Envisat FAQs

Envisat in a nutshell

Type Earth Observation ENVIronmental SATellite
Purpose To gather data to study and monitor the Earth and its environment. Envisat is a continuation and enhancement of the ERS missions.

An Advanced Synthetic Aperture Radar (ASAR) capable of imaging the Earth day and night

A Radar Altimeter (RA-2) primarily for topography of the ocean, ice and land surfaces with three complementary instruments for corrections and precise orbit provisions (MWR, DORIS and LLR)

Two imaging optical instruments (MERIS and AATSR) for analysis of the ocean and coastal water, marine biology, monitoring and precise surface temperature determination, both instruments also providing land vegetation monitoring

Three complementary instruments (MIPAS, GOMOS and SCIAMACHY) for the detailed observation of the atmospheric layers and the density of the gas traces in these layer


Launch mass


Deployed in orbit
Launch configuration (main body)

8.2 tons of which 2.2 tons of instruments

6.6 kW (end of life) of which 2.0 kW instruments

25m x 10m x7m
10m x 4m x 4m

Lifetime At least 5 years in orbit
Orbit Near polar sun-synchronous with a mean altitude of 800 km and a 35 day repeat cycle.

On board recording capability: 160 Gbits.

Ground links t 100 Mbit/sec direct to stations or relayed via ESA’s ARTEMIS satellite.

Launch Night between 28 February and 1 March 2002, by Ariane 5 from Europe’s spaceport in Kourou, French Guiana
Prime contracts

Satellite and polar platform: ASTRIUM Ltd (UK)

Instruments: ASTRIUM GmBH (D)

Payload data segment: ALCATEL (F)

Financial data Development programme, including launch 2000 MEuro
Industrial contracts for the space segment 1450 MEuro
Ariane 5 launch and services 140 MEuro
Ground segment development (flight operations and payload data segment) 160 MEuro
ESA internal costs (incl. testing) 250 MEuro
Operation (5 years), 60 Meuro/year 300 MEuro
Three instruments are financed nationally and provided in kind AATSR (UK), DORIS (F) and SCIAMACHY (D/NL)
Instruments ASAR Advanced Synthetic Aperture Radar
MERIS Medium Resolution Imaging Spectrometer
AATSR Advanced Along Track Scanning Radiometer
RA-2 Radar Altimeter 2
MWR Microwave Radiometer
DORIS Doppler Orbitography and Radioposit. Integrated by Satellite
LLR Laser Retro Reflector
MIPAS Michelson Interferometer for Passive Atmospheric
GOMOS Global Ozone Monitoring by Occultation of Stars
SCIAMACHY Scanning Imaging Absorption Spectrometer for Atmospheric Cartography

1. What is Envisat?

Envisat is the most powerful tool ever produced for observing the Earth and monitoring critical aspects of the environment – in particular those linked to global warming. It places Europe at the leading edge of environmental monitoring and will help both scientists and politicians to better understand and manage the earth’s climate, environment and resources.

It is, in effect, an orbiting observatory containing 10 instruments which between them will be able to monitor simultaneously what is happening on the land surface, in the oceans, in the atmosphere and to ice and snow coverage. 2. What makes it so important?

Envisat will be a comprehensive space observatory for measuring many different environmental parameters and their interaction. Among others, Envisat will :

  • Serve as a fundamental tool in monitoring global change and in the battle against global warming
  • Help us understand the hydrocarbon cycle and to what extent man’s activities are threatening the balance
  • operate as global watchdog on the activities of environmental polluters across the world.

In this role it will provide a continuation and enhancement of the data that has already been gathered by other satellites, including ESA’s ERS 1 and 2 satellites, and complement measurements taken through ground studies. This is important because much of the research into the environment depends on having continuous measurements over a period of years.

3. What specific benefits is it going to offer?

Envisat has three main unique benefits.

  • It is multi-dimensional – by combining data from different instruments Envisat can generate synergies and measure and study things not possible before.
  • It is accessible – more data will be available more quickly and more easily, using the world wide web, than has ever been possible before.
  • It offers 3 hours data delivery service for all data acquired all over the globe.

4. What are the major issues that Envisat will shed new light on?

  • Measurement of many different parameters, all affecting our global climate, its changes and the natural and anthropogenic parameters triggering such climate changes.
  • Estimate of the global content of Carbon dioxide and how much the forests and the ocean can absorb.
  • How badly damaged is the Ozone layer? And what are the impacts of the chemicals that we are now using to replace CFCs?
  • The rise in sea level - Envisat will improve our understanding of the causes and the impacts.
  • The colour of the ocean. This will give us a clearer understanding of how much carbon can be absorbed into the seas and permit reassessment of the oceans' health and condition.
  • The role of clouds and aerosols has not been investigated yet but scientists believe that they can have a high impact on the climate and the climate change.
  • Natural disasters - Earthquakes, floods and forest fires can be monitored. We will have greater clarity on the frequency of these events.

5. Why is it so “powerful”?

Envisat has 10 separate instruments on board. Most of them individually provide more accurate measurements than all of their predecessors. But more than this, these instruments can operate simultaneously and provide a unique possibility to combine their assets, providing more concentrated “monitoring firepower” than has ever been focussed on a single point before.

By combining data from different instruments Envisat can generate synergies and measure parameters at all layers of our planet, ranging from sea bottom topography, through sea surface temperature, ocean colour, sea surface height, land surface features, polar ice caps, up to trace gasses in the atmosphere.

N.B. the value of the measurements provided by Envisat is more than the sum of the individual readings, were they not working in synergy. For example, El Nino manifests itself through a change in the Sea Surface height (RA), the sea surface temperature (ATSR) and the ocean colour (MERIS). Only Envisat can combine this information.

6. What is it going to do that is different from its predecessors?

Apart from the fact that the instruments will provide more accurate measurement of individual geophysical parameters, its major difference is the fact that it has many more instruments on board than its predecessors. Some of these instruments are new and others are enhancements of instruments that were already on previous satellites. Because it has such a wide range of instruments, the spacecraft will be able to generate a multi-dimensional picture, enabling scientists to link various environmental factors and study in greater detail than has been possible before the interaction between the oceans, the earth and the atmosphere.

Envisat has the unique capability of observing the interactions between the atmosphere, oceans and land simultaneously. It will enable better understanding of the Earth system processes.

7. Are there instruments on board which are unique and can perform tasks that no other earth observation satellite can?

All instruments are unique in the sense that they will measure geophysical parameters from space at an unprecedented accuracy, especially after the extensive calibration and validation efforts.

For example:

  • The ASAR radar instrument is the first to have dual polarisation – something which gives much greater definition of surfaces than was possible before allowing the satellite to “see” things such as different types of ice.
  • The MERIS instrument can measure very precisely the colour of the oceans – allowing, amongst other things, the measurement of quantities of phytoplankton which are a major carbon sink and key factor in the equations of carbon dioxide emission and absorption.
  • By combining the data from the SCIAMACHY and MIPAS instruments, we can create a global, three-dimensional map of greenhouse gases in the atmosphere.
  • Through a combination of the ATSR sea surface temperature, the Radar Altimeter SSH, we will be able to monitor the effects of El Nino, for example.

8. Give me a list of the sort of unique results that can be expected from Envisat.

  • The most comprehensive modelling of the Earth’s atmosphere.
  • The most accurate picture of sea surface temperatures.
  • Measuring movement of the ground surface to a sensitivity of 1mm per year.
  • Measuring every three days the exact amount of forest coverage on the planet.
  • Measuring the heat transport effect of the ocean currents – the principle determinant of climate.
  • Calculating the exact amount of ice on the surface of the planet.
  • The most accurate map of ozone in the atmosphere.
  • The fastest available and most comprehensive data on earth observation.

9. How is Envisat’s data going to be used?

One of the unique features of Envisat is our ability to make the data from it available on-line in almost real time. This is a service which the scientific community has not had to date. We believe that by effectively making the provision of data a recognised service, this will transform both the scientific and commercial use of Earth observation data.

10. Are there other EO satellite’s, for example from NASA, which are similar to Envisat?

Envisat is unique. There is no other satellite which has such a wide ranging and flexible environment monitoring capability.

Obviously, other space agencies have programmes to launch and operate individual instruments similar to some of Envisat’s instruments (e.g. Modis, flown on a NASA satellite, is similar to Meris).

11. How much did it cost?

Envisat cost 2.3 BEuro (including 300 MEuro for 5 years operations) to develop and launch (launch price tag: 140 MEuro). This is equivalent to 7 Euro per head of population across all the ESA member states, or about one cup of coffee per year spread over its 15 year lifecycle.

12. Why is this something taxpayers' money should have been spent on?

Environmental change is no longer a remote concept. Even here in Europe, earthquakes and volcanic eruptions are no longer natural disasters faced in any developing countries. We are ourselves experiencing, almost daily, the impact of global change manifested through floods, landslides, air pollution in our towns and cities.

In recent years, these phenomena have taken a heavy toll, both in terms of human life and in a purely economic sense.

Whether such change and the resultant natural disasters are a consequence of human activity or a natural evolution, it is in our direct interest to fully understand such change. Only then will we be able to take the right decisions to minimise the impact of natural disasters or avoid them, as far as possible.

13. Are there risks and difficulties from putting all these instruments on one satellite?

The project was certainly a challenging one because there has never been a satellite with so many instruments on it – requiring great technical expertise to ensure they all work together. There are also always risks involved in sending anything into space, but we are confident that all the technical issues have been solved and everything has been very extensively tested.

14. What if it crashes on launch or if some critical aspect of the satellite fails?

We have done everything possible to minimise the risks as far as the satellite’s performance is concerned. As for the launch – we don’t believe there is a safer launch vehicle than the Ariane 5.

We will in any case continue all our efforts to provide environmental data from existing satellites, both from ESA (ERS) and its partners. And we will make every effort to anticipate future missions to ensure continuity.

15. The first Ariane 5 crashed – and there have not been many launches since. That means you are hardly using a tried and tested launch vehicle for such a precious cargo.

The first launch of Ariane 5 was effectively a test launch. The problems that led to the failure have been identified and there have been no similar problems with any of the subsequent launches.

16. Is the satellite insured?

The satellite is insured for an amount of 250 MEuro that would come in addition to the 250 MEuro set aside for the operations. Arianespace has guaranteed a free launch in case of failure.

17. Why is it such a big satellite, given that the earth observation satellites now being developed are smaller, cheaper and task specific?

It is the most cost effective solution to meeting the need, identified by the scientific community, of putting a large number of new instruments into a polar orbit. Also having all these instruments on one platform bring synergies and allows greater insights into the forces that shape the environment.

Because of this, Envisat will open up many new avenues of scientific investigation which can then be explored in more detail with smaller, mission specific satellites.

18. Are there commercial applications or benefits that follow on from this project?

Envisat is primarily a scientific satellite and it’s main objective is to gather data for scientific purposes. However, there are some commercial applications derived from this data, such as meteorology, marine navigation, exploration and mapping. For this reason we have appointed two commercial organisations (EMMA and SARCOM) to look after selling this data to those organisations who want it – and this will be aided by the fact that we have a very sophisticated data processing centre which can make data products available on-line in almost real time.

19. How is the data transmitted to Earth?

  • Envisat has a bigger on board recording provision than any of its predecessors. There are two solid state recorders, each capable of holding 70 Gigabits of data, and one tape recorder back up with 30 Gigabits capacity. This will allow in parallel:
  • Continuous recording of the global mission with data dump every orbit,
  • Recording of the high rate data of ASAR and MERIS in full resolution over the zones requested by the users.
  • Transmission via the ARTEMIS satellite, which will operate as a data relay satellite from a geostationary orbit position. It will be used to relay directly to the receiving station, located at ESA/ESRIN in Frascati (near Rome), the data recorded on board or taken in real time during operation of the relay link. With the use of ARTEMIS in complement to the Kiruna station direct reception, satellite data recovery can be ensured every 100 minute orbit and the data products made available to users in less than 3 hours from data take.

20. How will the Envisat data be distributed?

  • The decentralized Envisat Ground Segment is managed via the ESA centre ESRIN in Frascati, near Rome.
  • During the commissioning phase which is planned to last until six months after launch the data will be acquired at the ESA station in Kiruna only, and be distributed only to the Calibration /validation teams worldwide (these are 104 scientific teams). Following the end of the commissioning phase the data will be distributed according to the data policy defined by the ESA Member States. It foresees the distribution of the data to scientific users, selected through an announcement of opportunity system. 673 PI (Principal investigators) have been selected for Envisat. These users obtain the data at reproduction cost. Commercial users constitute the second user community and these will be served by the distributing entities, which have been selected by ESA, EMMA and SARCOM.
  • Dedicated national facilities (Processing and Archiving Centre, PAC) have been set-up in Europe: UK-PAC operated by former NRSC, now INFOTERRA, located in Farnborough.
    D-PAC operated by DLR, located at Oberpfaffenhofen- Germany.
    I-PAC operated by ASI, located in Matera- Italy.
    E-PAC operated by INTA, located at Maspalomas- Spain.
    F-PAC operated by CNES, located in Toulouse- France.
    S-PAC operated by SSC, located within the Kiruna-Salmijarvi station-Sweden.

21. Will Envisat continue former Earth observation measurements?

Yes, the research instruments data can be assimilated into existing models. Some scientists point out that many of the instruments on board Envisat are experimental in nature. But according to Prof. Hartmut Grassl, former director of the world climate research programme, the ESA way of working takes full advantage of these instruments. He says that “ESA anticipate operational usage from an experimental instrument. It puts scientists under pressure to make the instrument data work but it gives you an enormous head start if it does”. He quotes the example of the MERIS instrument. “With MERIS, we have product water vapour column content, so the entire amount of water vapour in the atmosphere can be measured over the sea. And this will be used by the European Centre for Medium Range Weather Forecasting, to assimilate this data into starting fields of the forecast - if it works it will have a major impact on weather forecasting in a very practical way. You will know whether it’s going to be light rain or a downpour. You will be able to decide if you need to bring an umbrella.”

Many of the instruments on board Envisat are experimental. But thanks to ESA’s unique way of working, the information from these instruments will become operational as the data is proven to be accurate. This relates particularly to the GOMOS, MIPAS and SCIAMACHY instruments. GOMOS for example will test for Ozone by using an occultation technique. MIPAS and SCIAMACHY will use limb sounding techniques which are quite innovative in space borne application. The novelty of the instruments is matched by the new data products they will supply. In particular the profiles of trace gas concentrations specifically by height, will be of greater use to atmospheric scientists.

22. How can these amounts of data be distributed internationally?

Using the two ESA stations at Kiruna, for direct data reception, and ESRIN, for data reception via ARTEMIS, all satellite acquired data will be recovered within 100 minutes and these two stations will generate Fast Delivery products available to users within 3 hours from data take. All data will be transferred to our European Processing and Archiving Centres (PACs), where data archives will be maintained and offline data products will be generated.

Both FD products from the two ESA stations, and off-line products from PACs, will be made available to the international user community by:

  • Direct Internet data access
  • Use of a 24 hours/7days a week data dissemination at 2 Mbits per second on an EUTELSAT satellite channel to be received by DVB stations (Data Video Broadcast receive stations, like TV receive antennas, hooked to a PC. 25 such receive Envisat stations are already installed at user sites supporting the Envisat calibration validation activities.
  • On physical media such as CD-ROM and later, DVD for offline distribution.

23. What is the orbit of Envisat?

The orbit is sun-synchronous at a mean altitude of 799.8 km with a repeat cycle of 35 days at the same location over the Earth. For most sensors, being wide swath, it provides a complete coverage of the globe within three days. The exceptions are the profiling instruments MWR and RA2 (Radar Altimeter), which do not provide real global coverage but span a tight grid of measurements over the globe. The grid is the same 35-day repeat pattern as for ERS-1 and ERS-2.

The maximum deviation of the orbit should be +/- 1 km from the reference defined ground track.

24. The commissioning phase

All instruments, with the exception of the ASAR imaging modes and of the MERIS full-resolution mode, are operated as part of the global mission.

Time since Launch (days) Phase Description
L-1d to L+1d LEOP
  • Countdown & launch by Ariane 5 from Kourou
  • Separation, release and deployment of solar array and release of reaction wheels and solar array drive mechanism locks
  • Acquisition of the earth pointing attitude and start of solar array rotation
  • Switch to Fine Pointing Mode (FPM) using reaction wheels
L+2d LEOP Orbit Control Manoeuvres to achieve drift orbit
L+3d LEOP Release of ASAR antenna and deployment of inner panel stacks
L+4d LEOP Deployment of ASAR antenna outer panels
L+4d to L+11d SODAP
  • PEB switch-on and start of check-out
  • Deployment of the KBS antenna (L+5d)
  • Instrument switch-on & start of functional check-out
  • AOCS activities (switching to the operational mode using star trackers, etc)
L+7d to L+49d SODAP Staggered switch on and check out of instruments
L+ 20d onwards CAL/VAL Start of Instrument Calibration Phase

25. Are there follow on missions planned?

The missions following Envisat will be more tailored to respond to a particular scientific subject or application need.

  • The Earth Explorer missions, such as CRYOSAT, GOCE, AEOLUS and SMOS, to develop the necessary experience for future Earth observation operational missions.
  • The Earth Watch missions which are operational missions, such as METOP and METEOSAT Second Generation, in partnership with operational entities such as EUMETSAT. Or the Radar and optical high resolution missions under discussion to support the GMES (Global Monitoring fro Environment and Security) initiative with the EU.

Last update: 11 March 2002
