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[aprssig] How gps works

Andrew Rich vk4tec at tech-software.net
Sun May 6 14:33:03 UTC 2012

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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