Envisat

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Envisat
Envisatmod.jpg
Model of Envisat
Mission type Earth observation
Operator European Space Agency
COSPAR ID 2002-009A
SATCAT № 27386
Website envisat.esa.int
Mission duration 5 years planned
10 years achieved
Spacecraft properties
Launch mass 8,211 kg (18,102 lb)
Start of mission
Launch date 1 March 2002, 01:07:59 (2002-03-01UTC01:07:59Z) UTC
Rocket Ariane 5G V145
Launch site Kourou ELA-3
Contractor Arianespace
End of mission
Last contact 8 April 2012 (2012-04-09)
Orbital parameters
Reference system Geocentric
Regime Polar low Earth
Perigee 772 km (480 mi)
Apogee 774 km (481 mi)
Inclination 98.40 degrees
Period 100.16 minutes
Repeat interval 35 days
Epoch 15 December 2013, 03:07:00 UTC[1]
Instruments
ASAR, RA-2, MWR, MIPAS, MERIS, AATSR, DORIS, GOMOS, SCIAMACHY

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 Guiana, 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.[2]

Envisat is the European Space Agency's (ESA) largest civilian Earth observation satellite put into space.[3][4]

Mission[edit]

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.[5] The mission is due to be 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.

Specifications[edit]

Dimensions

26 m (85 ft) × 10 m (33 ft) × 5 m (16 ft)

Mass

8,211 kg (18,102 lb), including 319 kg (703 lb) of fuel and a 2,118 kg (4,669 lb) instrument payload.[6]

Power

Solar array with a total load of 3560 W.

Instruments[edit]

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[edit]

Main article: MERIS

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[edit]

Main article: AATSR

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[edit]

Main article: SCIAMACHY

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.

RA-2[edit]

RA-2 (Radar Altimeter 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.

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.

MWR[edit]

MWR (Microwave Radiometer) was designed for measuring water vapour in the atmosphere.

DORIS[edit]

Main article: DORIS (geodesy)

DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) determines the satellite's orbit to within 10 cm (4 in).

GOMOS[edit]

Main article: GOMOS

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 (O
3
), 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 (NO
2
), nitrogen trioxide, (NO
3
), OClO), ozone (O
3
) 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[edit]

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 (NO
2
), nitrous oxide (N
2
O
), methane (CH
4
), nitric acid (HNO
3
), ozone (O
3
), and water (H
2
O
) 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 km (2 to 3 mi) depending on altitude (the larger at the level of the upper stratosphere).

ASAR[edit]

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.

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 × 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[edit]

ESA announced on 12 April 2012 that they lost contact with Envisat since 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.[7][8] Ground-based radar and the French Pleiades Earth probe were used to image the silent Envisat and look for damage.[9] ESA formally announced the end of Envisat's mission on 9 May 2012.[2]

Envisat was launched in 2002 and it operated five years beyond its planned mission lifetime, delivering over a petabyte of data.[2] ESA was expecting to turn off the spacecraft in 2014. [10]

Space safety[edit]

Space debris populations seen from outside geosynchronous orbit (GEO). Note the two primary debris fields, the ring of objects in GEO, and the cloud of objects in low earth orbit (LEO).

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.[11] Envisat is currently operating in an environment where 2 catalogued objects can be expected to pass within about 200 m (660 ft) of Envisat every year, which would likely trigger the need for a maneuver to avoid a possible collision.[12] 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.[12]

Envisat is a candidate for a mission to remove it from orbit. The spacecraft sent to bring down Envisat would itself need to have a mass of approximately 2 tonnes.[13]

Further information: Space debris

See also[edit]

References[edit]

  1. ^ "ENVISAT Satellite details 2002-009A NORAD 27386". N2YO. 15 December 2013. Retrieved 15 December 2013. 
  2. ^ a b c "ESA declares end of mission for Envisat". ESA. 9 May 2012. 
  3. ^ Flagship Envisat satellite stops communicating
  4. ^ EarthNet Online
  5. ^ European Space Agency web-site
  6. ^ Envisat – Overall configuration
  7. ^ "Breaking News | Flagship Envisat satellite stops communicating". Spaceflight Now. Retrieved 2012-04-21. 
  8. ^ "ESA Portal – Envisat services interrupted". Esa.int. Retrieved 2012-04-21. 
  9. ^ Huge, Mysteriously Silent Satellite Spotted by Another Spacecraft Space.com article, 20 April 2012
  10. ^ Contact lost with flagship Envisat spacecraft 12 April 2012.
  11. ^ "Envisat To Pose Big Orbital Debris Threat for 150 Years, Experts Say". Space News. 23 July 2010. Retrieved 2012-05-09.  |first1= missing |last1= in Authors list (help)
  12. ^ a b "Don Kessler on Envisat and the Kessler Syndrome". Space Safety Magazine. 25 April 2012. Retrieved 2012-05-09.  |first1= missing |last1= in Authors list (help)
  13. ^ Amos, Jonathan (2013-04-22). "'Urgent need' to remove space debris". BBC News Online. Retrieved 2014-04-12. 

External links[edit]