[aprssig] GPS problems in Los Angeles?

Alex Carver kf4lvz at yahoo.com
Sun May 22 13:41:38 EDT 2011

> From: "Stephen H. Smith" 
> On 5/21/2011 9:40 AM, Alex Carver wrote
> > I was driving up I-210 starting at the junction of
> I-210 and CA-134 (entered the highway just north of Colorado
> Blvd.)
> >
> Considering that Colorado runs parallel to the 210 for
> about 5 miles through 
> most of Pasadena (about 3 blocks south of the 210) this
> doesn't say much....

I think it says plenty.  I started at the junction of I-210 and CA-134 where I-210 makes the bend from being an E-W road to a NW-SE road.  There is an entrance right at that point just north of Colorado Blvd.  I didn't say I entered I-210 anywhere else.

> > I also happened to see a related problem (loss of
> satellite lock) in Burbank in the Empire Center parking
> lot.
> There is a string of 1.9 GHz PCS cell sites right by the
> side of the freeway, 
> along this stretch of the 210 through Pasadena and
> Glendale,  that can overload 
> and block 1575 MHz GPS receivers for about a 1/4-mile
> radius around each 
> site.   I have observed effects along this
> stretch ever since PCS came online 
> in the early 1990s.   Newer receivers with
> higher-dynamic-range front ends 
> don't seem to suffer as much as the older ones.

The problem I have with this is the fact that I've driven in many other cities with PCS all in close proximity to highways and surface roads and never had any ill effects with the very same receiver.  Not once.  I only have the problem in Pasadena, Glendale and Burbank.  If the source of the interference truly is the PCS towers then that would indicate that there is a major problem with the transmitters on the PCS towers covering Pasadena, Glendale and Burbank splattering out of band and not the front end of my receiver.  If the problem was only due to my receiver then I should have problems in other known high-density PCS areas.  For many years I used to live within visual range of several PCS towers perched on top of low rise buildings and skyscrapers and never once experienced this problem.

Sometimes the problem is so bad that the ephemeris data is scrambled and the GPS asks to be rebooted so that it can erase everything and start over.

> In downtown commercial districts, a combination of PCS
> overload and multipath 
> reflections from   metalized-glass 
> solar-reflective  high-e  windows on modern 
> all-glass office buildings can make GPS go nuts for a block
> or two.    I have 
> seen this effect both in downtown L.A. and in suburban
> monuments to the cubicle 
> culture like Walnut Creek, Irvine, Torrance, San Jose, etc
> that are dotted with 
> numerous glass office buildings and corporate
> headquarters.

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