Sunday, February 14, 2016

European Sentinel-3A Earth Observation Satellite Ready to be Launched from Plesetsk

Sentinel-3A upper composite being transferred to the launch pad on Feb. 12, 2016, at the Plesetsk Cosmodrome in northern Russia. Photo Credit: ESA–Stephane Corvaja, 2016

A Russian Rokot rocket stands tall on the launch pad at the Plesetsk Cosmodrome, located in northern Russia, ready to boost a European Earth-observing satellite into space on Tuesday, Feb. 16. The Sentinel-3A spacecraft is scheduled to liftoff from Plesetsk’s Site 133/3 at 12:57 p.m. EST (17:57 GMT).

The satellite arrived safely in Plesetsk in December 2015. It was then delivered to a clean room at the Rokot Integration Facility (MIK) to conduct various electrical test runs. The rocket’s Briz-M upper stage, together with payload fairing were also transported to MIK for extensive check-outs in preparation for a launch vehicle dry-run and eventually mating with the spacecraft. On Dec. 7, the propulsion system checks commenced, during which propellant tanks were pressurized to confirm their structural integrity before fueling the satellite.

After the series of demanding tests, the teams installed all the necessary equipment onto the spacecraft in mid-January. They’ve also carried out the launch and network countdown rehearsal with the European Space Operations Centre (ESOC) in Germany. While the spacecraft was ready for the liftoff initially scheduled for late January, it turned out that the launch pad required a ‘re-certification’ process, thus the launch was postponed to mid-February.

During the first week of February, the so-called combined operations started to mate the satellite with the launch vehicle adapter and to put this stack on top of the upper stage. The teams also carried out electrical checkouts on the adapter system. Finally, the spacecraft was encapsulated in the payload fairing.

“Preparation is continuing nominally here in Plesetsk. After having fueled both the Satellite and the Rokot last stage, the Sentinel-3A spacecraft has been mated first to the Flight Adapter, then to the upper stage,” Bruno Berruti, European Space Agency’s (ESA) Sentinel-3 Project Manager, told Astrowatch.net.

Rokot was rolled out to the launch pad on Friday, Feb. 12. The fully integrated launch vehicle, including the payload, is now standing on the launch pad undergoing the last preparatory activities.

Built by Thales Alenia Space, Sentinel-3A is a car-sized Earth observation satellite dedicated to globally monitor oceans and land-vegetation. Its dimensions are 12.1 by 7.2 by 7.2 feet (3.7 by 2.2 by 2.2 meters). The satellite weighs about 2,645 lbs. (1,200 kg), consumes about 1.1 kW of power and is expected to be operational for up to seven years.

The satellite is designed to globally monitor oceans, ice and land. It combines a multi-spectral optical mission providing sea/land color data and surface temperatures and an altimetry mission contributing to the determination of the sea surface, ice surface and in-land water topography. To fulfill its tasks, the spacecraft will be injected into a sun-synchronous orbit (SSO) at an altitude of 506 miles (815 kilometers), inclined 98.65 degrees.

Sentinel-3A is a part of the Copernicus program, funded jointly by the European Commission and ESA. The project uses accurate and timely data to provide key information services to improve the way the environment is managed, help mitigate the effects of climate change and ensure civil security. 

Sentinel-3 mission is based on a constellation of two satellites operating at the same altitude, optimizing coverage and global revisit. Sentinel-3A and Sentinel-3B (to be launched in 2017) will provide global coverage every two days. The majority of the data will be processed systematically and available for users within three to 48 hours after sensing.

Artist's rendering of the Sentinel-3A satellite in space. Image Credit: ESA–Pierre Carril
Artist's rendering of the Sentinel-3A satellite in space. Image Credit: ESA–Pierre Carril

All Sentinel missions carry a range of technologies, such as radar and multi-spectral imaging instruments for land, ocean and atmospheric monitoring. Sentinel-1 is a polar-orbiting, all-weather, day-and-night radar imaging mission for land and ocean services. The first Sentinel-1 satellite was launched on Apr. 3, 2014. Sentinel-2 is a polar-orbiting, multispectral high-resolution imaging mission for land monitoring to provide, for example, imagery of vegetation, soil and water cover, inland waterways and coastal areas. The first Sentinel-2 spacecraft was blasted off to space on June 23, 2015.

“While Sentinel-1 and Sentinel-2 are mission mono-instrument, respectively a radar mission and an high-resolution optical imaging mission, Sentinel-3 is today the only Copernicus Sentinel being multi-instrument and therefore multi-mission. It will provide systematic and global coverage of all land and ocean areas, providing continuity for a number of existing or previous missions,” Berruti said.

According to ESA, Sentinel-3 satellite will make a unique contribution to the paradigm shift in the quality and quantity of Earth observation measurements over our planet in the coming decades.

“A combination of increased spectral capabilities, wider swath and improved spatial sampling will allow an optical revisit time of two days for the entire surface of the Earth. Furthermore, unlike Sentinel-1 and Sentinel-2, whose specific acquisition scenarios have to be planned in advance, the innovative Sentinel-3 satellite operations concept is designed around the principle of a quasi-autonomous satellite requiring minimum human intervention,” Berruti added.

Sentinel-3A carries four instruments that will work in synergy. The medium-resolution Ocean and Land Color Instrument (OLCI) will provide multi-spectral data with a ground resolution of up to 984 feet (300 meters) per pixel and a swath of 790 miles (1,270 kilometers). The Sea and Land Surface Temperature Radiometer (SLSTR) will deliver high accuracy temperature readings of the Earth’s surface with a ground resolution of 1,640 to 3,280 feet (500 to 1,000 meters).

The third instrument is a dual-frequency (Ku and C band) advanced Synthetic Aperture Radar Altimeter (SRAL) based on experience from Envisat and CryoSat satellites. It will provide altimeter data with a spatial resolution of approximately 984 feet (300 meters) along the satellite track. The fourth instrument is a dual-frequency MicroWave Radiometer (MWR) based on heritage from Envisat. The primary purpose of this instrument is to correct the delay of radar altimeter signals travelling through the atmosphere. The MWR also serves to measure total column atmospheric water vapor.

In addition, the topography payload is supported by a Precise Orbit Determination (POD) package for precise orbit positioning. This package includes a GNSS receiver, a DORIS receiver and a Laser Retro Reflector (LRR).

“Sentinel-3 instruments can be commanded autonomously on board the spacecraft using time and position-tagged commands and commands can be stored for a period equivalent to the full 27-day satellite repeat cycle. Commands are linked to geographic location, orbital time and the selection of a particular measurement mode, depending on the surface over which the spacecraft is flying. This approach significantly simplify the management of Sentinel-3 operations,” Berruti noted.

Rokot launch vehicle that will be used to lift the Sentinel-3A satellite is a 95-feet tall (29 meters) liquid-fueled three-stage rocket manufactured by Eurockot Launch Services . With a total mass of 107 tons, the booster is capable of delivering up to two metric tons into a low-Earth orbit (LEO) and 1.2 metric tons to SSO. 

The Rokot launcher has been selected for this mission through an open competition. 

“The Sentinel-3A mission was contracted at a competitive rate compared to other available launch systems in this range of payload capacity following an open competition by ESA,” Anna Zorina of Eurockot Launch Services, told SpaceFlight Insider.

Rokot’s rival in the competition, the Vega rocket, at the time of the launcher selection, was still under development and had not yet performed its inaugural flight. Also the launch calendar for Vega launches was not yet consolidated at that time. According to Berruti, this was perceived as an unacceptable risk.

Briz-M is a liquid-propellant rocket orbit insertion upper stage manufactured by Khrunichev State Research and Production Space Center. It is composed of a central core and an auxiliary propellant tank that is jettisoned in flight following the depletion of the stage’s propellant. The Briz-M control system includes an onboard computer, a three-axis gyro stabilized platform, and a navigation system. The quantity of propellant carried is dependent on specific mission requirements and is varied to maximize mission performance.

Tuesday’s mission will be the first Rokot launch since Sept. 23, 2015, when it launched three Strela communications satellites into space. It will be also the second liftoff from Plesetsk in 2016.

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