Setting altimeters with no radio

"Neil Gould" wrote

From the Garmin site: "A WAAS-capable receiver can give you a position
accuracy of better than three meters 95 percent of the time. "

My experience with my 295 bears this out.

So you are trying to convince the town idiot, now?

You know better than that.
--
Jim in NC

Doug writes:

My IFR GPS, a King KLN90B is connected to the altimeter in my
transponder. It is also adjustable to the barometric setting.

It's the altimeter that provides the accuracy for measurement of
altitude in that case, not the GPS.

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

Neil Gould writes:

Ever hear of WAAS?

Yes. And it's not part of GPS.

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

Mxsmanic wrote:
"Jim Macklin" writes:

they give altitude, accurate to within a few feet.

Unfortunately, no, they do not. GPS is accurate for lateral
navigation, not vertical navigation. GPS altitudes can easily be off
by as much as 200 feet at ground level in comparison to a correctly
set altimeter, and at altitude the disparity can reach 500 feet.

The reason for this is that the angles used for triangulation of
lateral positions are large and permit a high level of precision, but
the angles for triangulation of altitude are very small and it's very
easy to be off by a wide margin.

This indicates a basic lack of understanding of GPS technology. The
GPS receiver never deals with measurement of any angles nor with
triangulation. What is measured are the precise times of arrival of
the signals from the satellites. Since the satellites encode the
signals with timing information from their sychronized atomic clocks
and also send detailed orbital data to define their own positions, the
receiver is able to determine the relative distances to the various
satellites based on the speed of light/radio and the observed relative
signal delays. Using this distance information together with the known
positions of the satellites then allows for a determination of the
position of the receiver. Note that this never involves a measurement
of any angles.

It is true that altitude measurements are generally somewhat less
accurate than horizontal position measurements due to the basic
geometry of receiving satellite signals from only the satellites that
are above you. Ideal measurement of altitude would also involve some
satellites below you but of course their signals are blocked by the
earth. Similarly, east-west positions are a bit better accuracy than
north-south since the satellites are equally likely to be east and west
of you but there's a greater likelihood of them being to the south
rather than the north (at least from the northern hemisphere).

My long-term evaluation of GPS altitude accuracy has shown that I get
values within 35' of accurately surveyed altitudes at least 95% of the
time ever since Selective Availability was turned off. Using the WAAS
correction data improves this to get the accuracy down to 20' with 95%
confidence. Both of these are based on having a reasonably
unobstructed view of the sky (which generally isn't hard in an aircraft
unless the antenna is poorly positioned).

So from a technical standpoint GPS altitudes these days are pretty good
although some care should be taken to check the actual satellite
geometry and reception at the time of any critical measurements.
However, there are good reasons why barometric measurements are used
instead for aviation to ensure consistency and uniform procedures.

You rarely get AIDS from a "real person of the opposite
gender" which is why GAY means "got aids yet."

This does not include the use of IV drugs, the other big
vector.


"Roy Smith" wrote in message
...
| In article ,
| Mxsmanic wrote:
|
| new_CFI writes:
|
| Unfortunatly you dont die in a sim.
|
| Why is that unfortunate? That's one of the advantages
to simulation.
| Indeed, when you clip a mountaintop east of Telluride
because you
| trusted the GPS altitude in your simulator, you learn a
valuable
| lesson that may keep you from getting killed in real
life.
|
| One of the advantages of masturbation is that you can't
get AIDS from it.
| That hardly makes it a complete substitute for a real
person of the
| opposite gender.

never said it would, said it was better than nothing.



"Peter Duniho" wrote in
message ...
| "Jim Macklin" wrote
in message
| ...
| they give altitude, accurate to within a few feet. just
set
| the altimeter to read the same. then you know adjusted
| pressure. considering the legal requirements, it is
| perfectly adequate.
|
| First of all, no they don't give altitude accurate to
within a few feet.
| Secondly, they give a completely different kind of
altitude measurement than
| the altimeter provides. Even if the GPS were accurate to
within feet for
| altitude, setting your altimeter to the GPS displayed
altitude would not be
| the same as having the current, local altimeter setting.
|
| Pete
|
|

In article ,
(Ron Lee) wrote:

Correction... Encoding altimeter input is the additional source for
RAIM determination.

Nit: encoding altimeter CAN be an additional input. It is not required.

--
Bob Noel
Looking for a sig the
lawyers will hate

".Blueskies." wrote in message
t...
What about GPS precision approaches...where does the glideslope
information come from?


WAAS?

WAAS is insufficient for a precision GPS approach. GPS precision approaches
use "LAAS", which is basically the same as WAAS except that the differential
station is much closer to the airport (I suppose in some or many cases it
may even be colocated...not sure). It's "local area" instead of "wide
area".

WAAS and LAAS are both a form of differential GPS, and it's true that both
increase the accuracy of GPS significantly, LAAS more so than WAAS.
However, a) Jim never restricted his claim to GPS using WAAS, and b) even
with WAAS (or LAAS, for that matter), the GPS does not indicate the same
altitude that a properly set altimeter would.

Pete

Peter Duniho wrote:

".Blueskies." wrote in message
t...
What about GPS precision approaches...where does the glideslope
information come from?


WAAS?

WAAS is insufficient for a precision GPS approach.

Nonsense. A fully deployed WAAS can provide precision GPS appoaches with
performance comparable to ILS (Cat 1).

GPS precision approaches
use "LAAS",

Not necessarily. In addition to WAAS, JPALS may be used in the future.

which is basically the same as WAAS except that the differential
station is much closer to the airport

LAAS transmits signals on the UHF band. WAAS does not. LAAS can eventually
provide more accuracy.

Jim Macklin wrote:
You rarely get AIDS from a "real person of the opposite
gender" which is why GAY means "got aids yet."

This does not include the use of IV drugs, the other big
vector.

Good lord, how freakin off topic can this get. Politics, religion and
miscellaneous hatred shouldn't be a part of aviation, can't we all just
get along.