I think you need 6 satellites for the FDE, fault detection and exclusion, function in RAIM to be working, otherwise the reciever cannot see which satellite to exclude, just something is wrong, FD.
I’ve kind of come to the conclusion that we don’t need to know exact time to find our position, we need the exact time between signals with the same timestamp to be recieved.
But we’re on basis of that, getting the exact time for free.