[aprssig] Poor GPS signal quality in Los Angeles County?

Stephen H. Smith wa8lmf2 at aol.com
Sun Oct 25 20:43:08 EDT 2009

Alex Carver wrote:
> Has anyone run into problems with GPS reception in and around Los Angeles County?  Until I moved out here, I never had a problem with reception unless I was driving through a thick tree canopy or into a parking garage.  But lately I've been having trouble even when I could see the whole sky (up near Pasadena).  I have an old Garmin 12XL with an external antenna.  My APRS performance has been spotty because the D700 will stop beaconing for a while (and I don't notice) until I observe that the GPS no longer has a lock.  In many cases, the GPS will even indicate it has no signals at all (RSSI bars on the constellation map screen are all zeros).  I never had this problem back east and it was all the same hardware, car, etc.

I live in Pasadena, and have experienced this problem with older GPS 
receivers that use external active antennas.  These active antennas 
(especially patch types) have virtually NO frequency selectivity before 
a 15-20 dB gain RF preamp, that is boosting the GPS signal to offset the 
horrendous loss of 10-15 feet of RG-174 (or even RG-58) cable at the 
1.575 GHz frequency of GPS. These preamps overload, desensitize and 
block from the extremely dense 1.9 GHz PCS cellular networks in L.A. 
that have a low-to-the-ground transmit site ever half mile or mile in 
most of the metro area. 

I have had much better results with all-in-one devices (like the eTrex 
or ForeTrex), or "hockey-puck"-type GPS receivers (like the Deluo or 
Garmin GPS-18) with integrated antennas. Since the antenna is connected 
directly to the receiver front end without any intervening coax & loss, 
the antennas are normally passive (not amplified) and FAR less prone to 
desensing and blocking from terrestrial sources. 



Stephen H. Smith    wa8lmf (at) aol.com
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