The 75 MHz beacons are going obsolete. An IFR GPS can substitute for an OM if it is in the database, which it will be. The FAA has been gradually decomissining the MMs on IFR approaches. The IM is only used for CAT1 or CAT 2 approaches. From a practical point of view, you really don't need a marker beacon receiver anymore for IFR flying with an IFR certified GPS.
