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[aprssig] GPS question

Andrew Rich vk4tec at tech-software.net
Sun Dec 9 03:45:00 UTC 2012


I have a survey gps on loan will try 

Sent from my iPhone
Andrew Rich

On 09/12/2012, at 12:55 PM, Greg D <ko6th.greg at gmail.com> wrote:

> Hi Steven,
> 
> Stephen H. Smith wrote:
>> 
>> 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.
> 
> While I agree with everything you said, one experience I had suggests that the particular constellation of satellites in view will make a difference in your apparent position...  
> 
> I was a passenger on a small corporate plane, with my Garmin GPS-III sitting in hand by a side window (half of the sky visible).  As we made a 180 degree turn to line up with the runway, my apparent position suddenly shifted some distance directly sideways - maybe a quarter of a mile or so.  I chalked it up to the GPS seeing a different set of satellites, and coming to a slightly different conclusion at where I was.  
> 
> There's always some uncertainty in your reported position, and in theory the real position should be inside of that uncertainty circle.  What I don't know is if there is a bias as to where in the uncertainty circle you will be reported to be, given a particular view of the sky.  This is two data points now that perhaps there is.
> 
> Greg  KO6TH
> 
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