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The Indian Space Research Organisation (ISRO) on Saturday began the countdown for the launch of its LVM3 rocket carrying 36 satellites of the UK-based Network Access Associated Ltd (OneWeb) scheduled for Sunday.

"The countdown started at 8.30 a.m and is progressing smoothly," a senior ISRO official told IANS.

During the countdown, the rocket and satellite systems will be checked, while fuel for the rocket will also be filled.

The Indian rocket LVM3 standing 43.5 metre tall and weighing 643 tonne will blast off at 9 a.m. from the second launch pad at the rocket port in Sriharikota in Andhra Pradesh carrying OneWeb's final instalment of 36 Gen1 satellites weighing 5,805 kg.

The satellites will be slung into low earth orbit (LEO).

The LVM3 is a three stage rocket with the first stage fired with liquid fuel, the two strap on motors powered by solid fuel, the second by liquid fuel and the third is the cryogenic engine.

The ISRO's heavy lift rocket has a carrying capacity of 10 ton to theALEO and four ton to the Geo Transfer Orbit (GTO).

The upcoming rocket mission code named as LVM3-M3/OneWeb India-2 Mission by ISRO.

Just over 19 minutes after the rocket's blast off, the process of satellite separation will begin. The separation of 36 satellites will happen in phases.

The OneWeb is backed by India's telecom major Bharti Group and with the successful launch of the satellites on March 26, the company will complete the global footprint of its Gen 1 constellation.

OneWeb has 582 satellites now in orbit. On March 26 the total number is expected to go up to 618.

By completing the constellation, OneWeb is taking a pivotal step forward in delivering global coverage including India, the company had said.

The upcoming launch will be the 18th one for OneWeb.

The first batch of 36 satellites was launched on October 23, 2022, from Sriharikota rocket port in Andhra Pradesh with the LVM3 rocket formerly known as Geosynchronous Satellite Launch Vehicle MkIII (GSLV MkIII).

ISRO's commercial arm, NewSpace India Ltd (NSIL) has signed a contract with OneWeb to launch 72 satellites in two phases for a launch fee of over Rs 1,000 crore, OneWeb Chairman Sunil Bharti Mittal had said last October.

OneWeb Gen-1 is a 150 kg class satellite.A The constellation comprises 648 individual satellites. Out of that 588 Active Satellites equally divided among 12 planes operate at an altitude of about 1200 km above the Earth's surface.

Each plane is separated in altitude by 4 km to prevent inter-plane collision.

The payload is a bent-pipe system operating in Ku and Ka band. The forward link receives Ka-band signals from the gateway via the satellite Ka antenna. The return link receives Ku-band signals from the User Terminals (UTs) via the satellite Ku antenna, ISRO said.