[ale] ot - FM transmitter makes good GPS jammer

[Ron Frazier (ALE)]

Hi Jim, and all,

I've tested this several times and confirmed the behavior.  It's 
definitely not power.  Today I tested this as follows with the tablet 
running on battery power.

I start the tablet on battery power and run a program called GPS test.  
I get a nice clean fix on 5 - 6 satellites, a positional accuracy of 15 
ft or so, and SNR values in the 26 - 31 range (this program doesn't 
specify the units).  Those satellites are color coded in yellow by this 
program.  I have a podcast playing in the background on the tablet's 
internal speakers.  I have the fm transmitter plugged into the cigarette 
lighter but the AUDIO cable is not plugged into the tablet.  So, sound 
is coming from the tablet's speakers, and nothing is coming from the 
car's speakers.  Everything is fine.  GPS works.

I plug the AUDIO cable in from the FM transmitter into the tablet's 
headphone jack.  Sound starts coming out of the car's speakers, as it 
should.  Immediately, SNR's for the satellites drop below 20, satellites 
with a fix drop to 0 - 1, and the position fix is lost.  I've confirmed 
this several times.  It is exceedingly strange.

I've officially fired the FM transmitter.  I don't think I have the 
packaging to take it back to Fry's.  It's a NAXA NA-3029 if anyone's 
interested.  I'd recommend avoiding this unit.

I've gone back to my old method.  I have a boom box with aux input 
sitting in the seat, and I run a wire from the tablet to the boom box.  
Messy, but it works.

Sincerely,

Ron



On 5/1/2012 8:19 AM, Jim Kinney wrote:
> that sounds more like an internal power issue. The device draws more 
> power on a bus that is available for both the gps and it to run to gps 
> falls down.
>
> To see if it's actually radio interference, check you cell phone gps 
> next to it and see if it also chokes.
>
> On Tue, May 1, 2012 at 12:20 AM, Ron Frazier (ALE) 
> <atllinuxenthinfo at techstarship.com 
> <mailto:atllinuxenthinfo at techstarship.com>> wrote:
>
>     I have one of those cheap fm transmitters that broadcasts audio from a
>     portable device to the car stereo.  It works great to send sound
>     from my
>     tablet to the car.  However, it also jams the tablet's GPS system.
>      When
>     I plug in the transmitter, the number of satellites the tablet has
>     a fix
>     on goes from 6 or more to 1 or zero immediately.  The GPS fails to
>     work.  If I unplug the fm transmitter, the GPS works again.
>      Anyone ever
>     seen that happen?
>
>     Ron
>



-- 

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Ron Frazier

770-205-9422 (O)   Leave a message.
linuxdude AT techstarship.com

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