MP3 Fm Transmitter interfering with GPS navigator

Feb 10, 2013
TomTom Model(s)
XL 340
I have recently discovered that GPS navigator can fail to operate if it gets interference from mp3 fm transmitters.

I have been using Tom tom GPS navigator (XL 340) for almost 3 years now and was very satisfied with the product until recently when all of a sudden my GPS stopped working and was always displaying ' waiting for a valid satellite .... ' . Tom tom support advised me to re-set and update software.
The update / resetting did not fix the issue and I found that the whole issue was caused by interference from mp3 Transmitter frequency (we recently change frequency on the mp3 transmitter to avoid interference from local FM stations and this change didn't agree with the GPS unit) . I have since change the frequency of the mp3 transmitter and GPS is working fine since then.

Just sharing this so other tom tom users can benefit from this.

Question is how can certain FM frequencies interfere with GPS operation ? Also could this be experienced in an area where local FM Radio stations are operating on such frequency ?
Strange. Back in 'the day' (e.g., GO 720, 930, etc), TomTom was using FM modulators for music and navigation right in the unit, and didn't seem to have any interference problems.

There is no particularly good reason why an FM modulator should interfere with the GPS receiver unless one of a couple of things happens. First, it could be overloading the front end of the GPS. This would require very close proximity or a rather higher (possibly than legal) power modulator.

The second and even far less likely is that the 15th harmonic of your modulator's ~105MHz output is actually strong enough to toss a signal up into the L1 GPS band (~1575MHz). Again, that would require extreme proximity to the GPS unit, and kind of an ugly transmitter.

In no case should proximity to an FM transmitter blow out GPS reception.
Yeah it is strange behavior and I must say the FM modulator I am using is not the best quality its one of the cheap ones from ebay.

The frequency which interferes with the GPS is 87.5 only and it works fine on 87.6 , 87.7 ....or any other for that matter (of-course I haven't checked them all) but 87.5 doesn't work for sure , change the frequency to 87.6 and everything works alright.

I have ordered a different modular and will pick one from Jaycar soon to see if a better modular unit makes a difference.

Its not biggie now as I can use a different frequency but before I thought the tomtom unit was faulty and was about to through it but it kept searching for satellite and that's why I kept experimenting till I found the problem..
Although these problems occur on odd harmonics in the VAST majority of cases, your example happens to be (may be coincidence, but those are hard to come by, too!) right on the 18th harmonic.

87.5MHz x 18 = 1575MHz.
The actual is 1575.42MHz, but close enough for 'government work' when it comes to wiping out the front end of a super-sensitive receiver such as a TomTom or other GPS receiver.

Who knows what the output of the device is .. could be over the typical 50mw power or even over the 100mW legal limit. All I can recommend until you get a new one is to keep as much separation between them as possible. If your existing device isn't well designed, it's also possible that a lot of that energy is being couple into the power connection for it .. unless it's battery operated, of course.
When I get home tonight, I'll try firing up my old Go720 (which has an internal FM transmitter) and see if, at that particular frequency, it interferes at all with GPS reception.
Bet it doesn't. The output of the TomTom is pretty weak compared to a lot of the standalone units out there, some of which (from China, of course) are substantially over the FCC limits.
This is 100% true that GPS signal interfare with mp3 FM transmitter. I had the experience myself and discover it myself on 21st August 2013. I always had good GPS signal in my Google navigator. Then in one day found that its loosing the GPS signal frequently and sometime do not get GPS at all. Then I thought there is something wrong with my GPS chip in phone. So bought Tomtom 720 and tested it inside home and it was getting 7 satellite. However when I tried it in my car, it was unable to get GPS at all. Then I started thinking, there must be something wrong in the car and then the new mp3 FM transmitter strike in my mind. Surprisingly it was set to 87.5 and I took it off from the car charger and presto! The tomtom and my phone got GPS signal straight away. I was surprised and that's why checking in internet today and now I am 100% sure of my discovery. Please spread this information to all. I know there are other people suffering from the same GPS signal issue due to those cheap mp3 FM transmitter.
As noted above, there's a good chance that the MP3 modulator has harmonics way over the allowed limits for an 'intentional emitter' per FCC regs. That said, it's either change out the unit, or don't use it. Bummer.
Hello people. Firstly let me apologize because I do not have a TomTom satnav but my experience is the same. The issue is with the factory satnav in my 2007 Nissan Pathfinder and a FIRSTe FM transmitter both set to 87:6. The FIRSTe FM transmitter is awesome but for some reason after one month of trouble free operation of both the FM transmitter and the Pathfinders satnav suddenly the car satnav stopped working, the message was 'out of range'. After trying other unsuccessful items I unplugged the FM transmitter and straight away my Pathfinders satnav started working. I don't understand why. It's a pity because I really like the FIRSTe FM transmitter. Anyone got any answers? Many thanks.

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