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RE: [aprsisce] Re: White Sat again..again

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  • Steve Daniels
    Having studied the NMEA spec and more GPS info than is healthy, I am trying to work out how to calculate the likely error in position, from what your GPS says
    Message 1 of 2 , Aug 27, 2011
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      Having studied the NMEA spec and more GPS info than is healthy, I am trying to work out how to calculate the likely error in position, from what your GPS says to your actual location. Handy to avoid GPS jitter and when you are doing things like geocaching, if anyone has a formula or bit of code to hand that would be good

       

      Anyway I know someone will eventually ask the question of only 12 satellites on the bottom bar when they have more than a 12 channel GPS

      The NMEA spec for the GSV sentences which is data for the satellites that the GPS thinks it should be able to see, limits the number of sentences to 3 with 4 satellites per sentence, so only 12 satellites.

      But some GPS units break this rule, my unit will output 4 sentences, so I should see up to 16 at least, in practice the most I have seen is 15

       

      So if you don’t see more than 12 that’s down to your GPS not Lynn’s software.

       

      The white satellite issue was a timing issue, where GSA had provided the data for satellites in use, but GSV had not yet provided all the data for the signal strength of those satellites, GSV sentences can be split between samples.

      The more satellites you try to display the more of an issue it became, at least that’s a simplistic explanation

       

       

      Steve Daniels

      G6UIM

      Torbay Freecycle Moderator


      From: aprsisce@yahoogroups.com [mailto: aprsisce@yahoogroups.com ] On Behalf Of Lynn W Deffenbaugh (Mr)
      Sent: 27 August 2011 11:46
      To: Keehan Dowd; aprsisce@yahoogroups.com
      Subject: [aprsisce] Re: White Sat again..again

       

       

      (Copying the group for learning about the two bottom Satellite display
      lines)

      Ok, those aren't white satellites, but WAD (Working As Designed). At
      least, they're not the white satellites that were the issue.

      The very bottom line on the screen shows the satellites in view
      according to the GPS ($GPGSV). The slots are colored and numbered based
      on the signal strength and PRN. The line has 12 entries, unless your
      GPS reports more than that at some point whereupon APRSISCE/32 will
      expand, but never contract, that number. If there is an empty slot at
      the end, the count of satellites is put there, with a white background.
      You can usually tell that this is a number and not a PRN because the
      line is sorted.

      The line above that is the satellites in use, again, according to the
      GPS ($GPGSA). The same 12 slots applies here, but it will never grow
      because the $GPGSA format doesn't support more than 12 satellites in
      use. The PRN comes from the GSA, but the color comes from a PRN lookup
      into the GSV results. Again, if the right most cell is empty, the count
      of satellites is put there in white.

      The issue we had before was that a satellite in the left side of the top
      line would go white because it's number wasn't found in the bottom
      line. I now have diagnostics that will force themselves into a
      non-enabled NMEA trace log to tell me what PRN isn't found. In an
      overnight run of my WebDT with a BT GPS, I didn't get this once, whereas
      in the past, I would have seen it (at least) several times per hour.

      So, unless there's something I'm missing in your screen caps, I think
      this one is actually fixed.

      Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32

      On 8/27/2011 1:41 AM, Keehan Dowd wrote:
      > Finally another screenshot. A bit different again.

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