The Indian Space Research Organisation (Isro) began the countdown for another satellite launch Tuesday evening. The countdown, which began 4.40 pm Tuesday, is for the launch of the RISAT-2BR1 Earth imaging satellite. The "spy satellite" will be carried onboard Isro's workhorse -- the Polar Satellite Launch Vehicle (PSLV) rocket. The PSLV rocket's launch is scheduled to take place from the Satish Dhawan Space Centre in Sriharikota at 3.25 pm Wednesday. This will be the rocket's 50th mission.The RISAT-2BR1 is a radar imaging Earth observation satellite that weighs around 628 kg. It is the second in the RISAT-2B series of satellites. The first RISAT-2B satellite was launched earlier this year to replace the ageing RISAT-2, which was decommissioned. Isro will launch two more RISAT-2B satellites in the coming days to complete a quartet of these "spy satellites". Not a lot is available in the public domain about the capabilities of the RISAT-2B satellites.Wednesday's launch of the RISAT-2BR1 comes days after the Isro successfully launched the CARTOSAT-3 Earth imaging satellite. On Wednesday, along with the RISAT-2BR1 Isro will also launch nine customer satellites -- six from the US and one each from Israel, Italy and Japan -- for which the space agency gets paid by other countries or companies.