Kein Folientitel

1
Monitoring of the GOME/ERS-2 Inflight Calibration
Parameters from GDP-4 Reprocessing M.
Coldewey-Egbers, S. Slijkhuis, B. Aberle, D.
Loyola
Wavelength Calibration In the framework of the
ESA-project Long-Term Monitoring of GOME
Calibration Parameters several spectral emission
lines of the PtCrNe hollow cathode lamp were
identified to be improper for an exact wavelength
calibration, and therefore have been removed from
the analysis. The lines did not meet the
well-defined statistical criteria for all
available lamp measurements. Figure 5 shows the
standard deviation of the wavelengths of all
emission lines for all available calibration
orbits between June 1995 and May 2003 for the old
and the new calibration analysis. Largest changes
can be found at the beginning of channel 3, where
three lines were excluded, and at the end of
channel 4 around 760 nm, where the very unstable
last line has been removed. The noise of the new
wavelengths is much smaller compared to the old
calibration, except in channel 2, where only one
line has been excluded.
Introduction In 2006 an update of the GOME
Level-0-1 processor (GDP-4) has been developed in
order to reprocess the entire data set. The main
driver for this updated version was the new sun
mean reference spectrum intensity check, and the
associated closing of the time gaps between sun
mean reference spectrum updates on the Level 1b
product. This opportunity has been used to
include other algorithm developments such as an
extension of the GOME on-fly calibration
parameter database, and a slightly modified
spectral calibration. For the first time a fully
homogeneous dataset is available that is used to
monitor the instrument performance and stability
over its lifetime from 1995 to 2006. Sun Mean
Reference Spectra, PMD Signals and
Q-factors Instrument degradation as well as the
ERS-2 pointing problem since 2002 lead to a
strong decrease in the measured intensity of GOME
spectral channels 1 and 2. Figure 1 shows the
ratio of the sun mean reference spectra from 1997
to 2006 to the corresponding reference spectrum
from 9th January 1996. The intensity in channel 1
is reduced by more than 90. In channel 2 the
decrease is still 40-50, and in channel 3 it is
0-40.
Figure 5 Standard deviation of the wavelengths
of all emission lines for the old (open circles)
and the new (red dots) calibration. Filled black
dots denote the lines that were removed from the
analysis.
Figure 1 Ratio of the sun mean reference
spectra from 9th January 1997 to 2006 to the
corresponding reference spectrum of 1996. Grey
shaded areas mark features caused by the dichroic
filter, which separates channels 3 and 4.
One of the key elements in the optical system of
GOME is the quartz predisperser prism. The
refractive index of quartz depends not only on
the wavelength of the light passing through it
but also on the temperature of the prism. It is
expected, that the temperature increases along an
orbit, partly due to warming by the sun and
partly because light passes through the
instrument. Those temperature changes may affect
the lamp measurements and therefore the
wavelength calibration. Figure 6 shows a
correlation between one single wavelength (759.96
nm) and the temperature. However, this
correlation is not existing in channels 1 and 2.
It is strongest in channels 3 and at the end of
channel 4.
Intensity decrease 90 at 240 nm and
50 at 325 nm
Figure 2 shows all sun mean reference spectra of
GOME from July 1995 to June 2006 for four single
wavelengths (290nm - channel 1, 330nm - channel
2, 430nm channel 3, and 760nm - channel 4).
Black curves denote uncorrected data. The low
periodic variation is due to the seasonality of
the sun-earth distance, which is maximum in July
and minimum in January. Large peaks in the time
series for all wavelengths at the beginning of
2001 are due to severe problems with the ERS-2
spacecraft. They can be directly assigned to data
gaps and GOME anomalies, such as instrument
switchoffs, as regularly documented in the GOME
yearly anomaly reports (see http//earth.esa.int/e
rs/gome/performance/). Besides the large peaks,
several small peaks can be identified in the
curves, which occur for different wavelengths at
different dates. They can be explained with
etalon structures. The red curves denote the sun
mean reference data which are first corrected for
the etalon effect Secondly, all spectra are
normalised to 1 A.U. (Astronomical Unit) in order
to remove the seasonal dependence. Finally, they
are normalised to the intensity of the reference
spectrum from 3rd July 1995 to calculate the
percentage decrease. The intensity decreased by
80 at 290nm and by 60 at 330nm until June 2006.
The drecease in channel 3 (430 nm) started in
2001 and reaches now 40. In channel 4 at 760nm
only minor changes are observed. A slight
decrease of 10 from 1995 to 2001, and then a
short increase of 5 until 2006. The
corresponding time series for the three PMD
signals are depicted in Fig. 3. The degradation
of the PMD signals show almost the same behaviour
as for the corresponding wavelengths.
Figure 6 One single wavelength (759.96 nm, black
curve) and temperature at the predisperser prism
(red curve) as a function of time.
Wavelength calibration Wavelengths are more
stable now using GDP-4. In channels 3 and 4,
wavelengths correlate with the temperature
measured at the predisperser prism.
Influence of the South Atlantic Anomaly on the
Leakage Current The four GOME detectors are
random access linear photodiode arrays. One
characteristic of these devices is a certain
amount of leakage current produced by thermal
leakage. The leakage current is monitored by
periodically taken dark-side measurements. The
South Atlantic Anomaly (SAA) is a region with
intense radiation in space near the Earth that
causes damage to many spacecrafts in low Earth
orbit. The GOME measurements are affected by
high-energy protons leading to large data spikes.
For this study, all GOME orbits crossing the SAA
region during night time have been separated.
Figure 7 shows the leakage current in channel 4
for an integration time of 30 seconds for 10
consecutive orbits in 1997. The third and fourth
orbit from top crossed the SAA. Data are much
noisier and contain large spikes. Figure 8 shows
the noise of the leakage current measurements
for 30s integration time and the year 1997. The
noise level inside the SAA increases by a factor
of two compared to the noise outside the SAA
region. The leakage current itself is slightly
larger inside the SAA than outside the SAA
(without figure), that is due to the expected
spikes on individual detector pixels. Calculation
of the dark signal using these measurements from
inside the SAA may yield to a slight
overestimation of the leakage, and therefore to
an underestimation of the real signal. The same
analysis for the year 2000 and the other time
patterns confirms these results. The influence of
the SAA on the darkcurrent and its noise level is
largest for the long integration times (e.g. 30
and 60 s). It becomes smaller for the shorter
ones of 1.5 s.
Figure 2 Sun mean reference intensity for four
different wavelengths (from top to bottom 290
nm, 325 nm, 502 nm, and 639 nm) from June 1995 to
June 2006. Red curves are corrected for etalon
structures and for 1A.U.
Figure 3 PMD Signals from June 1995 to June
2006.
Sun mean reference intensity and PMD
signals Large outliers and anomalies in 2001 can
be explained with GOME switch-offs. Low periodic
variation is due to the seasonality of the
sun-earth distance.

The so-called Q-factors are defined as relative
correction factors that transform the measured
signal with fractional polarisation to an
unpolarised signal (see GOME, 2000). Figure 4
shows the time series of all three Q-factors from
June 1995 to June 2006. The strong decrease of
Q-factor 1 is connected to the different
degradation of the PMD 1 signal and the measured
signal in channel 2. The PMD decreases faster
compared to the channel up to the year 1999 and
then from 2001 to 2006 the channel signal
decreases faster. Q-factor 2 increases slowly
from 1995 to 2006, that means the PMD signal is
larger than the corresponding channel, while the
channel decreases faster, respectively (see also
Figs. 2 and 3). Q-factor 3 is more or less stable
(0.15 to 0.2) over the entire period.
Measurements carried out during the calibration
of the GOME FM have shown that all three PMDs are
sensitive to light above 790 nm. Early in-flight
solar data showed that straylight appears to be
worst in PMD 3 (13), that explains the initial
non-zero Q-factor 3. The irregular large peaks
and outliers are due to GOME anomalies such as
cooler switch-offs, instrument or satellite
switch-offs, on-board anomalies, or special
operations.
Blickrichtung
Figure 7 Leakage current in channel 4 for 30s
integration time and 10 consecutive orbits from
1997. 3rd and 4th orbit from top cross the SAA
region.
Figure 8 Leakage current noise inside (red) and
outside (black) the SAA region for the year 1997.
Figure 4 GOME Q-Factors for each PMD from June
1995 to June 2006.
Leakage Current and South Atlantic
Anomaly Leakage current measurements are noisier
and contain large data spikes.
Q-factors Outliers and peaks due to cooler
switch-offs, instrument and satellite
switch-offs, and special operations. Decrease and
increase due to different degradation of PMD and
corresponding channel signal.
References 1 GOME Level 0 to 1 Algorithms
Descriptions, Techn. Rep., DLR,
ER-TN-DLR-GO-0022, 2000.