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CHENNAI: The countdown for the launch of PSLV-C28 carrying five satellites from the UK began at the Satish Dhawan Space Centre at Sriharikota at 7.28am on Wednesday.Sixty-two hours and 30 minutes later, at 9.28pm on Friday, the rocket will lift off, marking the biggest commercial launch by Indian Space Research Organisation ( Isro ).Isro scientists said fuelling began at 10am, starting with the fourth stage. "Mono Methyl Hydrazine propellant filling operation of the fourth stage commenced at 10am, Isro tweeted."The weather is fine and everything is going on well. Fuelling of the second stage will begin on Thursday evening," Isro spokesman Deviprasad Karnik told TOI. PSLV is a four-stage vehicle with its second and four stages powered by liquid propellants, and the first and third stages by solid propellants. In its 30th flight, PSLV this time will use, for the ninth time, an XL version, with additional strap-on.The Friday launch will take three identical optical earth observation satellites ( DMC3 ), each weighing 447kg to a 647km sunsynchronous orbit. Riding piggyback will be 9kg earth observation micro satellite called CBNT-1, and a 7kg technology demonstration nano satellite called De-OrbitSail, both from the UK. Together, the payload adds up to 1,440kg. The last big commercial launch by Isro was on June 30, 2014 when a PSLV put in orbit SPOT-7, a French satellite weighing 712g.Launched into a single Low-Earth Orbit plane and phased with a separation of 120° between them, these satellites can image any target on the Earth's surface every day. Major application areas include surveying the resources on earth and its environment, managing urban infrastructure and monitoring of disasters," Isro said on its website.The DMC3 satellites and CBNT-1 were built by Surrey Satellite Technology Limited (SSTL), while De-OrbitSail was put together by the Surrey Space Centre, UK.