See ICRB usually releases examination notification for Scientist posts in ISRO. Mainly this vacancies are for BTech/MTech as well as MSc Physics/Electronics students. Its been a long (as per the information on internet) that there are centralized recruitment in Mathematics. I would recommend you to visit the career section in the different ISRO centers such as:

• Vikram Sarabhai Space Centre (VSSC)
• Liquid Propulsion Systems Centre (LPSC)
• Satish Dhawan Space Centre (SDSC) SHAR.
• U R Rao Satellite Centre (URSC)
• ISRO Propulsion Complex (IPRC)
• Space Applications Centre (SAC)
• National Remote Sensing Centre (NRSC)
• ISRO Telemetry, Tracking and Command Network (ISTRAC)

In this websites you can find opening for the Mathematician although I would suggest you have to complete your MSc in order to be eligible for most of the opennings.

I hope this answer helps. All the very best for your future endeavors!