Paul Saccani wrote in
:

On Mon, 8 Sep 2008 20:46:32 +0000 (UTC), Bertie the Bunyip
wrote:

"Sr20goer" wrote in
:


"GB" wrote in message
...
Michael Henry wrote in
news:00d3a924$0 :
SNIP

It's not all that archaic. Aviation trails the bleeding edge by a
very long margin, and with good reason. The bleeding edge usually
draws blood at some point! I was initially surprised to learn, in
circa 1999, that the B767 didn't use GPS for navigation. I
subsequently learned that they don't need it... GPS isn't really
up to scratch. (I'm not gonna explain that here, that's fodder
for another post!)

GB

GB
You referring to INS?
But they do now use GPS for 'local' navigation (SID, STAR, flextrack
etc). Brian


No, they use GPS to update the INS.

Aren't both sources integrated via a fast Kalman filter? GPS errors
are rapid and errors tend to clump around an accurate position, whilst
INS errors are slow and tend to drift away from an accurate position.
Integrating the systems via a Kalman filter results in a far more
accurate and precise position.


No idea. Works well, lasts a long time.
When we dont use GPS, we use radio update of another type, usually. VOR
DME. Without it, even the best INS can drift appreciably over time. It's
not uncommon to see an error of a couple of miles after a long flight,
and errors of 20 miles are possible after an ocean crossing. Radio
update pretty much eliminates this and I've never seen any map shift
whatsoever on a GPS updated system.