GPS altitude again is close to actual

I made another flight to KFTG this afternoon and periodically checked
the GPS receiver provided altitude versus my aircraft altimeter. GPS
was within 40' every time I checked (closer at times). I need to
check my last altimeter check to see if it has any errors but it is
certainly within standards so GPS is proving to be close enough for
VFR in these cold weather conditions.

Ron Lee

In article ,
(Ron Lee) wrote:

I made another flight to KFTG this afternoon and periodically checked
the GPS receiver provided altitude versus my aircraft altimeter. GPS
was within 40' every time I checked (closer at times). I need to
check my last altimeter check to see if it has any errors but it is
certainly within standards so GPS is proving to be close enough for
VFR in these cold weather conditions.

Ron, is your GPS WAAS enabled?
My non-WAAS Garmin 195 will only get within 150 feet vertically.
Vertical is the least accurate of the three planes.

john smith wrote:

In article ,
(Ron Lee) wrote:

I made another flight to KFTG this afternoon and periodically checked
the GPS receiver provided altitude versus my aircraft altimeter. GPS
was within 40' every time I checked (closer at times). I need to
check my last altimeter check to see if it has any errors but it is
certainly within standards so GPS is proving to be close enough for
VFR in these cold weather conditions.

Ron, is your GPS WAAS enabled?
My non-WAAS Garmin 195 will only get within 150 feet vertically.
Vertical is the least accurate of the three planes.

Good question. It is a Garmin GNS 430 without WAAS. I should try my
Etrex as well.

Of course I am not advocating using GPS to establish flight altitudes.

Ron Lee

I thought the primary factor in determining the differences between
baro altitude and GPS altitude was the difference between "sea level"
and the WGS84 datum plane at the particular location? From my
understanding these two planes of reference are not the same but are
closer in some spots than in others.

Marco

Ron Lee wrote:
john smith wrote:

In article ,
(Ron Lee) wrote:

I made another flight to KFTG this afternoon and periodically checked
the GPS receiver provided altitude versus my aircraft altimeter. GPS
was within 40' every time I checked (closer at times). I need to
check my last altimeter check to see if it has any errors but it is
certainly within standards so GPS is proving to be close enough for
VFR in these cold weather conditions.

Ron, is your GPS WAAS enabled?
My non-WAAS Garmin 195 will only get within 150 feet vertically.
Vertical is the least accurate of the three planes.

Good question. It is a Garmin GNS 430 without WAAS. I should try my
Etrex as well.

Of course I am not advocating using GPS to establish flight altitudes.

Ron Lee

"Marco Leon" wrote in message
oups.com...
I thought the primary factor in determining the differences between
baro altitude and GPS altitude was the difference between "sea level"
and the WGS84 datum plane at the particular location? From my
understanding these two planes of reference are not the same but are
closer in some spots than in others.

Marco

Pressure based altitude measurements also assume a standard pressure change
as a function of altitude that will not always be correct. This will cause
the difference between the altimeter and the GPS to change with altitude.

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

Marco Leon writes:

I thought the primary factor in determining the differences between
baro altitude and GPS altitude was the difference between "sea level"
and the WGS84 datum plane at the particular location?

In theory, yes. But in practice there is also a lot of inaccuracy in
altitude readings via GPS. The system is not designed to provide
highly accurate altitude, as the original users (soldiers in the
field, ships) didn't require it. For most users, it's a lot more
important to know where you are on the map than to know how high you
are above sea level.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Mxsmanic wrote:
Marco Leon writes:
I thought the primary factor in determining the differences between
baro altitude and GPS altitude was the difference between "sea level"
and the WGS84 datum plane at the particular location?

In theory, yes. But in practice there is also a lot of inaccuracy in
altitude readings via GPS. The system is not designed to provide
highly accurate altitude, [..]

And a simple Google search with "gps altitude accuracy" will bring up
quite a few interesting write-ups on the topic. All urge extreme
caution about trying to use GPS as an altimeter.

Kev

Mxsmanic wrote:


In theory, yes. But in practice there is also a lot of inaccuracy in
altitude readings via GPS.

You are the absolute last person, flying a sim, to make this statement.
This is simply not borne out by those of us who actually do compare
the GPS to the altimeter.

Mxsmanic wrote:
Marco Leon writes:

I thought the primary factor in determining the differences between
baro altitude and GPS altitude was the difference between "sea level"
and the WGS84 datum plane at the particular location?

In theory, yes. But in practice there is also a lot of inaccuracy in
altitude readings via GPS. The system is not designed to provide
highly accurate altitude, as the original users (soldiers in the
field, ships) didn't require it.

Altitude has always been very important to field soldiers and "ships"
whose responsibilities include flinging large projectiles long
distances. Among the multitude of variable factors of which indirect
fire requires calculation, the accuracy of the weapon and target
constants x,y and z are most important. It's tough to hit targets
12-20 miles distant if you don't know the difference in altitude
between weapon and target.

GPS, being a military system, was finally derived (IMO, and
suprisingly, opinions vary) by combining the technologies being
developed by the Navy (Project Timation) and the Air Force (System
621B).

Since both services incorporate aviation and one goal of military
avaition is to operate in "all weather" the ultimate goal of GPS/WAAS
altitude accuracy is to permit pilots to land safely in zero-zero
visibility without the need, expense (and redundant technologies) and
limitations of the present ILS.
See: http://gps.faa.gov/programs/waas/que...swers-text.htm

The altimeter/GPS altitude accuracy debate is beyond my expertise but
my experience includes a different altimeter indication after landing
at an airport I had left earlier in the day while my handheld $150 WAAS
capable GPSr indicated its identical reading to 1/10ft.

Barometric altimeters are analog and the finest divisions of the scale
IME are 20', so I would not expect them to be any more accurate than +-
10'. I think WAAS combined with DGPS will soon provide digital
accuracy... far beyond any ordinary need.

"Both horizontal and vertical changes in position can be measured to an
accuracy of a few millimeters (horizontal) to several millimeters
(vertical).
http://volcanoes.usgs.gov/About/What...ation/GPS.html

The GPS altitude accuracy "problem" is mostly attributable to the
limitations and generality of the WGS84 datum geoid height
http://www.colorado.edu/geography/gc...gif/geoid2.gif
which could have easily been overcome by WAAS correction and simply
adding more satellites (and postioning them optimally). Economics have
precluded this rush to solution.

The bottom line is, IMO, WAAS altitude is far more accurate than
altimeter... depending on how many birds are in view. 4 (depending on
positional diversion and attitude relative to the receiver), I'd trust
GPS if my life depended on it.
-----

- gpsman