As pointed out by wickerman we always cross check the accuracy of the INS's by reference to a suitable VOR before trusting the accuracy of the map display.
This leads to the question that if and when VOR signals are turned off what reference are we going to cross check against? GPS? Not ground based either. At the moment I feel a lot happier checking the position against a known ground reference such as VOR.
Sure GPS is more accurate, but when it degrades it can surely not be used to cross check another system? What about microwave links? I understand that microwave approaches are already available so it would appear to me that ground based microwave signals would be the next step on from VOR.
Thoughts?
PP
ps Blacksheep, can you enlighten me as to GNSS/EGNOS? Not come accross the terms before.