How different is aviation GPS?

Paul Tomblin wrote:

As a former surveyor, I can tell you that the altitude requirements for
surveying are a lot more precise than for aviation - if my bridge abutment
is built 2 feet high, I'm getting fired. If my plane is 2 feet high,
nobody is going to notice.


Looks as if I'm missing something central here, as usual

If there is a 4-hour flight that passes over some 10 waypoints and if
the FMGS keeps getting data that's off by 50 meters or so, am I to
understand that the aircraft will still make heading changes, etc.
that'd be in accordance with the programmed flight plan and that none
of the waypoints will be missed or indeed the final destination
precisely arrived at?

And someone mentioned an acceptable accuracy of 0.1 foot in property
surveying. If surveyors in my industry had that much latitude, there'd
be a lot of equipment skids that'd get installed quite inappropriately,
with lots of patched modifications thereon

Ramapriya

In article .com,
wrote:

Looks as if I'm missing something central here, as usual

If there is a 4-hour flight that passes over some 10 waypoints and if
the FMGS keeps getting data that's off by 50 meters or so,

the errors don't accumulate.

wrote in message
oups.com...
Looks as if I'm missing something central here, as usual

Could be.

If there is a 4-hour flight that passes over some 10 waypoints and if
the FMGS keeps getting data that's off by 50 meters or so, am I to
understand that the aircraft will still make heading changes, etc.
that'd be in accordance with the programmed flight plan and that none
of the waypoints will be missed or indeed the final destination
precisely arrived at?

That is correct. The presumed 50 meter accuracy is constant throughout the
flight. It's not as though it's additive for each waypoint (or worse, as a
continuous function along the flight). Though frankly, even if it were,
you'd only be off by 500 meters after 10 waypoints which is still "no big
deal".

That's one of the many nice things about GPS. It is a continuous readout of
one's current position and any forward-looking navigation solution can be
derived from the instantaneous position information, without any previous
position information affecting the future calculations.

And of course, again...being 50 meters off in aviation just isn't that big
of a problem. Heck, being a mile off in aviation isn't that big of a
problem most of the time.

And someone mentioned an acceptable accuracy of 0.1 foot in property
surveying. If surveyors in my industry had that much latitude, there'd
be a lot of equipment skids that'd get installed quite inappropriately,
with lots of patched modifications thereon

Well, as that poster pointed out, it depends on the situation. But property
surveying for non-commercial purposes isn't likely to be used for any sort
of actual construction (except possibly locating a building, and for sure no
one is going to care if a building is off one inch one direction or another,
especially in a non-commercial situation).

As for equipment skids and such, since I don't know the details of your
industry I can't really comment on that. But it seems to me that if you
require that level of detail and are using GPS to accomplish it, you must be
dealing with positioning these skids at a significant distance from wherever
they are referenced to. Otherwise, I'd think one would use more
"conventional" surveying techniques to determine position, orientation, etc.

The only surveys I've ever hired were strictly property surveys, in which
property boundaries are determined, locations of roads, trees, terrain
contours, etc. Getting the results to within an inch is perfectly
sufficient for that type of survey. It's not hard to imagine a wide variety
of surveys for which the same holds true.

Just because some situations demand higher precision, that doesn't mean all
situations do.

Pete

Peter Duniho wrote:

That is correct. The presumed 50 meter accuracy is constant throughout the
flight. It's not as though it's additive for each waypoint (or worse, as a
continuous function along the flight). Though frankly, even if it were,
you'd only be off by 500 meters after 10 waypoints which is still "no big deal".

Now I know why planes occasionally land on the wrong runway in IFR
conditions )

As for equipment skids and such, since I don't know the details of your
industry I can't really comment on that. But it seems to me that if you
require that level of detail and are using GPS to accomplish it, you must be
dealing with positioning these skids at a significant distance from wherever
they are referenced to. Otherwise, I'd think one would use more
"conventional" surveying techniques to determine position, orientation, etc.

When I began my career, there were only conventional surveying
equipment. These days, however, rarely do you get a client in the oil
and gas industry who'll accept anything but a GPS survey. Btw, we
engineers are barred too from all surveying, although we're responsible
for supervision and copping attendant liabilities. The actual task
itself is carried out by qualified surveyors who do nothing else

Ramapriya

wrote in message
oups.com...
Now I know why planes occasionally land on the wrong runway in IFR
conditions )

Well, for what it's worth, wrong-runway (or even wrong airport) landings are
much more common in visual conditions, when the pilot is trusted to find the
runway himself.

Flying an ILS involves tuning a radio to a frequency that is specific to the
runway. If an airplane winds up in a position to land on the wrong runway,
then the ILS indication will be so far off that the pilot should be flying a
"missed approach" (that is, aborting the approach to try again).

In visual conditions, simple human error can result in landing in the wrong
place. In instrument conditions, there needs to be a series of poor
judgment decisions on the part of the pilot (or the simple error of tuning
the wrong frequency into the ILS receiver, of course ...but that's not an
"accuracy of guidance" issue, so it doesn't seem relevant in this
discussion).

Pete

wrote in message
oups.com...
Peter Duniho wrote:

That is correct. The presumed 50 meter accuracy is constant throughout
the
flight. It's not as though it's additive for each waypoint (or worse, as
a
continuous function along the flight). Though frankly, even if it were,
you'd only be off by 500 meters after 10 waypoints which is still "no big
deal".

Now I know why planes occasionally land on the wrong runway in IFR
conditions )

As someone already mentioned, the error is not cumulative. Then, too,
approach GPS is a different animal from enroute.

Now I know why planes occasionally land on the wrong runway in IFR
conditions


You do? I don't. In fact, I don't think they do. That happens in visual
conditions.

--
Thomas Borchert (EDDH)