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Old 29-05-2009, 22:09   #16
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Originally Posted by speciald@ocens. View Post
The GPS system in this country is "too big to fail" - Sound familiar?
Yes, it does and that is worrysome.
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Old 29-05-2009, 22:47   #17
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You folks who doubt GPS accuracy must have stopped flying commercial now that GPS approaches are approved (and flown - daily!) for most everywhere?
I fly and the aircraft systems are a fair bit more up spec and only certified in some places on some planes and only some systems and in certain circumstances. There is also a lot of double checks with other instrumentation. They don't fly on GPS alone, it's only one input of many and on big aircraft it's a 'big many'.

I'm not GPS rated but have mates who are and they still far prefer to see the ground with their eyes before getting wheels down. Up in the air and away from hard bits is fine and I use one myself, most do these days. To be honest I use my GPS more to make sure I have SAR times right.... OK I'm lazy All things aircraft are done in UTC.

I don't think GPS is wildly wrong but taking it as absolute gospel has killed people. GPS to get to a spot and Mk1 eyeballs to make sure you don't hit the spot is still the best option.
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Old 29-05-2009, 23:14   #18
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That may be true wherever you are, GMac, but the FAA (US) does indeed allow GPS-only approaches, generally down to 200 feet. Currently around 1500 airports have published procedures (as opposed to ca. 800 with ILS - which has been around for 60 years!). The only thing the certified units do that handhelds don't is tell you when the WAAS correction is out of limits (3m). I've made thousands of landings with my handheld running, and I'd happily follow it down through the clouds if I had no other options. Preferring to see the ground and needing to are thankfully very different things.
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Old 29-05-2009, 23:59   #19
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Exactly the same here (NZ) Dustymc.

I also pretty much trust my GPS to 200ft but not much less, just like the FAA
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Old 30-05-2009, 00:22   #20
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Hand Helds maybe not so accurate

I have done a little bit of Geo-Caching to test my handheld GPS (I have thus far only used it for a speedo on the boat at the lake). However......
GeoCaching with my brother, his Magellan handheld was waaaay out to lunch and mine, a cheap Garmin, was spot on to within 8-10 feet.

I have to agree, the eyeballs and maybe a hand-bearing compass or binos will be my first-line backed up by the GPS, when I finally get out on the salt-chuck. My belief is that I should learn the old systems first and then start doing things the "easy" way. If I don't learn how to do DR and celestial navigation and coastal navigation without GPS, and a chart-plotter.... it will be difficult when I am under stress. Just like I should learn to sail, before I learn to motor. IMHO.

On the other hand, my cell phone has Internet, so if I get lost maybe I could just ask you guys!!
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Old 30-05-2009, 12:09   #21
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That's 200 feet VERTICAL, and it's what the FAA feels is safe at 250KTS with inconveniences like rocks and buildings around. Boats are generally a fair bit slower and less concerned about that particular dimension. Horizontal accuracy must be below 3m for navigation. I've never seen it above that.
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Old 01-06-2009, 11:37   #22
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This whole GAO finding was (IMHO) aimed at telling the US Air Force to get on the ball and get the GPS upgrade back on track, or risk being dissolved and absorbed by the US Army and the US Navy. Yes, there's actually talk in government right now on that very subject. Remember that the F22, the most expensive weapons system ever created (I think), had not seen any action at all in the war with Iraq, and hasn't been used for post-war peace keeping in Iraq or Afghanistan. Other than a support role, the Air Force has been on the sidelines post-911.

There's lot's of turmoil in the Air Force acquisitions office right now, especially in lieu of the air tanker scandal a few years back (which resulted in fed pen time for the former head acquisitions official) and the current F22 program, among others. It's well recognized in the US DoD that most large acquisitions programs like the F22 take too long and is too expensive. For instance, technology passes them by, requiring updating of technical requirement, costing more money and time. Complex systems design is almost impossible to do when the regulations and procedures coded into US law don't recognize that technology marches on, while the acquisitions process is mired in a cold-war era mentality.

That being said, there's no danger that we'll lose GPS - it's the underpinning for a not-so-insignificant part of the US economy, as well as our national defense. As was said earlier, our government will not allow it to fail - unless there's a successor in place.

I've probably said more than I should, but I just had to get that off my chest...
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Old 01-06-2009, 12:38   #23
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Accuracy

I just want to point out that whatever error there is in the satelites and the calculations going on in the reciever---the biggest source of error is in the cartography, particularly in remote locations.
Just a few years ago, using Maptech charts, the harbor chart for Nantucket was offset by a few hundred feet. The good news is that the jump sideways was obvious when moving in.
Remote locations may rely on cartography that predates electronics, I mean RADIO.
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Old 01-06-2009, 13:00   #24
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That sounds like a geodetic datum issue, Highlander40. Your chart (paper or digital) and your GPS must have the same datum (geographical "starting point") to jive. You should be able to find the datum on most maps, and set it somewhere in your GPS. There are hundreds of datums in use, and the shift can be quite significant (up to about 1.3KM). Most digitization efforts transform maps to the WGS84 datum, so that's a reasonable place to start if you can't get that information.
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Old 01-06-2009, 13:50   #25
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Accuracy

charts and GPS were WGS84. Same chart disc. Different scale. Maptech made a Boo-Boo . Next release, the problem was fixed.
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Old 01-06-2009, 14:38   #26
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Maybe the issue is with Maptech. They still haven't put the open channel on Redfish pass, Captiva Island. It showed I was in 2 ft. of water when I was in a 15 foot deep channel. The channel was created by Hurricane Charley, but has never been put on the maps. They also have the old channel coming from the ICW- which is silted in.
Now I don't expect them to correct everything - the coast of SW florida is a ever changing thing. But I do expect them to update at least every 5 years or so, even if it is to say that the pass does appear to be open.
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Old 01-06-2009, 19:28   #27
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Quote:
Originally Posted by Maren View Post
Good idea in any case. Espically post lightning strike.

Anyway, I'm suprised no one has addressed Panbo's take on this, so I guess I will.

From the 20 May 2009 entry:
Maren...my take on this is that the GPS scare stories are being promoted by those who WANT eLoran. The scare stories appeared right after Obama cut funding for Loran. Coincidence?
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Old 01-06-2009, 20:24   #28
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Quote:
Now this is interesting: Google Earth, free on the Internet, is accurate where old charts are not.
Google Earth is grossly inaccurate where you can't be sure of. It's not a cartographic base by any stretch. They stretch the photos to make them edge match. Aside from some urban areas the images are the same ones that have been around for years. They have added only a few for the new 3D images they use in the city.
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Old 01-06-2009, 23:56   #29
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Google Earth has a mean accuracy of around 40m, with just over half that in the US and Europe. I'm not sure I'd call that "grossly inaccurate" - at any rate, it's as good as satellite imagery gets. Rasterization is a bit more detailed than "stretching the edges." They're constantly updating images, with a (yet unmet) goal of having everything <3 years old. Contrast that to paper charts, which are often based on decades old data. It has it's faults and is unsuitable for some applications, but it is nevertheless a revolutionary product and is more than adequate for most tasks.
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Old 02-06-2009, 01:50   #30
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It's healthy to look at the position from a GPS or any other electronic navigation aid with the same skepticism you would use when looking at you sextant readings or the readings from your hand bearing compass.

/Hampus
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