Fig-4: DC increase for the spectrometers A1, A2, B1 and B2.
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Fig-4: DC increase for the spectrometers A1, A2, B1 and B2. After 2 nd GOMOS major anomaly. Stabilization after new PSO algorithm for satellite pointing: Dec 2003. Fig-5: MIP position through the mission.

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Gomos

Fig-4: DC increase for the spectrometers A1, A2, B1 and B2.

After 2nd GOMOS major anomaly

Stabilization after new PSO algorithm for satellite pointing: Dec 2003

Fig-5: MIP position through the mission

Quality assessment of ENVISAT Atmospheric mission: implication for the scientific user community

[email protected]

Abstract

The objective of this poster is to highlight the importance of the instrument and products monitoring and to show the implication of this task for the scientific users community. We focus our attention on the GOMOS instrument on-board the ENVISAT ESA platform and we present some example of quality monitoring results which have significant impact on science data quality.

Monitoring activities in support to ENVISAT

The quality assessment activities of ESA Earth Observation missions (ERS, ENVISAT, GOCE in the future ) are today performed by the DPQC organization, a Serco-Datamat consortium of specialized companies which provides a service to ESA. The DPQC atmospheric team, which involves expertise from Serco, ACRI and DLR, is responsible for the monitoring of the three ENVISAT atmospheric mission (GOMOS, MIPAS and SCIAMACHY).

Introduction

  • The DPQC group is furthermore responsible for many other activities related to the operational mission support. The DPQC tasks for the ENVISAT mission support can be summarized in the following:

    • Daily and long term monitoring of instrument health

    • Daily and long term monitoring of products

    • Anomaly investigation and performance assessment

    • Calibration activities and delivery of auxiliary data files

    • Maintenance and support for IPF evolution and implementation

    • Mission support, reporting and attendance to progress meetings

    • Answer to general request from users

The satellite instruments are subject to performance degradation due to the aging of mechanical and electronics components or to the impact of platform and on-ground anomaly. Besides, the operational processor, which generates the products, is a compromise between extraction of optimal information from the measurement and the constraints of processing and dissemination resources. As a result the quality of the products arriving to the scientific users community can be variable due to several anomalies. The quick detection of instrument or products anomaly is then crucial for instrument safety and for optimization of data quality. In this poster we will address this problem considering the ENVISAT GOMOS instrument. The quality control baseline will be outlined and some examples of significant daily and long term monitoring will be reported; furthermore the implications of this activity for the scientific users will be underlined.

GOMOS

Performance assessment

Beside the daily monitoring, the evolution of key parameters is important for assessing the instrument performance and for mission evolution and calibration plan definition. Examples are shown here below. Further details can be found in the GOMOS monthly report available on-line: http://earth.esa.int/pcs/envisat/gomos/reports/monthly/

Mission and Processing Status

  • On 22nd March 2003 GOMOS suffered the anomaly ‘Azimuth Voice coil command saturation”. The anomaly occurred during the rallying of the telescope in the preparation for the star observation. The problem was in a wire and it was decided to switch the instrument and the ICU (Instrument Control Unit) to side B. GOMOS restarted the operations on 17th July 2003.

  • On 25th January 2005 GOMOS suffered the anomaly ‘Elevation Voice coil command saturation”. The anomaly occurred during the rallying of the telescope in the preparation for the star observation. GOMOS is in operations again since 29th August 2005 in a new operations scenario which foresees that in case of a failure the ICU ignores the error and rallies the telescope to the position of the following star observation. The azimuth angle range has been reduced in order to limit the anomaly occurrence.

  • The GOMOS mission is now on-going with the same product quality as before the anomalies. The main impact of the second anomaly on the GOMOS mission is the reduction of the data availability: from 35 occultations per orbit to 22 occultations due to the restriction in azimuth angular range.

  • Users are supplied with 2002-2005 data sets reprocessed by the last prototype processor GOPR_6.0c_6.0f developed and operated by ACRI. The next GOMOS operational ground segment version (GOMOS/5.00) foreseen to be in operations at mid June 2006 will be in line with the prototype version used for this second reprocessing. All reprocessed data can be retrieved via web query from http://www.enviport.org/gomos/index.jsp. FTP access to bulk reprocessing results (one tar file of GOMOS products per day) is allowed from the D-PAC: ftp:[email protected] See more details and latest status on http://www.enviport.org/boards/board_gomos.htm

  • Dark charge monitoring

  • The temperature dependence of the DC would make this parameter a good indicator of the DC behavior, but the hot pixels and the RTS are producing a continuous increase of the DC since the beginning of the mission.

  • “Hot pixels”, a pixel is “hot” when its dark charge exceeds its value measured on ground, at the same temperature, by a significant amount.

  • RTS phenomenon (Random Telegraphic Signal), it is an abrupt change (positive or negative) of the CCD pixel signal, random in time, affecting only the DC part of the signal and not the photon generated signal.

  • The trend of DC is of crucial importance for the final quality of the products, and is therefore subject to intense monitoring. Fig-4 shows the DC increase for the spectrometers A1, A2, B1 and B2. The modulation is given by the seasonal temperature variation of the detectors.

Instrument and Products monitoring

Daily checks on GOMOS telemetry, level 0, level 1b and level 2 data are performed in order to detect any instrument or products anomaly.

The MIP (Most Illuminated Pixel)

The MIPis the star position on the SATU CCD in detection mode. The nominal centre of the SATU is pixel number 145 in elevation and number 205 in azimuth. The detection of the stars should not be far from this centre. After the 2nd major anomaly the Azimuth and Elevation MIP have a drift that has no explanation (fig-5). Although it does not impact the data quality or the star location on the CCD array during the measurements, it may invalidate attitude monitoring by GOMOS and could represent a hidden anomaly.

Level 0

The level 0 monitoring consist in checking mainly the instrument missing packets. When many instrument source packets are missing in level 0 products it is probable that acquisition problems have occurred at the stations. This should be communicated as soon as possible in order to correct the error and avoid the acquisition of useless data.

An example of L0 monitoring is given in fig-1 where some level 0 files show acquisition problems.

Fig-1: Acquisition problems detected

Tracking performance

The SATU (Star Acquisition and Tracking Unit) health is essential for GOMOS instrument, which is based on star pointing and tracking mechanism. The SATU NEA monitoring consists in computing the average values of the SATU standard deviation above 105 km. On fig-6 it can be observed a change in trend between Sep-Dec 2005. The cause of this unexpected behavior is under investigation although the values were always below the thresholds.

Level 1

Concerning L1 monitoring, significant indicators are the ADF usage, the quality flags, the SATU noise equivalent angle, dark charge level, CCD temperatures and the altitude of star loss.

An example is given on fig-2 when a high percentage of cosmic ray flag was raised over the poles due to the eruption of a giant sunspot on 17th and 20th January 2005. The South Atlantic Anomaly is also visible.

Fig-6: SATU NEA through the mission

Acquisition probability

The number of star target failed during centering phase (before tracking phase) is monitored (fig-7). A high percentage of star loss will trigger the rejection of a specific star from the catalogue in order to guarantee the maximum number of stars measured and available for the users. The star id 115 was lost 40% of the times but it was planned to be occulted five times and was lost twice (high percentage of loss not statistically significant). For the moment, no stars have been removed from the star catalogue. Now with the instrument in a new operation scenario, users should be aware on the fact that the stars are also lost due to the anomaly “elevation voice coil command saturation” even if the instrument is not going anymore to Stand by / Refuse mode.

Fig-2: Percentage of cosmic ray hits per profile

Level 2

The monitoring of level 2 products consist mainly in checking the quality flags and the number of flagged points per profile.

An example is given on fig-3. The high percentage of flagged points was due to a high percentage of "geolocation flag" set. This means that a high percentage of the profile points are located outside the atmosphere. Indeed, special observations were performed at altitude range of 310-100 km (nominal is 130-5 km) for the study of the reflectivity.

Fig-7: Statistics of stars lost during centering phase

Fig-3: Percentage of flagged points per profile

Source of information

Important information for each instrument are publicly available on the web at the ESA-Product Control Service web page (see screen shot on the right). In particular, monthly reports are available at:

MIPAS

http://earth.esa.int/pcs/envisat/mipas/reports/monthly/

GOMOS

http://earth.esa.int/pcs/envisat/gomos/reports/monthly/

SCIAMACHY

http://earth.esa.int/pcs/envisat/sciamachy/reports/bimonthly/

Instrument unavailability

http://envisat.esa.int/instruments/availability/

Products disclaimer

http://envisat.esa.int/dataproducts/availability/disclaimers

General request

[email protected]


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