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Roger M. Derby writes:
 > Dire is a bit strong.  If Emerson's numbers are correct, DGPS gives you 33'
 > and raw gives you 50'.  For general aviation (no CAT II approaches) you need
 > 1800' of visibility to be legal for an ILS to 5R at Indianapolis.  That
 > gives you at least 18 seconds in which to sight the "runway environment" and
 > it should take you less than 1/4 second to correct that 17' offset, assuming
 > your hand and the winds were that steady to begin with.  If your safety is
 > compromised by that magnitude of error, then you made a major mistake in
 > commiting yourself to land at that time and place.  Airports for GA are less
 > than 10 minutes apart east of the Mississippi River.
[...]
Er, the 15 and 10 meter numbers I've cited apply to an absolute
position determination on the fly.  With averaging, allowing for
ionospheric conditions, etc. ,etc. at least centimeter results are
possible.  For aircraft, aside from positioning on a flight path,
attitude information is obtained by measuring the phase differences at
the nose, wing tips, tail and so on.  In that case, it's not where in
space the parts of the airplane are as where they are relative to each
other.  In this setting, millimeter resolution is possible.  Anyway,
it's my understanding that GPS-based landing systems are a very
localized version of DGPS.  In fact, one of the reasons US GPS landing
systems haven't been embraced by world commercial aviation is who
wants to think their air fleet is tied to the Yankee air pirate
navigation system?  By dropping SA, there's some hope that US makers
have a better chance to compete.
Rick
S/V One With The Wind, Baba 35

   

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