The core alone PSLV-C18, the 20th flight in the PSLV series, would lift off from the first launch pad at 11 am tomorrow morning.

The 50-hour countdown for the launch, which began at 9 am yesterday morning, was progressing smoothly, ISRO sources said.

''The countdown is progressing with butter smoothness. There are no hitches so far. The fourth stage has been fuelled up. Fuelling of the second stage will be taken up this evening,' the sources added.

Apart from the Megha Tropique satellite, which would study climatic and atmospheric changes in tropical regions, three other nano satellites would ride piggy back.    

The three nano satellites are the 10.4-kg SRMSAT built by students of SRM University, the three kg remote sensing satellite Jugnu developed by the Indian Institute of Technology-Kanpur and the 28 kg VesselSat from Luxumbourg to locate ships on high seas.

ISRO sources said about 22 minutes after lift off, the PSLV-C18 would first inject the Megha-Tropiques satellite into an orbit of 867 km altitude at an inclination of 20 deg with respect to the equator. It would be followed by the injection of SRMSAT, VesselSat and Jugnu.

The entire flight sequence and the satellite injection process was likely to last about 25 minutes. Megha-Tropiques is an Indo-French Joint Satellite Mission to study the water cycle in the tropical atmosphere in the context of climate change.

The sources said the satellite would study the water cycle and energy exchanges in the tropics. ''The main objective of this mission is to understand the life cycle of convective systems that influence the tropical weather and climate and their role in associated energy and moisture budget of the atmopshere in tropical regions,' the sources added.

Carrying four key scientific instruments -- MADRAS, SAPHIR, ScaRaB and ROSA -- Megha-Tropiques would provide scientific data on the contribution of the water cycle to the tropical atmosphere, with information on condensed water in clouds, water vapour in the atmosphere, precipitation and evaporation.    

While the Scanning Microwave Imager for Detection of Rain and Atmospheric Structures (MADRAS) was developed jointly by French Centre National d’Etudes Spatiales (CNES) and ISRO, the Sounder for Probing Vertical Profiles of Humidity (SAPHIR) and the Scanner for Radiation Budget (ScaRaB) were developed by the CNES.

The Radio Occultation Sensor for Vertical Profiling of Temperature and Humidity (ROSA) was procured from Italy.

MADRAS, a microwave imager, was to study the high level ice clouds associated with the convective systems, and would provide an estimation of rainfall, water vapour, liquid water, ice and surface wind.

While ScaRaB would study the radiation received by the earth and reflected by it, SAPHIR, a sounding instrument, would study the humidity present in the tropical atmosphere.

The 10.9 kg SRMSAT was aimed at addressing the problem of Global warming and pollution levels in the atmopshere by monitoring carbon-dioxide and water vapour.

It uses a grating Spectrometer, which would observe absorption spectrum over a range of 900 nm to 1700 nm infrared range.