Envisat

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Envisat
Type: Environmental satellite
Operator: European space agencyESA ESA
COSPAR-ID : 2002-009A
Mission dates
Dimensions: 8211 kg (at start)
Size: 25 × 10 × 7 m
Begin: March 1, 2002, 01:07 UTC
Starting place: ELA-3 , Center Spatial Guyanais
Launcher: Ariane 5G V145
Status: Failure on April 8, 2012
Orbit data
Rotation time : 100.2 min
Orbit inclination : 98.6 °
Apogee height 767 km
Perigee height 766 km

Envisat ( Envi ronmental Sat ellite ) is an approximately eight tonne environmental satellite of the European Space Agency (ESA), which was launched in 2002 and worked until 2012. Its most important tasks were the constant monitoring of the climate , the ocean , the land area and in general the earth's ecosystem . With a total cost of 2.3 billion euros, it was ESA's most expensive satellite to date and the largest earth observation satellite ever flown.

The development and construction of the satellite, which took more than ten years to complete, involved almost a hundred companies from fourteen countries, including Astrium locations in the UK, Germany and France. Astrium UK was the prime contractor for the Polar Platform and two of the main instruments; Astrium Germany, as Mission Prime, was responsible for the instruments, built two of them and delivered the electronics payload bay PEB to the polar platform. Astrium France supplied the service module and other tools.

Mission history

Envisat took off on March 1, 2002 from the European spaceport Kourou in French Guiana aboard an Ariane 5 rocket. With a weight of 8050 kg (including 300 kg of fuel for rail maneuvers), it was the heaviest payload for the Ariane to date.

After the successful launch, Envisat was placed in a polar sun-synchronous orbit at an altitude of 800 kilometers in a frozen orbit . With an orbit inclination of 98 °, ENVISAT flew over every location every 35 days. The satellite was controlled by the European Satellite Control Center ESOC in Darmstadt.

On April 8, 2012 - after more than double the originally estimated lifespan of five years - the satellite failed. Routine contact between the ground station in Kiruna and Envisat did not take place, and all subsequent contact attempts also failed. Radar observations from the ground showed that the satellite as a whole was still intact and in stable alignment with the sun and earth. However, the location of the satellite did not meet the expectations of the operators. On May 9, 2012, ESA announced the formal end of the mission. The reason for this is a disruption in communication, so that it is no longer possible to communicate with the satellite. So far, however, it is not clear how the disruption came about. A power regulator failure that would have blocked telemetry and remote control would be possible. Another possibility is a short circuit and a subsequent breakdown when automatically initiating the so-called "safe mode".

Given Envisat's orbit and its area-to-mass ratio, it will take approximately 150 years for the satellite to gradually crash through Earth's atmosphere. Envisat is currently in an environment in which approx. 2 encounters with cataloged objects within 200 m are expected within one year. A collision between a satellite the size of Envisat and an object weighing more than 10 kg could create a very large cloud of debris, which could trigger a self-sustaining chain reaction of collisions and fragmentation, a process known as Kessler syndrome .

Mission objectives

Envisat, model (for size comparison see work platform and person on the left solar collector).

Envisat started as a follow-up project for the ERS-1 and ERS-2 satellites , which had taken on similar tasks in a smaller version in the 1990s.

There are ten highly developed earth observation instruments on board. They were able to measure the chemical composition of the atmosphere, the temperature of the oceans, wave heights and directions, wind speeds, growth phases of plants and detect forest fires and environmental pollution.

Envisat was originally supposed to be in service until 2007. Due to the reliable work and the insightful data with a volume of around 280 gigabytes per day, the mission was initially continued until 2010. An extension of the mission beyond 2010 was technically possible and was decided by the member states in good time until 2013. As the fuel on board for positional maneuvers ran out, the orbit altitude of the satellite was lowered.

In addition to achieving research goals, the satellite was also used for the International Charter for Space and Natural Disasters .

Instruments

The redundancy of the systems made it possible to compare the various measurement results with one another. In addition, the measuring methods of the instruments differed, which means that the satellite's field of application was very diverse. With its enormous size and mass, ENVISAT will look for its equal for the foreseeable future. A high number of instruments and the capital tied up with them entails high financial risks in the event of a false start. For this reason, smaller satellites, which only specialize in one mission goal, are in use and in development.

X and Ka band

The X-band and Ka-band antennas were used for communication with the ground stations and with Artemis, respectively .

RA-2 (Radar Altimeter 2)

The RA-2 (Radar Altimeter 2) is an altimeter that emits pulsed radar beams via its antenna and receives their echo again. The altitude of the satellite could be determined from the transit time measurement of the reflection signal. A height profile of the earth could be created by comparing it with the data from the orbit. In addition, the characteristics of the echo allowed statements to be made about the surface properties, be it water, land or ice.

MWR (MicroWave Radiometer)

The main task of the microwave radiometer MWR (Microwave Radiometer) was the measurement of the humidity of the atmosphere . These values ​​are important for the evaluation of the RA-2 instrument, since its measurement results were strongly influenced by humidity in the atmosphere.

AATSR (Advanced Along-Track Scanning Radiometer)

AATSR (Advanced Along-Track Scanning Radiometer) is an instrument for measuring the surface temperature of the sea with an accuracy of 0.3 ° C. His results can be used over land areas to interpret the vegetation in the 500 km wide field of vision. Envisat has another instrument that was used to determine altitude, called DORIS ( Doppler Orbitography and Radiopositioning Integrated by Satellite ). However, this system was also able to carry out an exact orbit and position determination, which also includes the satellite speed. This was done by comparing two received signals. These signals are broadcast from a network of over 50 broadcasting stations all over the world.

LRR (Laser RetroReflector)

The LRR (Laser RetroReflector) is only a reflector that reflects a laser beam sent by the ground station and reflects it back exactly. This in turn enables the amount of ENVISAT to be calculated by measuring the runtime.

MERIS (Medium Resolution Imaging Specrometer)

The MERIS (Medium Resolution Imaging Specrometer) recorded images of a measuring strip 1150 km wide with the help of a CCD array. The recorded rays are exclusively due to the reflection of sunlight on the earth's surface, which is why no measurements in the eclipse (when Envisat moves through the earth's shadow) were possible. The recorded spectral range of MERIS was between 390 nm and 1040 nm, i.e. it covered the entire visible range and near IR. The spectroscopy of the image provides information about the content of chlorophyll and suspended particles as well as the properties of any sediment layers in the oceans. The investigation of the atmosphere with MERIS is of secondary importance, since other instruments are more suitable for this.

GOMOS (Global Ozone Monitoring by Occultation of Stars)

The composition of the atmosphere was investigated by GOMOS (Global Ozone Monitoring by Occultation of Stars). Stars of sufficient luminosity were sighted with a mirror just before they disappeared behind the horizon, but before the view of the star was obscured by the atmosphere. A spectrum is created from this image, which will later serve as a basis for comparison. If Envisat followed his career, the star slowly disappears behind the earth. Before it has completely disappeared, however, its light is dampened by the various layers of the earth's envelope. This is also reflected in the spectra that were created in this phase. Based on the element-specific absorption of different wavelengths, their concentration can be determined from this. The main focus here is on observing the distribution of ozone, hydrocarbons and nitrogen oxides. This principle for measuring the components of the atmosphere was first used by GOMOS, is extremely precise and also enables global and three-dimensional analysis.

ASAR (Advanced Synthetic Aperture Radar)

The ASAR (Advanced Synthetic Aperture Radar) used its five-panel antenna to send radar beams in the C-band to the earth and then received the reflected beams again. Using the computationally complex Synthetic Aperture Radar process , the antenna mimicked a very large (synthetic) antenna due to its constant movement, determined by the satellite speed of 7.5 km / s. This enabled the radar to achieve a high geometric resolution of up to 30 m. ASAR offers different recording modes, either small pictures with high resolution or large pictures with low resolution were created. Due to the active transmission of the radar beams and their properties, the instrument was independent of cloud cover and daylight. The areas of application of ASAR are mainly in environmental monitoring.

MIPAS (Michelson Interferometer for Passive Atmospheric Sounding)

MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is another spectrometer on board ENVISAT. In contrast to MERIS, however, it was exclusively aimed at investigating the atmosphere and can also operate without sunlight. For this purpose, its spectrum was shifted into the infrared range compared to MERIS, which functions in the visible wavelength range. MIPAS was able to record various trace gases, water vapor and the temperature of the atmosphere as well as their exchange processes. Height profiles up to a vertical resolution of 3 km could be recorded.

SCIAMACHY (SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY)

The SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY) is also as MIPAS a spectrometer to study the atmosphere. However, it worked in the visible, near infrared and a short part of the infrared wavelength range and was therefore dependent on the sun or weaker light sources such as the moon. Another difference between the two instruments is the orientation of the field of view. MIPAS viewing directions pointed sideways to the orbit and against the flight direction of ENVISAT tangential to the earth on the atmospheric layers. SCIAMACHY's field of vision, on the other hand, pointed both in the direction of flight and directly vertically downwards. When the satellite reached the column of air previously scanned horizontally, it could scan it again vertically. This made it possible to better determine the composition of the atmosphere and also to achieve a better spatial resolution.

Web links

Individual evidence

  1. Archive link ( Memento from July 21, 2015 in the Internet Archive )
  2. ^ Orbit data according to Chris Peat: Envisat - Orbit. In: Heavens Above. October 9, 2012, accessed October 9, 2012 .
  3. ESA: Envisat service interrupted . ESA news April 12, 2012
  4. Christoph Seidler: Possible breakdown: European satellite "Envisat" does not respond . In: Spiegel Online . April 12, 2012
  5. Christoph Seidler: "Envisat" -Havarie: Completely detached . In: Spiegel Online. April 14, 2012
  6. ESA: Envisat - ESA's efforts to re-establish contact with the satellite ( Memento of December 30, 2014 in the Internet Archive ). ESA news, April 19, 2012
  7. ESA declares end of mission for Envisat , ESA press release of May 9, 2012
  8. ^ Envisat To Pose Big Orbital Debris Threat for 150 Years, Experts Say. In: SpaceNews.com. July 23, 2010, accessed October 23, 2016 .
  9. ^ Don Kessler on Envisat and the Kessler Syndrome. In: Space Safety Magazine. April 25, 2012. Retrieved October 23, 2016 .
  10. Satellite image of the week , report on Spiegel Online from November 30, 2007
  11. Envisat flies lower , report on raumfahrer.net from October 28, 2010