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Calculating distance using GPS

Discussion in 'Electronic Design' started by jdhar, Sep 10, 2008.

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  1. jdhar

    jdhar Guest

    Using standard RMC NMEA strings from a GPS unit, I'm trying to figure
    out the best way to calculate distance traveled over time. The two
    methods I have are:

    - Use lat/long. coordinates and calculate distance using Haversine
    method (or similar) each sample, and keep a running total of distance
    traveled
    - Use velocity each sample, and multiply by sample interval to get
    distance traveled during that interval

    I have done some basic tests, and these two methods work out to be
    "almost" the same, but not quite... I don't know enough about the
    underlying GPS design to determine how lat/long vs. how speed are
    determined. Any comments on which method may be more accurate? (or are
    they both equally inaccurate).

    What is interesting however, is when going under a bridge structure,
    the velocity readings become distorted (there is a jump and then a
    dip). The lat/long readings don't seem to suffer from this same issue
    however. Does anyone know why this is??
     
  2. Rich Grise

    Rich Grise Guest

    First, you lose info from the leading satellite, then, you lose info from
    the trailing satellite?

    Cheers!
    Rich
     
  3. Rich Webb

    Rich Webb Guest

    The reported GPS velocity is likely to be the instantaneous velocity
    estimate at the "data valid" time rather than the effective mean
    velocity over the sample interval.
     
  4. PeterD

    PeterD Guest

    What am I missing. Take the starting point, and the ending point and
    compute distance? Or do you want road distance?
     
  5. jdhar

    jdhar Guest

    Right, I have read this before as well. I'm assuming that the
    instantaneous velocity will hold true until the next sample. Obviously
    not 100% accurate, but it's all I can do given the limitations of the
    system.
     
  6. jdhar

    jdhar Guest

    Ya, basically road distance. If I go around a circle of 1km
    circumference, I want to read 1km, not 0.
     
  7. Rich Webb

    Rich Webb Guest

    Not quite true. The discrete estimates hold true only for that instant.
    You have no information about what happens between estimates, although
    the physical constraints of the system may allow you to make some
    inferences.
    Not quite true, either. In addition to a simple linear approximation
    based on two preceding samples, you can do an n-th order approximation
    based on n+1 predecessors. Or, you can use an alpha-beta filter or go
    full bang and do a Kalman.
     
  8. dalai lamah

    dalai lamah Guest

    Un bel giorno jdhar digitò:
    Velocity is more accurate, because it is calculated measuring the doppler
    shift which is less affected by the ionospheric error. Here's an
    informative document:

    http://acl.mit.edu/papers/2002_MSEC_93.pdf
    I've noticed this too. Probably it's because the doppler calculation is
    "differential" (which is also the reason of its better accuracy): when the
    module loses the satellite signal, at the next good fix the difference in
    doppler phase will be bigger. For your purpose this is actually good,
    because the integral of the speed should remain almost correct (for small
    outages).
     
  9. LVMarc

    LVMarc Guest

    Changes in the local magnetic potion of the magnetic field we are
    naturally exposed to. By addding Iron structure the local magnetic field
    is disturbed by the ferrite material. This leaks into the velocity
    meters sensing.
     
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