Cluster Angewandte FernerkundungDLR Oberpfaffenhofen

www.caf.dlr.de

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/ers/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.

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.

Monitoring of the GOME/ERS-2 Inflight Calibration Parameters from GDP-4

Reprocessing M. Coldewey-Egbers, S. Slijkhuis, B. Aberle, D. Loyola

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. References

[1] GOME: Level 0 to 1 Algorithms Descriptions, Techn. Rep., DLR, ER-TN-DLR-GO-0022, 2000.

Figure 4: GOME Q-Factors for each PMD from June 1995 to June 2006.

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.

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.

Intensity decrease: 90% at 240 nm and 50% at 325 nm

Blickrichtung

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 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.

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.

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.

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.

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 3: PMD Signals from June 1995 to June 2006.

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.

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.

Figure 6:One single wavelength (759.96 nm, black curve) and temperature at the predisperser prism (red curve) as a function of time.

Leakage Current and South Atlantic Anomaly:Leakage current measurements are noisier and contain large data spikes.

Figure 8:Leakage current noise inside (red) and outside (black) the SAA region for the year 1997.