No, they really can't. At least not until GBAS is rolled out. GPS is notoriously unreliable for altitude data. GPS altitude is used for LPV approaches, provide

No, they really can't. At least not until GBAS is rolled out. GPS is notoriously unreliable for altitude data. GPS altitude is used for LPV approaches, provided you also have a WAAS correction signal, but those require higher weather minimums than the Cat II/III approaches that you'd use a radio altimeter for.