Dear Renu,

DRDO had a scheme for science/engineering PhD graduates called ROSSA (Registration Of Students with Scholastic Aptitude) in which a scholar of less than 35 years of age was eligible to apply for the post of Scientist C under this scheme throughout the year. Based on his/her area of research, DRDO used to shortlist the candidates for interview. But unfortunately now,  this scheme stands suspended .

There is no such scheme in ISRO as in my knowledge.

A PhD candidate can now apply only against the advertised vacancy in DRDO/ISRO. You can check out the links

www.rac.gov.in (http://www.rac.gov.in/)

ISRO - Government of India (http://www.isro.gov.in/)