Envisat
Envisat ( Environmental Satellite ) is an eight -ton environmental satellite of the European Space Agency (ESA ), which was launched in 2002 and worked until 2012. Its main functions were the constant monitoring of the climate, the ocean, the land surface or in general the ecosystem of the earth. With a total cost of 2.3 billion euros, he was the most expensive satellite of ESA and the largest ever flown Earth observation satellite.
The development and construction of the satellites that have taken more than ten years to complete, were nearly one hundred companies involved in fourteen countries, including Astrium sites in the UK, Germany and France. Astrium UK as the prime contractor responsible for the Polar Platform and two of the most important instruments; Astrium Germany held overall responsibility for the mission prime instruments, built two of them and delivered the electronics payload bay PEB for polar platform. Astrium France supplied the service module and other instruments.
Mission History
Envisat raised on 1 March 2002 from Europe's spaceport in Kourou, French Guiana on board an Ariane 5 rocket from. With a weight of 8050 kg (including 300 kg of fuel for orbital maneuvers ) he presented the then heaviest payload for Ariane dar.
After the successful launch of Envisat orbit was exposed in a Frozen on a polar sun-synchronous orbit at 800 km altitude. With an inclination of 98 ° ENVISAT flew over every place at intervals of 35 days. Was controlled, the satellite from the European Space Operations Centre, ESA - ESOC in Darmstadt.
On April 8, 2012 - after more than twice the originally estimated life of five years - fell from the satellite. The routine contact the ground station in Kiruna to Envisat did not materialize, and all subsequent contact attempts also failed. Radar observations from the ground showed that the satellite as a whole was still intact and was in stable orientation to the sun and earth. The attitude of the satellite, however, did not meet the expectations of the operator. On May 9, 2012, the formal end of the mission was announced by the ESA. The reason is a failure of communication, so that it is not possible to communicate with the satellite. So far, however, it is not clear how it has come to the disorder. Would be a failure of a power controller by which the telemetry and remote control would have been blocked. Alternatively apply a short circuit and a subsequent breakdown in the automatic introduction of the so-called "safe mode". This was supposed to ensure the survival of the satellite in case of error.
Mission Objectives
Envisat was launched as a follow-up project for the satellites ERS- 1 and ERS -2, which had adopted similar tasks in the 1990s in a slightly smaller package.
There are ten sophisticated instruments for Earth observation on board. You could measure the chemical composition of the atmosphere, the temperature of the oceans, wave heights and directions, wind speeds, growth stages of plants and detect forest fires and pollution.
Envisat was originally supposed to perform his duties until 2007. Due to the reliable work and insightful data with a circumference of about 280 gigabytes per day, the mission was initially continued until 2010. An extension of the mission beyond 2010 was technically possible and it was decided by the Member States in time to 2013. Since the fuel carried for position maneuver was running out, the orbit altitude of the satellite was lowered.
The satellite was only the achievement of research objectives for the International Charter for Space and Major Disasters.
Instruments
The redundancy of the system allowed the comparison of the different measurement results with each other. In addition, the measurement method of the instruments, thus the area of operation of the satellite was very varied distinguished. With its enormous size and mass ENVISAT is still looking for the foreseeable future, its same. A high number of instruments and thus committed capital involves high financial risk in the event of a false start. For this reason, again smaller satellites, which specialize only on an objective in use and in development.
X - and Ka-band
The X - and Ka-band antennas were used for communication with the ground stations or with Artemis.
RA-2 (radar altimeter 2)
The RA -2 ( Radar Altimeter 2) is an altimeter, the sending out pulsed radar signals through its antenna and its echo received back. From the travel time measurement of the reflection signal, the altitude of the satellite could be determined. By matching with the data of the orbit, therefore, a height profile of the ground could be created. In addition, allowing the characteristics of the echo statements about the surface finish, be it to make water, land or ice.
MWR (Microwave Radiometer)
The main task of the MWR (Microwave Radiometer) was in the measurement of the humidity of the atmosphere. These values are important for the evaluation of the RA-2 instrument, since its test results were influenced by moisture in the atmosphere.
AATSR (Advanced Along- Track Scanning Radiometer)
AATSR (Advanced Along- Track Scanning Radiometer ) is an instrument to detect the surface temperature of the sea at 0.3 ° C accuracy. Overland areas can be used its results to the interpretation of vegetation in the 500- km-wide field of view. Envisat has another instrument that was used to assess the height, and as DORIS (Doppler Orbitography and Radio Positioning Integrated by Satellite ) is called. However, this system was also able to perform more accurate orbit and attitude determination, which includes the satellite velocity counts. This was realized through the comparison of the two received signals. These signals are of a network of 50 stations which are distributed over the entire world, broadcast.
LRR (Laser Retroreflector )
The LRR (Laser Retroreflector ) is merely a reflector, which reflects one transmitted from the ground station and laser beam exactly back throws. This allows in turn by a period measurement, the calculation of the amount of ENVISAT.
MERIS ( Medium Resolution Imaging Specrometer )
The MERIS ( Medium Resolution Imaging Specrometer ) detected by using a CCD array, images of a gauge of 1150 km wide. The absorbed radiation are exclusively due to the reflection of sunlight on the Earth's surface, therefore no measurements are in eclipse ( when Envisat moves through the Earth's umbra ) have been possible. The detected spectral range of 390 nm was between MERIS to 1040 nm, that covered the entire visible region and near-IR. Due to the spectroscopy of the image resulting information on the chlorophyll and Schwebeteilchengehalt and the property of any sediment layers of the oceans. Secondarily is the study of the atmosphere with MERIS, since, other instruments are better suited.
GOMOS (Global Ozone Monitoring by Occultation of Stars )
From GOMOS (Global Ozone Monitoring by Occultation of Stars ), the composition of the atmosphere has been studied. With a mirror stars were targeted by sufficient luminosity just before they disappear behind the horizon, but before the view is clouded on the star through the atmosphere. From this image, a spectrum is created, which later serves as a basis for comparison. Followed now Envisat his career, the star is slowly disappearing behind the Earth. Before he is, however, entirely disappeared, its light is attenuated by the different layers of the earth's crust. This is also reflected in the spectra re- created in this phase. Can be determined using the element-specific absorption of different wavelengths from their concentration. The main focus here is on the observation of the distribution of ozone, hydrocarbons, and nitrogen oxides. This principle of measuring the atmospheric components was first used by GOMOS, is extremely precise and also allows a global and three-dimensional analysis.
ASAR ( Advanced Synthetic Aperture Radar )
The ASAR ( Advanced Synthetic Aperture Radar ) sent with his consisting of five panels antenna radar beams in the C-band to the ground and received the reflected rays back. Due to the computationally expensive processes Synthetic Aperture Radar antenna imitated due to their continuous movement, determined by the satellite velocity of 7.5 km / s, a very large (synthetic) antenna. Thus, the radar achieved a high geometric resolution of up to 30 m. ASAR offers various shooting modes, either small images with high, or large low-resolution images were created. By actively sending radar beams and their property, the instrument was independent of cloud cover and daylight. The areas of ASAR are primarily in the 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 but it was geared exclusively to the study of the atmosphere and can also operate without sunlight. For this purpose its spectrum was compared to MERIS, which acts in the visible wavelength range, shifted into the infrared region. From both MIPAS various trace gases, water vapor and the temperature of the atmosphere and their exchanges were recorded. It could height profiles are added to a vertical resolution of 3 km.
SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY )
The SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY ) is also MIPAS as a spectrometer to study the atmosphere. However, it worked in the visible, near infrared and a short portion 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 that the orientation of the field of view. MIPAS viewing directions showed lateral to the orbit and against the direction of flight of ENVISAT tangential to the earth atmosphere layers. The field of view of SCIAMACHY showed, however, both in the direction of flight as well as directly vertically downward. If the satellite reached in advance horizontally sampled air column, he was able to scan again vertically. This allowed the composition of the atmosphere better determined and also a better spatial resolution can be achieved.