How different is aviation GPS?

Matt Barrow wrote:

WAAS will provide about 10 feet, but that only for approaches with WAAS
augmentation.


WAAS actually works anytime the satellite correction signal is
available, you don't have to be flying an approach.

My Garmin 60CS, as well the eTrex Vista I previously owned, both use
WAAS corrections. I can't remember the last time I didn't get the
corrections.

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

B A R R Y wrote:
Matt Barrow wrote:

WAAS will provide about 10 feet, but that only for approaches with
WAAS augmentation.


WAAS actually works anytime the satellite correction signal is
available, you don't have to be flying an approach.

My Garmin 60CS, as well the eTrex Vista I previously owned, both use
WAAS corrections. I can't remember the last time I didn't get the
corrections.

A friend bought a 60CS recently and we noticed that my old Garmin-12 got
better reception and seemed more accurate. We called Garmin and the
fellow we talk with was surprised because both units use the same chips
inside!

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.

("Andrew Sarangan" wrote)
In addition to what every one else mentioned, aviation GPS also has a
feature known as RAIM which stands for Receiver Autonomous Integrity
Monitoring. It warns you of any problems with the satellite signals. Some
have RAIM prediction as well.


How does RAIM prediction work, I wonder?

The GPS unit knows where you are, knows where you're heading, and knows
where x number of signals will be - based on the satellites' "predictable"
orbits.... g

Ok. No clue.


Montblack

In a previous article, (Jay Masino) said:
Paul Tomblin wrote:
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
Paul,
Do you happen to remember what the keystrokes were to get to the 195's
diagnostic mode? I'd be curious to see what mine is saying.

Don't remember - it was probably 10 years ago, and probably on this
newsgroup. You might want to see if Google Groups has it. There were two
combinations of three buttons you could hold down as you started it - one
wiped all your routes, tracks and user waypoints, and the other started a
diagnostic mode. Don't get them confused.


--
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

In a previous article, Stubby said:
I had my property surveyed and asked what the accuracy was. The
surveyor replied that 0.1 foot is the standard for most applications and
0.01 foot is required for commercial, high-precision applications.
They use GPS (DGPS??) but I don't know how.

You need a better surveyor. When I was doing road construction layout, we
were expected to get the marks within 5-7 millimeters. And when they
actually did the construction, they were allowed to be within 2-3
centimeters. (You've heard the expression: measure with a micrometer,
mark with chalk, cut with an axe.) Legal surveyors were supposed to be
*far* more accurate than us. For instance, we just held the chain (that's
the "measuring tape" to you) or laid it down on the ground. Legal
surveyors had a special device to make sure they were holding exactly the
right amount of tension on the chain because that's what it was calibrated
for.

--
Paul Tomblin http://xcski.com/blogs/pt/
ALL programs are poems, it's just that not all programmers are poets.
-- Jonathan Guthrie in the scary.devil.monastery

Matt Barrow wrote:

Aircraft GPS systems are accurate to about 30 feet; how well do you think
that would work if your building a bridge, or more importantly, a tunnel?


If I was owner, I'd end up with two tunnels for the price of one

Ramapriya

In a previous article, B A R R Y said:
My Garmin 60CS, as well the eTrex Vista I previously owned, both use
WAAS corrections. I can't remember the last time I didn't get the
corrections.

I have a Garmin 296 with WAAS, and I get WAAS nearly every time when I'm
flying (but not always), but I only seem to get it about half the time
when I'm driving. But I live, fly and drive up near the Canadian border,
so maybe the coverage isn't as good up here.

--
Paul Tomblin http://xcski.com/blogs/pt/
Real Time, adj.:
Here and now, as opposed to fake time, which only occurs there and then.

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