Setting altimeters with no radio

Mxsmanic wrote:
Since it is possible to fly without voice radio equipment, and given
that (if I understand correctly) pilots are supposed to have their
altimeters set correctly to a reference located not more than 100
miles from their position, how does an aircraft without a radio keep
its altimeter properly set as it travels?

In my experience planes w/o radios can't go much more than 100 miles
between stops. It was a bit of a stretch in the Aeronca and worse in
the J-3

Mxsmanic wrote:

Newps writes:

Exactly, a system. WAAS is part of that system.

No, WAAS is a separate system.

GPS consists of three segments: Ground (control system), Space (the
satellites) and User (Receivers typically). WAAS is an augmentation
system to GPS. It is operated by the FAA whereas the GPS is operated
by the USAF.

Ron Lee

Mxsmanic wrote:

Steve Foley writes:

It's entirely possible he could give me an education on this subject, but
I'd rather go to a reliable source.

Better still, just don't depend on GPS for altitude.


Today I flew a real airplane and compared the GPS altitude from my GNS
430 with my altimeter. GPS was usually within 40' of the altimeter
which may not be the worst.

On the ground the GPS altitude was about 40' less than the GNS 430
indicated elevation of my airport but that number may be off since the
runway does slope.

Regardless, in this case GPS derived altitude was quite good.

Ron Lee

I have always found good agreement between GPS altitude and my altimeter.
And I've read all about the derivation of GPS altitude, and I wouldn't think
it would be so close.

"Ron Lee" wrote in message
...
Mxsmanic wrote:

Steve Foley writes:

It's entirely possible he could give me an education on this subject,
but
I'd rather go to a reliable source.

Better still, just don't depend on GPS for altitude.


Today I flew a real airplane and compared the GPS altitude from my GNS
430 with my altimeter. GPS was usually within 40' of the altimeter
which may not be the worst.

On the ground the GPS altitude was about 40' less than the GNS 430
indicated elevation of my airport but that number may be off since the
runway does slope.

Regardless, in this case GPS derived altitude was quite good.

Ron Lee

On Mon, 13 Nov 2006 18:07:31 -0500, LWG wrote:
I have always found good agreement between GPS altitude and my
altimeter. And I've read all about the derivation of GPS altitude, and I
wouldn't think it would be so close.

Part of the problem of comparing the GPS receiver output to the altimeter
output is that the GPS receiver output is digital and you end up seeing
the digits constantly changing... If you displayed the output of the GPS
receiver on an analog gauge, it might be a bit better... If you averaged
the altitude over multiple data point, it might even be even more
stable... The non-aviation GPS receivers usually update their position
every second... The Garmin GPSMAP 496 updates 5 times per second... From
what I understand, that 5 Hz update rate is considered the minimum for an
accurate aviation guidance system... I've seen others that operated at 10
Hz... The more messages that you can get per second, the more you can
average out the calculations to keep the altitude from radically jumping
around...

peter wrote:
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).

Your impression is supported by the published nominal accuracy of GPS:
+/- 10m horizontal, +/- 20m
vertical. The reduced vertical accuracy comes from the fact that
although the sats are at ~20,000 km
their horizontal spacing can be much larger than that. As you say, the
fix is 3D and it always is a 3D
solution once the minimum 4 sats are acquired for the solution. In
addition, if your receiver can hold more than 4 sats it may be able to
average the data to improve the fix. On my 12 channel boat GPS I see a
HDOP of ~1m these days. More important perhaps is that the aviation nav.
chart I use is not referenced to WGS84 which is strange as all my
sailing charts are. I understant that the "powers that be" are trying to
settle which geoid to use but I would imagine that the height
corrections could be quite large when it all gets sorted out. Is that
why the peak obstacle height over the sea is never marked as 0' (i.e.
MSL is not conforming to the geoid of reference)?

Cheers MC(student pilot)

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Mxsmanic wrote:

Ron Lee writes:


With SA off the vertical error is probably better that altimeter
accuracy requirements.


No. The figure of hundreds of feet is with SA OFF. It's even worse
with SA on.



The published accuracy of the vertical fix with 4 sats is +/- 20m. With
more sats and an averaging reciever I've seen the HD0P closer to 1m
which would imply a VDOP of just a few meters -without WAAS. I've read
somewhere that with new advances in data processing accuracies of the
order of 10cm should be possible without WAAS. I believe these methods
use propagation models to more accurately determine the GPS timings.

Cheers MC (student pilot)

Mxsmanic wrote:

Neil Gould writes:


Ever hear of WAAS?


Yes. And it's not part of GPS.


What do you think the WAAS is "augmenting"?

Cheers

Newps writes:

Wrong, as usual. It's all part of the same system.

That's not the way the DoD looks at it, and they built GPS.

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