The launch is scheduled at 6:00pm on Thursday, at Satish Dhawan Space Centre in the spaceport of Sriharikota.
"The 59-hour countdown activity of PSLV-C29/TeLEOS-1 Mission has started at 07.00 hours today," Isro said on Monday.
The Mission Readiness Review committee and Launch Authorisation Board have cleared the countdown, it said.
Isro's trusted workhorse Polar Satellite Launch Vehicle on its 32nd flight would launch the six satellites into a 550-km circular orbit inclined at 15 degrees to the equator.
Of the six satellites, TeLEOS-1 is the primary satellite weighing 400kg whereas the other five satellites include two micro-satellites and three nano-satellites.
TeLEOS-1 is the first Singapore commercial earth observation satellite and would be launched into a low Earth orbit for "remote sensing" applications.
To be launched from the first launch pad, this is the 11th flight of PSLV in 'core-alone' configuration without the use of solid strap-on motors.
Antrix Corporation Ltd, the commercial arm of Isro, has provided launch services in PSLV for 51 customer satellites from 20 countries.
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