Technical Bulletins

GOME-2 newsletter #38 September 2018

The GOME-2 newsletter provides information about the latest developments concerning the GOME-2 instruments and level 1 product status, as part of the EUMETSAT Polar System (EPS).

The Global Ozone Monitoring Experiment–2 (GOME-2) is an optical spectrometer on the Metop satellites.

It is used to get a detailed picture of the total atmospheric content of ozone and the vertical ozone profile in the atmosphere.

You find further details on the GOME-2 page.

Posts from the last eight newsletters can be found in this section. Older newsletters can be viewed in the Newsletter Archive (PDF, 8 MB)

Preparation for Metop-C launch and SIOV

Preparations for a launch of Metop-C, from the 'European Space Port' Centre Spatial Guyanais (CSG) in Kourou, French Guiana, on 7 November at 00:47 UTC, are ongoing at EUMETSAT. Three consecutive launch attempts over four days can be performed with the Soyuz launcher on the Fregat upper stage, once fuelled.

The preparation of Metop-C is well advanced at the launch site and a four week Stand-by Period had been introduced, before a restart of the launch campaign for the final activities to countdown.

The launch is followed by a LEOP phase of three days, after which the satellite is handed over to EUMETSAT for satellite operations. The satellite and instrument and in-orbit verification (SIOV) will start directly after the LEOP phase.

The GOME-2 In-Orbit Verification (GIOV) will start with an initial phase of functional tests and performance, as well as nominal timeline operations, all carried out with 'warm' detectors, to allow for the initial instrument and platform outgassing, without attracting contamination at the detector level.

The detector coolers will then be switched on, with a gradual decrease of detector temperatures during the first week of December. This will be followed by the next major milestone, the opening of the solar port shutter. We expect a first measurement of the Sun just before Christmas, around 21 December.

In the days over Christmas and New Year the instrument will acquire nominal and daily calibration spectra which should gradually lead to a stable nominal measurement conditions.

Commissioning and early data dissemination

The end of GIOV is marked by another throughput test, gradually increasing and then decreasing the detector temperatures (from their nominal un-cooled temperature plateau to their nominal low temperature plateau), to acquire temperature related signal responses as a reference early in the mission. Thereafter, during the course of January and February, the on-ground Level 1b processing is brought to a level of quality that is sufficient for early evaluation of Level 2 performance. Early dissemination of Level 1b data to selected users is planned for mid-February.

During the following months further adjustments in data quality are carried out involving the early feedback of Level 2 data users, such as the AC SAF who provide most of the operational GOME-2 level-2 products. The commissioning phase is expected to end with the release of the Metop-C GOME-2 Level 1b commissioning report, and the start of dissemination of Level 1b data to all users by May/June 2019.

The start of operational dissemination of GOME-2 Level 2 products by the AC SAF and the Polar Multi-Sensor Aerosol product (PMAp) is planned to follow with a couple of weeks of the start of Level 1b operations.

Three Metops

After the launch of Metop-C, there will be three Metop platforms. Metop-C will be placed in the same orbit as Metop-A and B at equal distance to both platforms, with LT equator crossing at 9:30 UTC as for Metop-A and B ('tristar' configuration – see Figure 1).

Figure 1
 
Figure 1: Left: Three Metops phasing with 120 degree separation (Tristar) adopted for SIOV and commissioning phase. Right: Trident phasing with 180 degree separation between Metop-B and C and Metop-A phased approx 90 degrees in-between.
 

This platform phasing is the favourable phasing for LEOP and SIOV, providing the best margin at the data downlink stations. During the course of commissioning, and based on early data-user feedback, the decision will be made to either remain in a tristar phasing configuration or to shift Metop-C 180 degrees opposite Metop-B (original A/B configuration) with Metop-A in-between (“trident” configuration – see Figure 1).

Milestones

7 November:        Metop-C launch
Launch+3days:    Start of SIOV
~21 December:    GOME-2 solar port opening
11 January:          End of SIOV
Mid-February:      Start of early data-dissemination
May/June:           End of Level 1b commissioning and start of dissemination

GOME-2 FM-1 (Metop-C) channel-3 anomaly

During an on-ground satellite test in March 2018 anomalous random 'spike' signals were detected in channel 3 of the GOME-2 flight-model 1 (FM1), to be launched on Metop-C. The spikes appeared in the dark-signal read-outs (see lower inlay Figure 2) at every 64th detector pixel, with the direct neighbouring pixels also receiving additional signal. In total 4% of the detector pixels in channel 3 are affected by the anomalous signals. All other GOME-2 channels are not affected. The spike positions appear to be stable by +- one pixel position since their first appearance in March 2018.

Figure 2
 
Figure 2: Bottom: Dark signal readout with every 64th detector pixel exhibiting a Spike (random read-out signal). Top: A GOME-2 reflectivity spectrum in the NO2 spectral region of channel 3 with background removed and every 64th (+-1) detector pixel set to NaN. The vertical lines indicate the position of the removed read-outs.
 

At its most recent meeting in Konstanz in May the Scientific and Technical advisory Group (STG) endorsed the decision to 'fly as is' and to remove the spike signatures by software processing on ground. The degradation was confirmed as present and stable at the last functional tests with GOME-2 FM-1 on the launch site in July.

A new version of the level-0 to 1b processor (version 6.3) is currently being developed and tested, and will be implemented during commissioning of Metop-C, after further tests of a recently developed spike-removal scheme (dead-pixel mask – DPM) prove successful. A prerequisite for a successful and stable removal of the anomalous signatures with minimal to negligible impact on the Level 2 products is that the spike positions remain stable over the course of at least one orbit. The spike positions will be detected using the daily white-light-source (WLS) calibration measurement, and monitored and, if needed, updated using dark-measurements carried out during every orbit.

The upper inlay in Figure 2 shows the NO2 spectral region between 420 and 500 nm with a reflectivity spectrum from GOME-2, for which a broad-band background has been removed and every 64th (+-1) detector pixel read-out has been set to not-a-number (vertical stem lines). Out of the 375 detector pixels covering the region 18 have been removed resulting in a loss of 4% of the nominal information content. As a result of dead-pixel screening, a small increase in retrieval noise, but a negligible impact on absolute accuracy, is therefore expected, providing the anomaly remains stable in orbit.

Last Updated:  Tuesday, 11 September 2018
 
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