Hi Dan,

Sorry, my questions could have been better posed.


Do most modern aircraft rely on the GPS for navigation, and to what
extent do they rely on it?

What kind of modern aircraft are you talking
about--bizjets...airliners...military?

I had primarily airliners and military aircraft in mind, but I didn't
state it (as would have been helpful).



And is it true that such aircraft's navigation systems use maps
'optimised' for GPS?

What does 'optimised' for GPS mean?

I'm not sure. Somebody with whom I am having a 'debate' has asserted
that there is an altitude-dependent error component (if I may call it
that) on positions determined by GPS (and I presume that this alleged
error component affects the horizontal component of a 3D position, as
well as vertical (altitude) component). In his own words:

"GPS is optimised for sea level, Blanchefort [a mountaintop ruined
castle] is 467 metres above sea level, couple this with a slant range
to a satellite of several thousand miles and the curvature of the earth
and you have error. At least up to 100 metres..."

I personally can find no evidence which supports his claim that a) GPS
is optimised for sea level, or b) GPS coordinates obtained at a few
hundred metres above (mean)
sea level are affected by his alleged altitude-induced error.

I replied (verbatim quote - please excuse the sarcastic tone!):

"This is really bad news. Modern aircraft - many of which rely heavily
on GPS for navigation - are in big trouble then, aren't they? I mean,
if the error at just 467 metres above MSL is ''at least up to 100
metres'', then surely it must be several kilometres by the time we get
up to altitudes like 30,000 ft., right?"

His reply (with some non-essentials removed):

"Aircraft ... are using a map optimised to the GPS system and this is
the key point which seems to be passing you by."


What's behind your questions; what are you trying to get at? If you posted
that, you might get more suitable answers.

I hope that that is sufficiently answered above. I didn't want to
burden this group with extensive background information and endless
quotes from this slightly silly debate!


Regards,

David,
England.