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gyre Regular Visitor
Joined: Aug 01, 2005 Posts: 98 Location: Bristol, UK
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Posted: Fri Feb 26, 2010 10:44 pm Post subject: fm transmitters and gps jamming |
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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 Pocket GPS Moderator
Joined: Jan 14, 2005 Posts: 19638 Location: Blackpool , Lancs
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Posted: Fri Feb 26, 2010 10:52 pm Post subject: |
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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 Pocket GPS Verifier
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
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Posted: Sat Feb 27, 2010 11:50 am Post subject: |
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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.... _________________ Gone fishing! |
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gyre Regular Visitor
Joined: Aug 01, 2005 Posts: 98 Location: Bristol, UK
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Posted: Sat Feb 27, 2010 1:06 pm Post subject: |
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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 Pocket GPS Moderator
Joined: Jan 14, 2005 Posts: 19638 Location: Blackpool , Lancs
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Posted: Sat Feb 27, 2010 1:43 pm Post subject: |
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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 Regular Visitor
Joined: Aug 01, 2005 Posts: 98 Location: Bristol, UK
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Posted: Sat Feb 27, 2010 2:18 pm Post subject: |
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It was somewhere around that frequency, yes.
Thanks.
-- gyre -- |
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gyre Regular Visitor
Joined: Aug 01, 2005 Posts: 98 Location: Bristol, UK
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Posted: Sat Feb 27, 2010 3:10 pm Post subject: |
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A quick google around the web found this snippit:
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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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