[aprssig] Poor GPS signal quality in Los Angeles County?
Stephen H. Smith
wa8lmf2 at aol.com
Sun Oct 25 19:43:08 CDT 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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