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GPS Speedo


 
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nixt
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Joined: 17 Aug 2005
Location: Atlanta/Athens, Georgia

PostPosted: Sun Dec 17, 2006 6:55 am    Post subject: GPS Speedo Reply with quote

Does anyone know of a GPS unit that will display nearly-instant correct speed in a MPH format and that has a display large enough for a normal person to be able to read from the driver's seat while the unit is mounted on the dash? Or is this just wishful thinking on my part? I can alternate between 3 different sizes of tires so I don't want to waste my time with one of those calibrator units.
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1998 ZJ 5.9 Limited W/242 swap
JKS disconnects & Garmin GPS
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with 2" coil spacers for 3.5-3.75 total lift
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TJ Apex
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Joined: 29 Jul 2005
Location: Mountains of SoCal. 6257 elev.

PostPosted: Sun Dec 17, 2006 8:57 am    Post subject: Reply with quote

I use my GPS all the time for the same reason, I found that my speedo is 12% off.
I have a Garmin E-Trex Vista GPS unit. 5 years ago it cost $400.00, (now it's probably $250 or so)
It's a small hand held unit that can be mounted to your windshield via a suction cup mount that you can buy separately along with other accessories.
When I have it in "Trip Computer" mode I’m able to toggle between large and small number displays. The large number screen shows only 4 lines of info and is very easy to read, the small number screen shows 8 lines of info that can be read, but is more difficult to read while driving.
Also, Garmin makes less expensive units that are in the E-Trex line that you can buy at Target for about $100 or less, and I believe they all show MPH.
http://www.garmin.com/products/etrexVista/index.jsp
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nixt
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Joined: 17 Aug 2005
Location: Atlanta/Athens, Georgia

PostPosted: Sun Dec 17, 2006 10:10 am    Post subject: Reply with quote

So what is the actual function called that displays the speed? The Foretrex 201 lists: Racing timers: Configurable start sequence, alert tones and large-number digital readout
Trip computer: Trip distance, trip timer, plus essential navigation data. Does that mean that it displays the speed? The foretrex and a few other models appear to be aimed more toward the ATV market- would those seem more likely to have the speed display?
_________________
1998 ZJ 5.9 Limited W/242 swap
JKS disconnects & Garmin GPS
Heavy duty OME Suspension kit
with 2" coil spacers for 3.5-3.75 total lift
235/85R16 Goodyear MTR's Measure a true 32"

Life has No Value,
But Death Has Its Price.
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TJ Apex
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Joined: 29 Jul 2005
Location: Mountains of SoCal. 6257 elev.

PostPosted: Sun Dec 17, 2006 11:45 am    Post subject: Reply with quote

That unit is for trail walking and jogging, but it should display your speed. I don't know of a GPS that doesn't show speed.
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Welfare recipients should pass a drug test to get paid…

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Drake69
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Joined: 26 Mar 2004
Location: Stormtroopin....

PostPosted: Sun Dec 17, 2006 9:53 pm    Post subject: Reply with quote

Any GPS designed for in-vehicle use, whether just for highway or both highway and offroad map storage, can display a speedometer. Garmin's Quest series, GPSII - GPSIV, nuvi, and Streetpilot series all can do this, as well as TomTom, Magellan, and others (I'm partial to Garmin myself). Each one should have a compass setting that displays everything from mileage to speed to heading, some even display road conditions.

Now, the reason domestic GPS units seem to be a "little off" when displaying moving updates like speed and direction, you have to understand how degrees, minutes, and seconds work to plot a location on the globe. For that, I'll let Wiki answer...

Quote:
The position calculated by a GPS receiver requires the current time, the position of the satellite, and the measured delay of the received signal. The position accuracy is primarily dependent on the satellite position and signal delay.

To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version. By comparing the rising and trailing edges of the bit transitions, modern electronics can measure signal offset within about 1% of a bit time, or approximately 10 nanoseconds for the C/A code. Since GPS signals propagate nearly at the speed of light, this represents an error of about 3 meters. This is the minimum error possible using only the GPS C/A signal.

Position accuracy can be improved by using the higher-speed P(Y) signal. Assuming the same 1% accuracy, the faster P(Y) signal results in an accuracy of about 30 centimeters.

Electronics errors are one of several accuracy degrading effects outlined in the table below. When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (50 ft). These effects also reduce the more precise P(Y) code's accuracy.


So, military application GPS units are more accurate than civi ones because they contain additional electronics to help compensate for additional errors like Ionosphere and Troposphere, Ephemeris and Satellite-clock errors (clock variances and time drift), and Multipath distortion (buildings and other structures), or other electronics that base themselves on precise movement and relative kinetics. Military GPS units are supposedly accurate down to the millimeter, meaning they can see a fly f@rt as it moves through the air. rofl

Now, you may ask yourself, "Why can't I be allowed to have a GPS that accurate? Why does the gov't get to play with all the nicer toys?" rofl

One of the nastier uses for GPS hardware? Missile Guidance.

Quote:
To help prevent (a) GPS (unit) from being used in improvised weaponry, the US Government controls the export of civilian receivers. A US-based manufacturer can not generally export a receiver unless it has limits on the velocities and altitudes at which it will report position and speed information. A general speed limitation of 515 m/sec applies and no GPS unit can function at an altitude of 18 kilometers or above.


Class dismissed.
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