WAAS 135 & 138 navigation message

Discussion in 'GPS Technical Discussion' started by ricky22, Feb 12, 2017.

  1. ricky22

    ricky22

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    Hello,

    I've read in multiple online sources that two WAAS satellites, PRN 138 and 135, transmit both DGPS correction data and standard GPS navigation message, which means they can be used for a fix, in addition to the "normal" GPS satellites. PRN 133 on the other hand only transmits correction data, but not the GPS navigation message. My question is, is that correct? If so, how is that possible? AFAIK, commercial GEO communication satellites don't have atomic clocks on board. Neither is their position maintained to the same accuracy as GPS satellites, using multiple ground tracking stations, radars, lasers and what not.

    My second question is, do SBAS systems like WAAS, EGNOS and MSAS transmit correction data for other GNSS constellations, like GLONASS and BeiDou?

    Many thanks,

    Patrick
     
    ricky22, Feb 12, 2017
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  2. ricky22

    ricky22

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    Hello,

    I think I found partial answer to my first question.

    "It is evident from their groundtracks that PRNs 135 and 138 are in more tightly controlled GEO orbits than that of PRN 133 whose orbital inclination is now at 3 degrees compared to the others each at less than 3 arc minutes."

    That would mean that precise tracking to the same standard as the MEO GPS satellites is not necessary. Secondly, unlike with the GPS satellites, apparently the WAAS navigation message is generated on the ground, at a precisely known location of the ground station, sent to the WAAS satellite in the C band and repeated using bent pipe (had to look that up). That might account for the apparent lack of need for the atomic clock onboard, although I'd appreciate if someone could confirm.

    The answer to my second question is as follows:

    Currently operating SBAS (WAAS, EGNOS, MSAS, GAGAN) only support GPS. In the future, EGNOS, SCDM (Russia) and SNAS (China) will support GPS plus the countries' own GNSS systems (Galileo, GLONASS and BeiDou respectively). KASS (South Korea, in development) will support only GPS initially, then expand to include GLONASS and GALILEO.

    Thanks,

    Patrick
     
    ricky22, Feb 18, 2017
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