[time-nuts] Distance between GPS Antennae

Brent Gordon time-nuts at adobe-labs.com
Sun Dec 18 19:54:49 EST 2011


   Tom,
   Yes, this is the same board that you played with.  The only difference
   is that the code on the GPS chip is NBCAA-1110 instead of CNAA1038.  I
   did not save any NMEA logs, just the VisualGPS diagrams.  When I
   collected the data I had just received the Sure GPS unit and wasn't
   doing any serious testing of it.  When this topic came up, I thought
   "Hey!  I've got some cool data that shows interference."
   Both boards were in their power-on default mode.  As you know, the Sure
   GPS defaults to factory settings after it has been powered down for a
   while.  From the documentation links on your web page, there doesn't
   seem to be a position hold mode for this GPS.
   A quick test shows that the first 20-30 points wander around before the
   unit locks the position.  Maybe K4CLE's comment "Some GPS receivers
   automatically go into "position hold" mode if less than 1 mph is
   detected for several seconds." applies to this board.
   I will be [DEL: playing with :DEL] testing these units over the next
   two weeks and see what I can learn.  I've got two patch antennas, two
   timing antennas, and a powered splitter.  I also have a couple of
   Thunderbolts.  However, because I have a metal roof, the Thunderbolts
   lock on to fewer than five satellites at a time.  The Sure and
   Copernicus both lock on to twelve.
   How did you measure the PPS jitter on your unit?  I'd like to do a
   similar measurement on mine.
   Brent
   On 12/18/2011 3:02 PM, Tom Van Baak wrote:

     From: "Brent Gordon" [1]<time-nuts at adobe-labs.com>

       The best answer is try it it and see what happens.  In my case two
       systems with patch antennas ten inches apart interfere
     significantly.
       Here are plots (using VisualGPS) of my position over a day or two.
       This is using a GPS from Sure Electronics.  Note that the standard
       deviation is 0 for over 100,000 samples.  As a side note, it must
     have
       some kind of heavy filtering going on to not show any position
       variation.  This kind of result is repeatable with this unit.

     Brent,
     That's really interesting. It looks like one receiver is in GPS
     hold mode (so you would expect the position to remain fixed)
     and the other is in a normal 2D or 3D mode. Can you look at
     the NMEA logs and tell which is which? Is this the same GPS
     receiver I played with (leapsecond.com/pages/MG1613S)?
     Thanks,
     /tvb
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