Thx so far for your replayīs;
Our maintanace told me that if the GPS is inop, you are not allowed to fly with your NAV recievers in Manual tuning mode; your radio updates are inhibited.
They checked all IRSīs, and they very completly in limits, now they think only the representaion on the PROG 2/4 is incorrect. The issue is still under investigtion, but it caused a big wave through Boeing and our FMS Manufacturer.
After the upgrade to EGPWS there was also an "upgrade" of the Position Software, and now they think the bug happened there. Looks like we are flying around with this error in our System since there.
The Ground speedīs of the IRSīs was never greater than 4 kt, even with an IRS(3) navigational performance of 20 nm.