The European Space Agency's latest satellite will monitor the Earth's ozone layer.
Tim Furniss/ LONDON
Europe's most complex environmental-monitoring satellite yet is scheduled for an Ariane 4 launch in April. The ERS 2 is the second of the European Space Agency's (ESA) remote-sensing satellites and, in addition to carrying the same payloads as its predecessor, launched in 1991, it will "...open a new chapter in ozone monitoring", says ESA.
This is the agency's attempt to answer worldwide criticism of the long lead times required by space agencies to develop sensors and craft to meet urgent environmental-monitoring needs. By the time they are flown, five years or more after the decision to build them, they may not be quite so relevant to the environmental priorities of the day and may even have been overtaken by new technologies.
The ERS 2 carries a much-vaunted, but not quite unique, global ozone monitoring experiment (GOME) - which has taken five years to develop, compared with the ten years it took for the ERS 1 to be flown. The GOME will generate a complete world-ozone map every three days.
Since the hole in the ozone layer was discovered 15 years ago - and first monitored and highlighted by the US satellite, Nimbus 7 in 1987 - chlorofluorocarbons (CFCs) have become infamous.
These "ozone killers" are by-products of gases used in refrigerators, air-conditioning systems and air-sprays and in 1987, many major industrial countries signed a protocol to phase them out. The recent environmental summit in Rio de Janeiro, Brazil affirmed the commitment. CFC content in the upper atmosphere, however, will remain a source of concern for over 50 years, even if all CFCs are phased out completely now.
There is a hole in the ozone layer over Antarctica, but not yet over northern latitudes where the concentration of ozone in the stratosphere has nonetheless fallen by up to 9% in some locations. A precise measurement of ozone depletion is therefore needed urgently.
Like those on US and Russian satellites, the GOME will measure characteristic peaks in the absorption spectrum of sunlight, scattered by molecules in the atmosphere, which indicate the presence of ozone.
A total-ozone-mapping spectrometer (TOMS) similar to the instrument flown on the Nimbus 7 - which ceased operating in 1993 - was fitted to a Russian Meteor 3 satellite in 1991. A dedicated US craft, the total-ozone-monitoring satellite, will be launched this year. This will be followed by the Japanese advanced Earth-observation satellite in 1996, which will also carry the TOMS instrument.
NEW OZONE TERRITORY
ESA says that the GOME will enter new ozone territory, with a differential optical-absorption spectroscopy (DOAS) system, which measures the solar spectrum, to increase ozone-measurement accuracy by three to five times. The GOME also covers, a broader spectral band than did previous satellites, in three relevant ozone-absorption bands, extending measurements to cover almost all seasons and latitudes, including, for example, the polar regions in late winter.
Ultra-violet instruments cannot make measurements in these regions at that time of the year, when the processes responsible for ozone depletion are particularly active, because the winter Sun is too low for ultra-violet light to penetrate. With the broader spectral range, the DOAS will also be used to monitor trace gases, in addition to the sulphur dioxide, which has already been routinely monitored by satellites. In the lower levels of the atmosphere, it will detect air pollution caused by aerosols and micro-particles.
The GOME may have taken ESA only five years to develop - fast by space standards - but it cannot provide a 100% accuracy guarantee, which would have required double the development and testing time, says ESA.
The ozone data will also be useless unless its reception is effective and it can be transformed into useful data products quickly.
When the ERS 1 was launched, the ground segment was not ready in time to make full use of the avalanche of data, which started to pour in. Now ESA is up and ready. Raw ozone data will be stored on the ERS 2 and transmitted, with data from other instruments, to four ERS ground stations in Europe and Canada.
GOME data will then be sent directly to the GOME Data Centre at Oberpfaffenhofen, near Munich, in Germany. Each total-ozone-content measurement will represent the content of a rectangular column of air on the Earth's surface, between 40 x 40km (25 x 25 miles) and 40 x 320km on the ground.
The data will be combined with information from other space and ground-observation sources, to produce regular ozone maps, ozone-depletion predictions and information on aerosols, sulphur dioxide and nitrogen oxide in the atmosphere.
Evaluation of raw data will be carried out under ESA control, by more than 50 principal investigators worldwide. The computer programmes are huge, with the algorithms for correcting, calibrating and checking raw data; extracting ozone values; and converting satellite-orbit information into map co-ordinates, amounting to 20,000 programme lines.
A tiny error in programming can have far-reaching consequences. Before a single ozone map can be produced, the GOME must be calibrated and the validity of its data checked by comparison with independent data. The first rough measurements are expected to come through six to eight weeks after launch, with detailed maps following some four to six months later - just before the hole in the ozone layer above Antarctica threatens to open up again.
Source: Flight International
