating that the MDSR is not empty (as there is no empty MDSR in the
GOMOS products).

Creation time

creation time of the data set record. Usually inserted at
the beginning of the DSRs corresponding to calibration data. Expressed in
Julian Date 2000 (transport format).

Spare

dummy bytes.

5.1.3.2 Level 1b product
5.1.3.2.1 Definitions related to the SPH and the DS
Definitions related to the SPH

Start time of the occultation

Starting time of the first
acquisition of the occultation.

Time of the end of the occultation

Ending time of the last
acquisition of the occultation.

Latitude and longitude of the tangent point at the
start time

Location of the tangent point
at the starting time of the occultation. Positive latitudes are North;
positive longitudes are East. Computed during the geolocation process.
Provided for the reference wavelength, which is equal to 500 nm.

Latitude and longitude of the tangent point at the stop
time

Location of the tangent point
at the ending time of the occultation. Positive latitudes are North; positive
longitudes are East. Computed during the geolocation process. Provided for
the reference wavelength, which is equal to 500 nm.

Occultation duration

Duration of the occultation
expressed in seconds. Computed during the datation process.

Sampling duration

Nominal value is 0.5 second.

Number of measurements

Number of acquisitions
performed during the occultation. This number is computed when the GOMOS
packets are read from the Level 0 product.

Status of the instrument

The Redundancy Definition
vector and the Instrument Configuration fields are used to determine in which
mode the GOMOS instrument was during the measurements. These two fields are
read in the Source Data Field Header of the first valid packet of the
occultation.
Status is set to 0 if the
instrument is in extended mode i.e. if:
 Bit 12 of the Redundancy
Definition vector set to 1
 Bit 13 of the Redundancy
Definition vector set to 0
 Bit 14 and 15 of the
Instrument Configuration set to 1
 Bit 4 and 5 of the Instrument
Configuration set to 0
and status is set to 1 in
another configuration

Occultation number in the orbit

This information is provided by
the Source Data Packet extraction processing.

Star name and associated information

These information are read from
the star catalogue auxiliary product once the star identifier of the current
occultation has been read in the GOMOS packets.

Star identifier in the star catalogue

Star identifier read in the
Source Data Field Header of the first valid packet of the occultation.

Star direction in the quasitrue of date frame (Cartesian)

Star direction computed in the
geolocation processing.

Dark/bright limb flag

Flag read in the GOMOS packets.
A value of 0 means that the electronic gain settings of the instrument has
been set to optimise the measurement of the star signal (this is usually
called dark limb settings), while a value of 1 means that the gain settings
has been set in order to perform the observation assuming some limb signal
(this is usually called bright limb settings). Note that this value is
defined in the mission planning and, although it is generally in line with
the actual illumination conditions, it may not reflect the actual
illumination conditions. The real illumination condition is given by the
illumination condition PCD and is computed from the sun zenith angle at both
the instrument and the tangent point locations.

Definitions related to GADS

Summary quality

The meaning of the possible
flag values stored in the Summary Quality GADS is detailed in section 5

Number of points of the spectra

The programmable number of
transmitted columns per CCD array. The values are read in the GOMOS packets.
In the Level 1b product, the
data coming from the 4 CCDs are joined together in a single vector. This is
true for the transmission and covariance spectra but also for the reference
star spectrum, the limb spectra and the wavelength assignment vectors. This
parameter must be used to distinguish between the different CCD arrays inside
these global vectors. For instance, if this parameter is equal to the current
nominal values: [450, 966, 420, 500], it means that in each global vector
(e.g. transmission) the 450 first samples belong to the UV CCD (SPA1), the
following 966 belong to the VIS CCD (SPA2), the 420 next to the IR1 CCD
(SPB1) and the last 500 to the IR2 CCD (SPB2).
The total number of transmitted
columns is currently equal to 2336.

Number of photometer output data per measurement

This is the number of fast
photometer output data per spectrometer measurement. Nominal value is 500.
This number will never change during the instrument life.

Number of SATU output data per measurement

This is the number of SATU
output data per spectrometer measurement. Nominal value is 50. This number
will never change during the instrument life.

Photometer central wavelength

This is the wavelength
corresponding roughly to the middle of the spectral range of each photometer.
The nominal values are 499.5 nm for the blue photometer and 672.0 nm for the
red one.

Time shift for ray tracing / geolocation

This time indicates the time
shift between what is called 'beginning of measurement' and 'half
measurement' in the ray tracing processing. Geolocation results are provided
at these two times for each measurement.

Spectrometer effective sampling time

Actual duration of one
spectrometer measurement. Nominal value is 0.4999639 second. This is a
constant value.

Reference wavelength for the ray tracing

All the geolocation information
is provided for this reference wavelength. For example the localisation of
the ray nodes and of the tangent point node are provided for this wavelength
although at one given time inside the atmosphere, and due to chromatic
refraction effects, each wavelength is looking at a different altitude. It is
also the reference wavelength for the aerosol products. It is equal to 500
nm.

Radiometric sensitivity curve (background)

This curve has been built from the radiometric
sensitivity of the instrument measured during its onground characterisation
and updated inflight. It allows the conversion of the background spectra
expressed in electrons in the product into physical units (ph/s/cm2/nm/sr).
It is given as a LUT of conversion factors. A linear interpolation of
the conversion factor is needed to use this curve for any sample of the
background spectra.

Radiometric sensitivity curve (star)

This curve has been built from the radiometric
sensitivity of the instrument measured during its onground characterisation
and updated inflight. It allows the conversion of the star spectra expressed
in electrons in the product into physical units (ph/s/cm2/nm). It is given as
a LUT of conversion factors. A linear interpolation of the conversion
factor is needed to use this curve for any sample of the star spectrum.

Thermistor temperature (SP and FP)

Thermistor temperature read
from the GOMOS packets and converted into degrees using a conversion LUT
calibrated onground. This temperature is used to adjust the dark charge map
to the actual temperature. The actual thermistor temperature are transmitted
once per occultation, in the first packet. The coding accuracy of the
thermistor temperature is roughly 0.4°.

Spectrometer dark charge at band level used for the
dark charge correction

Dark charge at band level for
each of the 3 bands (upper, central and lower) and for each of the
transmitted CCD column.

Mean dark charge for the spectrometers

The mean dark charge level over
the CCD array, expressed in electrons. This mean dark charge is assumed to be
constant during all the occultation. It is provided in this order: SPA1
upper, target and lower bands, SPA2 upper, target and lower bands, then SPB1
and SPB2.

Mean dark charge for the photometers

The mean dark charge level over
the CCD array, expressed in electrons. This mean dark charge is assumed to be
constant during all the occultation. It is provided in this order: FP1, FP2.

Offset between thermistor and CCD arrays temperature

Offsets directly read from the
Calibration auxiliary product. These offsets may be used to estimate the CCD
temperature. They are currently not used in the Level 1b processing which
uses directly the thermistor temperature.

Sun coordinates in the
geocentric equatorial inertial system

The Sun coordinates are
provided for the middle of the first measurement only.

Nominal wavelength assignment

Nominal wavelength assignment
of the transmission and covariance spectra written in this product. The limb
spectra can be directly associated with this spectral assignment because they
are not affected by pointing errors. The spectral assignment of the
transmission and covariance must take the spectral shift parameter into
account. This quantity, provided for each measurement, must be added to the
nominal wavelength assignment to obtain the effective wavelength assignment
of each spectrum. Note that if the spectrum resampling process has been
performed on the nominal spectral grid, then the effective wavelength
assignment is identical to the nominal assignment (in this case, the spectral
shift is equal to 0 for all the measurements in the product). The nominal
wavelength assignment is a global vector.
The nominal wavelength
assignment of the CCD columns of each spectrometer CCD array is coded in the
Level 1b product on 32 bits with an accuracy of 1.e6 nm. The spectra
provided in the Level 1b product are always in increasing spectral order CCD
per CCD whatever the original spectral orientation of the CCD is (for
example, the samples of the VIS CCD (SPA2) are reordered). There may be
configurations where there is a spectral overlap between the UV and the VIS
CCDs. In this case, the global vector is not in increasing order.

Effective number of star spectra used for the
computation of the reference star spectrum

This is the number of star spectra that have been averaged
to obtain the reference star spectrum. A maximum number of spectra is allowed
as well as a minimum altitude of the tangent point to be sure that the
observation is actually outside the atmosphere.

Reference star spectrum

The reference star spectrum is
computed by averaging several star spectra outside the atmosphere at the
beginning of the occultation. The current nominal value is 10 spectra. Note
that the 2 first spectra (see the PCD at measurement level) are not used for
pointing instability reason.
It is given in electrons and
must be converted into physical units (ph/s/cm2/nm) by multiplying the
electron values by the conversion factor inferred from the radiometric
sensitivity curve (star) provided as a LUT.

Reference atmospheric density profile

These variables fully describe
the reference density profile used during the ray tracing computation.
 Size of the reference
atmospheric profile
 First altitude of the
profile
 Altitude discretisation
 Reference atmospheric
density profile
The reference atmospheric
density profile is computed in the Atmosphere preparation process.

Definitions related to DS

Full transmission spectra and associated covariance

The transmission spectra are
computed as the ratio of each measured star spectrum by the reference star
spectrum, computed from star spectra measured outside the atmosphere. It is
said full because it is the actually measured transmission, not corrected,
neither from refraction effects (dilution, scintillation, chromatic
refraction) nor from the dynamic PSF. The covariance (here, in fact, the
variance of each pixel signal) is computed from analysis of S/N ratio. There
is one transmission spectrum and one covariance spectrum per spectrometer
measurement. The transmission and covariance spectra are global vectors (see
the "Number of points of the spectra" field).

Scaled estimated central background

Estimation of the background
signal measured by the target band. The estimated central background spectra
values are coded on 16 bits. Due to the high variations of these spectra with
the altitude, the coding is dynamic and uses an offset and a gain for each
measurement. The offset and the gain are stored in the Auxiliary data ADS of
the product. For each measurement, the offset is computed as the minimum
value of the spectrum to store (f_{min}) and the gain is computed as
65535/(f_{max}f_{min}) where f_{min} (resp. f_{max})
is the minimum (resp. the maximum) value of the spectrum for the current
measurement.
Note that the background
spectra are divided by the PRNU correction factor before being scaled.
The following table presents
the accuracy (in number of electrons) for several ranges of background
spectra:
Range (in e)

Accuracy (LSB)

100

< 0.1 e

1000

< 0.1 e

10000

0.2 e

30000

0.5 e

100000

1.5 e

300000

4.6 e

500000

7.6 e

1000000

15.0 e

To decode the value read in the
product, one must compute: Background (in electrons) = Offset + Background
Code / Gain
The decoded spectra values
obtained in electrons must then be converted into physical units
(ph/s/cm2/nm/sr) by multiplying them by the conversion factor inferred from
the radiometric sensitivity curve (star) provided as a LUT.

Error bar for the estimated central background

The error bars for the
estimated central background data are computed as the shot noise of the
background signal and expressed in relative percentage. The maximum error bar
is set to 6500%.
for each measurement f = 1, f_{size}
for each CCD column: k = 1, k_{size}
if ( #0 ) then
else
endif
/* truncate the error to 6500 percent (coding limit) */
if ( > 6500. ) = 6500.
end for k
end for f
Where f_{size}
is the number of spectrometer measurements and k_{size} is the total
number of transmitted pixels (global vectors)
They are
stored in the product in 16 bits. They represent the relative percentage of
error for each estimated central value. This coding allows an accuracy of at
least 0.1 percent.

Photometers engineering data

Fast photometer samples converted into electrons and
corrected from dark charge, straylight, background contribution and
flatfield.

Error bar for the photometers engineering data

The error bar vector (50 elements) is made of 25 couples
of (mean value, standard deviation) of the photometer shot noise. For each
spectrometer measurement, 500 photometer samples are available. They are
split into 25 sets. For each set, the mean value of the measurement shot
noise and its standard deviation (based on 20 samples) are computed and are
written in the product.

PCD at sample level (SP)

The meaning of the possible PCD values for the
spectrometers stored in this MDS is detailed in section 5.1.3.2.2

PCD at sample level (FP)

The meaning of the possible PCD values for the photometers
stored in this MDS is detailed in section 5.1.3.2.2

SATU mispointing angles

SATU data read from the Level 0
products and converted into angles (X and Y directions).

SFA angles measurements

SFA data read from the Level 0
products and converted into angles (azimuth and elevation).

Spectral shift of the star spectra

This quantity, provided for each
measurement, must be added to the nominal wavelength assignment to obtain the
effective wavelength assignment of each spectrum. Note that if the spectrum
resampling process has been performed on the nominal spectral grid, then the
effective wavelength assignment is identical to the nominal assignment (in
this situation, the spectral shift is equal to 0 for all the measurements of
the product).

Offset and gain for the background spectra coding

For each measurement, the
offset is computed as the minimum value of the background spectrum to store
(f_{min}) and the gain is computed as 65535/(f_{max}f_{min})
where f_{min} (resp. f_{max}) is the minimum (resp. the
maximum) value of the background spectrum for the current measurement.
This offset and this gain must
be used to decode the values of the background spectra written in the product
for both before and after straylight and vignetting corrections. To decode
the value read in the product, one must compute:
Background (in electrons) =
Offset + Background Code / Gain
There is one couple of gain and
offset values per spectrometer measurement.

PCD at measurement level

The meaning of the possible
flag values stored in this ADS is detailed in section 5.1.3.2.4.

Latitude,
longitude and altitude of the spacecraft

Spacecraft location computed
during the geolocation processing. These quantities are provided at the
beginning and during the measurement, generally at halfmeasurement (the
actual temporal shift is given by the time shift for the ray tracing). The
altitude is expressed in meters.

Latitude,
longitude and altitude of the tangent point and associated errors

Tangent point location and
associated errors computed during the geolocation processing. They are given
for the reference wavelength for ray tracing computation (equal to 500 nm and
given in the Occultation data GADS). These quantities are provided at the
beginning and during the measurement, generally at halfmeasurement (the
actual temporal shift is given by the time shift for the ray tracing). The
altitude is expressed in meters.

Distance
spacecraft  tangent point

Distance between the satellite
and the tangent point computed during the geolocation processing. It is given
for the reference wavelength for ray tracing computation (equal to 500 nm and
given in the Occultation data GADS). This quantity is provided at the
beginning and during the measurement, generally at halfmeasurement (the
actual temporal shift is given by the time shift for the ray tracing).The
distance is expressed in meters.

Instrument
pointing direction

Instrument pointing direction
(azimuth and elevation) in the GOMOS frame. These values are computed in the
geolocation process and do not use the SFA angles but only the geometrical
characteristics of the measurements: satellite and star location, GOMOS frame
definition, orbit properties.

Virtual
star direction in the quasitrue of date frame

Virtual star direction in the
quasitrue of date frame provided at the beginning and during the
measurement, generally at halfmeasurement (the actual temporal shift is
given by the time shift for the ray tracing ).

Number
of nodes of the ray tracing

The ray path is computed during
the ray tracing / geolocation process. The ray is made from 3 nodes outside
the atmosphere up to 150 nodes inside. If more than 150 nodes have been
necessary during the processing, the ray path is resampled in order to reduce
the number of nodes. This field indicates how many values are significant in
the following arrays.

Index
of the tangent point in the list of the ray tracing node

Index of the tangent point in the
list of the ray tracing node. There is one index per spectrometer measurement
(i.e. per ray tracing). This index must be used to locate the tangent point
information in the ray tracing output arrays.

Interpolation
factors P and Q for the law delta (lambda)

These factors may be used to
compute the chromatic deviation for a specified wavelength as: deviation
expressed in radians is equal to Q+P*EDLEN(l)
where EDLEN is the Edlen's function, with λ expressed in nm:
where the factor 1.00062 stands for temperature and
pressure correction.
These parameters are provided
during the measurement, generally at halfmeasurement (the actual temporal
shift is given by the time shift for the ray tracing).

Interpolation
factors P and Q for the law h_{0} (lambda)

These factors may be used to
compute the tangent point altitude at any wavelength in the spectral range of
the spectrometers. The altitude expressed in meters is equal to Q+P*EDLEN(l) where EDLEN is the Edlen's function
described above.

Latitude,
longitude and altitude of the ray tracing grid nodes

Location of the ray path nodes
computed during the geolocation process. The tangent point is always included
in these nodes. These parameters are provided during the measurement,
generally at halfmeasurement (the actual temporal shift is given by the time
shift for the ray tracing).

Air
density and atmospheric pressure at the tangent point

These values are given only for
the tangent point. They are computed from the ECMWF data and MSIS90 model.
The air density is expressed in cm^{3} and the atmospheric pressure
is expressed in Pa. They are provided during the measurement, generally at
halfmeasurement (the actual temporal shift is given by the time shift for
the ray tracing).

Air
temperature at the ray tracing grid nodes

Air temperature at the ray path
nodes computed by using the data from the ECMWF data and MSIS90 model. The
tangent point is always included in these nodes. These values are provided
during the measurement, generally at halfmeasurement (the actual temporal
shift is given by the time shift for the ray tracing).

Sunzenith angle at the spacecraft location

Angle computed at the middle of
each spectrometer acquisition

Sunzenith angle at the tangent point location

Angle computed at the middle of
each spectrometer acquisition

Sunazimuth angle at the tangent point location

Angle computed at the middle of
each spectrometer acquisition

Apparent altitude of the central background

Altitude computed at the middle
of each spectrometer acquisition

5.1.3.2.2 PCD and flags: PCD at Sample Level (SP)
The flags at sample level set during the Level 1b
processing are combined together in a 16bits integer before being written in
the Level 1b transmission product (corresponding field in the transmission MDS)
and in the limb product (corresponding field in the MDS). The way this
combination is done is described in the following table.
Bit #

Flag name

Comments

0

FL_{sat}(k,L,f)

saturation flag for the lower band (0: no saturation, 1:
saturation)

1

FL_{sat}(k,C,f)

saturation flag for the central band (0: no saturation, 1:
saturation)

2

FL_{sat}(k,U,f)

saturation flag for the upper band (0: no saturation, 1:
saturation)

3

FL_{bad}(k,L)

bad pixel flag for the lower band (0: no bad pixel in the
band, 1: at least one bad pixel in the band)

4

FL_{bad}(k,C)

bad pixel flag for the central band (0: no bad pixel in
the band, 1: at least one bad pixel in the band)

5

FL_{bad}(k,U)

bad pixel flag for the upper band (0: no bad pixel in the
band, 1: at least one bad pixel in the band)

6

FL_{cr}(k,L,f)

cosmic ray flag for the lower band (0: no cosmic ray
detected in the data, 1: cosmic ray has been detected in the data)

7

FL_{cr}(k,C,f)

cosmic ray flag for the central band (0: no cosmic ray
detected in the data, 1: cosmic ray has been detected in the data)

8

FL_{cr}(k,U,f)

cosmic ray flag for the upper band (0: no cosmic ray
detected in the data, 1: cosmic ray has been detected in the data)

910

FL_{bg}(k,f)

from the spectrometers central background estimation
processing
0: central background is computed with no flagged samples
1: less than 25 percent of flagged samples
2: less than 50 percent of flagged samples
3: more than 50 percent of flagged samples

1112

FL_{ft}(k,f)

from the transmission and covariance computations
processing
0: no problem identified
1: reference star spectrum value is zero
2: one of the spatial band is saturated

13

FL_{off}(k)

from the wavelength assignment processing
0: pixel is in a valid spectral range
1: pixel is in an invalid spectral range

14

FL_{rsp}(k,f)

from the reference star spectrum and transmission
computation processings
0: no problem
1: current data computed with flagged data

15


Not used

Note: bit 0 is the lowest significant bit of the byte
(bits are counted from right).
As for the spectrometer flags at sample level, the
photometer flags at sample level are combined together in a 16bits integer
before being written in the Level 1b product (corresponding field in the
transmission MDS). The way this combination is done is described in the
following table.
Bit #

Flag name

Comments

0


photometers saturation flags (0: no saturation, 1:
saturation)

115


Not used

Note: bit 0 is the lowest significant bit of the byte
(bits are counted from right).
Each flag is
affected to a whole spectrometer measurement i.e. gives global information
concerning all the spatial bands, all the CCD columns of the spectrometers and
all the photometer samples during the measurement. There are two kinds of flags
at measurement level: the first set is made of flags explicitly set at
measurement level such as the data valid flag (read in the GOMOS packets), the
flags indicating if SATU outputs have been used for correction or if the star
spectrum was outside the central band ... The second set of flags is built from
the flags at sample level and can be seen as a flag summary at measurement
level.
The PCD at measurement level are written in the auxiliary
data ADS of the Level 1b transmission product and in the ADS of the Level 1b
limb product.
Flags explicitly set at measurement level:
Variable

Description

Range  References

FLAG_{DV}(f)

data valid flag (measurement level)

from the SDP extraction processing
0: anomaly
1: timeout
3: fully successful
9: missing packet

FLAG_{time}(f)

datation flag

from the datation processing
0: no datation problem
1: problem in the datation processing
2: invalid measurement (FLAG_{DV}(f)=0 or 1)
9: missing packet

FLAG_{rt}(f)

raytracing flag

from the geolocation processing
0: no problem
1: wrong pointing (not in the direction of the atmosphere)
2: ray path across the Earth

FLAG_{gl}(f)

geolocation flag

from the geolocation processing
0: no problem
1: geolocation problem

FLAG_{irv}(f)

vignetting correction flag

from the spectrometers vignetting correction processing
0: no vignetting
1: vignetting occurs for this measurement

FLAG_{out}(f)

indicates if the star spectrum falls outside the central
band during the measurement

from the star spectrum computation
0: star spectrum is inside the central band
1: centre line of star spectrum image is outside the central band


indicates the number of fast photometers saturation during
the spectrometer measurement

from the photometers saturated samples processing
fc: FP1, FP2

FLAG_{stab}(f)

indicates instable measurements

The number of instable measurements at the beginning of
the occultation (f_{stab}) is read from the Calibration auxiliary
product.
For all measurements before f_{stab} (including
it), FLAG_{stab}(f) is set to 1 and for all following measurements
FLAG_{stab}(f) is set to 0.

FLAG_{mod}(f)

modulation correction flag

from the spectrometer demodulation processing
0: no problem
+1: problem of structure for SPA1 upper band
+2: problem of structure for SPA1 lower band
+4. inconsistent upper and lower band structures for SPA1
+10: problem of structure for SPA2 upper band
+20: problem of structure for SPA2 lower band
+40. inconsistent upper and lower band structures for
SPA2
Note that the flag values between SPA1 and SPA2 may be
mixed, ie flag values like 11, 21, 32, 23, 41, 24 can exist

FLAG_{UC}(f)

ratio between the averaged upper band signal and the
averaged star signal over the SPA2 spectral range

from the star spectrum computation
n<65535: computed ratio expressed in %
65535: star signal is equal to 0

Note: FLAGDV(f) and FLAGDB are read in the GOMOS packets.
They are not generated by the Level 1b processing.
Flags at measurement level computed from flags at sample
level:
The following flags at measurement level are computed as
the sum over the number of CCD columns of the corresponding flags at sample
level.
Variable

Description

Range  References

FLAG_{sat}(f)

number of saturated samples per measurement

from the spectrometers saturated samples processing

FLAG_{cr}(f)

number of cosmic ray detections per measurement

from the spectrometers cosmic rays detection processing

FLAG_{bg}(f)

number of background flagged data per measurement

from the estimated central background processing

FLAG_{ft}(f)

number of samples per measurement for which
FL_{ft}(k,f) has been raised

from the transmission and covariance computations
processing

FLAG_{sum}(f)

PCD at measurement level (summary)

16 integer values  filled with the various flags at each
measurement (see next table); written in the Level 1b and Limb products

For each spectrometer measurement, each component of the
working array FLAG_{sum}(f) (16 integer values) is set to one of
the flags at measurement level as shown in the following table. The array FLAG_{sum}(f)
is written in each Auxiliary data ADSR of the Level 1b and Limb products.
The order of the flags is described in the table below.
Array index

Flag name

Comments

FLAG_{sum}(f)(1)

FLAG_{DV}(f)

data valid flag

FLAG_{sum}(f)(2)


not used

FLAG_{sum}(f)(3)

FLAG_{time}(f)

datation flag

FLAG_{sum}(f)(4)

FLAG_{rt}(f)

raytracing flag

FLAG_{sum}(f)(5)

FLAG_{gl}(f)

geolocation flag

FLAG_{sum}(f)(6)

FLAG_{sat}(f)

saturation flag (SP)

FLAG_{sum}(f)(7)

FLAG_{cr}(f)

cosmic ray flag

FLAG_{sum}(f)(8)

FLAG_{irv}(f)

vignetting flag

FLAG_{sum}(f)(9)

FLAG_{bg}(f)

background flag

FLAG_{sum}(f)(10)

FLAG_{out}(f)

star spectrum out of band

FLAG_{sum}(f)(11)

FLAG_{ft}(f)

transmission flag

FLAG_{sum}(f)(12)


saturation flag (FP1)

FLAG_{sum}(f)(13)


saturation flag (FP2)

FLAG_{sum}(f)(14)

f_{stab}

stability flag

FLAG_{sum}(f)(15)

not used

set to 0

FLAG_{sum}(f)(16)

not used

set to 0

5.1.3.2.5 PCD and Flags: PCD at occultation level
Each flag
characterises the whole occultation. There are three kinds of flags at occultation
level:
 the first set is made of flags
explicitly set at occultation level (e.g. dark/bright limb flag,
star reference spectrum fatal flag...).
 the second set of flags is built
from the flags at measurement level (e.g. saturated samples, cosmic
rays...) and can be seen as a flag summary.
 the third set of flags is built
from flags at sample level (e.g. samples containing bad pixels).
Flags explicitly set at occultation level:
Variable

Description

Range  References

PCD_{ist}

PCD for internal straylight correction processing

from the internal straylight correction processing
0: the internal straylight correction has been applied
1: the internal straylight correction has not been applied

PCD_{earth}

PCD for external earth straylight correction
processing

from the external straylight correction processing
0: the external earth straylight correction has been
applied
1: the external earth straylight correction has not been
applied

PCD_{sun}

PCD for external sun straylight correction
processing

from the external straylight correction processing
0: the external sun straylight correction has been applied
1: the external sun straylight correction has not been
applied

PCD_{slit}

PCD for slit transmission correction processing

from the slit transmission correction processing
0: the slit transmission correction has been applied
1: the slit transmission correction has not been applied

PCD_{ref}

PCD indicating that the reference star spectrum is
computed with a small number of star spectra

from the reference star spectrum computation
processing
0: no problem
1: the reference star spectrum has been computed with a
small number of measurements
2: no valid measurements in the occultation to compute the
reference star spectrum (see PCD_{sbd} and PCD_{fatal})

PCD_{sdb}

PCD indicating if the star spectrum has been read
in the Stellar Spectra Database

from the reference star spectrum computation
processing
0: the reference star spectrum is computed from the
measurements
1: the reference star spectrum is read in the Stellar
spectra database
2: the reference star spectrum has not been found in the
Stellar spectra database (see PCD_{fatal})

PCD_{fatal}

PCD indicating that the reference star spectrum has
not been computed

from the reference star spectrum computation
processing
0: the reference star spectrum has been computed
1: the reference star spectrum has not been computed (no
transmission computed)

PCD_{SATU}

PCD indicating that the SATU data are used for
flatfield correction

from the Level 1b processing configuration database
(same value as OK_{SATU})
0: the SATU data are not used
1: the SATU data are used

PCD_{lv0}

PCD read from the Level 0 product

from the SDP extraction processing
0: standard occultation
1: first part of a tangent occultation
2: last part of a tangent occultation

PCD_{atm}

PCD for atmosphere file

from the preparation of the Atmosphere model
54: one ECMWF file used. Time record is
included in the occultation period.
102: one ECMWF file used. Time record
before the beginning of the occultation (
Dt
Δ≤ 24h.
103: one ECMWF file used. Time record
after the beginning of the occultation (Dt£24h).
106: one ECMWF file used. Only one time
record in the validity interval (Dt£24h).
155: two ECMWF files used.
201: no ECMWF file available. Use MSIS
model alone.
202: only old ECMWF files (Dt>24h). Use
MSIS model alone.
203: only future ECMWF files (Dt>24h). Use
MSIS model alone.
206: no ECMWF
file available in the validity interval (Dt>24h). Use MSIS model
alone.
In case of no
convergence of the ray tracing, the PCD_{atm} flag is increased by
10. Example, an initial value of 106 of PCD_{atm} will be
actually set to 116 in case of no convergence.

PCD_{dc}

PCD for dark charge correction

from the dark charge correction processing
indicates the dark charge correction processing applied
and if problems have occurred
0: dark charge map used
1: first measurements used
2: no dark charge correction applied
11: first measurements not available  DC correction with
DC maps
12: first measurements not available and missing 1^{st}
packet  DC correction with DC maps without temperature dependence
21: dark charge map computed from DSA observation has been
used

ok_{back}

Background correction activation switch

from the level 1b processing configuration or modified by
the background correction processing.
indicates which background correction has been effectively
performed.
Note that in full dark limb condition, no correction is
applied to SPA samples, whatever the value of the activation switch read from
the level 1b processing configuration.
0: no correction
1: background correction applied (linear interpolation)
2: background correction applied (exponential
interpolation)
3: background correction applied (general method)

Flags at occultation level for the photometers:
Variable

Description

Range  References


PCD for dark charge correction of the photometers
samples

from the photometers dark charge correction
indicates the dark charge correction processing applied
(0:DC correction performed; 1:no DC correction)


PCD for saturation: number of photometers saturated
samples

from the photometers bad pixels processing

Flags at occultation level computed from flags at measurement
level:
The following flags at occultation level are computed as
the sum over the number of measurements of the corresponding measurement flags.
Variable

Description

Range  References

PCD_{fvalid}

No valid data flag

set to 1 if PCD_{DV }= f_{size}
set to 0 otherwise

N_{err}

Number of source packets containing errors

from the SDP extraction processing
number of invalid or missing measurement

PCD_{DB}

PCD for dark/bright limb conditions

from the SDP extraction processing
0: dark limb
1: bright limb

PCD_{illum}

Observation illumination
condition

from the geolocation processing. This is the actual illumination
of the observation, computed from geometrical configuration between the Sun,
the satellite and the GOMOS instrument pointing direction.
0: full dark limb condition
1: bright limb condition
2: pure twilight condition
3: straylight condition
4: twilight+straylight condition
This flag is not dependent on the dark/bright limb flag
that is read in the Level 0 product; as this latter comes from the mission
scenario and may not fully reflect the actual observation conditions.

PCD_{DV}

PCD of the SDP extraction processing

from the SDP extraction processing
number of invalid or missing measurements

PCD_{time}

PCD for datation: indicates the number of
measurements where flags have been raised

from the datation processing

PCD_{rt}

PCD for ray tracing: indicates the number of
measurements with invalid ray path

from the geolocation processing

PCD_{gl}

PCD for geolocation: indicates the number of
measurements performed outside the atmosphere

from the geolocation processing
note: set to 1000 if the occultation is completely
performed outside the atmosphere

PCD_{sat}

PCD for saturation: indicates the number of
measurements containing saturated samples

from the spectrometers saturated samples processing
number of measurements where saturated samples have been
detected

PCD_{cr}

PCD for cosmic rays: indicates the number of
measurements where cosmic rays has been detected

from the cosmic rays detection processing
number of measurements where cosmic rays have been
detected

PCD_{mod}

PCD for modulation correction processing

from the modulation correction processing
number of measurements where the modulation correction has not been applied

PCD_{irv}

PCD for vignetting correction processing

from the vignetting processing
number of measurements where the vignetting correction has
been applied

PCD_{bg}

PCD of the central background computation

from the estimated central background processing
number of measurements where FLAG_{bg}(f) has been
raised

PCD_{out}

PCD for flatfield correction: indicates the number
of measurements where the star falls outside the central band

from the star signal computation processing
number of measurements where part of the star spectrum is
outside the central band

PCD_{ft}

PCD indicating that a problem has occurred during
the computation of the transmission or of the covariance

from the transmission and covariance computations
processing
number of measurements where a problem has been detected
during the full transmission computation

Flags at occultation level computed from flags at sample
level:
The following flags at occultation level are computed as
the sum over the number of CCD columns of the corresponding flags at sample
level.
Variable

Description

Range  References

PCD_{bad}

PCD for bad pixels

from the spectrometers bad pixels processing
number of bad pixels (same value for all the measurements)

The PCDs at occultation level are written in the Summary
quality GADS of the Level 1b transmission products, of the Level 1b limb
products, of the Level 2 atmospheric constituents products, and of the Level 2
residual extinction products.
5.1.3.3 Limb product
Most of the fields included in the limb product are also
included in the GOMOS level 1b products. Only the different fields are
explained in the table below. For the other fields, please refer to the
previous level 1b description chapter.
Definitions related to
DS

Summary quality

The meaning of the possible
flag values stored in the Summary Quality GADS is detailed in section 5.1.3.2.1

Radiometric sensitivity curve (background)

This curve has been built from
the radiometric sensitivity of the instrument measured during its onground
characterisation and updated inflight. It allows the conversion of the
background spectra expressed in electrons in the product into physical units
(ph/s/cm2/nm/sr). It is given as a LUT of conversion factors. A linear
interpolation of the conversion factor is needed to use this curve for any
sample of the background spectra.

Scaled upper & lower background spectra without
straylight correction

Estimation of the background
signal measured by the upper and lower bands before applying the straylight
and the IRvignetting corrections. The estimated background spectra values
are coded on 16 bits. Due to the high variations of these spectra with the
altitude, the coding is dynamic and uses an offset and a gain for each
measurement. The offset and the gain are stored in the Auxiliary data ADS of
the product. For each measurement, the offset is computed as the minimum
value of the spectrum to store (f_{min}) and the gain is computed as
65535/(f_{max}f_{min}) where f_{min} (resp. f_{max})
is the minimum (resp. the maximum) value of the spectrum for the current
measurement. To decode the values read in the product, one must compute: Background
(in electrons) = Offset + Background Code / Gain
Note that the background signal
is divided by the average PRNU map before being scaled.
The following table presents
the accuracy (in number of electrons) for several ranges of background
spectra:
Range (in e)

Accuracy (LSB)

100

< 0.1 e

1000

< 0.1 e

10000

0.2 e

30000

0.5 e

100000

1.5 e

300000

4.6 e

500000

7.6 e

1000000

15.0 e

The decoded spectra values
obtained in electrons must then be converted into physical units (ph/s/cm2/nm/sr)
by multiplying them by the conversion factor inferred from the radiometric
sensitivity curve (star) provided as a LUT.

Scaled upper & lower background spectra after
straylight and IR vignetting corrections

Estimation of the background
signal measured by the upper and lower bands after applying the straylight
and IRvignetting corrections. The estimated background spectra values are
coded on 16 bits. Due to the high variations of these spectra with the
altitude, the coding is dynamic and uses an offset and a gain for each
measurement. The offset and the gain are stored in the Auxiliary data ADS of
the product. For each measurement, the offset is computed as the minimum
value of the spectrum to store (f_{min}) and the gain is computed as
65535/(f_{max}f_{min}) where f_{min} (resp. f_{max})
is the minimum (resp. the maximum) value of the spectrum for the current
measurement. To decode the values read in the product, one must compute:
Background (in electrons) = Offset + Background Code / Gain
Note that the background signal
is divided by the average PRNU map before being scaled.
The following table presents
the accuracy (in number of electrons) for several ranges of background
spectra:
Range (in
e)

Accuracy (LSB)

100

< 0.1 e

1000

< 0.1 e

10000

0.2 e

30000

0.5 e

100000

1.5 e

300000

4.6 e

500000

7.6 e

1000000

15.0 e

The decoded spectra
values obtained in electrons must then be converted into physical units
(ph/s/cm2/nm/sr) by multiplying them by the conversion factor inferred from
the radiometric sensitivity curve (star) provided as a LUT.

Error bar for the upper & lower background spectra
after straylight and IR vignetting corrections

The error bars for the upper and lower background spectra
after straylight and vignetting corrections are computed as their square root
and are expressed as a relative percentage. The maximum error bar for the
upper and lower background spectra is set to 255%.
for each measurement f = 1, f_{size}
for each CCD column: k = 1, k_{size}
if ( #0 ) then
else
endif
/* truncate the error to 255 percent (coding limit) */
if ( > 255. ) =
255.
end for k
end for f
Where f_{size}
is the number of spectrometer measurements and k_{size} is the total
number of transmitted pixels (global vectors)

PCD at sample level

The meaning of the possible PCD values stored in this MDS
is detailed in section 5.1.3.2.2.

Offset and gain for the background spectra coding

For each measurement, the
offset is computed as the minimum value of the background spectrum to store
(f_{min}) and the gain is computed as 65535/(f_{max}f_{min})
where f_{min} (resp. f_{max}) is the minimum (resp. the
maximum) value of the background spectrum for the current measurement.
This offset and this gain must
be used to decode the value of the background spectra written in the product
for both before and after straylight and vignetting corrections. To decode
the value read in the product, one must compute:
Background (in electrons) =
Offset + Background Code / Gain
There is one couple of gain and
offset values per spectrometer measurement.

PCD at measurement level

The meaning of the possible PCD values stored in this MDS
is detailed in section 5.1.3.2.4.

5.1.3.4 Level 2 product
Definitions related to GADS

Summary Quality

The meaning of the possible flag values stored in the
Summary Quality GADS is detailed in section 3.3.2.

Level 1b PCD check

This PCD is initialised when the Level 1b product is read,
following the logic:
First of all, read the summary quality GADS in the Level
1b product:
C1 : no valid data
test if PCD_{fvalid}=1 (field 1 of the GADS)
C2 : second part of a tangent occultation
test if PCD_{lv0}=2 (field 12 of the GADS)
C3 : geolocation is not valid
test if PCD_{gl}=1000 (field 20 of the GADS)
C4 : reference star spectra missing, i.e.
transmission terms not valid for the whole occultation
test if PCD_{fatal}>0 (field 8 of the GADS)
Initialise PCD_{lv1}=0
If C1 is true set PCD_{lv1} to 1
If C2 is true set PCD_{lv1} to 2
If C3 is true set PCD_{lv1} to 3
If C4 is true set PCD_{lv1} to 4

Chromatic refraction mode for the measured transmission

Switch read from the Level 2
processing configuration auxiliary product and copied in the Level 2 product.
The possible values are:
0 : no correction
1 : correction is performed

Chromatic refraction mode for the transmission model
(second spectral inversion)

Switch read from the Level 2
processing configuration auxiliary product and copied in the Level 2 product.
Two numbers are read in the auxiliary product: first value is for the second
spectral inversion and second value is for the third and subsequent spectral
inversions.
0: no correction
1: correction is performed

Chromatic refraction mode for the transmission model
(third and further spectral inversions)

Switch read from the Level 2
processing configuration auxiliary product and copied in the Level 2 product.
Two numbers are read in the auxiliary product: first value is for the second
spectral inversion and second value is for the third and subsequent spectral
inversions.
0: no correction
1: correction is performed

Instrument function mode for the transmission model
(second spectral inversion)

Switch read from the Level 2
processing configuration auxiliary product and copied in the Level 2 product.
Two numbers are read in the auxiliary product: first value is for the second
spectral inversion and second value is for the third and subsequent spectral
inversions.
0: no correction
1: correction is performed

First altitude where the ratio U/C is greater than 25%

This value is computed from the
PCD at measurement level provided in the Level 1b product (see Section 2.1.1.1.3).
It is specified in km,
truncated to the nearest integer value. If the ratio is lower than 25% at all
altitudes, this altitude is set to 0.

Vertical inversion mode

Switch read from the Level 2
processing configuration auxiliary product and copied in the Level 2 product.
The possible values are:
2: linear (only one mode is available in the processor)

Smoothing mode (after the spectral inversion)

Switch read from the Level 2
processing configuration auxiliary product and copied in the Level 2 product.
Smooth or not the density column used by the vertical inversion. If the
smooth mode is not activated only the flagged value of the density column are
filled.
0: no smoothing
1: Gaussian filter
2: Tikhonov's regularisation

Time mode for the transmission model (second spectral
inversion)

Switch read from the Level 2
processing configuration auxiliary product and copied in the Level 2 product.
Two numbers are read in the database value: first value is for the second
spectral inversion and second value is for the third and subsequent spectral
inversions.
0: zero order term
1: second order term

Time mode for the transmission model (third and further
spectral inversions)

Switch read from the Level 2
processing configuration auxiliary product and copied in the Level 2 product.
Two numbers are read in the database value: first value is for the second
spectral inversion and second value is for the third and subsequent spectral
inversions.
0: zero order term
1: second order term

Number of iterations for the main loop (also called q
loop)

Number of main loop iterations actually performed.

Number of iterations for the inversion loop (also
called p loop)

Number of inversion loop iterations actually performed.

Number of points in profile column densities where chi2
> chi2_warn

chi2_warn is a parameter read
in the Level 2 processing configuration auxiliary product (LUT versus
altitude).

Number of flagged points in profile for the column
densities

Number of points where the corresponding PCD (tangent line
density PCD) is not equal to 0.

Number of flagged points in profile for the local
densities

Number of points where the corresponding PCD (local
density PCD) is not equal to 0.

Definitions related to DS

Local density product

This is the result of the
vertical inversion of line densities, assuming local spherical symmetry. The
local density of each species is expressed in cm3. One value is provided for
each measurement. The datation and the geolocation of each point of the local
density profile is also provided in the product.

Standard deviation of the local density product

Errors are estimated throughout
the processing chain and are propagated along the chain to the final data
products. It is assumed that the error statistics follow a normal Gaussian
distribution. Values given correspond to 1. The error bar associated
with the local density of the different species is expressed in % of the
local density. The maximum value of the error bar is set to 6553.5%.

PCD summary (for the local density profiles)

The PCD summary contains flags
dedicated to the validity of the outputs of the vertical inversion, i.e. the
retrieval of the local density profiles. After vertical inversion, local
densities are checked for flag setting. There is one flag per acquisition and
per species: parameter PCDV(i,j), where index j denotes the acquisition
number, and index i denotes the species among the following species list in
this order: O_{3}, NO_{2}, NO_{3}, air, O_{2},
H_{2}O, OClO.

Tangent line density

Tangent line density of each
species (expressed in cm^{2}). One value is provided for each
measurement. The tangent line density profiles are computed during the
spectral inversion process.

Standard deviation for the tangent line density

Errors are estimated throughout
the processing chain and are propagated along the chain to the final data
products. It is assumed that the error statistics follow a normal Gaussian
distribution. Values given correspond to 1. With the current
operational processor (IPF 5.00), an empirical error estimate has been added
after inversion to the error bar of O_{3} line densities to account
for the effects of turbulence (incomplete scintillation correction). The
error bar associated with the tangent line density of the different species
is expressed in % of the tangent line density. The maximum value of the error
bar is set to 6553.5%.

PCD summary (for the tangent line density profiles)

The PCD summary contains flags
dedicated to the validity of the outputs of the spectral inversion, i.e. the
retrieval of the tangent line density profiles. After spectral inversion,
tangent line densities are checked for flag setting. There is one flag per
acquisition and per species: parameter PCDV(i,j), where index j denotes the
acquisition number, and index i denotes the species among the following
species list in this order: O_{3}, NO_{2}, NO_{3},
air, O_{2}, H_{2}O, OClO.

Aerosol product

The wavelength dependence of aerosol extinction may vary,
even over one occultation. A polynomial expression is used to describe this
wavelength dependence of the aerosol extinction; for a polynomial in degree
n:
For each measured transmission, it is calculated:
o
the extinction coefficient at the reference wavelength ,
corresponding to d_{0}, given in km^{1}
o
the other spectral parameters corresponding to the coefficients
d_{1}(z) to d_{n}(z) in the polynomial expression, given in
nm^{1}.km^{1}, ..., nm^{n}.km^{1}
The reference wavelength value
is stored in the SPH of the product ("Reference wavelength used for the
ray tracing"); it is equal to 500 nm. It is also given in the SPH of the
residual extinction product and in the Occultation data GADS of the Level 1b
product.
The extinction coefficient at is
given in the dedicated MDSR "Extinction coefficient at "
and as the first spectral parameter of the MDS "Spectral parameters of
the extinction coefficients". In the case of a polynomial in degree 2 as
in the current operational processor (IPF 5.00), two other coefficients are
given in the MDS "Spectral parameters of the extinction
coefficients": d_{1} (nm^{1}.km^{1}) and d_{2
}(nm^{2}.km^{1}).
The wavelength dependence of the aerosol optical thickness
may be described as:
The MDS "Tangent
integration extinction profile at
λ=λ_{ref}
"
stores the value τ
calculated
at
λ=λ_{ref}
(dimensionless). The MDS
"Spectral parameters of tangent integrated extinction profile"
stores the coefficients r_{0} to r_{n}, retrieved by the
spectral inversion processing; r_{0} is the integrated retrieved
density column of aerosols given in cm^{2}; the other parameters r_{1}
to r_{n} are given in cm^{2}.nm^{1}, ... cm^{2}.nm^{n}.
In the case of a polynomial in degree 2 as in the current operational
processor (IPF 5.00), three coefficients are stored: r_{0}
(dimensionless), r_{1} (cm^{2}.nm^{1}) and r_{2}
(cm^{2}.nm^{2}).

Extinction coefficient

The extinction coefficient is
given at the reference wavelength, in km^{1}. The reference
wavelength is given in the SPH of the product and in the Occultation data
GADS of the Level 1b product; it is equal to 500 nm.

Standard deviation of the extinction coefficient

The values of the standard
deviation are expressed in % and correspond to 1. An empirical
error estimate has been added after inversion to account for the effects of
turbulence (incomplete scintillation correction).

Spectral parameters of the extinction coefficients

Coefficients of the polynomial
expression used to describe the wavelength dependence of the aerosol
extinction. The first spectral parameter is the extinction coefficient at the
reference wavelength, given in km^{1}; for a polynomial in second
degree, two other parameters are given, in nm^{1}.km^{1}
and in nm^{2}.km^{1} respectively.

Standard deviation of the spectral parameters of the
extinction coefficients

The values of the standard
deviation are expressed in % and correspond to 1.

Tangent integrated extinction profile

The tangent integrated
extinction (aerosol optical thickness, dimensionless) profile is given at the
reference wavelength. The reference wavelength is given in the SPH of the
product and in the Occultation data GADS of the Level 1b product; it is equal
to 500 nm.

Standard deviation of the tangent integrated extinction
profile

The values of the standard
deviation are expressed in % and correspond to 1.

Spectral parameters of the tangent integrated
extinction profile

Coefficients of the polynomial
expression used to describe the wavelength dependence of the aerosol optical
thickness. The first spectral parameter is the integrated retrieved density
column of aerosols given in cm^{2}; for a polynomial in second
degree, two other parameters are given, in cm^{2}.nm^{1}
and cm^{2}.nm^{2} respectively.

Standard deviation of the spectral parameters of the
tangent integrated extinction profile

The values of the standard
deviation of the polynomial coefficients of the tangent integrated extinction
profile are expressed in % and correspond to 1.

PCD summary

The PCD summary provides the spectral and vertical PCD
(first and seventh values) of the extinction coefficient at the reference
wavelength; the other values are set to 0.

Turbulence product

From the analysis of fast
photometers, the tangent altitude of LOS, and the temperature and density
vertical profiles at High Resolution (40 Hz) are retrieved. They are
basically derived from the time delay between the peaks of red and blue Fast
Photometers, when the time delay between the two photometer signal is
significant enough to be analysed. There is no high resolution profiles when
the occultation is made in bright limb.

Tangent altitude including fluctuations

Altitude of the high resolution
temperature profile.

High resolution temperature vertical profile

High resolution T vertical
profile obtained by analysis of the photometer signals. The output frequency
of the HRTP is 40Hz. This corresponds to 20 values for each processed
measurement during the Level 2 processing (frequency 2Hz). There is no high
resolution temperature profile when the occultation is made in bright limb or
if the time delay between the two photometer signals is not significant
enough to be analysed.

Error bar of the high resolution temperature profile

The values of the error bar are
expressed in % and correspond to 1. The maximum error bar is
set to 6500%.

High resolution vertical profile of the local density

High resolution vertical
profile of air local density obtained by analysis of the photometer signals.
The output frequency of the HRTP is 40Hz. This corresponds to 20 values for
each processed measurement during the Level 2 processing (frequency 2Hz). There
is no high resolution air density profile when the occultation is made in
bright limb or if the time delay between the two photometer signals is not
significant enough to be analysed.

Error bar of the high resolution density profile

The values of the error bar are
expressed in % and correspond to 1. The maximum error bar is
set to 6500%.

Geolocation and atmospheric profile

Geolocation data are the same
as for Level 1b and are also provided for the reference wavelength (equal to
500 nm).

Latitude, longitude and altitude of the spacecraft

Spacecraft location read from
the Level 1b product. These quantities are provided during the measurement,
generally at halfmeasurement (the actual temporal shift is given by the time
shift for the ray tracing).The altitude is expressed in meters.

Latitude, longitude and altitude of the tangent point
and associated errors

Tangent point location and
associated errors read from the Level 1b product. These quantities are
provided during the measurement, generally at halfmeasurement (the actual
temporal shift is given by the time shift for the ray tracing).
The altitude is expressed in meters.

Tangent point atmospheric pressure (from external
model)

Pressure profile obtained by
the combination of the ECMWF data in the lower part of the profile and of the
MSIS90 data in the upper part of the profile (smooth transition altitude
range around the pressure level 1hPa). The values of the profile are read in
the Level 1b product. The values are given in Pa. They are provided during
the measurement, generally at halfmeasurement (the actual temporal shift is
given by the time shift for the ray tracing).
Note: if the ECMWF file is
not available for the Level 1b processing, only the MSIS90 model is used. In
this case, a specific flag is raised during the Level 1b processing and
copied in the Level 2 product.

Tangent point temperature (from external model)

Temperature profile obtained by
the combination of the ECMWF data in the lower part of the profile and of the
MSIS90 data in the upper part of the profile (smooth transition altitude
range around the pressure level 1hPa). The values are given in K. They are
provided during the measurement, generally at halfmeasurement (the actual
temporal shift is given by the time shift for the ray tracing).
Note: if the ECMWF file is
not available for the Level 1b processing, only the MSIS90 model is used. In
this case, a specific flag is raised during the Level 1b processing and
copied in the Level 2 product.

Tangent point density (from external model)

Air density obtained by the
combination of the ECMWF data in the lower part of the profile and of the
MSIS90 data in the upper part of the profile (smooth transition altitude
range around the pressure level 1hPa). The values are given in cm^{3}.
They are provided during the measurement, generally at halfmeasurement (the
actual temporal shift is given by the time shift for the ray tracing).
Note: if the ECMWF file is
not available for the Level 1b processing, only the MSIS90 model is used. In
this case, a specific flag is raised during the Level 1b processing and
copied in the Level 2 product.

Local air density from GOMOS atmospheric profile

Local air density profile
computed from the measurements. In the current operational IPF version (IPF
5.00), the vertical inversion on air is not activated, and the local density
values, the standard deviation and the vertical resolution for this species
are set to 0 in the products (as well as the terms related to air in the
covariance matrix for local densities after the vertical inversion).

Standard deviation for the local air density

The values of the error bar are
expressed in % and correspond to 1. The maximum error bar is
set to 6553.5%. In the current operational IPF version (IPF5.00), all values
are set to the maximum error bar.

Local temperature

Local temperature profile
computed from the measurements. In the current operational IPF version (IPF
5.00), the profile values are all set to 0.

Standard deviation for the local temperature

The values of the error bar are
expressed in % and correspond to 1. The maximum error bar is
set to 6553.5%. In the current operational IPF version (IPF5.00), all values
are set to the maximum error bar.

Accuracy estimate product

For the spectral inversion, the
c^{2} (final best fit) is
given. The 12 x 12 covariance matrix (symmetrical) is given, corresponding to
the 6 gas species, 5 aerosol parameters and one spare (influence of one gas on
the others).
For the vertical inversion the
covariance matrix is also given (influence of one altitude on the other).
A scale factor is applied on
the covariance matrix terms before recording.

Chi2 final value

c^{2}
final value from the spectrometer A spectral inversion.

Scale factor for the elements of the covariance matrix
(spectral/vertical inversion)

The scale factor is the power of 10 to be applied to the
associated covariance matrix elements in order to store/read them in the
product. More details are given in the text below.

Covariance matrix for line densities after spectral
inversion

The covariance matrix for the
spectral inversion provides information on the influence of each gas on the
others. It is a 12 x 12 matrix whose terms are in the following order: O_{3},
NO_{2}, NO_{3}, air, OClO, aerosols (a maximum of 6
parameters for aerosol spectral dependency) and a spare gas. The matrix is
symmetrical: only half of the matrix (78 values) is written in the product.
More details on the storage of these values are given in the text below.

Covariance matrix for local densities after vertical
inversion

The covariance matrix for the
vertical inversion provides information on the influence of each altitude on
the other. It is symmetrical. 12 species are given in the following order: O_{3},
NO_{2}, NO_{3}, air, O_{2}, H_{2}O, OClO,
aerosols and 4 spare gases. Only the diagonal terms and 6 offdiagonal terms
are given in the product, as a 12 x 7 matrix for each acquisition. More
details on the storage of these values are given in the text below.

The scale factor for the elements
of the covariance matrix DS gives the power of 10 to be applied to the
covariance matrix elements read in the product in order to interpret them (same
value for all species):
V
= V_{p} x 10^{fact}
where:
V: the
covariance matrix element calculated in the Level2 processing
V_{p}:
the scaled covariance matrix element read in the product
fact: the
scale factor read in the product.
The covariance matrix for
spectral inversion is a 12 x 12 matrix whose terms [A_{x,y}] are in the
following order: O_{3}, NO_{2}, NO_{3}, air, OClO,
aerosols (a maximum of 6 parameters), and a spare gas. This matrix is
symmetrical, only the relevant part (78 terms) is written in the product; that
is:
For x=1,12
write A(x,y)=C_{SIi,i',j} y ≥ x,
with the species i and i' being those
provided by the order x and y defined above
End For

The covariance matrix for
vertical inversion is, by construction, symmetrical. Only the diagonal + 6 off
diagonal terms are written in the product. 12 species are considered in the
following order: O_{3}, NO_{2}, NO_{3}, air, O_{2},
H_{2}O, OClO, aerosols and 4 spare gases. Thus, a matrix 12 x 7 (noted B
hereafter) is written in the product for each acquisition.
The procedure for the creation of this part of the Level 2
product is the following:
 One row is used for each species
 For each species (row), the diagonal element of the
original covariance matrix is written in the last column.
 elements taken from the left of the diagonal in the
original matrix are written in the first six columns
 An exception is made for the first 6 acquisitions, where
less than 6 elements are available on the left.
For these first 6 acquisitions, the elements are taken
instead from the right of the diagonal in the original matrix and are inserted
into the matrix B in the first six columns, but in reverse order (so that the
elements closest to the diagonal in the original are also closest in the
matrix B).
This procedure is more explicitly
described below:
For the current acquisition j
if (j>6) then
for i = 1,12
B (i,7+p) = C_{Vli,j,j+p} with p = 6 to 0
end for
else
for i = 1,12
B (i,7p) = C_{Vli,j,j+p} with p = 6 to 0
end for
end if
Note 1 : for all spare gases (or non retrieved
species), B(i,7p)=0 with p=6 to 0
Note 2 : the scaling is applied to the matrix term
prior to this recording

Explicit illustration is given on
the following pictures:
5.1.3.5 Residual extinction
Definitions related to the SPH

Number of processed measurements during the Level 2
processing

This is the actual number of
measurements included in the product. This number is always lower than the
total number of measurements of the occultation (read in the Level 1b
product). It may be smaller if problem occurred (e.g. invalid data at the
beginning of the occultation).

Reference wavelength used for the ray tracing

All the geolocation information
is provided for this reference wavelength. For example the localisation of
the ray nodes and of the tangent point node are provided for this wavelength
although at one given time inside the atmosphere, and due to chromatic
refraction effects, each wavelength is looking at a different altitude. It is
also the reference wavelength for the aerosol products. It is equal to 500
nm.

Time shift for the ray tracing / geolocation

This time indicates the time
shift between what is called 'beginning of measurement' and 'half
measurement' in the ray tracing processing. Geolocation results are provided
at these two times for each measurement (actual value read from the Level 1b
product)

Mean wavelength of the photometer used for the
scintillation correction

Value computed from the
photometer spectral range and index of the photometer used for the
scintillation correction.

Note: most of the SPH fields are identical to those of the
Level 1b product and are not detailed here.
Definitions related to GADS

Summary quality

The meaning of the possible flag values stored in the
Summary Quality GADS is detailed in section 3.3.2.

Level 1b PCD check

This PCD is initialised when the Level 1b product is read,
following the logic:
First of all, read the summary quality GADS in the Level
1b product:
C1 : no valid data
test if PCD_{fvalid}=1 (field 1 of the GADS)
C2 : second part of a tangent occultation
test if PCD_{lv0}=2 (field 12 of the GADS)
C3 : geolocation is not valid
test if PCD_{gl}=1000 (field 20 of the GADS)
C4 : reference star spectra missing, i.e.
transmission terms not valid for the whole occultation
test if PCD_{fatal}>0 (field 8 of the GADS)
Initialise PCD_{lv1}=0
If C1 is true set PCD_{lv1} to 1
If C2 is true set PCD_{lv1} to 2
If C3 is true set PCD_{lv1} to 3
If C4 is true set PCD_{lv1} to 4

Chromatic refraction mode for the measured transmission

Switch read from the Level 2
processing configuration auxiliary product and copied in the Level 2 product.
The possible values are:
0 : no correction
1 : correction is performed

Chromatic refraction mode for the transmission model
(second spectral inversion)

Switch read from the Level 2
processing configuration auxiliary product and copied in the Level 2 product.
Two numbers are read in the auxiliary product: first value is for the second
spectral inversion and second value is for the third and subsequent spectral
inversions.
0: no correction
1: correction is performed

Chromatic refraction mode for the transmission model
(third and further spectral inversions)

Switch read from the Level 2
processing configuration auxiliary product and copied in the Level 2 product.
Two numbers are read in the auxiliary product: first value is for the second
spectral inversion and second value is for the third and subsequent spectral
inversions.
0: no correction
1: correction is performed

Instrument function mode for the transmission model
(second spectral inversion)

Switch read from the Level 2
processing configuration auxiliary product and copied in the Level 2 product.
Two numbers are read in the auxiliary product: first value is for the second
spectral inversion and second value is for the third and subsequent spectral
inversions.
0: no correction
1: correction is performed

First altitude where the ratio U/C is greater than 25%

This value is computed from the
PCD at measurement level provided in the Level1b product (see Section 2.2.3.2).
It is specified in km,
truncated to the nearest integer value. If the ratio is lower than 25% at all
altitudes, this altitude is set to 0.

Vertical inversion mode

Switch read from the Level 2
processing configuration auxiliary product and copied in the Level 2 product.
The possible value is:
2: linear (only one mode is available in the processor)

Smoothing mode (after the spectral inversion)

Switch read from the Level 2
processing configuration auxiliary product and copied in the Level 2 product.
Smooth or not the density column used by the vertical inversion. If the
smooth mode is not activated only the flagged value of the density column are
filled.
0: no smoothing
1: Gaussian filter
2: Tikhonov's regularisation

Time mode for the transmission model (second spectral
inversion)

Switch read from the Level 2
processing configuration auxiliary product and copied in the Level 2 product.
Two numbers are read in the database value: first value is for the second
spectral inversion and second value is for the third and subsequent spectral
inversions.
0: zero order term
1: second order term

Time mode for the transmission model (third and further
spectral inversions)

Switch read from the Level 2
processing configuration auxiliary product and copied in the Level 2 product.
Two numbers are read in the database value: first value is for the second
spectral inversion and second value is for the third and subsequent spectral
inversions.
0: zero order term
1: second order term

Number of iterations for the main loop (also called q
loop)

Number of main loop iterations actually performed.

Number of iterations for the inversion loop (also
called p loop)

Number of inversion loop iterations actually performed.

Number of points in profile column densities where chi2
> chi2_warn

chi2_warn is a parameter read
in the Level 2 processing configuration auxiliary product (LUT versus
altitude).

Number of flagged points in profile for the column
densities

Number of points where the corresponding PCD (tangent line
density PCD) is not equal to 0.

Number of flagged points in profile for the local
densities

Number of points where the corresponding PCD (local
density PCD) is not equal to 0.

Nominal wavelength assignment

Nominal wavelength assignment
read from the Level 1b product and reordered in increasing wavelength value.
Note that the effective spectral assignment of the transmission spectra
written in the product is the combination of the nominal wavelength
assignment and of the spectral grid correction.

Definitions related to DS

Transmission corrected for scintillation and dilution
effects

Transmission read in the Level
1b product and corrected from the scintillation and dilution effects.

Transmission model function

The transmission model function
is a forward model of the transmission with the "best fit" values
of the parameters

Latitude, longitude and altitude of the spacecraft

Spacecraft location read from
the Level 1b product. These quantities are provided during the measurement,
generally at halfmeasurement (the actual temporal shift is given by the time
shift for the ray tracing). The altitude is expressed in meters.

Latitude, longitude and altitude of the tangent point
and associated errors

Tangent point location and
associated errors read from the Level 1b product. These quantities are
provided during the measurement, generally at halfmeasurement (the actual
temporal shift is given by the time shift for the ray tracing).
The altitude is expressed in meters.

Tangent point atmospheric pressure (from external
model)

Pressure profile obtained by
the combination of the ECMWF data in the lower part of the profile and of the
MSIS90 data in the upper part of the profile (smooth transition altitude
range around the pressure level 1hPa). The values of the profile are read in
the Level 1b product. The values are given in Pa. They are provided during
the measurement, generally at halfmeasurement (the actual temporal shift is
given by the time shift for the ray tracing).
Note: if the ECMWF file is
not available for the Level 1b processing, only the MSIS90 model is used. In
this case, a specific flag is raised during the Level 1b processing and
copied in the Level 2 product.

Tangent point temperature (from external model)

Temperature profile obtained by
the combination of the ECMWF data in the lower part of the profile and of the
MSIS90 data in the upper part of the profile (smooth transition altitude
range around the pressure level 1hPa). The values are given in K. They are
provided during the measurement, generally at halfmeasurement (the actual
temporal shift is given by the time shift for the ray tracing).
Note: if the ECMWF file is
not available for the Level 1b processing, only the MSIS90 model is used. In
this case, a specific flag is raised during the Level 1b processing and
copied in the Level 2 product.

Local air density from GOMOS atmospheric profile

Local air density profile
computed from the measurements. In the current operational IPF version
(IPF5.00), the vertical inversion on air is not activated, and the local
density values are set to 0 in the products.

Spectral grid correction for the transmission model
function

This spectral grid is the same
as the one stored in the Level 1b product except that it is reordered in
increasing wavelength (in the spectral overlap between the spectrometers A1
and A2). This quantity is the difference between the reference spectral grid
and the actual spectral grid of the transmissions written in this product.

5.1.3.7 Near Real Time meteo product
All fields included in the NRT meteorological products are also included in the GOMOS level 2 products. Please refer to the previous chapter for the explanation of these fields.
