* Now with the MIKO Company, Lystrup, Denmark
GOME is an across-track scanning optical spectrometer, covering the wavelength range 250 790 nm. This spectral range is split into four channels, each equipped with a 1024-pixel linear array detector. The resulting spectral resolution is 0.2 nm in the ultraviolet and 0.4 nm in the visible/near-infrared parts of the spectrum.
GOME's task is to sense the sunlight being reflected or scattered in the Earth's atmosphere and at its surface. The measured spectrum contains absorption features, which can be used to derive quantitative information on the amount of ozone present, and a number of other atmospheric species. GOME is the only new instrument on ERS-2 compared with ERS-1. A full technical description of it was published in ESA Bulletin No. 73 (February 1993).
As early as 1988, the Agency's Executive started with the preparatory work for ERS-2 as the follow-on to ERS-1 which, at that time, was just starting its assembly phase. It was felt necessary to complement the capabilities of ERS-1 with instrumentation that could contribute to the growing discussions taking place in the public arena about such contentious issues as global warming and ozone depletion. In November 1988, selected European scientists involved in atmospheric chemistry instrumentation were therefore approached with a request to submit proposals for such an instrument to be included in ERS-2's payload, possibly replacing the Infrared Radiometer part of the ATSR instrument.
Among the proposals received was one prepared jointly by J. Burrows and P. Crutzen, called 'Sciamini', being derived from the 'Sciamachy 'instrument concept proposed for flight on the Polar Platform (which later became part of the Envisat project). An in-house assessment of the technology involved confirmed the feasibility of such a concept in principle, and the authority was given to proceed with a more detailed instrument concept study. Some simplifications were, however, already introduced at this point: no limb viewing and only one unit, rather than two side by side. The modified instrument was also renamed the 'Global Ozone Monitoring Experiment', or GOME.
By the end of 1989, a contract had been placed with the Dutch firm TPD to develop the optical concept in more detail. In parallel, ESA performed in-house studies on the possible accommodation of the instrument inside the ERS-2 Service Module, and on the details of the detector's analogue electronics. In February 1990, the first contacts were made with the Italian firms Officine Galileo and Laben, to complement TPD in conducting a Phase-B study beginning in July that year. In parallel, ESA's Earth Observation Programme Board endorsed the ERS-2 Programme, including the GOME instrument.
The industrial Phase-B activities cumulated in a Design Baseline Review in March 1991 in Noordwijk (NL). The outcome was quite significant in a number of areas:
Another major outcome, initiated at the DBR but confirmed only later in terms of its feasibility, was the change in the accommodation of the instrument from inside the Service Module to the outside of the Payload Module.
The main development phase for GOME was prepared and negotiated on this basis, and began in April 1992. A separate contract was placed with RAL and BAe for the necessary modifications to the ATSR's DEU hardware and software.
The political and technical boundary conditions as set forth by the Programme Board for the inclusion of GOME into the ERS-2 Programme can be summarised as follows:
The development programme for the GOME instrument, as initially envisaged, implied some breadboarding activities for critical subunits and a bench model for scientific testing, but was essentially a protoflight programme aiming for instrument delivery in early 1993 (Fig. 1).
Soon after starting the detailed definition of the breadboard model, it became obvious that some critical performance parameters could only be evaluated in vacuum. Hence, the first upgrade to the breadboard was to make it suitable for thermal-vacuum testing. In a next step, it was realised that the critical spectral-stability aspect could only be thoroughly evaluated if the structure were in close to final form. As a result of these concerns, the final step to producing a full engineering model was taken and this was subjected to a full environmental test programme at qualification vibration, EMC, and thermal-vacuum levels. In addition, this model was used for interface testing with the entire payload and was also subjected to a full calibration programme to exercise all necessary setups and procedures.
Whilst these activities were still in process, the flight-model programme was started. For schedule reasons, after the instrunent-level vibration and EMC tests, the flight model was used for the satellite- level alignment, vibration, acoustic and EMC tests and was then returned to the contractor for thermal- vacuum testing. The GOME flight model was declared flight-ready just in time for transport to the launch site together with the ERS-2 spacecraft. The major steps in this development programme are summarised in Figure 2.
Figure 1. The initial GOME programme planning
Figure 2. The actual GOME development schedule
No financial provisions were made in the ERS-2 Programme for the processing of the GOME data. Still, it was recognised that, in order to optimally exploit the sensor's capabilities, a ground processing system was necessary and that it must be comparable in capability to those for the other instruments on ERS-2.
Early in the GOME programme, scientists had started to work on some specific ground-processing issues, such as radiativetransfer modelling and an instrument simulator. In 1991, the German Deutsche Forschungsanstalt für Luft- und Raumfahrt (DLR) (Fig. 3) volunteered to implement, within the framework of the German Processing and Archiving Facility (D-PAF) and with national funds, the core of an operational GOME Data Processor. This comprised the conversion of the raw data to geo-located, wave- length and radio-metrically calibrated radiances (level 0 to 1) and the retrieval of ozone total column amounts (level 1 to 2). This proposal was endorsed by the ESA Programme Board in November 1992. Additional level-2 products, related to the retrieval of ozone profiles and cloud/aerosol parameters, were earmarked for being generated at the UK and Italian PAFs, respectively.
Figure 3. The German Aerospace Research Establishment (DLR) in Oberpfaffenhofen, which hosts the D-PAF facility, including GOME data processing
Compared to many space- and ground-segment development programmes, the time needed for the GOME instrument's development was extremely short. The obvious question is why is this not generally possible? The answer is that there were numerous favourable boundary conditions in GOME's case that are not generally valid:
Although every project differs in terms of its particular boundary conditions and constraints, some worthwhile lessons can be learnt from the GOME Project experience:
Although at first glance costly and leading to many activities having to be conducted in a short time, the breadboard model, which ultimately became a fully-fledged engineering model, proved to be an invaluable tool in the overall GOME programme. Not only did it enable the discovery of difficult- to-predict effects, such as straylight and electronic crosstalk, in time to implement remedies on the flight model, but it also served as a 'place holder' in many system-level tests where the presence of 'a GOME', but not necessarily the flight model, was required.
Another benefit of the breadboard model was that it passed through the entire calibration programme. The main benefit was that the acquisition of these breadboard calibration data allowed the necessary software tools for data analysis and processing to be written and debugged. The experience gained allowed the time needed for the entire calibration campaign to be reduced, from more than six months in the case of the breadboard model to less than two months for the flight and flight- spare models.
Few results are yet available from GOME, due mainly to the outgassing time of about one month needed after launch. During this initial period, certain instrument functions could be tested, but no performance evaluations or onboard calibrations could be performed.
When the closed-loop coolers were activated for the first time, and the first solar spectrum was acquired a few days later, it was proved that all of the hardware is functioning correctly.
The temperatures of the four detectors and of the optical bench are very stable and within the uncertainty range of the predictions. From the wavelength calibration, one can conclude that spectral stability is excellent: the measured wavelength drift as a function of the orbital temperature is of the order of just 1/50th of a detector pixel (Fig. 4).
As expected, GOME shows some sensitivity to the space radiation environment. To quantify this effect, the instrument was left for three days in 'dark-current mode' and the results mapped; Figure 5 clearly shows the location and extent of the South Atlantic Anomaly (SAA). The radiation has two effects: a general increase in noise level, which is evident when comparing Figures 6a and 6b, and the high sharp spikes evident in Figure 6c. The ground-processing software has been configured to cope with the latter.
On 15 May, GOME recorded its first solar spectrum, which is being used both for instrument calibration and in the retrieval of ozone data in the ground processing chain. The spectrum was largely as expected, except that for the wavelength range 289 307 nm the detector was in saturation. This was corrected by adjusting the integration time for the affected band, and the next Sun acquisition was then within the nominal range. Detailed investigations and fine tuning of the processing are still going on, but first impressions are that the GOME measurements compare very well with the external references.
The acquisition and initial processing of earth-shine spectra is currently in progress, together with the optimisation of integration-time settings, stepping through different scan patterns, and fine-tuning of operational procedures.
Figure 4. Wavelength shift as a function of orbital temperature variations. The plot shows the relative shifts of three selected lines of the wavelength calibration lamp, in fractions of a detector pixel. One pixel in channel 1 corresponds to 0.1 nm. Also shown is the temperature at the disperser prism, as the most temperature-sensitive optical element (yellow curve)
Figure 5. GOME radiation impact mapping as a function of geographical location. The plot shows the highest occurring peak (see Fig. 6c also) in any of the four channels
Figure 6. Dark-current readouts of one channel:
(a) at a location inside the South Atlantic Anomaly;
(b) outside the SAA, showing a less noisy behaviour;
(c) the spikes on individual (or very few) detector pixels, as plotted in Figure 5.
It has to be pointed out that the whole GOME programme has truly been a team effort, involving many engineers, scientists, managers and support staff.
The instrument Prime Contractor was Officine Galileo (Florence, I), with major contributions from Laben (Milan, I) for the DDHU and EGSE, TPD-TNO (Delft, NL) for the Calibration Unit, and Dornier (Friedrichshafen, D) for the Thermal-Control Subsystem.
Under separate contracts, TPD-TNO performed the instrument calibration, and Rutherford Appleton Laboratory (Chilton,UK), with the support of BAe (Bristol, UK), made the necessary modifications to the ATSR-DEU.
Credit has also to be given to the ERS-2 Prime Contractor Dornier, and the team of the AIT subcontractor Fokker (Leiden, NL) for their continuous support and their flexibility in coping with the special needs of GOME.
The Deutsche Forschungsanstalt für Luft- und Raumfahrt (DLR), in Oberpfaffenhofen (D), developed the GOME Data Processor with only a limited financial contribution from ESA, the majority of the funding being provided by the German Space Agency (DARA).
In addition, the GOME project has enjoyed the continuous support of a large number of scientists from all over Europe and the USA, with J. Burrows from the Institut für Fernerkundung, Bremen (D) acting as the lead scientist. He has been supported by the members of the GOME Science Advisory Group, the subgroups for calibration, data processing and algorithm development, and validation: K. Chance (USA), A. Goede, S. Slijkhuis, P. Stammes (NL), R Guzzi (I), B. Kerridge, R. Munroe (UK), D. Perner, U. Platt, H. Frank, D. Diebel (D), J-P. Pommereau (F) and P. Simon (B).
Last but not least, the specific contributions of the various ESA establishments - ESA Head Office, ESOC, ESRIN and ESTEC - to the overall success of the GOME project are gratefully acknowledged.