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  1. #31
    Join Date
    Apr 2011
    Location
    McBride BC Canada
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    We've got radar speed signs on the highway as you come into the village.
    Big flashing numerical displays. So far, all 6 that I encounter on a regular
    basis display exactly what my speedo says.

    What about the odometers?
    For many years, I drove a battleship, a Chev Caprice station wagon
    with fantastic load space. From the city house to here was always 221km.
    Now, my GMC Suburban show 216km, all the time!

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  3. #32
    Mobyturns's Avatar
    Mobyturns is offline In An Instant Your Life Can Change Forever
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    Jul 2012
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    "Brownsville" Nth QLD
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    Back to the GPS - two things here the GPS device & the information supplied with the device i.e. mapping and data re speed zones. Neither is infallible because of the human & environmental components involved.

    Global Navigation Satellite System (GNSS) is the more widely accepted term for these devices now as many utilize data from the GPS, Glonass, and other soon to be commercialized navigation satellite systems.

    Humans compile the data that is used to generate the mapping and info systems displayed like - speed zones, road works, road centre line data, road surface, classification, traffic lights etc.

    Humans also install, set up & operate the GNSS units. The unit’s antenna requires an unobstructed view of the sky to receive data from orbiting satellites. As vehicles travel in less than ideal environments for GNSS units with overhanging trees, tall buildings, obstructions from the vehicle itself etc it becomes more difficult to maintain a reliable & direct signal from each of the satellites to calculate position and secondary data like speed.

    By the nature of the system a 2D position calculation requires ranged distance (time signal takes from satellite to device * speed of light) from a minimum of 3 satellites. http://www.novatel.com/an-introducti...gnss-overview/ The timing standard used in devices is far superior to any other timing standard available to the general public. As a vehicles speed is distance / time taken between two points we only require two position fixes and the time taken to travel between them to calculate speed. The difficulty arrises in that the GNSS devices position/s may be corrupted from multi-pathing or skewed because many satellites are not visible to the unit due to environmental obstructions or from the mounting position in the vehicle.

    Instantaneous velocities (speed) may be heavily skewed from one or both positions being incorrect – timing is not an issue.

    GNSS averaged velocities (speed) will generally be more more reliable & more accurate than a vehicle speedometer in ideal environments, however they still can be skewed due to obstructions & multi-pathing.

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