The various satellites have different orbits and different altitudes. One constellation may do better with TDOA and others may be better with doppler. Different techniques are not mutually exclusive and actually tend to augment each other. The problem isn't the satellites or techniques (math is math) it is the sporadic nature of the target.

The fact that at this late date they haven't gotten the search area much smaller than it is tells me something stinks about the entire evolution, starting at Day One. There's a lot we're not being told, governments can be so secretive... But that's just my opinion.