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The first signals from Sentinel 1A have been detected through a ground station in Svalbard, Norway. All parameters look normal, says mission control in Germany.
It is currently 5:52 p.m. in French Guiana.
Coming up at T-minus 10 minutes, the Sentinel 1A spacecraft is due to be configured for launch by switching to an on-board battery power source.
The Soyuz countdown sequence begins 6 minutes, 10 seconds prior to liftoff, then the Fregat upper stage will transition to internal power five minutes before launch.
The umbilical arm servicing the upper stage and payloads will pull away at T-minus 2 minutes, 25 seconds. The Soyuz rocket is operating on internal power at T-minus 40 seconds, and the final servicing mast retracts from the rocket 20 seconds later.
The ignition sequence of the Soyuz rocket's kerosene-fueled core stage and four strap-on boosters begins 17 seconds before liftoff, and all engines should be at full thrust three seconds before launch.
See our launch timeline for more details.
The satellite will almost immediately turn on its radio transmitter and radio its status to a ground station in Svalbard, Norway.
A complex sequence of solar array and antenna deployments will begin around 2354 GMT (7:54 p.m. EDT), first with the partial deployment of Sentinel 1A's C-band radar instrument.
Part of the five-panel, 40-foot-long C-band instrument must move out of the way to allow Sentinel 1A's solar arrays to extend and begin charging the spacecraft's batteries.
"The sequence is a little bit strange," said Guido Levrini, ESA's Copernicus space segment program manager, in an interview earlier this week. "You first open half of the antenna wing just to get it out of the way, then you open the first solar array wing, you turn it toward the sun, and this is the point in time it is safe. This point will be reached five-and-a-half hours after the launch."
Once the spacecraft is generating power to stay alive, Sentinel 1A will complete the deployments of its two solar array wings and rectangular radar antenna.
The deployments take such a long time because ground controllers designed the sequence to occur only when Sentinel 1A is in view of a tracking station, giving engineers full insight into the activities on-board the satellite.
"The various deployments -- and there are some six individual deployments --will end only about 10 hours after the launch," Levrini said.
The first data from the radar instrument should be available Sunday, assuming an on-time launch today.
The first radar images from Sentinel 1A should be released within about three weeks.
The European-funded, Russian-built pad is located about eight miles northwest of the Ariane 5 and Vega launch pads at the Guiana Space Center. Engineers selected the Soyuz launch site based on terrain, geology and a location away from Ariane facilities to ensure they did not interfere with each other.
It took three years and cost European governments $800 million to build the Soyuz launch facility, which is known by its French acronym ELS. Other than the 17-story mobile servicing tower and four lightning masts, the launch pad is modeled after the Soyuz launch pad at the Baikonur Cosmodrome in Kazakhstan.
This morning's launch is the seventh Soyuz to fly from ELS.
The Soyuz pad includes blue and yellow umbilical arms and hold-down petals at the base of the rocket. On the back side of the pad is a deep flame trench dug out of granite bedrock. The facility also houses living quarters for Russian workers and a launch control center.
The Soyuz site lies closer to the town of Sinnamary than Kourou, which is more typically associated with the spaceport.
The next milestone in the countdown will be retraction of the Soyuz rocket's mobile gantry. Engineers are currently configuring the servicing tower to move to a point about 260 feet from the Soyuz rocket.
The Soyuz rocket with Sentinel 1A on-board is a modernized version of the venerable Russian launcher with an automated digital control system. It also has a flight termination system that can receive commands from safety officials on the ground in the event of a mishap, a key difference between the Soyuz rockets flying from French Guiana and Russian launch sites.
The Soyuz launching this evening is known as the Soyuz ST-A or Soyuz 2-1a configuration.
Other upgrades for Soyuz launchers based in French Guiana include an S-band telemetry system, modifications to cope the the humid tropical climate, and valves in the rocket's fuel tanks to allow empty stages to sink in the Atlantic Ocean. Soyuz launches from Kazakhstan or Russia drop their stages on land.
After liftoff, the rocket will go through pitch and roll programs to align with an northerly trajectory from the launch pad near Sinnamary, French Guiana. After a nearly 10-minute flight powered by the Soyuz rocket's three core stages, a Fregat-M upper stage will take over for a 10-minute burn to put Sentinel 1A into a 430-mile-high orbit with an inclination of 98.2 degrees.
See our launch timeline for more details.
Some statistics on today's flight:
The launch team has completed electrical checks after turning on the Soyuz rocket's avionics systems, and fueling of the three-stage launcher with liquid oxygen and kerosene will begin in about 30 minutes.
Coming up in about one hour, the Russian State Commission will meet to approve the start of fueling of the Soyuz three-stage launcher.
The launch is scheduled for 2102:26 GMT (5:02:26 p.m. EDT; 6:02:26 p.m. local time) from the Guiana Space Center in South America.
The weather looks favorable for launch this evening, with little chance of thunderstorms or rain. Winds are also predicted to be acceptable.
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The review confirmed the readiness of the Soyuz launcher, Sentinel 1A spacecraft, the launch site and support infrastructure at the Guiana Space Center, and a network of downrange communications and tracking stations, according to Arianespace.
The launch is set for precisely 2102:26 GMT (5:02:26 p.m. EDT; 6:02:26 p.m. local time) from the Soyuz launch site near Sinnamary, French Guiana.
It will take about 23 minutes for the three-stage Soyuz ST-A launcher and its Fregat-M upper stage to inject Sentinel 1A into a 430-mile-high orbit with an inclination of 98.2 degrees.
Thursday's launch will mark the seventh Soyuz mission to originate from French Guiana since the Russian rocket began flying there in October 2011.
The rollout occurred at dawn Monday, and the three-stage Soyuz rocket was on the ELS launch pad by midday, ready for the arrival of the Sentinel 1A spacecraft Monday evening.
Ground crews attached the rocket's "upper composite" overnight, which includes the Sentinel 1A satellite and its Fregat upper stage enclosed inside the Soyuz payload fairing.
The launch team also completed a mission rehearsal in a final exercise before Thursday's launch.
Liftoff is set for 2102:26 GMT (5:02:26 p.m. EDT; 6:02:26 p.m. French Guiana time) from the European-run Guiana Space Center on the northern coast of South America.
Sentinel 1A is the first in a series of Earth observation satellites developed by the European Commission and the European Space Agency. The Copernicus program is a multi-billion euro initiative to supply European governments with critical data to serve environmental and security applications.
"Everything is perfectly on schedule," said Guido Levrini, ESA's Copernicus space segment program manager. "The spacecraft is inside the fairing and sitting on top of the launcher, which is now in the vertical position on the launch pad.
"It has been 10 years of professional life to build up to this moment," Levrini said in an interview Tuesday. "Of course, this moment is not the only one. It's just the launch of the first satellite in a series, but still the first one is the most emotional one for sure."
The Soyuz launch comes during an especially busy few weeks at the spaceport, falling between the March 22 launch of Europe's heavy-lift Ariane 5 rocket and a liftoff of a lightweight Vega launcher set for April 25.
The 12-day turnaround between the Ariane 5 and Soyuz launches is a record and marks a reduction from the two-week standard gap between Ariane and Soyuz flights previously advertised by officials.
It usually takes a couple of weeks to reconfigure antennas, range assets and downrange tracking stations between missions from French Guiana, especially launches of different types of rockets and going to different orbits.
Sentinel 1A will be deployed by the Soyuz rocket's Fregat upper stage in a 693-kilometer (430-mile) orbit with an inclination of 98.2 degrees.
The rocket will head north from French Guiana before releasing Sentinel 1A into polar orbit.
Thursday's launch will mark the seventh Soyuz rocket mission to lift off from French Guiana since the Russian booster began operations there in October 2011.
Sentinel 1A is a joint project between the European Space Agency and the European Commission. It is the first satellite to launch in the Copernicus program, a multi-satellite initiative to monitor Earth's resources and environment for scientists and European governments.
The spacecraft is equipped with a C-band synthetic aperture radar for day-and-night imaging, replacing a radar mapping capability lost when Europe's flagship Envisat satellite failed in 2012.
Sentinel 1A will map floods, classify types of land cover, track sea ice, detect ships, measure sea surface winds and monitor ocean currents, according to ESA.