In what was likely to be its last commercial launch, Russia’s Rokot rocket gave the European Sentinel-3b satellite a ride to orbit Wednesday. Liftoff, from the Plesetsk Cosmodrome in northwest Russia, occurred at 20:57 Moscow Time (17:57 UTC).
Wednesday’s launch deployed Sentinel-3B, a spacecraft dedicated to the study of Earth’s oceans and vegetation. Sentinel-3B is part of the Copernicus programme, a partnership between the European Space Agency (ESA), EUMETSAT and the European Commission to perform Earth science and environmental research from orbit. The launch of Sentinel-3b completes the first-generation satellite constellation for Copernicus.
Originally named Global Monitoring for Environment and Safety (GMES), Copernicus is managed by the European Commission, while ESA and EUMETSAT are responsible for the Sentinel missions that make up its space component. Copernicus is a successor to the research done by the Envisat satellite, which operated from 2002 to 2012. Deployed by an Ariane 5 rocket, at over 8,200 kilograms (18,000 pounds) in mass Envisat remains the largest dedicated Earth science satellite ever launched. Instead of a single replacement satellite, Copernicus uses smaller satellites specializing in a particular field of research.
Six different types of satellite will make up the Copernicus constellation, with most operating in pairs to increase the rate of data collection and reduce the time between passes over individual points of interest. Two Sentinel-1 satellites – launched in 2014 and 2016 – perform radar imaging of the Earth’s surface, while a pair of Sentinel-2 satellites that launched in 2015 and 2017 are used for multispectral optical imaging.
Sentinel-4 and 5 are yet to launch. They will consist of hosted payloads aboard EUMETSAT’s geostationary Meteosat Third Generation (MTG) and polar-orbiting MetOP Second Generation (MSG) satellites respectively. These will both look for trace gases in Earth’s atmosphere but are not expected to launch until the 2020s. As a gap-filler, a free-flying precursor mission – Sentinel-5p – was deployed into low Earth orbit late last year.
Sentinel-6, also known as Jason Continuity of Service (Jason-CS), is a follow-on to the successful series of Jason satellites that use radar altimetry to map and study variations in the height of the sea surface. Sentinel-6 is the only part of the Sentinel constellation that is not expected to use multiple satellites for concurrent observations, with its satellites instead expected to operate sequentially.
Wednesday’s launch carries to orbit the second Sentinel-3 satellite – the seventh Copernicus spacecraft to launch overall. Sentinel-3b will join the Sentinel-3A satellite on orbit, which was launched aboard a Rokot rocket in February 2016. The Sentinel-3 spacecraft carry a suite of instruments to study Earth’s oceans and vegetation on land.
Sentinel-3B was built by Thales Alenia Space around its Prima satellite bus – a platform that was also used for the Sentinel-1 satellites, Italy’s COSMO-SkyMed constellation and Canada’s Radarsat-2 spacecraft. The satellite has a mass of 1,150 kilograms (2,535 lb) and is designed to give at least seven years of service. It carries enough fuel to operate for twelve years, contingent upon the health of other systems aboard the spacecraft.
The two Sentinel-3 satellites will operate in the same orbital plane – separated by 140 degrees. This is a circular sun-synchronous orbit at an altitude of 814.5 kilometers (506.1 miles, 439.8 nautical miles) and an inclination of 98.65 degrees. The orbit will have a local time of 10:00 at its descending node.
The Sentinel-3B satellite carries the same instrument suite as Sentinel-3A. The Ocean and Land Colour Instrument (OLCI) is a push-broom imaging spectrometer that follows on from the Medium Resolution Imaging Spectrometer (MERIS) that flew aboard Envisat. OLCI has five cameras imaging in twenty-one spectral bands between 400 and 1,020 nanometres – five more than MERIS – which cover the visible spectrum and part of the near infrared.
The 150-kilogram (339 lb) instrument provides an imaging resolution of up to 300 meters (980 feet), with a swath width of 1,270 kilometers (790 miles). OLCI’s data will be used to monitor the color of Earth’s oceans and coastal regions – monitoring pollution, ecosystems and currents. Over land, OLCI will be able to monitor agriculture and land usage.
The Sea and Land Surface Temperature Radiometer, or SLSTR, is another multispectral imaging payload that covers nine visible-light and infrared bands at wavelengths between 550 and 12,000 nanometres, with resolutions of between 500 and 1,000 meters (1,640 to 3,281 feet). The instrument has two fields of view – looking downwards it can cover a swath of 1,400 kilometers (870 miles), while a swath of 740 kilometers (460 miles) to the satellite’s aft can also be imaged.
The data SLSTR collects will be used to produce a temperature map of the Earth’s surface accurate to within 0.3 of a degree on the Celsius or Kelvin scale (0.54 Fahrenheit/Rankine). Its infrared bands will also be used to help detect and monitor fires, aiding emergency responses. SLSTR was developed by Anglo-Italian firm Selex ES and is the successor the Advanced Along-Track Scanning Radiometer (AASTR) aboard Envisat – itself a follow-up to instruments aboard the ERS-1 and ERS-2 satellites of the 1990s.
As well as its imaging payloads, Sentinel-3B carries two instruments to study Earth’s topography. The SAR Radar Altimeter (SRAL) uses C and Ku-band radar to measure the height of land of the surface of the sea. The instrument also allows wind speeds over the sea to be measured. A Microwave Radiometer (MWR) which was developed by Airbus (formerly EADS CASA) will measure the content of water in the atmosphere – in the form of water vapor or clouds – in order to calibrate SRAL measurements. The presence of water vapor can affect measurements from the radar altimeter, increasing the time taken for a signal to propagate back to the satellite and making the ground appear to the instrument to be further away than it is.
To support its topographical instruments, Sentinel-3B carries a Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) transmitter and satellite navigation receivers that will be used to plot its orbit with a high degree of accuracy. DORIS consists of a radio beacon aboard the satellite broadcasting ultra-high frequency (UHF) signals at predetermined wavelengths that are picked up by ground stations. By analyzing how the frequency of these signals is shifted by the Doppler effect, the satellite’s position and orbit can be computed.
The launch of Sentinel-3B was contracted by German-based Eurockot Launch Services – a partnership between European ArianeGroup and Russia’s Khrunichev State Research and Production Centre. The Rokot vehicle that will be used to carry Sentinel-3B into orbit is derived from the UR-100N missile that was withdrawn in service in the 1990s as a result of the START-1 treaty and the introduction of the upgraded UR-100NUTTKh. Rokot – meaning “rumble” – can also be spelled Rockot under methods of Romanising the Russian language used in parts of Europe.
The UR-100N – also known as the RS-18A or by the NATO reporting name SS-19 Stiletto – was a derivative of Vladimir Chelomei’s Universal Rocket 100 (UR-100) missile, part of a series of rockets that his design bureau planned during the 1960s to accommodate all of the missile and space launch needs of the Soviet Union with a single family of rockets. While most of these designs never flew, the UR-100 was successful as a missile while the larger UR-500 would become the basis for the Proton rocket.
Rokot is a three-stage vehicle, consisting of the first two stages of a UR-100N with a Briz-KM upper stage. The Briz-KM – Briz meaning Breeze – is a restartable, storable-propellant upper stage that uses the same propulsion module as the larger Briz-M stage, but without the latter’s drop tank that allows it to carry additional propellant. The combination of Rokot and Briz-KM is usually known as Rokot/Briz-KM or just Rokot-KM.
The Rokot made its first flight in November 1990, a suborbital test flight launched from a silo at the Baikonur Cosmodrome. For its first three launches, Rokot was silo-launched and incorporated the earlier Briz-K upper stage instead of the Briz-KM. Operations moved to a surface launch pad at the Plesetsk Cosmodrome at the end of the 1990s in order to provide a better acoustic environment at launch for payloads, but not before Rokot’s first orbital launch carried the Radio-ROSTO satellite to space in 1994.
Rokot’s first launch from Plesetsk was originally planned for 1999, with the RVSN-40 satellite aboard. However, during prelaunch tests the vehicle’s payload fairing separated while it was still standing on the launch pad. Following inspections of the rocket, the planned launch was canceled and RVSN-40 later flew aboard a Kosmos-3M rocket as Mozhayets 3.
Instead, Rokot’s debut from Plesetsk came in May 2000, when the rocket carried two mass simulators representing Iridium communications satellites, named SimSat-1 and 2. This launch also marked the first test flight of the Briz-KM – the RVSN-40 launch was to have used a Briz-K.
Rokot’s launch complex at the Plesetsk Cosmodrome is Site 133/3. Originally built as 133/1 for the Kosmos-2I, the pad was known as Raduga, meaning Rainbow. The pad served the Kosmos-2I rocket from 1967 to 1977 before returning to service with the Kosmos-3M rocket in 1985. With its conversion to support Kosmos-3M, the pad was renumbered 133/3, reflecting the two Kosmos-3M launch pads then operational at nearby Site 132. The final Kosmos launch from Site 133/3 was made in 1994. The name 133/3 was retained when the complex was modified to support Rokot.
Eurockot’s launch campaign for the Sentinel-3B launch began on 17 March, with the arrival of the satellite in Arkhangelsk and its loading onto a train for transport to Plesetsk. The spacecraft arrived at its launch site the following day and began final preparations for launch in cleanroom 101A of the integration building, or MIK.
Following testing and fit checks between the satellite, Rokot’s Briz-KM upper stage and the payload fairing, both Briz-KM and Sentinel-3B underwent fuelling in the second week of April. On 14 April the satellite was installed atop the adaptor that will connect it to the rocket for launch. This includes the CASA CRSS-937 clamp bands that will separate it from the Briz-KM after reaching the planned orbit.
The combined satellite and launch adaptor were transported to cleanroom 101B for integration with the Briz-KM upper stage, which took place on 16 April. Rokot’s payload fairing was installed around the satellite and upper stage the next day.
On 21 April the encapsulated Briz-KM and satellite – termed the “upper composite” or “space head” – were transported to the launch pad for integration with Rokot. A crane within the launch pad’s mobile service tower was used to lift the combined unit into position atop the rocket. Among Russian rockets, Rokot is unusual in that this integration takes place with the rocket vertical at the launch complex – most other Russian vehicles are integrated horizontally and then transported to the launch complex.
It took about one hour and twenty minutes for Rokot to place Sentinel-3B into orbit. This began with ignition of the RD-0233 first stage engine shortly before liftoff. Burning hypergolic propellants – unsymmetrical dimethylhydrazine (UDMH) and dinitrogen tetroxide – the first stage operated for about two minutes. At the end of the first stage burn, the second stage vernier engines ignited, the spent stage separated, and retrorockets pushed the two stages apart. The second stage main engine then ignited to continue boosting Sentinel-3B towards orbit.
The second stage burns the same hypergolic propellants as the first stage. It is powered by an RD-0253 engine and four RD-0236 vernier motors to provide attitude control. During second stage flight – at about three minutes and three seconds mission elapsed time – Rokot’s payload fairing separated from around the satellite. The second stage burnt out about five minutes and five seconds into the mission, separating at the five-minute, 19 second mark.
Following second stage separation, Briz-KM fired its S5.92 engine twice to inject Sentinel-3B into its planned orbit. Its first burn began about 1.7 seconds after separating from Rokot’s second stage, lasting a little over nine minutes to reach an initial transfer orbit. Just over an hour later, a second burn of about thirty seconds circularized the orbit. Spacecraft separation occurred a little before one hour and 20 minutes mission elapsed time.
After spacecraft separation, Briz-KM is expected to make two further burns to lower its orbit. These maneuvers will ensure that the stage reenters quickly, reducing the amount of debris in low earth orbit.
Wednesday’s launch was Rokot’s thirty-first launch and the twenty-eighth to use the Rokot/Briz-KM configuration. Two of these launches have failed: the first of these, in October 2005, saw the rocket’s second stage continue burning past its planned cutoff time, preventing stage separation from occurring and resulting in the loss of ESA’s CryoSat satellite.
In 2009 an upper stage issue left a military geodesy satellite, Geo-IK-2 No.11 – in a lower than planned orbit. Rokot also suffered a partial failure during a January 2013 launch, when an off-nominal spacecraft separation resulted in the loss of a military communications satellite – one of three aboard the rocket.
It is the first Rokot launch since last October’s successful deployment of the Sentinel-5p satellite. While Russia’s TASS news agency had previously reported that the Sentinel-3B mission would mark Rokot’s last flight, it now appears that the rocket has three Russian Government missions remaining on its books. These should be completed by the end of the year, with the next launch scheduled for June with Geo-IK-2 No.13.
A further military launch is planned for the third quarter of the year – likely with Rodnik communications satellites – before a final launch currently slated for October carries three Gonets-M satellites into orbit.