European Sentinel-3A Earth observation satellite ready for launch from Plesetsk
A Russian Rokot launch vehicle stands ready on the launch pad at the Plesetsk Cosmodrome, located in northern Russia. It is primed to boost a European Earth-observing satellite into space on Tuesday, Feb. 16. If everything goes as advertised, the Sentinel-3A spacecraft is scheduled to lift off 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 tests. The rocket’s Briz-M upper stage, together with its payload fairing were also transported to the MIK for extensive checkouts in preparation for a dry-run of the launch. After this has been completed the booster was mated 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 a series of demanding tests, the teams installed all of the necessary equipment onto the spacecraft in mid-January. Mission managers also carried out the launch and network countdown rehearsal within the European Space Operations Centre (ESOC) located in Germany. The spacecraft was ready for the original launch date, initially scheduled for late January. However, it turned out that the launch pad required a ‘re-certification’ process; thus, the flight was pushed back to no-earlier-than mid-February.
During the first week of February, the so-called ‘combined operations’ started with the mating of 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 its payload, is now standing on the launch pad undergoing the last activities to prepare it for flight.
Built by Thales Alenia Space, Sentinel-3A is a car-sized Earth observation satellite dedicated to monitoring the world’s 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 approximately 2,645 lbs. (1,200 kg), and consumes about 1.1 kW of power. If everything goes as planned, it is expected to be operational for up to seven years.
As noted, the satellite is designed to globally monitor Earth’s oceans, its ice, and land masses. It combines a multi-spectral optical mission which provides 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 some 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, to help mitigate the effects of climate change, and to ensure civil security.
The Sentinel-3 mission is based on a constellation of two satellites operating at the same altitude, optimizing coverage. Sentinel-3A and Sentinel-3B (scheduled to be launched in 2017) should provide global coverage every two days. The majority of the data will be processed systematically and will be made available for users within three to 48 hours after sensing.
All Sentinel missions carry a range of technologies – such as radar and multi-spectral imaging instruments for land, ocean, and atmospheric monitoring – with them to orbit. The first Sentinel-1 satellite was launched on April 3, 2014. Sentinel-1 is a polar-orbiting, all-weather, day-and-night radar imaging mission for land and ocean services. Sentinel-2 is a polar-orbiting, multispectral high-resolution imaging mission for land monitoring. It provides imagery of vegetation, soil, and water cover, as well as inland waterways and coastal areas. The first Sentinel-2 spacecraft was launched into 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 officials, the Sentinel-3 satellite will make a unique contribution to the paradigm shift in 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 with a swath of 790 miles (1,270 kilometers). The Sea and Land Surface Temperature Radiometer (SLSTR) is designed to 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) which was developed from experience gained from the 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 tech from Envisat. The primary purpose of this instrument is to correct the delay of radar altimeter signals traveling 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 orbital 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 [simplifies] 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 metric 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 that has been selected for this mission was chosen 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 European-produced Vega rocket, at the time of the launcher selection, was still under development and had yet to performed its inaugural flight. The launch calendar for Vega had not been consolidated at that time of the competition. According to Berruti, this was perceived as an unacceptable risk.
Briz-M is a liquid-propellant fueled launch vehicle with an upper stage designed to boost payloads into orbit (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 mission to take to the skies from Plesetsk in 2016.
Tomasz Nowakowski is the owner of Astro Watch, one of the premier astronomy and science-related blogs on the internet. Nowakowski reached out to SpaceFlight Insider in an effort to have the two space-related websites collaborate. Nowakowski's generous offer was gratefully received with the two organizations now working to better relay important developments as they pertain to space exploration.