The 33-hour countdown for the launch of two earth observation satellites on-board PSLV from the space port of Sriharikota, about 110 kilometres from here, began at 1.08 PM on Saturday.

The Indian Space Research Organisation's (ISRO) trusted workhorse Polar Satellite Launch Vehicle-C42 (PSLV) would carry the satellites, NovaSAR and S1-4, together weighing over 800 kilograms, at lift off on Sunday at 10.07 PM, ISRO said in a release.

The foreign satellites, meant for forest mapping and flood and disaster monitoring, among other uses, would be released into sun synchronous orbit at a height of 583 km, it added.

They have been developed by Surrey Satellite Technologies Limited, United Kingdom.

The mission is a commercial arrangement between the company and Antrix Corporation Limited, which is the commercial wing of ISRO.

This would be the 44th flight of the PSLV and the third launch by ISRO this year.

In January, PSLV-C40 launched India's weather observation satellite CARTOSAT-2 Series and PSLV-C41 launched IRNSS - 1I navigation satellite in April. 

The IRNSS launch happened even as ISRO was trying to re-establish communication with the launched GSAT-6A communications satellite. While the IRNSS space segment is in place, India still has to wait a few months more before utilising it as the ground segment and receivers are yet to be deployed. Also, it remains to be seen if ISRO will order more spare satellites as the budgeted ones are already used up.

The IRNSS project had begun with the construction of the space segment in 2013 when the first satellite IRNSS-1A was launched and the seventh and last satellite was placed in orbit in 2016. With the space segment complete, ISRO had set out to calibrate the system. The CSIR-National Physical Laboratory is helping with synchronising atomic clocks on-board the satellites with Indian Standard Time (IST). A variety of receivers are being developed for providing information and services based on this system. Educational institutions also became part of this project, helping ISRO validate the system at various geographic locations of the country.