EGPWS terrain database in arc-seconds?
A question has come up regarding FMS position accuracy and the performance of enhanced GPWS.
One answer from the manufacturer uses the term arc-seconds, and I am having trouble comparing that to FMS position accuracy (normally expressed in nm). Can someone help me understand the following statement?
The IRS provides position to the FMS which in turn provides the
information to the EGPWS in cases where the EGPWS is not connected to a
GPS source directly.
You are absolutely right the correct aircraft position is mandatory
for the EGPWS to operate properly. However the position data provided to
the EGPWS does not have to have the utterly precise position given the
terrain database is in 6 arc seconds. We can allow up to a 2 degree
shift and still get it right.