How different is aviation GPS?

As part of my work (civil engineer), I routinely use GPS equipment in
surveying. And construction specifications usually calls for this
equipment to be held stationary for as much as three hours where
crucial transition points are to be located, and for up to twenty
minutes at less important locations.

I guess since you folk use GPS to navigate all across the globe and
requires to be both very precise and instantaneous, my equipment is
very much inferior to what's used in aviation.

Has anyone here used the type of equipment I'm mentioning? You should
see the time the thing I use takes to stabilize itself to show the
elevation... you'd comfortably CFIT if you had that in your airplane
)

Ramapriya

In article .com,
wrote:

[snip]
Has anyone here used the type of equipment I'm mentioning? You should
see the time the thing I use takes to stabilize itself to show the
elevation... you'd comfortably CFIT if you had that in your airplane

yeah, but what is the accuracy of your GPS?
I suspect it is much more accurate than needed for aviation.

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

You don't need much acuracy to find an airport. They tend to be the
large things with the long areas of pavement. As of today, the use of
GPS as sole naviation for preceision approachs in low visibility (fog)
is limited to "non precision" approaches, just good enough to get you
down to about 500 feet and near the end of the runway. At some point it
should be good enough to actually touch down on the runway with (LAAS).

-Robert


wrote:
As part of my work (civil engineer), I routinely use GPS equipment in
surveying. And construction specifications usually calls for this
equipment to be held stationary for as much as three hours where
crucial transition points are to be located, and for up to twenty
minutes at less important locations.

I guess since you folk use GPS to navigate all across the globe and
requires to be both very precise and instantaneous, my equipment is
very much inferior to what's used in aviation.

Has anyone here used the type of equipment I'm mentioning? You should
see the time the thing I use takes to stabilize itself to show the
elevation... you'd comfortably CFIT if you had that in your airplane
)

Ramapriya

No real difference. The stabilization that you refer to is getting to an
accuracy that is not needed for ground-based or aviation-based navigation.
Typical near-instanstaneous accuracy for even cheap GPS receivers is a few
meters. Not good enough for surveying but certainly good enough to find a
60-foot wide (or better) runway.


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

wrote in message
oups.com...
As part of my work (civil engineer), I routinely use GPS equipment in
surveying. And construction specifications usually calls for this
equipment to be held stationary for as much as three hours where
crucial transition points are to be located, and for up to twenty
minutes at less important locations.

I guess since you folk use GPS to navigate all across the globe and
requires to be both very precise and instantaneous, my equipment is
very much inferior to what's used in aviation.

Has anyone here used the type of equipment I'm mentioning? You should
see the time the thing I use takes to stabilize itself to show the
elevation... you'd comfortably CFIT if you had that in your airplane
)

Ramapriya

Bob Noel wrote:

[snip]
Has anyone here used the type of equipment I'm mentioning? You should
see the time the thing I use takes to stabilize itself to show the
elevation... you'd comfortably CFIT if you had that in your airplane

yeah, but what is the accuracy of your GPS?
I suspect it is much more accurate than needed for aviation.


Not really Bob. Most building, refinery and pipeline surveys accept a
+/- 0.8 centimeter variation, so it isn't pinpoint like in aircraft, I
think.

Btw, just out of interest - does TCAS use GPS data in some ways too?

Ramapriya

In article .com,
wrote:

Not really Bob. Most building, refinery and pipeline surveys accept a
+/- 0.8 centimeter variation, so it isn't pinpoint like in aircraft, I
think.

+/- 50+ meters is plenty accurate enough for aviation use in determining
lateral position. +/- 10 meters vertically would be wonderful but not always
obtained.


Btw, just out of interest - does TCAS use GPS data in some ways too?

No.

ADS-B does, but not TCAS.

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

Bob Noel wrote:

+/- 50+ meters is plenty accurate enough for aviation use in determining lateral position. +/- 10 meters vertically would be wonderful but not always obtained.


I confess I couldn't have guessed that )


Btw, just out of interest - does TCAS use GPS data in some ways too?

No.

ADS-B does, but not TCAS.


Thanks, but how exactly does the TCAS get the coordinates of the
transponder it talks to, in determining whether or not an evasive
mechanism needs setting in? Anything like a +/- 50 meter accuracy there
would be disastrous.

Ramapriya

wrote in message
ups.com...
Thanks, but how exactly does the TCAS get the coordinates of the
transponder it talks to, in determining whether or not an evasive
mechanism needs setting in? Anything like a +/- 50 meter accuracy there
would be disastrous.

TCAS typically uses the transponder output to determine altitude of the
other traffic, and basic radio direction and range-finding processing to
determine direction and distance.

For the purpose of TCAS, +/- 50 meters is perfectly sufficient. Alerts are
provided at distances much greater than that, and an error of 50 meters
would affect the timing of the alert by a second or less. When in a
congested area, where TCAS is most typically important, airplanes are
traveling on the order of 50-100 meters per second.

You seem to be under the impression that flying involves a very high degree
of accuracy with respect to position information. That's simply not true.
Even in a differential GPS precision approach or a Cat III ILS, accuracy is
only to within a meter or so, and for anything else tens, hundreds, or even
thousands of meters is sufficient (depending on the exact situation).

In addition, while you are relying on your GPS receiver for altitude,
airplanes almost never do. They have barometric altimeters that provide a
different reference (not necessarily more accurate...just different) that is
more appropriate for the operation of an airplane (generally speaking). So
the GPS data is used only in two dimensions, in which the instantaneous
accuracy is generally much better, as compared to 3D accuracy including
altitude.

Your demand for accuracy in construction is FAR greater than any need
aviation has. As far as the difference in the GPS receiver goes, it is
likely that the basic operation of the GPS receiver you are using is
identical to that of an aviation handheld.

That said, I'm surprised you are using sample averaging to gain the accuracy
you need. Surveyors do use differential GPS with great success, and it
provides similar accuracy to what you're getting, only in a matter of
seconds rather than hours. (DGPS used for surveys has better accuracy than
that used for aviation because the reference point is so close to the
measured location in the case of surveying, whereas it may be quite a
distance away for aviation).

Pete

I guess since you folk use GPS to navigate all across the globe and
requires to be both very precise and instantaneous, my equipment is
very much inferior to what's used in aviation.


Well, several wrong assumptions here.

1. Most airline aircraft do not or not regularly use GPS to "navigate
across the globe". They use inertial navigation. Some use inertial
navigation with position updates from GPS. GPS is typically used in
general aviation aircraft - because it is cheap.

2. For enroute navigation, GPS accuracy of the standard signal (50
meters or so) is plenty accurate, altitude is measured with
barometrics, not GPS. For approaches to airports, the prevalent method
of navigation is not GPS, but other means (google ILS and VOR). GPS
approaches can make do with standard GPS signals, however, in the US
more and more approaches using WAAS (a method of differential GPS) as
an enhanced signal. However, the approaches down to an automatic
landing are never done with GPS, they use ILS.

2. The key problem in aviation with GPS is immediate feedback to the
airplane in case the signal goes bad, aka signal integrity monitoring.
Certified aviation GPS receivers have higher standards in that regard.

--
Thomas Borchert (EDDH)

In a previous article, said:
I guess since you folk use GPS to navigate all across the globe and
requires to be both very precise and instantaneous, my equipment is
very much inferior to what's used in aviation.

Years and years ago, I was told how to access some diagnostic modes on my
Garmin GPSMAP 195. The diagnostic mode showed the state of all sorts of
internal stuff, and from that it was apparent that one major difference
between the aviation 195 and the marine 175 is that the 195 had
temperature and pressure sensors. I suspect they put that in there so
that they could correct the results from the GPS radio for those factors.
The other major difference was that the 195 had WAY more flash memory to
store all those aviation waypoints.

But the fact of the matter is that aviation or not, GPS doesn't do
altitude as precisely as position because of the basic geometry. There
are numerous explanations of why on-line. So for aviation use, we either
need WAAS (which might or might not be available) or use a barometric
altimeter for altitude, and for surveying you let it sit so it can
accumulate a lot of data and integrate it.

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.

--
Paul Tomblin http://xcski.com/blogs/pt/
"All life is transitory. A dream. We all come together in the same place at
the end of time. If I don't see you again here, I will see you in a little
while in the place where no shadows fall." - Delenn