Setting altimeters with no radio

peter writes:

This indicates a basic lack of understanding of GPS technology.

It reveals exactly the opposite. That's how GPS determines position.

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.

Surprising though it may be, those "precise times of arrival" are the
sides of a triangle.

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.

Actually it does. The arrival times define spheres in 3D space around
the satellites (the geoid can also be used as a reference sphere).
The intersections of these spheres effectively isolate the position of
the receiver. It's just a fancy version of good old triangulation,
and it works very well.

Unfortunately, however, it is optimized for lateral positioning, not
vertical positioning. To achieve the same vertical accuracy as
lateral accuracy, a much higher measurement precision is required.
For this reason, vertical measurement accuracy is very poor.

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.

More than "somewhat" less accurate: they are usually unusable,
certainly for aviation.

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.

How were you able to accurately survey your altitude in the air?

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.

It's hard to do that in the air.

However, there are good reasons why barometric measurements are used
instead for aviation to ensure consistency and uniform procedures.

The main reason is that it's more accurate.

GPS altitude data is so poor and so variable that I've given up using
it even on the ground. It's almost never anywhere near surveyed
altitudes, and it drifts all over the place. Indeed, you can watch it
change as you stand still on the ground, and that's with SA turned
off. I definitely would not want to depend on that in the air.

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

Mxsmanic wrote:
peter writes:

This indicates a basic lack of understanding of GPS technology.

It reveals exactly the opposite. That's how GPS determines position.

No, your statement before was that it required measurement of angles
and "triangulation" whereas the actual procedure does not measure any
angles at all and is closer to "trilateration" or determining the
distances to the satellites at known positions

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.

Surprising though it may be, those "precise times of arrival" are the
sides of a triangle.

Not sure what you mean by times being equal to sides (the units don't
match for one thing), but again, no angles are measured by the GPS
receiver.

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.

Actually it does. The arrival times define spheres in 3D space around
the satellites (the geoid can also be used as a reference sphere).

The geoid is not a sphere but rather a complex empirically determined
surface that closely approximates MSL on the earth (i.e. it is
certainly not anything like a sphere around the satellites as you state
above). It is not used by the GPS in the initial position
determination but may later be used in converting the calculated height
above the WGS-84 ellipsoid to an equivalent height above MSL.

The intersections of these spheres effectively isolate the position of
the receiver. It's just a fancy version of good old triangulation,
and it works very well.

I agree it works well, but it doesn't involve measuring angles and is
therefore not "triangulation." I suggest you read the GPS tutorial at
Trimble's website.

Unfortunately, however, it is optimized for lateral positioning, not
vertical positioning.

No, the somewhat better horizontal vs. vertical accuracy is an inherent
consequence of not being able to receive signals from satellites that
are below us (and therefore blocked by the earth). That's not a
deliberate engineering optimization decision but just the way things
are.

To achieve the same vertical accuracy as
lateral accuracy, a much higher measurement precision is required.

No again. As the accuracy of GPS continues to improve, both the
horizontal and vertical accuracy gets better, but horizontal will
always be somewhat better so we won't achieve "the same vertical
accuracy." However, we can continue to improve both accuracies so that
they are good enough for most applications.

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.

More than "somewhat" less accurate: they are usually unusable,
certainly for aviation.

The FAA doesn't seem to think so since Garmin recently indicated that
600 GPS LPV approaches have been approved by the FAA providing for
certified GPS with WAAS to be used down to 200' (same as Cat 1 ILS).
See
http://gps.faa.gov/programs/waasnews.htm

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.

How were you able to accurately survey your altitude in the air?

I do my surveying on terra firma, but it is frequently also reasonably
high "in the air" i.e. on top of mountains. (Neither a GPS nor a
barometric altimeter cares if the 10000' below is occupied by a
mountain or by empty air.)

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.

It's hard to do that in the air.

Really? I find it very easy to do since the GPS receiver itself
indicates the satellite geometry and reception conditions.

However, there are good reasons why barometric measurements are used
instead for aviation to ensure consistency and uniform procedures.

The main reason is that it's more accurate.

You might want to check what instruments are used by surveyors to get
accurate altitudes. E.g. the altitude of Mt. Everest was revised
fairly recently based on use of GPS. A barometric altimeter would have
been useless for that task.

GPS altitude data is so poor and so variable that I've given up using
it even on the ground. It's almost never anywhere near surveyed
altitudes, and it drifts all over the place. Indeed, you can watch it
change as you stand still on the ground, and that's with SA turned
off. I definitely would not want to depend on that in the air.

Either your receiver is broken or you are using it incorrectly. (The
lack of knowledge about the fundamentals and ability to check on
satellite geometry suggests the latter possibility.). Of course
locations with poor GPS reception due to obstructions are far more
likely to be found on the ground than in the air.

peter writes:

No, your statement before was that it required measurement of angles
and "triangulation" whereas the actual procedure does not measure any
angles at all and is closer to "trilateration" or determining the
distances to the satellites at known positions

It does indeed measure angles, rest assured. Of course, it does a lot
more than that.

Not sure what you mean by times being equal to sides (the units don't
match for one thing) ...

Then let's just leave it at that. This is not a sandbox, and I don't
have time to play.

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

In article ,
Mxsmanic wrote:

This is not a sandbox, and I don't
have time to play.


You've got to be kidding. Count your posts.

--
Scott Post

"Mxsmanic" wrote in message
...
It does indeed measure angles, rest assured. Of course, it does a lot
more than that.

It is complex. But it's not triangulation. If it were, you would only need
two well-placed satellites to fix a position. You need three satellites to
fix a 2-D position and four to fix a 3-D.

-------------------------------
Travis
Lake N3094P
PWK

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)

DR wrote:
On my 12 channel boat GPS I see a
HDOP of ~1m these days.

HDOP is unitless.

Ron Lee

Ron Lee wrote:

DR wrote:

On my 12 channel boat GPS I see a
HDOP of ~1m these days.


HDOP is unitless.


Good point. I'm sorry that I was being so loose. But you could descibe
the 1 sigma dilutuion of precision in terms of distance at your position
and time -right? I was actually amazed to see that my Ryatheon GPS put
my boat right in the center of the correct dock -implying 2m accuracy
(or better)!

Cheers MC

DR wrote:

On my 12 channel boat GPS I see a
HDOP of ~1m these days.

HDOP is unitless.

Good point. I'm sorry that I was being so loose. But you could descibe
the 1 sigma dilutuion of precision in terms of distance at your position
and time -right? I was actually amazed to see that my Ryatheon GPS put
my boat right in the center of the correct dock -implying 2m accuracy
(or better)!

Cheers MC

I have to think about it. Position accuracy is given by this generic
equation:

DOP x UERE x 2 = position accuracy (2 sigma or 95%) where UERE is the
combination of the space and user equipment error sources.

Ron Lee

"Mxsmanic" wrote in message
...

No, it is not. It's especially dangerous for RVSM flight, but it's so
inaccurate that it should never be used for anything, except as a last
resort (if the altimeters disintegrate, or whatever).


It was posed as a possible means to adjust a pressure altimeter with no
radio. I'd say that it is better than nothing, in that case. Since you can't
be NORDO in the IFR system, separation is the responsibility of the pilot's
eyes anyway.

-------------------------------
Travis
Lake N3094P
PWK