topic has become very intereting.
ICAO doc 4444 refers two words-1. Radar Service 2. Radar Vectors
In Radar service the acft is identified and monitered. ATC shall give help if pilot asks for it or for some other reasons. But clearly the acft flies with its own navigation and is resposible for terrain clearance even if it has been allowed to go from one point to other as requested by pilot.
During radar vectors the navigation of acft is been taken over by ATC and hence he is wholly resposible for all terrain clearance. This is one of the reason why before when acft is asked to fly direct to a point after being radar vectored he must be given his position with reference to a navaid or way point so that now pilot can navigate of his own and hence become resposible for terrain cleance.
Regarding MSA
MSA is valid within 25NM of Navaid and acft can desend to MSA on QNH. The MSA is decided by the IAP expert based on hight of highest obstruction within 30(25+5 buffer) nm+1000feet. There maybe many other obstruction lower than it near approach path. Hence radar controller is authorised to descend acft to MSA or MVA only and make acft establish on final approach track may it be ILS, VOR or visual approach. In no case he shall descend the acft below MSA or MVA.
The Acft shall descend below MSA once established on Final approach track or Intermediate segment. The FAT and segments are designed by IAP experts taking into consideration obstructions near approach path which radar controller may not be aware of. The acft shall strictly follow the FAT while descending below MSA on her own navigation if unable shall immediately go around.
I wonder if I am able to clear few queries. Apologies for a big writeup.