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d(at)cfisher.com Guest
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Posted: Wed Feb 21, 2007 10:44 am Post subject: GPS info |
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There was some talk about GPS and Atlitudes lately,
What is the thoughts on GPS and flight planning ,altitudes etc ?
I find mine fairly accurate ( garmin 196) and wondered why anyone would think that they are not. I think GPS are used for failry precision approaches. I also use WAAS which seems alot more accurate than without.
Dave
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Michel
Joined: 10 Jan 2006 Posts: 966 Location: Norway
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Posted: Wed Feb 21, 2007 12:33 pm Post subject: GPS info |
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On Feb 21, 2007, at 7:43 PM, D. Fisher wrote:
Quote: | I also use WAAS which seems alot more accurate than without.
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Yes, it certainly is a big advantage on your continent, Dave.
Regarding the accuracy of altitude (non WAAS, that is) my observation
is that it is not that bad either. But, as the geometry of the
satellites get unfavourable, the first accuracy to go is the altitude.
That's why a 3D position requires more satellites than a 2D.
Now, while most of the time the altitude is within reasonable values,
it may - and will, with time - become inaccurate in relation to the 2D
positioning. And that "not knowing" factor is exactly what was behind
the now inactive "Selective Availability" (SA).
Until May 2000 non-NATO GPS had to suffer the SA. The idea was: 99% of
the time, it was accurate, but one percent of the time, it could be as
much as 600 feet off position. Now, would you e,g. sail your ship in no
visibility in a narrow waterway, knowing that there is one percent of
chance that you may run aground? I think not.
The same applies for e.g. GPS software simulating a virtual ILS with
glide slope. No one wants to take the responsibility to make it because
one could have bad luck and be far away from the actual altitude; even
if most of the time it would work.
At least, that what the guys who write the PocketFMS program told me.
An interesting thing is that, while everyone thought the GPS SA would
be screwed to the highest level during Desert Storm (1991) it was the
opposite. The reason being that some of the allied forces (Saudi
Arabians) were not members of NATO and had to use the "civil" version
of the GPS. But the US DoD meant that, in any case, they had the
military superiority over the Iraqi that it didn't matter if they were
also able to position themselves accurately.
Cheers,
Michel
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Float Flyr
Joined: 19 Jul 2006 Posts: 2704 Location: Campbellton, Newfoundland
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Posted: Wed Feb 21, 2007 2:25 pm Post subject: GPS info |
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Michel:
As much as 600 feet off...Gwaaan.
I was on my way over the highway one day (pre 911)and had the GPS set up on
the dash. I happened to notice that I was no where near the indicated
location so I stopped the car to check it out. And when I was stopped I
noticed the receiver gave me a speed in excess of 100 mph! The location
displayed was around thirty miles south west of my actual location. I
waited a few minutes to see if things would change... Boy did they! In less
than a minute I was reported as being over twenty miles East of the same
location. At that time I figured the receiver was fried so I continued on
my way. When I got to my destination (30 Mi South East) This was a
difference of almost 65 Mi. in about a minute in a stopped car! I noticed
the GPS was reading within ten feet or so of the same location from the day
before. I am convinced that somebody was playing hide and go seek with the
satellites.
That never happened to me before and I've never seen it happen again. After
about a year that receiver just stopped working one day. The manufacturer,
Magellan, Replaced it with an upgrade. I've had it upgraded once again
since then. Their receivers seem to have a relatively short lifespan.
Noel
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pwmac(at)sisna.com Guest
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Posted: Wed Feb 21, 2007 4:06 pm Post subject: GPS info |
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Noel,
Did you know that a GPS has to be moving to give accurate speed readings. Your distance errors using a highway as a reference is not a good way to see accuracy. Your location jumping is a function of a slow processor and the fact that you stopped. This always will yield bad data. Again the GPS is not moving and just sits there and dithers. My old Etrex exhibits this behavior. Its slow and it takes as much as 15 minutes to get accurate results. And it still dithers, but the dithers are small after that time.
I use a simple Etrex for hiking. On top of a mountain with a USGS bench mark of known elevation the GPS read the correct altitude + or - 20 feet in elevation and the plot of the coordinates when I got home were dead on using a 7.5' map. In my truck I have a 2 year old single processor (1 compute / second) GPS with WAAS, but I cannot give you any accuracy data since I have yet to drive to a high altitude bench mark. The newer the GPS the better. Some come with more than 12 channels and dual processors. The result is faster and better accuracy. The remote antenna is highly desirable. I did a test with my hand held and even my body reduces the signal and gives poor results. If your GPS is used in a car expect poor data unless you have an antenna on the roof. However, some are pretty good looking thru the windshield even though half or more of the sky is missing.
Remember how GPS works - your GPS looks at satellites so if it sees several above then X and Y coordinates will be accurate and It has to see satellites near the horizon to triangulate the altitude. WAAS is a great help with regard to location/altitude but it is still necessary for the unit to see satellites at low angles to get accurate altitude. This is seldom an issue in an airplane since the horizon is visible. Also remember that a GPS contains a computer that recalculates your location/altitude frequently 1 to 4 times a second. This causes what I call dither where it displays jumps the location/altitude. This is especially frustrating when you are not moving. Its the satellites that are moving changing the calculations at each recalculation.
Don't worry about using a GPS in your plane as the internal antenna can see thru the plastic overhead and to the side. My tests show no degradation inside a plastic or fiberglass container.
If you want to find out if your GPS is giving bad data, go to a known surveyed place at the airport and see what the result is for coordinates and altitude, assuming you are not at a podunk place with high mountains near by to mask the horizon.
Don't be mislead by a GPS that says you are not located on the highway you are driving on. The error observed is not from the GPS but are the errors created when the aerial photos were digitized to real maps. Then the real maps were loaded into the GPS background and more errors are created. What is going on is 50 year old map data done by real people compared to state computerized locating device. It makes the cartographers look real bad.
Regards, Paul
=======================
At 02:25 PM 2/21/2007, you wrote:
Michel:
As much as 600 feet off...Gwaaan.
I was on my way over the highway one day (pre 911)and had the GPS set up on
the dash. I happened to notice that I was no where near the indicated
location so I stopped the car to check it out. And when I was stopped I
noticed the receiver gave me a speed in excess of 100 mph! The location
displayed was around thirty miles south west of my actual location. I
waited a few minutes to see if things would change... Boy did they! In less
than a minute I was reported as being over twenty miles East of the same
location. At that time I figured the receiver was fried so I continued on
my way. When I got to my destination (30 Mi South East) This was a
difference of almost 65 Mi. in about a minute in a stopped car! I noticed
the GPS was reading within ten feet or so of the same location from the day
before. I am convinced that somebody was playing hide and go seek with the
satellites.
That never happened to me before and I've never seen it happen again. After
about a year that receiver just stopped working one day. The manufacturer,
Magellan, Replaced it with an upgrade. I've had it upgraded once again
since then. Their receivers seem to have a relatively short lifespan.
Noel
[quote] --
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Michel
Joined: 10 Jan 2006 Posts: 966 Location: Norway
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Posted: Sun Feb 25, 2007 2:12 pm Post subject: GPS info |
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On Feb 22, 2007, at 1:05 AM, paul wilson wrote:
Quote: | What is going on is 50 year old map data done by real people compared
to state computerized locating device. It makes the cartographers
look real bad.
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Don't forget that prior to WGS84, the standard reference for GPS,
cartographers were using national reference systems, Paul. And there
are many, many of them. When the only available way to define a
position was astro-navigation, it wasn't that important. But with the
precision of satellite navigation, we came in need of a standard
reference. On most maritime charts, there is now an overprint that
says, Add so many minutes and seconds of latitude and longitude to
correct to WGS84. Many of the Norwegian charts may have as much as
several hundred yards offset.
Cheers,
Michel
do not archive
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Bob
Joined: 24 Oct 2006 Posts: 89 Location: Damascus, Maryland, USA
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Posted: Mon Feb 26, 2007 5:11 am Post subject: Re: GPS info |
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I've seen several hundred feet of difference between altimeter and GPS. Living in the big ADIZ/FRZ of Baltimore/Washington, and flying frequently within 100-200 feet under the Class B ceiling, this would be more than enough to easily get busted. Also, the transponder works on pressure, not actual/corrected altitude, so an encoder would still be needed under Class B.
Bob
Vixen in work
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