Arianespace chairman and CEO Stephane Israel says it will take another 30 minutes or so to verify the satellites are in the correct orbit and confirm the success of tonight's mission.
Ground controllers will analyze signals from the satellites to determine their location in space.
The long wait to confirm the outcome of the launch stems from an embarrassing premature announcement of a successful flight in August 2014, when a closer look at data revealed the Fregat stage placed two Galileo satellites in an off-target orbit.
The second Fregat burn will begin at about T+plus 3 hours, 39 minutes to place the Galileo satellites into a circular orbit at an altitude of 23,522 kilometers (14,619 miles) at an inclination of 57.39 degrees.
It is currently 10:58 p.m. in French Guiana.
The two 715-kilogram (1,576-pound) Galileo satellite are being configured for launch.
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.
The 1,577-pound satellites mounted side-by-side on top of the Soyuz rocket were manufactured by OHB System of Bremen, Germany. They carry L-band navigation payloads supplied by Surrey Satellite Technology Ltd.
Once deployed from the launcher's Fregat-MT upper stage, the satellites will extend their solar arrays and begin a week-long phase of initial activations and tests.
The satellites will be allowed to drift into their final positions within Plane A of the Galileo constellation and will enter service in about three months.
The satellites join four other Galileo satellites launched in 2011 and 2012 to validate the network's design -- in space and on the ground.
Two Full Operational Capability, or FOC, satellites launched into the wrong orbit by a Soyuz/Fregat booster in August 2014 are also being incorporated into the Galileo fleet. The satellites should be able to be full-fledged members of the Galileo system even though their orbits do not match the orbits of the network's other spacecraft.
Another pair of satellites launched in March brought the total number of Galileo spacecraft in orbit to eight. Tonight's launch adds the ninth and 10th members.
Fourteen more Galileo satellites are being built and tested to launch on Soyuz and Ariane 5 rockets through 2018 to populate the constellation, which will eventually consist of 30 satellites in orbit at a given time, including spares.
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 evening's launch is the 12th 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 Galileo satellites on-board is a modernized version of the venerable Russian launcher with an automated digital control system and an upgraded RD-0124 third stage engine. 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-B or Soyuz 2-1b 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 a northeasterly 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-MT upper stage will take over for two burns before releasing the pair of Galileo satellites into a circular orbit 14,615 miles (23,522 kilometers) above Earth at an inclination of 57.39 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 the process to fill the three-stage launcher with liquid oxygen and kerosene has begun.
Fueling should be complete about two hours before liftoff.
The Soyuz ST-B rocket will blast off at 0208:10 GMT Friday (10:08:10 p.m. EDT; 11:08:10 p.m. French Guiana time Thursday), soaring into orbit from the European-run spaceport in French Guiana.
It will mark the 12th launch of the venerable Russian rocket from the Guiana Space Center, and its fifth mission from the tropical space base to serve the Galileo network. The launch is the second Soyuz flight of the year from French Guiana.
Two 1,577-pound (715-kilogram) Galileo navigation satellites built by OHB of Bremen, Germany, are packaged inside the Soyuz rocket's nose cone.
Upgrades to the Soyuz rocket featured on Thursday's launch include a modernized digital flight control system, an RD-0124 third stage engine, and a ST-type payload fairing with a diameter of nearly 13.5 feet.
Following a state commission meeting of mission managers, the Soyuz rocket will be filled with liquid oxygen and kerosene propellants beginning about four hours before liftoff, according to information released by Arianespace, the commercial operator for Soyuz rocket missions in French Guiana.
The launch pad's 174-foot-tall mobile gantry will be retracted about an hour before launch, moving into position about 260 feet from the Soyuz rocket.
After a computerized sequence, the 151-foot-tall launcher will ignite its main engines and blast off, turning northeast from the French Guiana spaceport and shedding strap-on boosters less than two minutes into the mission.
The Soyuz rocket's second and third stages will accelerate a Fregat upper stage and the four Galileo satellites on a suborbital trajectory before giving way to the Fregat engine for two burns to put the spacecraft in the proper orbit.
Deployment of the satellites from a specially-designed dispenser on the Fregat upper stage will occur simultaneously at 0556 GMT (1:46 a.m. EDT), according to Arianespace.
The launch is targeting a circular orbit with an altitude of about 14,615 miles (23,522 kilometers) and an inclination of 57.39 degrees.
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