
WAAS Up? - naish
http://www.cringely.com/2009/05/waas-up/
======
stratomorph
Minor points in the comments:

 _The reason this doesn’t matter is that we don’t land airplanes with GPS
really. We direct them down a path to the airport and then the pilot flys at
minimum the last 200 feet with the runway in sight. If he can’t see it, he
doesn’t land. Airliners are a different animal but they don’t use GPS for that
either._ (Mike C)

 _I couldn’t imagine anyone being silly enough to think that planes rely on
GPS to figure out their altitude above the runway._ (P)

 _WAAS-qualified avionics like Garmin’s 430W, 480W and 530W receivers are
approved for precision instrument approaches based entirely on GPS._ (Robert
X. Cringely)

There's some truth mixed in with some misunderstanding there. While GPS is
able to measure altitude, aircraft systems don't rely on it for a primary
altitude measurement. They already have perfectly good barometric systems
which measure air pressure and are adjusted for field conditions every hour
(or so). On a GPS precision approach (that is, one with both horizontal and
vertical guidance) the computer combines the (x,y) information from the GPS
with the altimeter's data to produce ILS-type course and glideslope deviation
indicators, showing how far from the desired approach course the aircraft is
in the left/right and up/down axes.

So, while you can in fact fly an approach where your position data is based
solely on GPS, that z-axis is still being measured by an altimeter. (Unless
Mr. Cringely is certified to fly cat II or III approaches, in which case he
needs a LAAS-certified receiver and suitably equipped airport, as unlike WAAS,
it is ground-based:
<http://en.wikipedia.org/wiki/Local_Area_Augmentation_System>)

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lutorm
Except that the WAAS geostationary satellites are so widely separated that you
can't see both at the same time, and in most locations the one visible is also
very low in the sky and easily obstructed.

Edit: apparently my info is out of date, the two new WAAS satellites have
better positioning.

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bcl
Sounds like dueling bureaucracies. There is also the LORAN system, which was
resurrected a few years back and is reportedly now on the chopping block
again. Even if the GAO's dire predictions aren't true it would probably be
wise to have a redundant system in place. Shipping also depends heavily on
GPS, now that it has been certified as a primary navigation tool.

~~~
lucumo
_> Even if the GAO's dire predictions aren't true it would probably be wise to
have a redundant system in place._

Good thing then that the EU is building its own system.
<http://en.wikipedia.org/wiki/Galileo_(satellite_navigation)>

~~~
Create
despite:

<http://www.hajnalka-vincze.com/Publications/132>

[http://74.125.77.132/search?q=cache:bbg0mOuVSmgJ:www.hajnalk...](http://74.125.77.132/search?q=cache:bbg0mOuVSmgJ:www.hajnalka-
vincze.com/Publications/91+European-American+space+battles)

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eli
_But I couldn’t find any stories that put the whole thing together and
questioned whether there was any news value at all._

Isn't that kinda like proving a negative? Why would anyone write a story about
something they have concluded has no news value?

