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Lloyd M. Mitchell wrote:
<blockquote cite="mid:41797.479.qm@web53906.mail.re2.yahoo.com" type="cite">
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<td style="font-family: inherit; font-style: inherit; font-variant: inherit; font-weight: inherit; font-size: inherit; line-height: inherit; font-size-adjust: inherit; font-stretch: inherit;" valign="top">If you're using a Garmin GPS V, I can jam it with my FM
broadcast radio on 94.3MHz at 50 ft. I imagine the IF freq's are
mixing. Took me the longest to figure out why in the morning drive I
could not get a fix, but the afternoon drive no problem (different
stations morning and evening) of course this was pre-Sirus...<br>
<br>
Lloyd KO4L<br>
<br>
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<br>
<br>
Most likely, it's the 15th harmonic of the FM broadcast radio's local
oscillator landing on the GPS operating frequency. <br>
<br>
94.3 + 10.7 = 105 MHz [10.7 MHz is the standard IF frequency of
most FM receivers; thus the LO will be above the operationing frequency
by this amount.}<br>
<br>
105 MHz x 15 = 1575 MHz which is the center frequency of the GPS
signal. <br>
<br>
<br>
I had this kind of problem years ago during some GPS experiments with
Los Angeles Sheriff Dept vehicles. My GPS-driven radio-coverage
mapping system worked fine in my car, but would absolutely not work in
LASD black&whites. Finally discovered that the third harmonic of
the local osc of the GE Ranger 506/507 MHz T-UHF mobiles were landing
directly on the GPS frequency and wiping out GPS RX completely. <br>
<br>
[Bear in mind that GPS signals on the ground are INCREDIBLY WEAK (i.e.
-130 to -140 dBm). It is only the sophisticated spread-spectrum
de-processing at the receive end that allows these signals (that are
10-20 dB weaker than anything a conventional narrowband FM receiver
could ever hope to hear) to be detected at all. And with a very
minimal no-gain antenna to boot. It doesn't take much to cover them
up. <br>
<br>
Aircraft of course do have a huge advantage over land vehicles. Since
the desired signal (the satellites) is coming from above, antennas
mounted on the top of the vehicle with a hemispheric directional
pattern upward can ignore a pretty hefty jamming signal originated on
the ground. On the other hand, I have no idea what sort of antenna
pattern a JDAM (GPS-guided bomb) would have... ]<br>
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