- Press Release
- Feb 5, 2023
Astrium completes Envisat, Europe’s largest environmental satellite
Mathias Pikelj, [email protected]
Noordwijk — Europe’s largest and most complex Earth observation satellite, the environmental satellite Envisat — with a mass of approximately eight tons and a stowed height of ten meters — is currently completing its final tests at the technical centre of the European Space Agency, ESA, in Noordwijk (the Netherlands). The space-borne environmental Earth monitoring spacecraft, developed and built under the industrial lead of Astrium will be shipped to the Kourou launch site in French Guyana in April. It is schedule for launch into a Low-Earth Polar orbit by the end of July 2001 on an Ariane 5 rocket, the only launch vehicle capable of lifting this spacecraft.
The industrial contract for Envisat was worth about 1.5 billion Euro in total and the development and construction of the spacecraft has taken more than ten years, with the involvement of almost 100 companies in 14 countries. Astrium sites in all three countries, the UK, Germany and France, have participated in the programme with Astrium (UK) as prime contractor to ESA for the Polar Platform and two of the major instruments, Astrium (Germany) as Mission Prime to ESA with overall responsibility for the instruments, the Payload Equipment Bay and two instruments, Astrium (France) as supplier of the Service Module and other instruments.
Following launch the Envisat will orbit the Earth approximately every 100 minutes. Every part of the Earth’s surface will be revisited every 35 days throughout the satellite’s mission lifetime. During the mission of at least five years, the ten advanced instruments on board Envisat will help scientists gain a better understanding of global warming, climatic changes and the depletion of the ozone layer as well as changes in the oceans, the ice caps, vegetation and the composition of the atmosphere.
"Envisat’s data will be of unprecedented quality and will thus open up a new era in space-borne climate research", stated Dr. Klaus Ensslin, Member of the Board of Astrium. "The companies which merged to form Astrium have already been working together as partners in this programme for many years. Now, as an integrated European company, we are delivering, in Envisat, a technical masterpiece." Dr. Ensslin is convinced that the Envisat programme will be of enormous value — especially in combination with the meteorological satellites Meteosat and Metop. "The extent of our knowledge will take a giant leap forward — as it did when we launched Envisat’s predecessors, the ERS satellites."
The technical backbone of the satellite is the Polar Platform, which consists of two main assemblies: the service module (SM) and the payload module (PLM). The SM includes, for example, the components for the power supply, the orbit control system and the telemetry/telecommand, whilst the PLM carries the instruments and associated systems including instrument data transmission for Earth observation and atmospheric research. The Polar Platform is a further development of Astrium’s highly successful SPOT satellite platform. ESA’s successful ERS satellites were also based on developments of the SPOT family of satellites.
The complete payload comprises seven ESA-supplied instruments and three provided by the national space agencies of Germany/the Netherlands, France and Great Britain. From a scientific and application point of view, the Enivisat instruments form an ideal payload combination to monitor and examine our Earth and its atmosphere. They will gather a wealth of information for scientific analysis to give a better understanding of the complex systems that govern the well being of Earth’s fragile environment.
Astrium was responsible for the design, development and manufacture of five of the ten major on-board instruments:
* ASAR, the Advanced Synthetic Aperture Radar supplied by Astrium (UK), is the largest instrument on board. A high resolution imaging radar instrument with resolutions down to 30m, it can be used for a wide range of commercial and scientific purposes. including the measurement of sea state, sea ice, surface topography, soil moisture and the extent of wetlands.
* AATSR, the Advanced Along Track Scanning Radiometer supplied by Astrium (UK), will monitor sea surface temperature and land vegetation to assist in the early detection of global climate change.
* GOMOS, the Global Ozone Monitoring by the Occultation of Stars instrument supplied by Astrium (France), will provide valuable information on ozone depletion.
* MIPAS, the Michelson Interferometer for Passive Atmosheric Sounding supplied by Astrium (Germany) will observe a number of photochemically interrelated trace gases in the middle atmoshere.
* Sciamachy, the Scanning Imaging Absorption Spectrometer for Atmosheric Cartography supplied by Astrium (Germany) in Dutch cooperation is an imaging spectrometer, scanning the atmoshere between Earth’s surface and 90 kilometers of tangential height to determine trace gases, aerosols, cloud height and coverage.
During a three-month launch campaign at the European launch centre in Kourou, an Astrium technical team of about 70 people will prepare the giant satellite for its lift-off into space.
Mathias Pikelj, [email protected]
Envisat and its Instruments
Gomos (Global Ozone Monitoring by Occultation of Stars), an instrument of Astrium SAS, ensures, with a high long-term precision, the observation of the processes in the Earth’s atmosphere which cause ozone depletion in the stratosphere. Gomos is therefore especially suited for the highly important long-term trend measurements of the ozone concentration. Seasonal and local variations as well as variations in the ozone concentration caused by specific events make trend measurements difficult and require a high measuring accuracy over a long period of time. The high long-term stability is achieved by means of a special method: The instrument tracks a star which is high above the horizon and undisturbed by the Earth’s atmosphere and continuously tracks the occultation of that star. The spectrum of the undisturbed star light is compared with the spectrum disturbed by the atmosphere. The instrument thus receives a current, self-calibrated measurement signal. Gomos enables the simultaneous measurement of ozone and other trace gases, thereby contributing to the exploration of the ozone chemistry of aerosol. In addition, temperature distributions in the stratosphere as well as the analysis of atmospheric turbulences in the altitude range between 20 and 100 kilometers will be measured.
The Medium Resolution Imaging Spectrometer (Meris), for which Alcatel, France, is the prime contractor, will primarily serve oceanographic and secondarily atmospheric and land-oriented applications. Meris measures the solar radiation reflected by the Earth in the optical and near infrared spectrum. Observations can be made in parallel in 15 freely-programmable, very close spectral ranges so that imaging of the requested individual objects can be performed in the optimal spectral range. Meris requires suitable daylight for these measurements and it provides a spatial resolution of 250 x 250 meters. The wide range of observation possibilities offered by Meris is remarkable: in addition to cloud distribution and height, this instrument also collects data on ocean contamination, algae bloom, icebergs, the discharge of pollutants into the ocean, the state of vegetation maturity or damage on land, etc. These applications are only a descriptive excerpt from the diverse observation spectrum provided by this highly interesting instrument. Meris, complemented by RA-2 and AATSR, forms a unique combination for the bio/geophysical characterization of the ocean and coastal zones and thus for climate and global environment exploration and monitoring.
The Radar Altimeter 2 (RA-2), built by Alenia Spazio, Italy, as the prime contractor, is derived from the ERS-1/2 radar altimeters providing markedly improved measurement performance and new capabilities. The main objectives of this instrument are high-precision measurements of the ocean surface and its short and long-term variations. Operating over ocean, these measurements are used to determine the ocean topography, thus supporting research of ocean circulation, sea bottom surface and marine geoid characteristics. Evaluation of the radar echo data enables the determination of wind speed and wave height in the observed sea area, thus supporting weather and sea state forecasting. RA-2 will permit the extension of measurements over oceans to land regions. These measurement data will be used for the determination of altitude changes, geological structures and surface characteristics.
The MicroWave Radiometer (MWR), for which Alenia Spazio, Italy, is the prime contractor, will measure the atmospheric integrated water content (clouds, vapor and rain). These data are required to calculate correction values for the evaluation of the RA-2 measurements by means of which RA-2 altitude values can be improved with an accuracy of a few centimeters.
A Laser Retro-Reflector (LRR) originating from Alcatel, France, will be used to support satellite ranging and RA-2 altitude measurement calibration. Using the data delivered by LRR and Doris, the spacecraft orbit can be determined to within a precision of a few centimeters.
The ESA-developed instruments are ideally complemented by three additional Announcement of Opportunity (A.O.) instruments supplied by national institutions to optimally achieve the mission objectives.
The Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (Sciamachy) is an imaging spectrometer, scanning the atmosphere between the Earth’s surface and 90 kilometers of tangential height to determine trace gases, aerosols, cloud height and coverage. Trace gases absorb sunlight in a characteristic way. The measurement of absorption lines allows Sciamachy to determine the gas type and concentration with high precision. By virtue of its special measuring principle, Sciamachy ideally complements the other atmosphere measuring instruments on-board Envisat. The Sciamachy measurements, among other things, also permit conclusions on the concentration of important trace gases in the lower atmosphere on which the impact of anthropogenic influences is particularly high. On top of this, Sciamachy is able to measure individual trace gases which, which on account of their measurement characteristics, cannot be captured by the other on-board instruments. Astrium GmbH, Friedrichshafen, in cooperation with the Netherlands has developed Sciamachy under contract to the German Space Agency DLR.
The Advanced Along Track Scanning Radiometer (AATSR) is a nadir and along track scanning visible/infrared radiometer imaging a 500-kilometer wide swath centered at the subsatellite track with a spatial resolution of 1 x 1 kilometer. The AATSR for instance is able to measure sea surface temperatures with an accuracy of 0.1 degree. The British National Space Centre is the customer for this instrument built under the management of Astrium Ltd.
The range finder Doris (Doppler Orbitography and Radiopositioning Integrated by Satellite), developed under contract to CNES, will be employed on Envisat to exactly determine the satellite’s position.
The total payload of Envisat with 2,050 kilograms of highly ambitious technology, requires about two kilowatts of electrical power, an onboard recording capacity of approximately 140 gigabits per Earth orbit — this is about five times the capacity of ERS — and powerful downlink performance. The data can be transmitted either directly to Earth or via ESA’s telecommunications satellite Artemis
Mathias Pikelj, [email protected]
Envisat and the Polar Platform
The carrier of the ten instruments of the environmental satellite Envisat is a novel satellite bus, the Polar Platform (PPF). The development of this modular platform was started in 1989 under the industrial lead of today’s Astrium. About 50 companies from twelve countries are participating in this development.
In comparison to ESA’s previous Earth observation satellites, the multitude and complexity of Envisat’s instruments require a large platform, as well as larger and more advanced subsystems and equipment for power supply, altitude and orbit control, data processing, storage and transmission to ground. In conjunction with the instruments this resulted in a spacecraft of over 8 tonnes and 10 metres in its stowed, launch configuration. For Envisat, Astrium was able to call on the wealth of experience it gained both in the design, construction and test of the SPOT satellite platforms, as well as in the ERS missions to successfully complete this challenging development.
The PPF consists of two main modules: the Service Module (SM) and the Payload Module (PLM), the latter consisting of the Payload Carrier and Payload Equipment Bay. The Payload Module accommodates the Instruments and the associated subsystems for Instrument control and monitoring, power switching, thermal control, data collection, processing and transmission to ground. These supporting subsystems are accommodated in the Payload Equipment Bay, with the Instruments and high accuracy attitude measurement sensors are accommodate on the Payload Carrier The SM contains the equipment for power generation and distribution, attitude and orbit control, thermal control and communications with the ground control station. The SM is also the interface to Ariane 5.
Many units and subsystems of the Service Module are repeat builds of those used on the SPOT series of spacecraft. It was, however, necessary to make some adaptations, such as providing accommodation of eight SPOT type batteries and four tanks holding 300 kilograms of hydrazine fuel required for a lifetime of at least five years.
The Payload Module equipments and the Service Module S-band transponder and the solar array are new developments. The Solar Array consists of 14 panels one by five meters, which generate a total of 6.6 kilowatts of electrical power, end of life. During the launch, the individual panels are folded and stowed on the spacecraft side wall Following separation from Ariane 5 the solar array stack is deployed on an arm followed by deployment of the solar array stack. The in orbit spacecraft length is then 25 metres.
The Polar Platform has a completely modular design allowing adaptations — by various sections — for other missions. Envisat is configured in four sections, each having a length of 1.60 meters. The PPF technology is also the basis for the platform of the Metop meteorological satellites, which are being built by Astrium.