Originally Posted by IO540
They will be going down to 250ft without vertical guidance or WAAS/EGNOS??
No. System minimum is not the same as OCH or MDH. The MDH will be the higher of system minimum and OCH. The point is that the system minimum for NDB is 300', so if GPS approaches are based on a 250' system minimum then there is the "potential for lower MDAs".
Originally Posted by IO540
No nonprecision approach can go that low.
Where the obstacle situation allows the system minimum and MDH can be close. For example, the VOR/DME system minimum is 250' and the Cat A MDH for the VOR/DME RWY 21 at Biggin Hill can be as low as 300'.