WAAS Airport Costs?

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.

"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

The operative word is 'protection' and refers to what the system can
guanrantee it can bound the error to with a very high degree of
certainty. The integrity requirement says that there must be no HMI
99.several 9's% of the time. The actual error will vary (due to
satellite geometry, ionospheric activity, satellite(s) going out of
tolerance or being taken out of service for maintenance). WAAS must
detect and either correct or flag.

For more details than most would probably ever want to know, check out
the Tech Center PAN reports.
http://www.nstb.tc.faa.gov/reports/pan47_1004.pdf for example. Amongst
the plethora of statistics and charts, there's some good explanations
of the phrases being tossed about, such as "protection level" and
"HMI".

Regards,
Jon

Regards,
Jon

Dave Butler writes:
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.

That certainly clears that up!

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.


That certainly clears that up!

You're welcome. Maybe an example will be more helpful. Suppose that your true
altitude above the WGS-84 ellipsoid is 1000 feet, and the VPL is 100 feet. The
altitude as indicated on your TSO 146 WAAS-enabled navigator is assured to lie
between 900 and 1100 feet. The WGS-84 ellipsoid is the mathematical model of the
earth's surface used in GPS position calculations.