Scientists at the Indian Space Research Organisation (ISRO) on Sunday commenced the 27.5 hour countdown for the launch of a navigation satellite on board a GSLV rocket here on May 29. The Bengaluru-headquartered space agency has drawn up plans to launch a second generation navigation satellite series which would ensure continuity of NavIC (Navigation with Indian Constellation) services. The satellite would provide real-time positioning and timing services over India and a region approximately 1,500 km around the mainland. The countdown for the lift-off commenced at 7.12am on Sunday, ISRO sources said. The 51.7 metre tall Geosynchronous Satellite Launch Vehicle, on its 15th flight, would carry the navigation satellite NVS-01 weighing 2,232 kg on Monday at 10.42 am from the second launch pad at the Satish Dhawan Space Centre (SHAR) here, about 130 km from Chennai.