GPS message: compute ephemeris data and pseudo-ranges from subframes

Discussion in 'GPS Technical Discussion' started by Ash, Apr 4, 2017.

  1. Ash

    Ash

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    Hi people!

    I have raw GPS data (see end of the question for details if you think the protocols are relevant), and I need to extract/compute ephemeris and pseudoranges (my goal is to replace the recursive least squares that the receiver solves to compute the position with a home-brewed method based on sensor fusion). Concerning the ephemeris, I have access to the 3 first subframes of each of the 25 frames of the GPS message. Documentations/books/etc that I have found just vaguely mention that the first subframe contains clock data, and the two others contain ephemeris data. However, none of them precisely says what the words in these subframes are, and how I can use them to compute orbital information (I'm assuming that I only want satellite positions?). Can anyone give me pointers to some references on how to do this? Or (even better), is there any open-source code that already implements this?

    I really appreciate your help.

    Details on the data: They have been transmitted by a Ublox EVK-7p using the UBX RXM-EPH and RXM-RAW protocols.

    RXM-EPH: satellite ID (4 byes), HOW word (4 bytes), followed by three 32-byte arrays corresponding to subrames 1 to 3 of the GPS message.

    RXM-RAW: time of week, week, num of satellites, reserved (?) , carrier phase, pseudo-range, doppler, and so on....
     
    Ash, Apr 4, 2017
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  2. Ash

    Nuvi-Nebie Moderator

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    Here is a U-Blox document that includes the protocol Spec., it may be useful if you have not already seen it (Click HERE)
     
    Nuvi-Nebie, Apr 4, 2017
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  3. Ash

    Ash

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    Thanks a lot! I had seen this documentation but it describes the receiver output in term of subframes. However, I found a specification (it is really old, though...1995) which seems to answer a lot of my questions.

    (See attached file below)

    Seems that http://www.gps.gov/technical/icwg/ naturally has a lot of resources that I was unaware of.

    Thanks for you time.
     

    Attached Files:

    Ash, Apr 4, 2017
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  4. Ash

    ndoggac

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    GPS L1 NAV message hasn't changed since then ;-) gps.gov is the go to location for all standards regarding signal data structure (L1, L2, L5, CNAV, OCS, OCX).
     
    ndoggac, May 4, 2017
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