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fm transmitters and gps jamming

 
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gyre
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Location: Bristol, UK

PostPosted: Fri Feb 26, 2010 10:44 pm    Post subject: fm transmitters and gps jamming Reply with quote

I chose a new fm frequency to transmit music from my iphone today... and noticed I was unable to get a gps lock.

When I changed to a different frequency, I got a lock.

Changed it back, lost the lock.

Total coincidence, or accidental gps jamming?

Anybody hazard a guess?

-- gyre --
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mikealder
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Joined: Jan 14, 2005
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Location: Blackpool , Lancs

PostPosted: Fri Feb 26, 2010 10:52 pm    Post subject: Reply with quote

Considering the difference in frequency I wouldn't have thought an FM transmitter would knock out GPS but if I can find a post by Eldar (one of the other moderators) from about four years ago I am sure there is a connection between a specific FM frequency and the GPS carrier frequency.

FM uses typically 87,5 to 108MHz whereas GPS uses 1,5GHz - If I can find the post I will add a link to it but it was from some time ago (strange how the brain can remember a single thread but the search engine can't find it!)

Out of interest what frequency have you got your FM Tx set transmitting on? - Mike
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Skippy
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PostPosted: Sat Feb 27, 2010 11:50 am    Post subject: Reply with quote

Could be that the GPS is on a harmonic of the FM frequency but 90 Mhz is quite a long way removed from 1500 Mhz.... Confused
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gyre
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Location: Bristol, UK

PostPosted: Sat Feb 27, 2010 1:06 pm    Post subject: Reply with quote

I can't remember the exact frequency now, as I've retuned, unfortunately.

Was most odd tho... unable to get a lock at all, all afternoon.

-- gyre --
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mikealder
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PostPosted: Sat Feb 27, 2010 1:43 pm    Post subject: Reply with quote

As I can't find the post by Eldar having spent an hour last night looking for it I do remember it was harmonic related so a simple calculation shows that 98,27 FM will produce a fourth harmonic the same frequency as used by the GPS, it can't have been a very good FM Tx unit though if its throwing out RF harmonics of the magnitude required to kill the GPS receiver - Mike
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gyre
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Location: Bristol, UK

PostPosted: Sat Feb 27, 2010 2:18 pm    Post subject: Reply with quote

It was somewhere around that frequency, yes.

Thanks. Smile

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gyre
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Location: Bristol, UK

PostPosted: Sat Feb 27, 2010 3:10 pm    Post subject: Reply with quote

A quick google around the web found this snippit:

Code:

FM radio stations, while lower in radiated power, can cause GPS jamming also. Table 3 lists the
potentially problem radio frequencies and their respective GPS harmonic.
FREQUENCY HARMONIC
104.8 - 105.2 15th
98.3 - 98.7 16th
92.5 - 92.9 17th
87.3 - 87.7 18th
TABLE 3. FM RADIO FREQUENCIES WITH GPS JAMMING POTENTIAL
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