[aprssig] GPS QRM

Stephen H. Smith wa8lmf2 at aol.com
Sat Feb 5 13:32:18 EST 2011

On 2/5/2011 9:20 AM, Keith VE7GDH wrote:
> R
> Some further reading...
> http://en.wikipedia.org/wiki/LightSquared
> I know one APRS user sees inaccurate positions on Georgia St. in
> downtown Vancouver BC. It could be the "concrete jungle" and
> multi-path, but some kind of interference is suspected.
> 73 es cul - Keith VE7GDH
> -- 

I've seen this on a smaller scale for many years.    When 1.9 GHZ PCS cellular 
service appeared here in Los Angeles, I started noticing that my GPS devices 
with active antennas would go out of lock when I got within about 1 block of a 
PCS cell site.

The active antennas, used when the antenna is separated from the actual 
receiver by 15-20 feet of coax, have a 15-20 dB-gain preamp built-in to offset 
the astronomical loss of small coax at 1575 Mhz.   There is very little 
selectivity in front of the amplifier (typically just a low-Q patch antenna) 
making the amp susceptible to blocking and overload by nearby (physically and 
frequency-wise) terrestrial sources.   [The GPS signal is around -145 to -150 
dBm on the ground, while the PCS signal is probably  -70 dbm or greater around 
the site.]  Fortunately the PCS sites are relatively low powered and low to the 
ground so the blackout exists for only a block or two.

This problem is far far less of an issue with car navigator and all-in-one 
"mouse" or "hockey puck" GPS units, since these devices have a passive antenna 
attached directly to the receiver front end, with no preamp to overload.



Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

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