The 29-hour countdown for tomorrow's launch of navigation satellite 'IRNSS-1H' to augment the existing seven satellites of the NavIC constellation began today.

To be launched on board PSLV-C39, IRNSS-1H will be a 'back-up' navigation satellite for IRNSS-1A, one of the seven satellites in the constellation, as its three rubidium atomic clocks on board had stopped functioning.

"The 29-hr countdown operations of PSLV-C39/IRNSS-1H  mission have started on Wednesday at 14:00 hrs IST (2 pm)," the Indian Space Research Organisation said.

The launch is scheduled at 7 pm from the Second  Launch Pad of the Satish Dhawan Space Centre at Sriharikota  in Andhra Pradesh.

The Mission Readiness Review committee and Launch  Authorisation Board had yesterday cleared the 29-hour  countdown.

The launch vehicle PSLV-C39 will use the 'XL' version of  PSLV equipped with six strap-ons, each carrying 12 tons of  propellant.

The over 1,400 kg spacecraft was built and tested by ISRO along with a consortium of six small and medium  industries.

The Indian Regional Navigation Satellite System is an independent regional navigation satellite system  developed by India on par with US-based GPS.

The system that offers services like terrestrial and  marine navigation, disaster management, vehicle tracking and  fleet management, navigation aide for hikers and travellers,  visual and voice navigation for drivers, was named 'NavIC'  (Navigation with Indian Constellation) by Prime Minister  Narendra Modi.

ISRO had launched seven satellites - IRNSS-1G on April  28, 2016, IRNSS-1F (March 10, 2016), IRNSS-1E (January 20,  2016), IRNSS-1D (March 28, 2015), IRNSS-1C (October 16, 2014), IRNSS-1B (April 4, 2014) and IRNSS-1A on July 1, 2013.

According to ISRO officials, the total cost of all the  seven satellites was Rs 1,420 crore. -- PTI