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None
14th Dec 2009, 00:16
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.

hugel
14th Dec 2009, 11:54
An arc second is one sixtieth of a nm measured at the equator. This is therefore about 31m between each of the terrain samples in the grid. So he is saying that the terrain samples are about 185m apart.

The next part makes no sense to me: "We can allow up to a 2 degree
shift and still get it right." Two degree shift in what ? I speculate that he could be saying that your that true position can be 2 arc seconds wrong, and still relate to the same terrain point.

Which function is the author describing ? It is also not clear from the information provided the sensitivity of the different EGPWS functions to the accuracy of the aircraft position.

hugel

None
14th Dec 2009, 13:36
The next part makes no sense to me: "We can allow up to a 2 degree
shift and still get it right." Two degree shift in what ?

Sorry about that...I copied only a portion of the text. I believe he is referring to the FMS position. However, that seems like quite a large error. I'm hoping he will reply to my latest e-mail for clarification.

Thanks

None
14th Dec 2009, 15:17
More info:

I'm not familiar with arc-seconds (sounds like an angular measurement)). Our IRS and FMS position errors are displayed in nautical miles, and rarely exceed 2 nm.

arc seconds is a measure of geography in mapping. The 2 nm
is a good number as a max deviation - we like to say 2 degrees but only
because it changes in and out of the airport area.