The 51.30 hour countdown for the April 28 launch of the country’s seventh and final navigation satellite, IRNSS-1G, on board Polar Satellite Launch Vehicle— C33 from the spaceport of Sriharikota, about 110 km from here, began today.

The Mission Readiness Review Committee and the Launch Authorisation Board gave its approval on Monday for the countdown that began at 9.20 am today for the launch scheduled on Thursday at 12.50 pm from the first launch pad at the spaceport of Sriharikota, ISRO said.

IRNSS-1G on board PSLV-C33 in its 35th flight is the last in the series of the Regional Navigation Satellite System (IRNSS), comprising seven satellites that would give a fillip to Indian space agency’s tryst to set up its own independent regional navigation system on par with the US based Global Positioning System.

While four satellites would be sufficient to start operations of the IRNSS system, the remaining three would make it more “accurate and efficient”.

ISRO had launched the sixth navigation satellite IRNSS—1F on March 10, the other five being IRNSS—1A on July 1, 2013, IRNSS—1B (April 4, 2014), IRNSS—1C (October 16, 2014), IRNSS—1D (March 28, 2015) and IRNSS—1E (January 20, 2016).

ISRO scientists are using the same XL variant, used in previous launches of IRNSS satellites, for the launch of IRNSS—1G, given its capacity to carry load.

Occasions where rocket with XL configuration was used were during launch of Chandrayaan—1, Mars Orbiter Mission, ASTROSAT besides the six IRNSS satellites.

The 44.4 metre tall IRNSS—1G has a lift—off mass of 1.425 kg and would be launched in sub—Geosynchronous Transfer Orbit (sub GTO). It has a 12 year mission life.

With the operationalisation of six satellites, India has demonstrated the system’s targeted position accuracy which is much better than 20 metres over 24 hours of the day.

With the launch and operationalisation of IRNSS—1G, the seventh in the constellation, the completion of IRNSS constellation will be achieved, ISRO said.

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