[pct-l] GPS Altimeters

[elee at microsoft.com (Eric Lee (GAMES))]

Mike wrote:
>
WAAS enabled units include the ability to receive signal from LAND based
signal stations.
Since land based stations are relatively unaffected by atmospheric
disturbances, they provide a more reliable/accurate additional reference
for GPS's to locate themselves on. 
>

Since this thread is overly geeky to begin with, I don't mind being
overly geeky myself: WAAS signals are actually generated at land-based
stations but are relayed through geo-stationary satellites.  There
aren't many of them and they're positioned over the equator so to people
in the Northwest, the closest WAAS satellite is almost on the horizon
and is often obstructed.

The idea here is that one major source of error in the GPS system is
that the signals from the GPS satellites can bent by the ionosphere on
their way to the surface.  This changes the time they take to travel to
a given point and thus can make the receiver believe its location is
different than it actually is.  WAAS uses land-based stations that
already know precisely where they are.  They monitor the incoming GPS
signals and note any discrepancies in the measured signal vs. the known
location, then they package up that discrepancy information and
broadcast it via the WAAS satellites to any GPS receives that happen to
be listening.  The information basically says, "Here is the way the GPS
signals are being distorted right now, so when you calculate your
position, take that into account."

It really improves GPS accuracy, if you can get it.

Eric