[aprssig] GPS question

Andrew Rich vk4tec at tech-software.net
Sat Dec 8 16:49:53 EST 2012

Thanks - will go find a hill 

- Andrew - 
  ----- Original Message ----- 
  From: Stephen H. Smith 
  To: Andrew Rich ; TAPR APRS Mailing List 
  Sent: Sunday, December 09, 2012 7:36 AM
  Subject: Re: [aprssig] GPS question

  On 12/8/2012 4:26 PM, Andrew Rich wrote:

    I am not sure I have asked this before but I will ask it again.

    I have a GPS on the side of a building and it can only see half the sky - to the north

    What is curious - is that when plotted on google earth - the postion is north of where it should be 

    I will move it into full view of the sky and see what that does 

    - Andrew - 

  A gps receiver should be able to function normally WITHOUT seeing the entire sky; they do this all the time on car dashboards.  

  Most likely you either have the unit set for a datum other than WGS84 (the GLOBAL grid system created by and for GPS).  

  Or the device is being confused by reflections from nearby structures.  Since GPS works by timing the difference in time of arrival of signals from multiple satellites, multipath reflections that add to path length (and thereby delay the time of arrival of signals) will translate directly into a shifted position.  In downtown large-city "urban canyons" of glass-faced high-rise buildings that bounce the 1575 MHz GPS signals every which way, it's not uncommon to see GPS "go nuts" and jump back and forth several blocks. 

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