Envisat
Envisat is a large inactive Earth-observing satellite which is still in orbit. Operated by the European Space Agency, it was the world's largest civilian Earth observation satellite.
It was launched on 1 March 2002 aboard an Ariane 5 from the Guyana Space Centre in Kourou, French Guiana, into a Sun synchronous polar orbit at an altitude of 790 ± 10 km. 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 cost 2.3 billion Euro to develop and launch. The mission has been replaced by the Sentinel series of satellites. The first of these, Sentinel 1, has taken over the radar duties of Envisat since its launch in 2014.
Mission
Envisat was launched as 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, agriculture and arboriculture, natural hazards, digital elevation modelling, monitoring of maritime traffic, atmospheric dispersion modelling, cartography and study of snow and ice.
Specifications
;Dimensions× × in orbit with the solar array deployed.
;Mass
, including of fuel and a instrument payload.
;Power
Solar array with a total load of 3560 W.
Instruments
Envisat carries an array of nine Earth-observation instruments that gathered information about the Earth using a variety of measurement principles. A tenth instrument, DORIS, provided guidance and control. Several of the instruments were advanced versions of instruments that were flown on the earlier ERS 1 and ERS 2 missions and other satellites.MWR
MWR was designed for measuring water vapour in the atmosphere.AATSR
AATSR 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, 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.
MIPAS
MIPAS is a Fourier transforming infrared spectrometer which provides pressure and temperature profiles, and profiles of trace gases nitrogen dioxide, nitrous oxide, methane, nitric acid, ozone, and water 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 depending on altitude.MERIS
MERIS measures the reflectance of the Earth in the solar spectral range and transmits 15 spectral bands back to the ground segment. MERIS was built at the Cannes Mandelieu Space Center.SCIAMACHY
SCIAMACHY 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 for wavelengths between 240 and 1700 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, to the limbus, and during solar or lunar eclipses. SCIAMACHY was built by Netherlands and Germany at TNO/TPD, SRON and Airbus Defence and Space Netherlands.
RA-2
RA-2 is a dual-frequency Nadir pointing Radar operating in the Ku band and S bands, it is used to define ocean topography, map/monitor sea ice and measure land heights.Mean sea level measurements from Envisat are continuously graphed at the web site, on the .
ASAR
ASAR 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.| Mode | Id | Polarisation | Incidence | Resolution | Swath |
| 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 km × 5 km | |
| Suivi global | GM | HH, VV | 1000 m | 405 km | |
| Wide Swath | WS | HH, VV | 150 m | 405 km |
These different types of raw data can be given several levels of treatment :
- RAW, which contains all the information necessary to create images.
- S, images in complex numeric form, the real and imaginary parts of the output of the compression algorithm
- P, amplified image with constant pixel width
- M, amplified radiometry image with a resolution greater than P
- G, amplified image to which simple geographical transforms have been applied to show relief.
DORIS
DORIS determines the satellite's orbit to within.GOMOS
GOMOS looks at stars as they descend through the Earth's atmosphere and change color, allowing measurement of gases such as ozone, including their vertical distribution.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, nitrogen trioxide,, ), ozone and aerosols present between about altitude. It has a resolution of.
Loss of contact
announced on 12 April 2012 that they lost contact with Envisat on Sunday, 8 April 2012, after 10 years of service, exceeding initially planned life span by 5 years. 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.Envisat was launched in 2002 and it operated five years beyond its planned mission lifetime, delivering over a petabyte of data. ESA was expecting to turn off the spacecraft in 2014.
Space safety
Envisat poses a hazard because of the risk of collisions with space debris. Given its 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 orbiting in an environment where two catalogued space debris objects can be expected to pass within about of it every year, which would likely trigger the need for a manoeuvre to avoid a possible collision. A collision between a satellite the size of Envisat and an object as small as 10 kg could produce a very large cloud of debris, initiating a self-sustaining chain-reaction of collisions and fragmentation with production of new debris, a phenomenon known as the Kessler Syndrome.Envisat is a candidate for a mission to remove it from orbit, called e.Deorbit. The spacecraft sent to bring down Envisat would itself need to have a mass of approximately 1.6 tonnes.