[ale] ot - FM transmitter makes good GPS jammer

[Jay Lozier]

On 05/07/2012 03:47 PM, Ron Frazier (ALE) wrote:
> Hi Jim,
>
> OK, I did one more test.  This time, I tested both my tablet GPS and 
> my TomTom GPS.  Both devices were running on battery power and had no 
> connection to the car at all.  I plugged the fm transmitter into the 
> headphone output of my boombox.  I started the test with 10 - 11 
> satellites being tracked by the TomTom and about 7 being tracked by 
> the tablet.  The moment I plugged the fm transmitter into the 
> cigarette lighter, the TomTom went down to tracking only 3 satellites 
> and the signal levels on those decreased substantially, so it would 
> sometimes lose the fix.  The tablet went down to tracking no 
> satellites.  Once I remove the fm transmitter from the outlet, both 
> units return to normal.  I do think the GPS receiver in the tablet is 
> weak, for whatever reason, but this problem is not the tablet.  It's 
> definitely the fm transmitter and it's definitely RADIO based.  This 
> thing is SO fired from my collection of gadgets.
>
> Sincerely,
>
> Ron
>
<snip>
Ron,

One of the for GPS receivers is that they are tracking very weak 
signals. It is possible for a radio transmitter to emit such a strong 
signal that the very weak GPS signals are masked when the transmitter is 
near. The situation is even worse if the transmitter is not properly 
tuned. Another problem is that a GPS receiver may be optimized to 
receive very weak signals with a greater possibility of interference 
problems from very close radio transmitters.

Lightsquared ran into problems with this with the FCC because the 
frequencies they wanted to use and transmitter power levels would 
interfere with GPS signals. They tried to blame the GPS manufacturers 
but the FCC said no dice. One of the reasons was the original frequency 
allocation of the frequencies Lightsquared wanted to use was for use 
that would not interfere with GPS; I believe it was originally for 
satellite communications. The FCC allowed some testing of the 
frequencies for cell phone use but required that Lightsquared must prove 
this would not interfere with GPS.


-- 
Jay Lozier
jslozier at gmail.com

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