Chennai: The 51-hour countdown for the 100th mission of the Indian space agency, slated for September 9, began on Friday at 6.51 am at the Sriharikota satellite launch centre in Andhra Pradesh, around 80 km from Chennai, an official said.
Carrying French and Japanese satellites, an Indian rocket - Polar Satellite Launch Vehicle (PSLV) - will blast off at 9.51 am on Sunday.
"The countdown started at 6.51 am and is progressing normally. Everything normal now," an Indian Space Research Organisation (ISRO) official said.
During the countdown, liquid fuel in the fourth stage/engine and also in the control systems would be filled. On Saturday the fuel for the second stage/engine will be filled, the official said.
The 715-kg French satellite, SPOT 6, which is an earth observation satellite, will be the heaviest foreign satellite to be launched so far by PSLV.
The Japanese satellite weighing 15 kg is called Proiteres.
ISRO's commercial arm Antrix Corporation and French company Astrium SAS compete in the global market for vending remote sensing satellite imageries.
The SPOT and Indian remote sensing satellites are the two leading earth observation satellite series.
According to ISRO, the satellite launch agreement between Antrix and Astrium is part of a long-term agreement signed between the two agencies Sep 2008.
In November 2010, a communication satellite HYLAS was built for a European customer under an agreement between Antrix and Astrium.
The PSLV rocket has successfully launched a total of 53 satellites out of 54 it has carried; ISRO has launched all its 27 foreign satellites successfully.
According to ISRO, India has built 62 satellites and launched 37 rockets, starting from its first satellite Aryabhatta and rocket satellite launch vehicle (SLV).
"ISRO counts each of its rocket flights as a single mission. If our satellite is launched by a foreign rocket then it is counted as single mission. Whereas if an Indian rocket (PSLV) launches multiple satellites built and owned by ISRO then each satellite is counted individually as a mission," an ISRO official said.
Launching foreign satellites or those owned by other Indian organisations are not counted as a mission.
The Sunday flight of PSLV will be counted as single mission.
According to ISRO officials, Prime Minister Manmohan Singh is expected to witness the 100th mission at Sriharikota. He will reach the launch centre Saturday evening.
India began its space journey in 1975 with the launch of Aryabhatta satellite using a Russian rocket.