"Dave Butler" wrote in message
news:1108735101.414417@sj-nntpcache-5...
See http://www.nstb.tc.faa.gov/vpl.html for a map that is updated every

six

minutes.

What is the 'vertical protection level' on the map? Is this the altitude
(in
meters!) that waas is guaranteed? If so, it looks like over 120 meters
coverage is everywhere. This can't be, can it? What am I missing?

From the WAAS Terms & Definitions link at the bottom of the page:

Vertical Protection Level (VPL). The Vertical Protection Level is half the
length of a segment on the vertical axis (perpendicular to the horizontal
plane
of WGS-84 ellipsoid), with its center being at the true position, which
describes the region that is assured to contain the indicated vertical
position.
It is based upon the error estimates provided by WAAS.

Ok. I have read this perhaps 6 times. Well, now 8. So it relates to the
probability of my altitude readout being within a certain range of actual.
That may not be quite it, but the chart makes more sense now. There are
areas with more or less uncertainty, which varies over time.

Thanks,
Harvey