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Local Weather
Oberpfaffenhofen

GOME Product Survey

Abstract

The purpose of this document is to give a short but comprehensive overview on data products derived from the Global Ozone Monitoring Experiment, GOME, at DLR-DFD. Data products are briefly described and examples for application are given. Different ways to get access to the data products are presented. However, for a much more detailed description of data products and processing of higher level data products please refer to the GOME product guide.

Please send questions, comments, ideas or criticism to wdc@dlr.de.

Global Ozone Monitoring Experiment (GOME)

GOME is a passive imaging spectrometer providing high spectral resolution. It was launched in April 1995 on the ERS-2 satellite and marks the beginning of a long-term European ozone monitoring effort. Scientists expect from this experiment high quality data on the global distribution of ozone and several other climate-influencing trace gases such as BrO, OCIO and NO2 in the Earth's atmosphere.

GOME was suggested as a down-scaled version of SCIAMACHY (scheduled to be launched onboard ESA's ENVISAT -satellite in November 1999) by the Max Planck Institute of Chemistry in Mainz and the (Institute for Environmental Physics) of Bremen University to be flown by ESA on ERS-2.

GOME spectrum

Figure 1: GOME spectral ranges and trace gas information content

GOME is a nadir-sounding instrument, that means, the instrument looks down on the Earth's surface. Spectrometers, operating in the ultraviolet and visible spectral range (240 - 793 nm) are used to measure solar radiation as it is scattered back from the atmosphere. Spectral resolution is excellent and ranges from 0.2 - 0.33 nm, depending on frequency. Depending on the measurement mode, GOME provides a horizontal resolution from 40 km along track and 320 km across track to 40 km along track and 40 km across track (Figure 2). This gives the opportunity to detect even smaller scale structures in the atmosphere. DLR's contribution to GOME, besides archiving and distributing data, is to perform an operational data processing starting with the satellies bits and bytes and ending up with value added scientific data products.

Scanning Geometry

Figure 2: GOME scan pattern. GOME scans the Earth normal to flight direction with the help of a mirror. During the 4.5 sec forward movement of the mirror, three ground pixels 40 x 320 km2 in size are generated. The back motion of the mirror occurs at threefold speed.

GOME data products are divided into different levels depending on the processing step. Level 0 data are the satellites raw data as they are down-linked to the ground-stations. This information is transferred to DLR via tape and ISDN (see below). In a first step, raw data are converted into radiances using information from calibration measurements (Level 1, see Figure 3). In a second step, a radiative transfer model is applied to convert the radiances into trace gas concentrations. Level 2 data therefore show up as footprints of trace gas concentrations along the satellite's orbit (Figure 4).

Level 1 Product

Figure 3: Typical spectrum as an example for level 1 data products

One day of earth orbits

Figure 4: Typical footprint-pattern of GOME orbits for one day (level 2)

The processing chain up to this level is performed in order by ESA. Processing beyond this level results from DLR's responsibility.


Availability of GOME Data

GOME data are recorded globally and are relayed to the ground stations once per orbit (100 min). As a consequence of the orbit geometry, about ten orbits per day are received at the ESA station in Kiruna (Sweden), and the remaining orbits, about four, in Gatineau, Prince Albert (both Canada), and Maspalomas (Spain). At these locations the data are copied onto magnetic tape cassettes, and reach DFD in Oberpfaffenhofen by mail in about two weeks.

GOME fast delivery services was initiated with the installation of the GOME Data Processor system at the Kiruna station (Sweden) in January 1997. This was done by DLR’'s Deutsches Fernerkundungsdatenzentrum (DFD) in collaboration with the Bremen Institute of Environmental Physics and Remote Sensing (ife/uip) and ESA. The generation of earthshine radiances (level 1) and ozone total column (level 2) NRT products from Kiruna is done using the GOME Data Processor (GDP) system, the ground segment of the GOME sensor developed and operated at the German D-PAF in DFD. The NRT products from Kiruna have the same precision as the off-line products and are available one hour after acquisition.

Figure 5: Data flow from the ERS-2 satellite (level-0) to the GOME Data Processor (level-1 and level-2) in Kiruna. Ozone data is digitally transmitted to the DFD where global maps are generated (level-3) and distributed via internet.

The Kiruna station receives up to ten out of the daily fourteen orbits. The remaining orbits are collected by Maspalomas, Gatineau and Prince Albert stations. The optimal solution for having global fast delivery ozone data, would be the installation of GDP in the remaining stations, but this is not possible due to financial and technical constrains. The alternative solution is the usage of the so called "extended EGOI (Extracted GOME Instrument Header Data)", a subset of the normal GOME science data (EGOC), send from the ground-stations to ESRIN for instrument monitoring.

EGOI data from Maspalomas and Gatineau has been interfaced at DFD to be processed using the standard GDP system since August 1998. One orbit from Prince Albert is lost because the station can not generate EGOIs. The processing of the EGOI data is done using the on-line calibration parameters computed by GDP in Kiruna and transmited to DFD using database replication techniques.

Figure 5.1: GOME NRT Ground Segment, Level 2 ozone data and Level 1 calibration parameter are transmitted to DFD from Kiruna. EGOI data comming from ESRIN is processed at DFD and the global maps are distributed via internet. DFD does the remote monitoring and control of GDP in Kiruna.

The EGOI ozone total column has only 0.2% difference with respect to the standard EGOC products. The EGOI data is available at ESRIN two hours after acquisition.

GOME level 2 and level 3 near-real-time data processed at DFD are distributed to the users via internet. The source of the near-real-time data, EGOC or EGOI, is totally transparent for the user.