Calculating GPS position from RAW data

Discussion in 'GPS Technical Discussion' started by Sherest, Aug 19, 2014.

  1. Sherest


    Aug 19, 2014
    Likes Received:

    I got an GPS receiver which sends me the data that it receives from the GPS satellites.
    So, I get a bunch of Bits/Bytes.
    I want to calculate my position using 4 satellites - as usual.

    I read the Ephemerid data and parse them.
    With them I can calculate the GPS satellite position. (As far as I know I dont need the Almanach data - right ?)
    My main problem is that some things are defined recursive. page 38

    For example:
    tk = t - toe
    It says here that "t is GPS system time at time of transmission, i.e., GPS time corrected for transit time (range/speed of light)."

    t = tsv - dtsv (d - Delta sign)

    Where I get tsv from ? It's not transmitted by the satellite ?

    and now
    dtsv = af0 + af1 * (t - toc) + af2 * (t - toc)^2 + dtr
    that's already recursive: t is calculated by dtsv and dtsv is caltulated by using t
    dtr = F * e^sqrt(A) * sin (Ek)
    Mk = Ek - e * sin Ek
    Mk = M0 + n * tk
    which is recursive again ...

    So I am a bit confused. How shall I calculate all of this.

    Then the next step would be to solve the non linear equotion system.
    (x1 - x)^2 + (y1 - y)^2 + (z1 - z)^2 = c * (timetransmitted - timereceived + clockcorrection)
    clockcorrection will be the 4th unknown in the 4 equotions.
    I guess that the timetransmitted will be the t from above.
    timereceived though ... I guess it will be sufficient to use something like Time.getActualTime()

    Thanks in advance, big thanks,

    maybe someone can help,
    Sherest, Aug 19, 2014
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