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Envisat operations 
Envisat provides continuous observation and monitoring of the Earth's land, atmosphere, oceans and ice caps. Due to its near-Earth polar orbit (782 km mean altitude) and fast cycle time (one orbit in 100 minutes), the mission presents unique challenges for ground communications and data delivery. Envisat data are providing a wealth of information on processes of the Earth's systems, including insights into factors contributing to climate change. More than 2700 scientific projects are working with Envisat data as well as and other operational users, who are using data from Envisat for non-scientific activities on a routine, operational basis (e.g. earther services) or commercial users. The Envisat Flight Operations Control Centre (FOCC) is located at ESOC, Darmstadt, Germany.
The Flight Control Team operates from the combined Earth observation Dedicated Control Room located at ESOC. Spacecraft Operations Manager (SOM) Frank-Jürgen Diekmann, from Germany, manages the team, which is responsible for command and control, on-board software and database maintenance and mission planning for the satellite. "I joined the Envisat team in 1995 and have been SOM since 2006. Besides all the necessary technical and administrative obligations involved in managing the operations of an EO satellite, I am still enthusiastic about the complexity and the beauty of our blue planet," says Diekmann. The team operates a 7-day-per-week on-call service for engineering and mission planning, ensuring a fast reaction in case of any on-board anomalies, ground segment problems or unforeseen re-planning requests from the Envisat user community.
Envisat was launched on Ariane-5 Flight V145 on 1 March 2002 from Europe's Spaceport in Kourou, French Guiana. After a perfect launch and injection into its Sun-synchronous polar orbit, the critical LEOP (Launch and Early Orbit Phase) began with an autonomous sequence, including solar array deployment. Launch and Early Orbit Phase The solar array drive mechanism and reaction wheel release pyros were then fired and the Earth-pointing attitude was acquired as planned. On Day 2, the satellite performed orbit control manoeuvres to start moving toward the planned orbit. On Days 3 and 4, the ASAR (Advanced Synthetic Aperture Radar - the largest single instrument on Envisat) antenna was successfully deployed, marking the end of LEOP. Commissioning Phase The commissioning phase began with the Payload Switch-on and Data Acquisition Phase (SODAP) on 5 March 2002. The Payload Module Computer was switched on first. During this phase, all operational modes of the Payload Equipment Bay (PEB) and the instruments were successfully exercised. The remaining payload deployments of the Ka-band antenna and instruments (GOMOS, MIPAS, SCIA) were performed. SODAP finished in mid-April 2002, at least for the majority of the instruments. Cal/Val Phase The second half of the commissioning phase, the so-called 'Cal/Val' (calibration and validation) activities, started next. In September 2002, the first calibration review occurred at ESA/ESTEC, confirming the promising scientific results already being received. A second validation review, to mark completion of the commissioning phase, took place in December 2002. Routine operations Since 1 January 2003, Envisat's flight operations and ground segment have operated in the 'routine operations' phase. After nine years in-orbit operations, the satellite has completed over 46 000 orbits, covering 2 billion km since launch. Over 280 gigabytes (GB) of science data are generated each day, contributing to the growing 750-terabyte (TB) database.
More than 2000 nominal and contingency procedures have been developed and tested by the Flight Control Team. As with any complex satellite, different types of anomalies of the satellite platform and payload have to be investigated and require ongoing meetings, training and reporting, as well as upgrading of operational procedures. Mission planning systems at ESOC and ESRIN plan and schedule operations for the payload and the data management system on a routine basis. The planning cycles are about 5 days for nominal operations and less than two working days for emergency requests. Commands are uplinked to the on-board queue up to 24 hours before their execution. Up to 230 000 commands are transmitted to the satellite every month. Mission extension In June 2009, ESA Member States unanimously voted to extend the Envisat mission through to 2013. The operational scenario for extending the mission had already been defined in 2008 and was based on altitude lowering and a new orbit control concept, allowing a continuation of the routine operations until the end of 2013. The orbit change became necessary, to avoid further fuel consumption (the main life= limiting resource onboard ENVISAT) for inclination manoeuvrers. The Flight Control Team at ESOC was responsible to implement the orbit lowering manoeuvrers and resume nominal operations afterwards. These activities were carefully planned, tested and finally successfully executed in October and November 2-1-. After a series of manoeuvrers ENVISAT is now orbiting the Earth at an average altitude of 782 km with a repeat cycle of 30 days and a slowly drifting inclination.
Kiruna, Svalbard Envisat, following a Sun-synchronous low-Earth orbit of about 782 km altitude with a 30-day repeat cycle, presents unique challenges for telecommanding and data download. For routine operations, mission controllers use the Kiruna ground station - part of ESA's ESTRACK global network - for command, control and tracking of the satellite during visible orbits. Additional capacity is provided by the Svalbard ground station and others. During blind orbits over Kiruna, engineers monitor the satellite using two Svalbard passes during the night.
Envisat also communicates using a direct Ka-band link to the advanced Artemis data-relay satellite. Data is relayed in space from Envisat to Artemis, and then downloaded to the payload data handling stations at ESA/ESRIN in Frascati, Italy. This relay enables on-board instruments to continuously gather high-rate data (which cannot be stored on board) and permits engineers to optimally manage the satellite's solid-state memory.
The Envisat ground segment at ESOC uses the SCOS-1b mission control system, based on ERS-2 heritage. The ground segment comprises:
The FOS is responsible for:
The Flight Operations Control Centre hosts the following major subsystems:
The platform and payload The platform Envisat's total mass is 8.1 tonnes including a payload mass of around 2.1 tonnes and around 314 kg of hydrazine thruster fuel (at beginning of life). A 14x5-metre solar array made of silicon cells and eight nickel-cadmium batteries supply an average power of 6 kilowatts in sunlight and 3 kilowatts during eclipse, of which on average 2 kilowatts is available for instruments. Envisat's major system elements:
The solar array is pointed at the Sun using a solar-array drive mechanism. A hydrazine propulsion system provides the control torque for attitude acquisitions as well as the thrust required for orbit control manoeuvres. An independent safe mode electronics system is implemented on board; if there is a satellite emergency, this ensures that the satellite can acquire a safe attitude that allows the solar array to point to the Sun. More than 50 on-board processors are available in a decentralised on-board architecture with different software packages maintained by the Flight Control Team when in orbit. For Telemetry (TM), Telecommand (TC) and Tracking, an S-band transponder with a TM bit rate of 4 kbps and a TC bit rate of 2000 bps is available. Science data can be stored on two solid-state recorders having 70 Gigabits of memory each. The data can then be downlinked either in X-band (2 x 100 Mbps) direct-to-ground or in Ka-band (2 x 100 Mbps) via Artemis. The payload Envisat's payload comprises:
Last update: 16 March 2011
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