The 27-hour countdown for Thursday's launch of an Indian rocket carrying strategic communication satellite GSAT-6A was progressing smoothly, an ISRO official said.

According to the Indian Space Research Organisation, the real time systems check operations have been completed.

The countdown began at 1.56 p.m. on Wednesday.

During the countdown, the rocket will be fueled up and its systems would also be checked.

The 415.6-tonne GSLV rocket measuring 49.1 metre tall is scheduled to blast off on Thursday at 4.56 p.m. from the second launch pad in Sriharikota, 105 km from here.

The GSAT-6A satellite will be put into orbit around 17 minutes after the rocket's lift off.

According to the ISRO, two improvements -- induction of high thrust Vikas engine and electromechanical actuation system -- have been made in the rocket's second stage this time around.

The ISRO said the GSAT-6A was similar to the GSAT-6.

However, the two satellites became a subject of controversy as 90 per cent of transponders were to be leased to Devas Multimedia by the ISRO's commercial arm Antrix Corporation under a deal which was annulled in February 2011 on the grounds that the country's defence needs had to be met.

Under the controversial deal, the Bengaluru-based Devas was to use the transponders of GSAT-6 and GSAT-6A in the

crucial S-Band wavelength (that was primarily kept for the country's strategic interests) for its digital multimedia service for 12 years.

Antrix had signed the $300 million contract with Devas in January 2005 and obtained the sanction of the Space Commission and the union cabinet for the two satellites without informing the government that the bulk capacity would be leased to the
multimedia service provider.

When the controversy broke in December 2009, the ISRO ordered a review of the deal and subsequently the Space
Commission had recommended its annulment on July 2, 2010. Antrix terminated the deal on February 25, 2011.

Subsequently, the GSAT-6 was launched in 2015.
"We have slightly tweaked the configuration of GSAT-6 from the original one to suit the needs of strategic users," ISRO's
then Chairman A.S. Kiran Kumar told IANS after the launch of GSAT-6.

The controversy and the legal fight is continuing between the parties.