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GPS
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PostPosted: Mon Oct 06, 2008 6:55 pm 
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Just bought my 1st GPS. Pretty decent little outfit. Have used it on a couple of short trips and got me to the destintation properly.

I have taken it to work recently and it is not as accurate as I first had hoped. The calculation of my vehicle speed is off by 2km/hr (GPS reads high). When stationary in many locations, it does not place me exactly where I am parked. On 2 different times it placed me about 15m away from where I was actually parked. Area was flat, level, no trees.

Overall, I would use it to get to destinations, but would not rely on it for precision/pinpoint location, but merely a guide only.

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PostPosted: Mon Oct 06, 2008 9:16 pm 
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You nailed the GPS accuracy bang-on. A lot of people mistakenly assume consumer GPS equipment has "pin-point" accuracy but the best it can do is about 15m. The GPS signal is managed by the US Air Force (50th Space Wing) which encrypts it. The pin-point accuracy is reserved for the military.

Land surveying uses techniques to correct this but it's a cumbersome process involving a base station and multiple receivers.

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PostPosted: Tue Oct 07, 2008 2:45 am 
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As ticketcombat explains, GPS for consumer use is never 100% accurate.

I had the same frustration when I first used my GPS. It is a TomTom which is not bad IMO, but I used it while walking lol, while visiting relatives in Florida, in a new neighbor hood.

The GPS didn't exactly get me to my pin pointed location, but nevertheless it got me to the intended place quite accurately. Meaning I could see the place.

But overall my TomTom is not too bad. It counts down the meters remaining to make a left turn for example, and I find it quite accurate. Maybe a meter or two off of my intended turn.

Just thought I would share :D

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PostPosted: Tue Oct 07, 2008 9:09 am 
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I've had a TomTom for a couple of months now, and my experience is much like those above. It's very useful for navigation on a trip, much less so for hiking. I wonder if the speed over the ground is a factor; i.e., the little computer likes a relatively high rate of angular change between it and the satellites, and at a walking pace the rate of change is too slow. If I walk around my block carrying the thing it is nothing but confused, flailing around, trying to figure out what direction to point its compass: if I give it a ride around the same block in my car it's perfectly happy.

I find that mine disagrees with my car's speedo by 2km/hr, like hwybear's, but mine is reading lower. I just assumed that my car's indicator was the one that was off, but hwybear's experience makes me wonder.

Overall, though, a very useful gadget. Wish I'd had it in June and July when I was going to rugby games at addresses in foreign (to me) places like Wingham, Kincardine and Chatham from home here in Norfolk County. That's the sort of thing it does best.


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PostPosted: Tue Oct 07, 2008 11:54 am 
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Proper1 wrote:
I wonder if the speed over the ground is a factor; i.e., the little computer likes a relatively high rate of angular change between it and the satellites, and at a walking pace the rate of change is too slow.
The error is called "Multipath". Basically GPS receivers calculate location based on the satelite signals. But the signals also reflect off the surrounding terrain but are a little bit delayed than the direct signal. Your GPS picks up both and can't filter out the true signal. In your car, the GPS antenna is moving and the false reflected signals quickly fail to converge and only the direct signals result in accurate and stable calculations which the GPS uses to determine position.

So I guess in theory, if you run really, really fast, you GPS would become more accurate. Perhaps use a neighbour's pitbull for motivation :-)

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PostPosted: Tue Oct 07, 2008 11:07 pm 
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One thing that helps GPS accuracy is ensuring that the antenna has a clear view of the sky. While the GPS won't be 100% accurate because Precise Positioning Service is only available to the US military, basically the more satellites it can see, the more accurate the position. When it's in a car, even if it is on the dashboard, it won't be able to get all of the possible satellites because the metal roof of the car acts to block some of the signals, even if you're in an open area. Even if you're out of the car, though, it will be off by a few metres one way or the other.

Almost every modern airliner, and most General Aviation aircraft manufactured these days, comes with GPS. The GPS antenna is installed on top of the fuselage so that no part of the aircraft blocks its view of the satellites, and those systems have very good accuracy. They also have RAIM (Receiver Autonomous Integrity Monitoring) which is a self-check that provides a warning if there aren't enough satellites in view, if there is a system fault or if there is a satellite fault or error. No hand-held GPS unit has RAIM, so if your GPS unit seems to be telling you that you're in Texas :shock: it's probably picking up a bad satellite but it won't tell you.


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PostPosted: Thu Oct 09, 2008 2:06 am 
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The GPS works on a principle of intersecting circles - your GPS receives the signal from 3 satellites, and they all send out a time stamp at the same time. The GPS receives those signals, compares the time it took to arrive from satellite to the GPS, and maps 3 circles, and they all intersect at one point. A fourth signal may be used to pinpoint your location in 3D. Small mistake in the computer using a floating-point precision where a double-floating-point precision should have been used and you are off 15 meters. You gotta realize that the satellite is some kilometers up in the sky. Also, the receiver is a point receiver, therefore it cannot determine the DIRECTION of your GPS. However, when you are in motion on a car/motorcycle, the software will put you on a proper lane and proper road. When you make a turn onto a mapped street the software will recalibrate the GPS receiver thus you have (apparently) more accurate reding. The software will also know which way you are headed, therefore it will show you the proper compass (although this is relative to your car's motion, again, as the GPS can be tilted.
For walking around use something like Garmin eTrex or Geko or 60 or 72/76, as the software there is slightly more accurate.

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PostPosted: Thu Oct 09, 2008 2:54 pm 
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I wonder why my better half just bought me this!
http://gps-snitch.com/
No more 6 hour rides looking for that perfect loaf of bread :cry:

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PostPosted: Sat Oct 11, 2008 2:26 pm 
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Hwybear-did you calculate the speed accuracy from your cruisers odo, or radar? Just wondering, because at the local racetrack(1/4mile), my Garmin GPS was very accurate in giving me my top speed compared to the readout from the tracks timing system.


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PostPosted: Sun Oct 12, 2008 7:45 pm 
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dntmstr wrote:
Hwybear-did you calculate the speed accuracy from your cruisers odo, or radar? Just wondering, because at the local racetrack(1/4mile), my Garmin GPS was very accurate in giving me my top speed compared to the readout from the tracks timing system.


On 3 different cruisers, 3 different radar units, the GPS reading was 2km/hr higher than what the actual speed showed on radar. One cruiser has cruise control so I could confirm my speed remained constant.

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Re: GPS
PostPosted: Mon Nov 24, 2008 11:54 pm 
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hwybear wrote:
Just bought my 1st GPS. Pretty decent little outfit. Have used it on a couple of short trips and got me to the destintation properly.

Hello Bear,

I hope your GPS does not include an MP3 player, a JPEG viewer, a calculator or a bluetooth module, otherwise bill 118 forbids its use in Ontario.

Quote:
78. (1) No person shall drive on a highway a motor vehicle that is equipped with, carries, contains or has attached to it a television, computer or other device with a display screen if the display screen is visible to the driver.

Exceptions

(2) Subsection (1) does not apply in respect of,

(a) a global positioning system navigation device that has no other function than to deliver a global positioning system for navigation;


I suspect that it was not the intention to ban most of the models being sold (all Garmin models and all PDAs with a built-in GPS) but that is the what the wording says.

By the way, you're OK as long as you don't use it in your private car.

Quote:
(3) Subsection (1) does not apply to the driver of an ambulance, fire department vehicle or police department vehicle.


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Re: GPS
PostPosted: Tue Nov 25, 2008 2:37 pm 
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alexo wrote:
By the way, you're OK as long as you don't use it in your private car.

Quote:
(3) Subsection (1) does not apply to the driver of an ambulance, fire department vehicle or police department vehicle.


I just let my dispatch "tell me where to go" :wink: Sometimes others tell me as well :o

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