Garmin Glo - How many satellites?

Discussion in 'Garmin GPS' started by Capn666, Apr 10, 2019.

  1. Capn666

    Capn666

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    Hi. Newbie question.
    My Garmin Glo detects up to 20/21 GPS and/or GLONASS birds with good S/N but it never seems to use more than 12 in calculating a fix according to the BluetoothGPS app used for mock position on android.
    Garmin support say there's no limit to the number it can use.
    Can anyone give me an explanation please?
     
    Capn666, Apr 10, 2019
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  2. Capn666

    Nuvi-Nebie Moderator

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    I can't find a spec. that details how many satellites the Glo will use in its calculations, it may be set to 12, however this is not such a problem as the Glo will be selecting the 'best' 12 from the 24 or so visible GPS + Glonass constellations, the extra accuracy supplied by the other 12 is minimal.

    The Garmin Glo vastly out perform GPS only units like the GPSmap60 and the Montana 600 because it can select 12 'better' satellites and is pobably roughly as good as any other GPS + Glonass unit
     
    Nuvi-Nebie, Apr 11, 2019
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  3. Capn666

    Capn666

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    Thanks for the reply. It ties in with what I thought but Garmin Support are saying there is no limit.
    I've tried to upload a screenshot and would appreciate any comments on whether the selected 12 sats (solid green bars) are indeed the "best" ones.
    Screenshot_20190409-161227.png
    I'm very happy with the Glo but just curious to understand how it works.
    TIA.
    Ken
     
    Capn666, Apr 11, 2019
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  4. Capn666

    Nuvi-Nebie Moderator

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    Basically satellite numbers from 1 to 32 are GPS and numbers above 36 are Glonass, it isn't really GPS vs Glonass (although Glonass is reguarded as slightly better at higher latitudes) , it is more to do with where the satellites are at any given time, 12 satellites all grouped together will produce less accuracy than 12 well spaced ones
     
    Nuvi-Nebie, Apr 12, 2019
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  5. Capn666

    Capn666

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    Thanks again.
    It seems to me though that some of the satellite choices are a bit strange in the example I attached. There are a couple of overlaps that seem to offer no advantage that could be swapped for other satellites and give an improved fix.
    Or am I missing something else?
     
    Capn666, Apr 12, 2019
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  6. Capn666

    Nuvi-Nebie Moderator

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    It is probably just because the GPS hasn't yet got all (or any) data from the others yet, your unit downloads an almanac which is a list of satellites that should be 'visible', this means it knows about satellites it hasn't managed to download from yet, also they are moving all the time so the ones near the horizon may have recently become visible or are going out of view
     
    Nuvi-Nebie, Apr 13, 2019
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  7. Capn666

    Capn666

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    I've watched the dynamic situation and it seems that if one of the 12 satellites "in use" is lost, one of the others is immediately substituted and its block goes solid colour. So its data must already be acquired. If the satellite is visible but no data is acquired it appears as shown on the attached. See numbers 30 15 18 & 50.
    Screenshot_20190412-192043.png

    So it appears that the Glo just picks what it regards as the best 12 and keeps the others on hot standby.
    It just seems to me that it isn't always selecting the best 12!
    It's a pity that Garmin don't seem to have this publicly documented and indeed their own support seem to think there is no limit to the number of satellites it uses in computing the fix.
    There is a noticeable improvement in accuracy between using 8 and 12 satellites so I'd expect some further, albeit smaller, improvements going from 12 to say 16 or 18 and still have some in reserve.
     
    Capn666, Apr 13, 2019
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