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[aprssig] How gps works

Stephen H. Smith wa8lmf2 at aol.com
Sun May 6 14:53:54 UTC 2012


On 5/6/2012 10:12 AM, Andre wrote:
>
> 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.

Here in the US at least,  a lot of cellular phone systems operate in the  1.9 
to 2.1 Gz range (the so-called "PCS - Personal Communications Service" 
second-generation cell networks).     Since the range of these 
microwave-frequency systems is inferior to the older 800 MHz systems, PCS 
networks tend to have far more sites spaced much closer together for the same 
amount of coverage.  In high-traffic urban areas the base-station sites are 
often less than a KM apart.

Using GPS receivers with external antennas in these areas is often 
problematic.  To offset the astronomical loss in small-diameter coax at 1.575 
GHz, many of the external patch antennas have 20 dB gain preamps built-in, but 
virtually no selectivity.  The result id that every time you get within a third 
to a half a KM of a PCS site, the GPS receiver front end overloads and loses lock.

(The previous use of the 2 GHz band had been point-to-point microwave links, 
which was far less of a problem since these use tightly focused beams 
dish-to-dish that were NOT trying to spray the ground below with as much RF as 
possible.   Not to mention that there were far far fewer of these links than 
there are PCS sites today.)

This is one reason I far prefer all-in-one (receiver and antenna together) 
faceless "hockey puck" receivers with passive antennas.wired directly to the RX 
front end.   Without 20 dB of gain in the front end, they are far far less 
likely to block and overload.


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Stephen H. Smith    wa8lmf (at) aol.com
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