Hey there !
There are four ways to get admission in IIA : GATE, CSIR NET , JEST and IIAST.
You need minimum 96 percentile in GATE to get selected for the Interview round in IIA.
NET and JEST qualified students can also apply for IIA.
IIAST is a national level exam conducted by IIA. The forms for IIAST comes in September and the exams are in major cities across India on second Saturday of December.
There are two courses available Ph.D and Int. M.Tech- PhD.
For engineers, in the IIAST written exam,they basically ask basic astronomy, EMT, Signals, Digital & Analog electronics, Control systems, Optics and basic 12th standard physics.
Almost 4-5 students selected each year for M.Tech -PhD.
If you have qualified IIAST then you need not to qualify GATE or any other test. At the time of interview, it doesn't matter from which National level test you have come.
To know how to prepare for IIA Interview, you can ask any student of IIA Int. Mtech-PhD programme. Their email IDs are available on IIA website.

All the best !