How different is aviation GPS?

"Stubby" wrote in message
...
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!

What antenna types do they both use?

When I bought a GPS (basic hiking handheld), I discovered that the smaller
GPS units used a flat square plate antenna that worked okay when one had a
perfectly clear view of the sky, but otherwise was noticeably inferior to
the "helix" antenna (the thumb-sized antenna that usually sticks out from
the top of a GPS unit).

Either should be fine in an airplane, where one gets a great view of the sky
all the time, but on the ground one would notice significant differences.

It's too bad, because I really liked the form-factor of the smaller GPS.
But it just didn't perform as well as the larger ones that can use the
better antenna.

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

("Peter Duniho" wrote)
It's too bad, because I really liked the form-factor of the smaller GPS.


I live in a cave.

I have not seen the term "form-factor" used before.


Montblack

Stubby wrote:

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!

I'm totally surprised, and a tad skeptical for a few reasons.

The 12 used a patch antenna, similar to the eMap and eTrex series. The
60 series uses a quadrafilar, which usually works much better. The 12
was very similar to the eMap in processing ability, which in my personal
hands-on experience is far better on the 60. The published accuracy of
the two units is different, notice that the 60 has a velocity spec much
better than the 12 in the following Garmin specs. Please note that the
velocity spec is so different, they don't even bother to use the same
unit of measurement.

60CS:
Update rate: 1/second, continuous
GPS accuracy:

Position: 15 meters, 95% typical*

Velocity: 0.05 meter/sec steady state

WAAS accuracy:

Position: 3 meters, 95% typical

Velocity: 0.05 meter/sec steady state

12CX:
Update Rate: 1 second, continuous

* Accuracy:
o Position: 15 meters (49 feet) RMS*
o 1-5 meters (3-15 feet) RMS with Garmin® GBR 21 DGPS
receiver (optional)
* Velocity: 0.1 knot RMS steady state


Furthermore, the 12 series didn't use WAAS. Is something wrong with
your 60CS? G

Paul Tomblin wrote:
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.


I'm in CT, so being right on the east coast, and not right against the
Canadian border might have a positive effect. AFAIK, the two special
satellites are over the Atlantic and Pacific.

In a previous article, B A R R Y said:
Paul Tomblin wrote:
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.
I'm in CT, so being right on the east coast, and not right against the
Canadian border might have a positive effect. AFAIK, the two special
satellites are over the Atlantic and Pacific.

They're also on the equator, so the further north you go the more likely
they're going to be occluded by buildings and terrain when you're on the
ground.

--
Paul Tomblin http://xcski.com/blogs/pt/
Usenet is a co-operative venture, backed by nasty people -
follow the standards.
-- Chris Rovers

You also attach a thermometer to the tape because there is a
correction for expansion. If the tape was supported by the
ground, a tension of 10 pounds was required if I remember
correctly.


"Paul Tomblin" wrote in message
...
| 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

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

"Montblack" wrote in message
...
("Peter Duniho" wrote)
It's too bad, because I really liked the form-factor of the smaller GPS.

I live in a cave.

I have not seen the term "form-factor" used before.

Sorry...I've seen the term in a variety of industries, but I admit it's
probably not yet part of the common vernacular. It simply means how the
device is packaged, and how that packaging affects the user-interface and
utility of the device.

Basically, I just mean that the smaller GPSs seem "handier". But they don't
perform as well, so I wound up with a larger one anyway.

Pete

"Paul Tomblin" wrote in message
...
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.

Yup. Even within the coverage area of WAAS, if you are not directly below
the satellite (or nearly so) you start running into signal blockage due to
the terrain. Here in the Pacific Northwest, where we are both far to the
north of the satellites and also on the eastern/western boundaries of the
coverage), WAAS is essentially unusable on the ground (though it works quite
well in the airplane).

For the curious, here are some web pages with information about where the
satellites are, and what the theoretical coverage is:
http://gpsinformation.net/exe/waas.html
http://gps.faa.gov/programs/waas/for_pilots.htm

I note that they are in the process of adding a new WAAS satellite, and have
for the moment moved the North America coverage somewhat to the west,
leaving the northeasternmost area of the US without WAAS coverage. They say
that by the Fall, the new satellite will be up, restoring coverage.
Presumably everyone will have somewhat better performance after that (and
some of us should probably be getting better performance now).

Pete