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Thread: GPS performance meter on in-car PC

  1. #21
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    Sep 2007
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    22
    Quote Originally Posted by Scouse Monkey View Post
    ROFL! how do you think missiles and jet planes navigate?

    GPS?

    Inertial systems can be incredibly accurate and fast. Look at the speed of missiles and what if you loose the GPS signal? Aircraft NEVER use GPS as their main for of navigation - it is too risky.
    oh yeah

  2. #22
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    Join Date
    Nov 2007
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    1
    All of the GPS devicese discussed and sold via MP3CAR are considered "recreational" grade units: at best, with WAAS or SBAS correction, they can achieve 5 meter circular accuracy, in an open field, standing still, with no horizon obstruction. If you doubt this claim, and believe the manufacturers "claims" or believe that your GPS device puts you on the street centerline every time, place your GPS antenna over any NGS control station benchmark (www.ngs.gov) and calculate the difference. Make sure you factor in the height of your antenna. Also note that street center line data used by most consumer navigation software has been found to show a good portion of street centerlines with a horizontal accuracy of about 50 feet (some are dead on, some can be hundreds of feet off). Therefore, a single "channel" (can receive L1 code from up to 12 platforms) unit is good enough for most recreational uses, but is not going to report speed, acceleration, or fuel use with any degree of accuracy, or usefulness. You can use a single channel "mapping" grade unit, to get slightly better velocity results, PROVIDED the unit has SBAS and real time correction capabilities through the use of a radio beacon. A Trimble GeoXM or XH will do this, starting at 6,000$ for all the hardware. But, with accuracy comes a steep price: the "rules" the unit firmware applies to acceptable satellite signal strength are MUCH stricter, and mappig grade unit set to only read at 1m circular accuracy will drop 75% of satellite signals at 5 mph. This is why Garmins pretty much work everywhere, and you often "lock" 8 satellites. A garmin will accept a signal reflected from the bottom of a dumpster (multipath). There are units that come with additional signal processors to account for rapid acceleration and high-speeds, they use complicated algorithms and dynamic positioning to account for directions, speed, and last known position: they can be found in the botton of every 105 mm shell fired in Bagdahd. You can probably get one of the hundreds of suppliers of OEM GPS hardware to supply you with a "test" box to round out your installation: I'm using a board from Thales that gives me 1 m circular accuracy at 55 mph along a 1 mile test course, with an obnoxious antenna high up on the roof...consumer patch antennas do little to amplify/log signals...

    http://tl.gpsworld.com/gpstl/Vehicle...tegoryId=25186

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