The 71-hour countdown for the launch began at 6.47 AM on Monday and "it is going on perfectly", the official said.
The satellite is said to the heaviest in its category to be lifted by the country's workhorse rocket, the PSLV.
"Everything is going as planned for the launch scheduled for Thursday at 5.47 AM," he added.
India currently depends on images from a Canadian satellite as domestic remote sensing spacecraft cannot take pictures of the ground during cloud cover.
This is the third time that ISRO is using an PSLV-XL (Extra Large) rocket after first using it in October 2008 to put Chandrayaan-1 in orbit and again in July 2011 during the communication satellite GSAT-12 launch.
The approved cost of RISAT-1, including its development,is Rs 378 crore, while Rs 120 crore has been spent to build the rocket (PSLV-C19), making it a Rs 498-crore mission.
RISAT-1 will be launched into a 536-km orbit by PSLV at an altitude of 536km.
The satellite carries a C-band Synthetic Aperture Radar (SAR) payload, operating in a multi-polarisation and multi-resolution mode to provide images with coarse, fine and high spatial resolutions. PTI VIJ BN
