"The countdown for the launch started at 9 AM today. The countdown will be for 50 hours. All the mandatory checks will be made in PSLV-C18, before the launch. The launch is scheduled at 11 AM on October 12," ISRO spokesman S Sathish told PTI.
Megha-Tropiques will carry three payloads - two by France's space agency CNES and one jointly by ISRO and CNES - and a complementary scientific instrument.
ISRO has built Megha-Tropiques, an advanced tropical climate monitoring satellite, at a cost of Rs 80 crore with "equal contribution" from French government space agency, Centre National d'Etudes Spatiales.
Megha-Tropiques (Megha meaning cloud in Sanskrit and Tropiques denoting tropics in French) will investigate the contribution of water cycle in the tropical atmosphere to climate dynamics.
The other three nano satellites are one each from Luxembourg, the indigenous 'Jugnu' of IIT-Kanpur and SRM University, Chennai.
Data received from 'Jugnu' will be studied with a tracking system installed at IIT-Kanpur and pictures and information received from it will be used for research.
Jugnu will also help gather information on floods, drought and disaster management.
This will be the fourth ISRO mission this year, including one launched from French Guyana.