Model of Envisat
|Operator||European Space Agency|
|Mission type||Earth observation|
|Launch date||1 March 2002, 01:07UTC|
|Carrier rocket||Ariane 5|
|Mission duration||5 years (design)
10 years (achieved)
|Ceased operations||8 April 2012|
|Mass||8,211 kg (18,100 lb)|
|Apoapsis||791 km (492 mi)|
|Periapsis||785 km (488 mi)|
|Orbital period||100.6 min|
|Repeat interval||35 days|
Envisat ("Environmental Satellite") is an inoperative Earth-observing satellite still in orbit. It was launched on 1 March 2002 aboard an Ariane 5 from the Guyana Space Centre in Kourou, French Guyana, into a Sun synchronous polar orbit at an altitude of 790 km (490 mi) (± 10 km (6.2 mi)). It orbits the Earth in about 101 minutes with a repeat cycle of 35 days. After losing contact with the satellite on 8 April 2012, ESA formally announced the end of Envisat's mission on 9 May 2012.
Envisat was an Earth observation satellite. Its objective was to service the continuity of European Remote-Sensing Satellite missions, providing additional observational parameters to improve environmental studies.
In working towards the global and regional objectives of the mission, numerous scientific disciplines currently use the data acquired from the different sensors on the satellite, to study such things as atmospheric chemistry, ozone depletion, biological oceanography, ocean temperature and colour, wind waves, hydrology (humidity, floods), agriculture and arboriculture, natural hazards, digital elevation modelling (using interferometry), monitoring of maritime traffic, atmospheric dispersion modelling (pollution), cartography and study of snow and ice.
Envisat cost €2.3 billion (including €300 million for 5 years operations) to develop and launch. The mission is due to be replaced by the Sentinel series of satellites. The first of these – Sentinel 1 – is supposed to take over the radar duties of Envisat when it is launched in 2013.
26 m (85 ft) × 10 m (33 ft) × 5 m (16 ft)
8.211 metric tons, including 319 kilograms (700 lb) of fuel and a 2,118 kilograms (4,670 lb) instrument payload.
Envisat carried an array of nine Earth-observation instruments that gathered information about the Earth (land, water, ice, and atmosphere) using a variety of measurement principles. A tenth instrument, DORIS, provided guidance and control. Several of the instruments are advanced versions of instruments that were flown on the earlier ERS 1 and ERS 2 missions and other satellites.
MERIS (MEdium Resolution Imaging Spectrometer) measures the reflectance of the Earth (surface and atmosphere) in the solar spectral range (390 to 1040 nm) and transmits 15 spectral bands back to the ground segment. MERIS was built at the Cannes Mandelieu Space Center.
AATSR (Advanced Along Track Scanning Radiometer) can measure the sea surface temperature in the visible and infrared spectra. Because of its wide angle lens it is possible to make very precise measurements of atmospheric effects on how emissions from the Earth's surface propagate.
AATSR is the successor of ATSR1 and ATSR2, payloads of ERS 1 and ERS 2. AATSR can measure Earth's surface temperature to a precision of 0.3 K (0.54 °F), for climate research. Among the secondary objectives of AATSR is the observation of environmental parameters such as water content, biomass, and vegetal health and growth.
SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) compares light coming from the sun to light reflected by the Earth, which provides information on the atmosphere through which the Earth-reflected light has passed.
SCIAMACHY is an image spectrometer with the principal objective of mapping the concentration of trace gases and aerosols in the troposphere and stratosphere. Rays of sunlight that are reflected transmitted, backscattered and reflected by the atmosphere are captured at a high spectral resolution (0.2 to 0.5 nm) for wavelengths between 240 to 1,700 nm, and in certain spectra between 2,000 and 2,400 nm. Its high spectral resolution over a wide range of wavelengths can detect many trace gases even in tiny concentrations. The wavelengths captured also allow effective detection of aerosols and clouds. SCIAMACHY uses 3 different targeting modes: to the nadir (against the sun), to the limbus (through the atmospheric corona), and during solar or lunar eclipses.
One interesting result is the low rate of global mean sea level rise measured over the first eight years of the mission: just 0.5 mm/year, which is about 1/4 the rate of GMSL rise measured over the same period by the Jason-1 satellite. Mean sea level measurements from Envisat are continuously graphed at the Centre National d'Etudes Spatiales web site, on the Aviso page.
DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) determines the satellite's orbit to within 10 centimetres (4 in).
GOMOS (Global Ozone Monitoring by Occultation of Stars) looks to stars as they descend through the Earth's atmosphere and change color, which also tells a lot about the presence of gases such as ozone (O3), and allows for the first time a space-based measurement of the vertical distribution of these trace gases.
GOMOS uses the principle of occultation. Its sensors detect light from a star traversing the Earth's atmosphere and measures the depletion of that light by trace gases nitrogen dioxide (NO2), nitrogen trioxide, (NO3), OClO), ozone (O3) and aerosols present between about 20 to 80 km (12 to 50 mi) altitude. It has a resolution of 3 km (1.9 mi).
MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is a Fourier transforming infrared spectrometer which provides pressure and temperature profiles, and profiles of trace gases nitrogen dioxide (NO2), nitrous oxide (N2O), methane (CH4), nitric acid (HNO3), ozone (O3), and water (H2O) in the stratosphere. The instrument functions with high spectral resolution in an extended spectral band, which allows coverage across the Earth in all seasons and at equal quality night and day. MIPAS has a vertical resolution of 3 to 5 kilometres (2 to 3 mi) depending on altitude (the larger at the level of the upper stratosphere).
ASAR (Advanced Synthetic Aperture Radar) operates in the C band in a wide variety of modes. It can detect changes in surface heights with sub-millimeter precision. It served as a data link for ERS 1 and ERS 2, providing numerous functions such as observations of different polarities of light or combining different polarities, angles of incidence and spatial resolutions.
|Alternating polarisation||AP||HH/VV, HH/HV, VV/VH||15 – 45°||30 – 150 m||58 – 110 km|
|Image||IM||HH, VV||15 – 45°||30 – 150 m||58 – 110 km|
|Wave||WV||HH, VV||400 m||5 × 5 km|
|Suivi global (ScanSAR)||GM||HH, VV||1 km||405 km|
|Wide Swath (ScanSAR)||WS||HH, VV||150 m||405 km|
These different types of raw data can be given several levels of treatment (suffixed to the ID of the acquisition mode: IMP, APS, and so on):
- RAW (raw data, or "Level 0"), which contains all the information necessary to create images.
- S (complex data, "Single Look Complex"), images in complex numeric form, the real and imaginary parts of the output of the compression algorithm
- P (precision image), amplified image with constant pixel width (12.5 m for IMP)
- M (medium precision image), amplified radiometry image with a resolution greater than P
- G (geocoded image), amplified image to which simple geographical transforms have been applied to show relief.
Data capture in WV mode is unusual in that they constitute a series of 5 km × 5 km spaced at 100 km.
Loss of contact 
ESA announced on 12 April 2012 that they lost contact with Envisat since Sunday, 8 April 2012. The spacecraft was still in a stable orbit, but attempts to contact it were unsuccessful. Ground-based radar and the French Pleiades Earth probe were used to image the silent Envisat and look for damage. ESA formally announced the end of Envisat's mission on 9 May 2012.
Space safety 
Given Envisat's orbit and its area-to-mass ratio, it will take about 150 years for the satellite to be gradually pulled into the Earth’s atmosphere. Envisat is currently operating in an environment where 2 catalogued objects can be expected to pass within about 200 metres (660 ft) of Envisat every year, which would likely trigger the need for a maneuver to avoid a possible collision. A drifting satellite as big as Envisat is more than just a huge space wreck: in case of collision, given its mass, volume and shape, it might generate a cloud of smaller debris large enough to populate the orbit, initiating a self-sustaining chain-reaction of collisions and fragmentation with production of new debris. This phenomenon, known as the Kessler Syndrome, would eventually make space operations difficult or even impossible, and may prevent access to space to future human generations.
- "ESA declares end of mission for Envisat". ESA. 9 May 2012.
- Flagship Envisat satellite stops communicating
- EarthNet Online
- European Space Agency web-site
- Envisat – Overall configuration
- "Breaking News | Flagship Envisat satellite stops communicating". Spaceflight Now. Retrieved 2012-04-21.
- "ESA Portal – Envisat services interrupted". Esa.int. Retrieved 2012-04-21.
- Huge, Mysteriously Silent Satellite Spotted by Another Spacecraft Space.com article, 20 April 2012
- Contact lost with flagship Envisat spacecraft 12 April 2012.
- "Envisat To Pose Big Orbital Debris Threat for 150 Years, Experts Say". Space News. 23 July, 2010. Retrieved 2012-05-09.
- "Don Kessler on Envisat and the Kessler Syndrome". Space Safety Magazine. 25 April 2012. Retrieved 2012-05-09.
- Envisat-homepage at ESA
- Envisat operations page at ESA
- Miravi – Meris Image Rapid Visualization. MIRAVI shows the gallery of images generated on the Level0 (raw data) Meris Full Resolution (300m) products, few seconds after their availability.
- SRRS – Satellite Rapid Response System. Like MIRAVI but including also ASAR, MERIS Full and Reduced Resolution and ALOS AVNIR2 images.
- Earth Snapshot – Web Portal dedicated to Earth Observation. Includes commented satellite images, information on storms, hurricanes, fires and meteorological phenomena.
- Next ESA SAR Toolbox for viewing, calibrating and analyzing Envisat ASAR Level 1 data and higher
- Four years on, Envisat hailed for its contribution to Earth science Physorg.com (2006-02-28)
- ESA Envisat DDS (Data Dissemination System)
- gcs Global Communication & Services – Manufacturer of the Envisat DDS Commercial Receiver Kit for ENVIHAM Home Users
- Dossier Envisat: 10 years of climate and environmental research at Astrium