Calculating position question

Discussion in 'GPS Technical Discussion' started by gps1234, Jan 8, 2018.

  1. gps1234

    gps1234

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    Hi. I'm quite new to this.

    I am faced with a task of calculating a position given Satellite, Pseudo range,Carrier Phase and Doppler frequency.

    I think I have a basic understanding of the calculations, but don't I need to know where the satelites was positioned at the time of the measurement?
    Where do I find data for this time? Is there a feed for old data?

    /newbie
     
    gps1234, Jan 8, 2018
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  2. gps1234

    Nuvi-Nebie Moderator

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    The data transmitted from each satellite contains the time and it's Ephemeris data (location) above the earth, using 4 of these locations taken at the same time, together with their calculated distance to the GPS receiver (pseudo-range), it is possible to calculate the GPS's location
     
    Nuvi-Nebie, Jan 8, 2018
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  3. gps1234

    gps1234

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    Okay. That's what I belived.
    But where do I find these data?
    I do not have the transmitted Ephemeris.Is there a way of looking it up, If I know the date and time?
     
    gps1234, Jan 8, 2018
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  4. gps1234

    Nuvi-Nebie Moderator

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    I don't have any info. on the ephemeris data transmitted by the satellites, but GPS receivers output data in the form of NMEA sentenses, this data contains a mixture of received data and calculated data, the GPGSV sentence outputs elevation and azimuth data for each satellite e.g. :-

    GPGSV

    GPS Satellites in view

    $GPGSV,3,1,11,03,03,111,00,04,15,270,00,06,01,010,00,13,06,292,00*74
    $GPGSV,3,2,11,14,25,170,00,16,57,208,39,18,67,296,40,19,40,246,00*74
    $GPGSV,3,3,11,22,42,067,42,24,14,311,43,27,05,244,00,,,,*4D

    $GPGSV,1,1,13,02,02,213,,03,-3,000,,11,00,121,,14,13,172,05*67

    1 = Total number of messages of this type in this cycle
    2 = Message number
    3 = Total number of SVs in view
    4 = SV PRN number
    5 = Elevation in degrees, 90 maximum
    6 = Azimuth, degrees from true north, 000 to 359
    7 = SNR, 00-99 dB (null when not tracking)
    8-11 = Information about second SV, same as field 4-7
    12-15= Information about third SV, same as field 4-7
    16-19= Information about fourth SV, same as field 4-7

    http://aprs.gids.nl/nmea/#gsv
     
    Nuvi-Nebie, Jan 8, 2018
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  5. gps1234

    Mark Hunter

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    Almost the right answer but a there is a slight difference that can make a lot of difference (quite literally!). You need to know where the SV was positioned at the time of transmission, not where it was at the time of reception.

    Try the IGS for historical ephemeris data; use the final version for the most precise ephemeris:

    Code:
    http://www.igs.org/products
     
    Mark Hunter, Apr 16, 2018
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