EMIT is correct. ANP is simply based on a standard value for the available sensors and their time in use. This is a probability value.
Your IRU's will tell you their error. It will be the IRU value you use when comparing it against the charts.
If the IRU's were showing a 20nm error, then I would check this against your charts. Our charts show that it is right on the edge of the limit where you would have to observe the IRUs on the next sector (20nm is not enough to reject it on the first flight).
If your aircraft is fitted with Enhanced GPWS, there should be a GPS card within the EGPWS computer.
Regards.
NSEU