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[aprssig] How gps works

Joe Jesson jjesson at voyager.net
Sun May 6 17:05:09 UTC 2012

  A great GPS freeware tool to analyze GPS data is u-Center, 
NMEA data is sent to this tool and the data attributes are shown graphically 
as multiple cockpits..   This is a tool I have used over several years in my 
commercial tracking biz to solve many GPS problems (excessive DOP errors was 
one of the most recent problems which u-Center helped diagnose).. The latest 
version has KML integration Google Earth links and a video input to capture 
the environment the antenna resides..

Joe Jesson, KC2VGL

----- Original Message ----- 
From: "Andrew Rich" <vk4tec at tech-software.net>
To: "TAPR APRS Mailing List" <aprssig at tapr.org>
Sent: Sunday, May 06, 2012 10:33 AM
Subject: Re: [aprssig] How gps works

> You might have been getting interference from DME ?
> SSR RADAR is TX on 1030 and RX on 1090 MHz
> GPS is 1575 MHz
> - Andrew -
> ----- Original Message ----- 
> From: "Andre" <aprs at pe1rdw.demon.nl>
> To: "TAPR APRS Mailing List" <aprssig at tapr.org>
> Sent: Monday, May 07, 2012 12:12 AM
> Subject: Re: [aprssig] How gps works
>> On Sun, 06 May 2012 08:34:19 +0200, Stephen H. Smith <wa8lmf2 at aol.com> 
>> wrote:
>>> On 5/6/2012 1:49 AM, Andrew Rich wrote:
>>>> Does seeing "part" of the sky make a difference ?
>>>> If I sit beside a buidling were I can only see 1/2 the sky does my 
>>>> position skew ?
>>> NO.  It's a very binary process.  Either the part of the sky you can see 
>>> has enough satellites in view for a fix.  Or, if there are not enough 
>>> receivable, the unit "spins it's wheels" endlessly in the acquisition 
>>> phase with no fix.
>>> Typically as the pattern of satellites visible from your location 
>>> changes, and the number visible fluctuates above and below the magic 
>>> minimum, the unit will keep shifting between a 4-satellite 3-D fix 
>>> (altitude as well as lat/long), a 3-satellite 2-D fix (no altitude info) 
>>> and no fix at all.
>>> When you get into the "urban canyons" of high-rise big city downtown 
>>> areas, GPS units frequently "go nuts" due to signals being blocked, and 
>>> by apparent time delays for given satellites abruptly changing due to 
>>> multipath reflections off glass-faced buildings.
>>> The same thing happens frequently on winding mountain roads, especially 
>>> deep in canyons.   The receiver is constantly acquiring satellites only 
>>> to lose them again when you go around a curve, forcing the receiver to 
>>> search for and acquire other satellites, as the visible part of the sky 
>>> keeps changing.
>> It's not just visabilety of the sky that can make gps go nuts, I have 
>> worked as an assistant surveyer for a while in Rotterdam and everytime we 
>> where in range of the airport we would get big deviation errors, 
>> sometimes more then half a meter and this was with a DGPS system and at 
>> least 8 satellites locked, waiting a few minutes would bring the 
>> deviation back to a few CM making at acceptable again for surveying 
>> pipes.
>> We never figured out what it was but we assumed it was the airport radar.
>> -- 
>> 73 Andre PE1RDW
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