Hi D1,
What do we need for the GPS to become the primary means of navigation
GPS is not the prime means of Navigation -but it can be the primary means of updating the GPIRS position.
The IRS are the primary navigators. Once they are aligned, the GPIRS compares the GPS position with the sensed IRS LAT (and possibly the last remembered LONG), and if within tolerance, will be accepted as primary position updating.
If it fails the test - then runway position will update the FMS position on take off and DME/DME position updates will be accepted when airborne.
Edit. A few places have had the incorrect ILS coordinates in the data base. Once locked onto the ILS LOC with the DME, the MAP position was "updated" to somewhere else.
