Chennai, Dec 16 : The Indian space agency said the countdown for the December 17 evening launch of communication satellite CMS-01 (formerly GSAT-12R) began at 2.41 p.m on Wednesday and the fuel for the rocket PSLV-C50 has been filled. The Indian Space Research Organisation (ISRO) will be using its Polar Satellite Launch Vehicle (PSLV) rocket's XL variant numbered as PSLV-C50. During the countdown, the rocket and satellite systems will be checked while the liquid fuel for the rocket will also be filled. According to ISRO, the PSLV-C50 rocket carrying CMS-01 is tentatively scheduled to blast off at 3.41 p.m. on December 17 from the second launch pad at the rocket port in Sriharikota, in Andhra Pradesh. The launch is subject to weather conditions, the ISRO had earlier said. CMS-01 is a communication satellite envisaged for providing services in Extended-C Band of the frequency spectrum, which will include the Indian mainland, Andaman and Nicobar and Lakshadweep Islands. The 42nd communication satellite of India, it will have a mission life of seven years. The 44-metre-high four staged/engine PSLV-C50 is the 22nd flight of PSLV in 'XL' configuration (with six strap-on motors hugging the first stage). The PSLV in normal configuration is a four stage/engine expendable rocket, powered by solid and liquid fuels alternatively, with six booster motors strapped on to the first stage to give higher thrust during the initial flight moments. The Indian space agency has PSLV variants with two and four strap-on motors, larger PSLV-XL and the Core Alone variant without any strap-on motors. The choice of the rocket to be used for a mission depends on the weight of the satellite and the orbit where the satellite has to be orbited. About 20 minutes into its flight, PSLV-C50 will eject CMS-01 into geosynchronous transfer orbit (GTO) and from there, the satellite will be taken up and positioned at geosynchronous stationary orbit. The CMS-01 will be a replacement for GSAT-12 that weighed 1,410 kg and was launched on July 11, 2011 with a mission life of eight years. ISRO Chairman K. Sivan had earlier said the PSLV-C50 rocket will be followed by the launch of new small rocket Small Satellite Launch Vehicle (SSLV) carrying EOS-02 (Earth Observation Satellite), and Geosynchronous Satellite Launch Vehicle-F10 (GSLV) carrying EOS-3. The other Indian satellites that are ready for launch are GISAT and Microsat-2A. The launch of the GISAT-1 satellite slated for March 5 this year was postponed due to technical reasons a day before the launch. The GISAT-1 satellite will be carried by a GSLV rocket. The GSLV rocket was dismantled after the launch was called off, and is being refurbished. The rocket's cryogenic engine has been brought down and it is being readied again. The GSLV carrying GISAT-1 is expected to fly after PSLV C50. /IANS