[time-nuts] WAAS.....

[Joe Leikhim]

Brian;

Regarding mobile jammers..

Many years ago I was faced with finding the cause of sporadic 
interference to a new 800 MHz trunked LMR system in Miami. This problem 
dogged several engineers and myself for months as the customer was 
reluctant to make final payment on the $8million system. The 
interference was received nicely by the many remote receivers throughout 
the city and disturbed the system audio and caused alarms to be 
reported.  At street level, little was heard and when heard, it 
disappeared quickly. We could get weak intermittent signals from various 
rooftops and after a while it became obvious than _many_ emitters were 
responsible.  Finally I had permission to bring a spectrum analyzer and 
antenna aboard a Miami PD helicopter while a co-worker with a spectrum 
analyzer took to the streets. About 10 minutes after  taking off, I got 
a strong hit near a downtown parking lot. My coworker arrived and 
confirmed same hit. To make a long story short, it was the local 
oscillator of a Motorola MOSTAR radio. The problem was both a design 
problem of the mobile radio and more importantly a network design 
problem of the trunked system to be unable to deal with the momentary 
illegal carriers. (A point I argued with the product manager from the 
start).

Once we had the first interfering radio captured, we determined that 
they belonged to a radio system two counties away, and whenever they 
arrived in Miami, they would scan for a missing control channel and 
create havoc. To confirm this, I drove to the other county, and parked 
at a major intersection and took note of the commercial vehicles that 
drove by with their LO's leaking. This confirmed the model radio 
involved was limited to the one initially found.

This is documented in Chapter 2.7 of Gary C. Hess' book titled "Land 
Mobile Radio System Engineering. "


GPS jamming, intentional or not is pretty serious, and the FCC takes this seriously, but unless you have some pretty hard evidence they may not find it.

If you have the time and equipment, you should monitor the L1 frequency from a high vantage point with a spectrum analyser to see if either it is a fixed emitter or mobile. If the latter, I would suggest doing as I did, set up a monitoring point at a traffic choke point to see if mobiles drive by that are emitting energy.

A possible source is harmonic energy from mobile radio transmitters in the VHF, UHF, 700 and 800 MHz bands, or strong fundamental energy overloading the amplifed antenna to the point harmonics occur, or the MMIC amplifer is saturated.

Someone on timenuts mentioned a GPS vendor who made a metal shield can to put over the GPS antenna that was essentially a waveguide slot/ bandpass filter. It was to test for out of band interference. This you might try at one or two sites.


-- 
Joe Leikhim


Leikhim and Associates

Communications Consultants

Oviedo, Florida

JLeikhim at Leikhim.com

407-982-0446

WWW.LEIKHIM.COM