GPS altitude again is close to actual

Ron Lee writes:

Altitude accuracy is not as good as horizontal for reasons already
mentioned...not because of a design issue.

It's also a design issue. GPS was designed to provide highly accurate
lateral positioning; vertical positioning was just a fringe benefit
and no great effort was put into making it accurate. The intended
users of GPS didn't need much vertical accuracy, as they were
typically on the ground or on the sea.

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Newps writes:

You are the absolute last person, flying a sim, to make this statement.

There isn't any connection between flight simulation and GPS.

This is simply not borne out by those of us who actually do compare
the GPS to the altimeter.

Do what you want, as long as I'm not your passenger and not in your
airspace. Some people learn best through experience.

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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

N2310D wrote:

"Ron Lee" wrote in message
Never say never, as the old saw goes....
Having said that, I would never use my GPS altitude indication for
anything other than driving my car over the Rockies or Sierras.
On the other hand, the KLN-89B in the 172SP I used to fly had the
ability to dial in the local baro setting. I don't recall the delta between
the 89B and the legal altimeter being more than 20 feet with the baro date
set in. Biggest problem was the 89B altitude info was all but useless for
anything other than audible altitude deviation alert. That and having to dig
out the book and flip through the pages to remember how to input the baro
data. And, as I recall the altitude alerting was not legal for Part 91
either

You input the baro by hitting the ALT button. Not sure much page flipping
is required for that. But entering in the current pressure is to allow
an input pressure altitude to be compensated for, not the altitude calculated
from the GPS satellites. Finding the GPS altitude does require a certain
amount of buttonage. You must have had MODE S , my MODE C will only be within
100 feet of my "legal" altimeter.

--
Don Poitras

I think WAAS combined with DGPS will soon provide digital
accuracy...

What is "digital accuracy"?

Jose
--
"Never trust anything that can think for itself, if you can't see where
it keeps its brain." (chapter 10 of book 3 - Harry Potter).
for Email, make the obvious change in the address.

gpsman writes:

Altitude has always been very important to field soldiers and "ships"
whose responsibilities include flinging large projectiles long
distances.

The altitude of a ship is sea level. The altitude of an infantry unit
is marked on its maps.

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.

WAAS isn't part of GPS.

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.

There's nothing inherently accurate about digital systems. No digital
system can be more accurate than the best analog system.

"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

Not in a tenth of a second. You can average readings over long
periods and get extremely accurate measurements, but that technique is
useless to aircraft.

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 problem is linked to the basic design of the system.

The bottom line is, IMO, WAAS altitude is far more accurate than
altimeter... depending on how many birds are in view.

WAAS isn't part of GPS.

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Jose writes:

I think WAAS combined with DGPS will soon provide digital
accuracy...

What is "digital accuracy"?

It's a marketing term, which translates roughly to "fleecing the
uninformed."

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"Mxsmanic" wrote in message
...
gpsman writes:


WAAS isn't part of GPS.


WAAS isn't part of GPS.


For the purposes of GPS's use in aircraft it most certainly is.

Mxsmanic wrote:

Jose writes:

I think WAAS combined with DGPS will soon provide digital
accuracy...

What is "digital accuracy"?

It's a marketing term, which translates roughly to "fleecing the
uninformed."

I am not sure of all of MX's biases but GPS provides a far better
navigation, positioning and timing service globally, free than
anything he or his country has done.

Ron Lee

Ron Lee wrote:
GPS provides a far better
navigation, positioning and timing service globally, free than
anything he or his country has done.


He's American.

Are you talking about GLONASS being better?