Chennai: The countdown for the Monday midnight launch of an Indian navigation satellite is progressing smoothly with the liquid fuel filling for the fourth stage engine getting completed, an official said Sunday.

"Saturday evening, the filling of the liquid fuel in the fourth stage was completed and the fuel for the second stage will be filled during the countdown. In addition, pressurisation of the stages with gases will also be done," a senior official at the Indian Space Research Organisation (ISRO) told IANS Sunday.

The 64.5 hour countdown began at 7:11 a.m. Saturday.

The 44-metre-tall Polar Satellite Launch Vehicle-XL (PSLV-XL) weighing 320 tonnes at lift-off is a four-stage rocket powered by solid and liquid propellants alternatively.

The solid fuel hydroxyl-terminated-poly-butadiene comes readily cast while the liquid fuel -- unsymmetrical dimethyl-hydrazine-hydrate and 25 percent of nitrogen tetroxide for second stage and mono-methyl-hydrazine and mixed oxide nitrogen for fourth stage -- are filled during the countdown.

The PSLV will blast off into the dark skies from Sriharikota, around 80 km from here, Monday night around 11.41 p.m. carrying India's first navigational satellite the 1,425 kg IRNSS (Indian Regional Navigational Satellite System)-1A.

"We have had late evening and early morning launches. But this is the first time ISRO is launching a rocket around midnight," the ISRO official said.

The official said the launch time has been fixed taking into account the orbit and inclination at which the satellite will be injected into the space.

According to him, the weather at Sriharikota is fine and it may not pose any hindrance for the rocket launch.

He said ISRO chairman K. Radhakrishnan is expected to have a brief meeting with the media at the rocket port post-launch which will be around 12.45 a.m. Tuesday.