Sira completes delivery of Europe's global ozone-watch units

Citation

(1998), "Sira completes delivery of Europe's global ozone-watch units", Aircraft Engineering and Aerospace Technology, Vol. 70 No. 5. https://doi.org/10.1108/aeat.1998.12770eaf.002

Sira completes delivery of Europe's global ozone-watch units

Sira completes delivery of Europe's global ozone-watch units

Keywords Environmental monitoring, Ozone layer, Sira, Spectrometer

Sira Electro-Optics has delivered all of the final flight models of the global ozone-watch units it has designed and built for the European Space Agency's space-based GOMOS instrument.

GOMOS will monitor the earth's ozone layer and the layer's long-term depletion, from ESA's ENVISAT environmental monitoring satellite, which is planned to be launched by the Ariane launcher in 1999 into a sun-synchronous polar orbit about 800km above the earth.

The global ozone monitoring by occultation of stars (GOMOS) instrument consists of an ultra violet/visible spectrometer and a near-infra red spectrometer, both fed by a single, fixed telescope mounted behind a steerable front mirror.

Chislehurst-based Sira has built GOMOS' near-infra red and ultra violet/visible spectrometer detection modules, its star acquisition and tracking units, and a specially-designed fast photometer.

GOMOS will enable accurate mapping of the ozone layer and will watch closely long-term trends in the amount and distribution of ozone around the earth. Over the period of its mission, the instrument will be able to measure ozone concentration decreases as small as 0.1 per cent per year, well below depletions expected from model calculations.

Detailed worldwide daily maps at every altitude between 15 and 60km or more will be produced by GOMOS, which will provide full earth coverage and as much data as 360 ground-based stations.

Measurements will be made by orienting GOMOS' line of sight, using the steerable mirror, towards preselected stars while they are setting behind the atmosphere at the horizon. By observing the full ultra violet-visible-near-infra red spectrum of the star and employing a self-calibration method, GOMOS will provide an absolute measure of the concentration of atmospheric molecules present.

The location of each of these concentration measurements will be accurate to ± 30 metres through the atmosphere and stratosphere extending up to 60km from the earth's surface.

In addition to monitoring seasonal, latitudinal and long-term trends in ozone depletion, GOMOS will also measure other atmospheric molecules and parameters, such as NO₂, NO₃, H₂O aerosols and vertical temperature distribution, to provide a better understanding of the ozone depletion mechanism.

Sira has been working as part of an international team headed by Matra Marconi Space, reporting to ESA's prime contractor, Dornier.

Sira Electro-optics Division is also looking at how conditions found on missions to the moon and the planet Mars could affect the use of fibre-optic systems in spacecraft.

Under the terms of a contract awarded by the European Space Agency (ESA), Sira is reviewing the worldwide availability and suitability of fibre-optic components which ESA could potentially use in its space programmes (Plate 1). Part of the work includes testing selected components under the conditions they will experience during launch, in orbit, and when they are located in a planetary base on the moon or on Mars.

Fibre-optic components are likely to be used in space for high-speed data transfer, communication networks and fibre-based measurement systems. During the launch of a spacecraft, however, they will need to withstand mechanical shocks typically of up to 2,000 × g at frequencies above 1.5kHz ­ low-frequency and random vibrations, rapid declines in pressure of about 10mbar/second, and peak ambient temperatures in the payload area of up to 100°C for several minutes.

When the fibre-optic components are in space, they will have to withstand ionising radiation, including high-energy cosmic ray particles.

The major problems for optical-fibre-based systems will be posed by geomagnetically-trapped electrons and protons. These usually originate from solar flares and the solar wind and have energies ranging from a few thousand electronvolts (keV) to several million (typically 5MeV).

According to Sira project manager Dr Mike Cutter, a spacecraft going into geostationary orbit at about 36,000km above the earth's surface experiences very significant irradiation.

Plate 1 Fibre optic components

"Exact levels are difficult to predict", he says, "as they depend on the amount of solar activity. In finding out the ability of key fibre-optic components to survive and operate under the actual conditions peculiar to space, we have made allowances for wide variations in radiation levels by applying large safety margins."

At the end of the two-phase contract, Sira will be providing ESA with written guidelines and recommendations on how to select fibre-optic components for specific duties in space and on other planets.

Sira to build tiny spectrometer for better environmental monitoring

Sira Electro-Optics recently won financial support from the British National Space Centre (BNSC) to build the world's smallest satellite-based imaging spectrometer, which is expected to give the UK a technological lead in monitoring pollution and the environment from space.

The instrument, known as CHRIS (compact high-resolution imaging spectrometer), is capable of observing the earth from a miniature satellite (minisat) with a ground resolution of just 25 metres, yet it uses less power than a camcorder and could fit into a briefcase.

BNSC's support was confirmed as John Battle, Minister for Science, Energy and Industry, was visiting the Chislehurst-based advanced instrumentation and space hardware company.

He said: "This project is an important step forward in the UK's ability to monitor pollution and climate change. Future designs of the instrument could also be used closer to earth for aerial monitoring of vegetation and our use of natural resources, and even in a portable ground-based capacity".

"Sira's proposal was judged the best in Europe and we are delighted to be providing support for such a worthwhile project."

Sira won a competition organised by the European Space Agency (ESA) for the opportunity to provide an advanced, but highly compact instrument to be taken on board a new type of ESA minisat, which will be much smaller and more agile than its predecessors.

Called project for on-board autonomy (PROBA) and intended to be launched in the year 2000, the ESA satellite will demonstrate how future spacecraft could become increasingly autonomous, especially in the areas of platform control data handling and storage systems.

A separate agreement between Sira and ESA will cover the supply and operation of the CHRIS payload.

Sira's chief executive, Richard Brook, said winning the competition was a terrific opportunity to demonstrate Sira's expertise in designing and building optical instruments for pollution and environmental monitoring.

"PROBA will allow us to demonstrate CHRIS, which in the longer term has enormous commercial potential and could represent an important technology lead for the UK. But without BNSC's support we could not have afforded to shoulder the full financial risk of building the instrument ourselves," he said.

"This mission could give rise to a new generation of compact instruments providing cost-effective access to widespread data about land-use, pollution and other environmentally-important parameters," Brook added.

BNSC and Sira will be co-funding the construction, testing and deployment of CHRIS, which will have a total cost of about £1.5 million.

Consuming less than 10W of power and weighing under 15kilograms, CHRIS will fit neatly on PROBA without compromising the satellite's power and mass constraints. It will operate across 19 spectral bands within the visible region of the spectrum (400-1,050 nanometres). CHRIS will also be capable, however, of a ground resolution of just 25 metres while observing a strip of the earth up to 19 kilometres wide as PROBA passes overhead.

Furthermore, future CHRIS payloads could operate at wavelengths out to approximately 300 nanometres in the infra-red region of the spectrum.

Two key design features make the combination of CHRIS and PROBA potentially more powerful for earth-observation, particularly of land use, than previous satellite-based instruments.

1. 1.

   PROBA will be capable of pointing itself to view particular areas of interest. Target areas of vegetation will be viewed from different orbital positions, thereby giving a better indication of ground conditions. More traditional earth-observation instruments are not capable of adjusting their attitude as they fly over a target area, and are only capable of fixed viewing angles.

2. 2.

   PROBA's attitude will be constantly adjustable from the ground, providing CHRIS with the flexibility to review targets in response to its findings. Furthermore, the spectral bands in which CHRIS is observing back-scattered solar radiation from different types of vegetation will be selectable from the ground.

On-board programmable digital electronics will control the charge-coupled device (CCD) imager and its data output.

Sira has patented the spectrometer design, which makes use of a novel geometric layout. The resulting configuration is simple, passive and compact, measuring just 260 × 200 × 800mm.

Imaging spectrometers generate more data faster than any other type of space-based instrument. Concurrently, they are increasingly being used in scientific missions to measure parameters of environmental concern.

Sira played a significant part in designing MERIS, an imaging spectrometer for monitoring land and water pollution, which will soon be deployed on ESA's ENVISAT environmental-monitoring satellite.

For further details, Tel: +44 (0)181 467 2636.