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On 05/07/2012 03:47 PM, Ron Frazier (ALE) wrote:
<blockquote cite="mid:4FA826DE.5050901@techstarship.com" type="cite">
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Hi Jim,<br>
<br>
OK, I did one more test. This time, I tested both my tablet GPS
and my
TomTom GPS. Both devices were running on battery power and had no
connection to the car at all. I plugged the fm transmitter into
the
headphone output of my boombox. I started the test with 10 - 11
satellites being tracked by the TomTom and about 7 being tracked
by the
tablet. The moment I plugged the fm transmitter into the
cigarette
lighter, the TomTom went down to tracking only 3 satellites and
the
signal levels on those decreased substantially, so it would
sometimes
lose the fix. The tablet went down to tracking no satellites.
Once I
remove the fm transmitter from the outlet, both units return to
normal. I do think the GPS receiver in the tablet is weak, for
whatever reason, but this problem is not the tablet. It's
definitely
the fm transmitter and it's definitely RADIO based. This thing is
SO
fired from my collection of gadgets.<br>
<br>
Sincerely,<br>
<br>
Ron<br>
<br>
</blockquote>
<snip> <br>
Ron,<br>
<br>
One of the for GPS receivers is that they are tracking very weak
signals. It is possible for a radio transmitter to emit such a
strong signal that the very weak GPS signals are masked when the
transmitter is near. The situation is even worse if the transmitter
is not properly tuned. Another problem is that a GPS receiver may be
optimized to receive very weak signals with a greater possibility of
interference problems from very close radio transmitters.<br>
<br>
Lightsquared ran into problems with this with the FCC because the
frequencies they wanted to use and transmitter power levels would
interfere with GPS signals. They tried to blame the GPS
manufacturers but the FCC said no dice. One of the reasons was the
original frequency allocation of the frequencies Lightsquared wanted
to use was for use that would not interfere with GPS; I believe it
was originally for satellite communications. The FCC allowed some
testing of the frequencies for cell phone use but required that
Lightsquared must prove this would not interfere with GPS.<br>
<br>
<br>
<pre class="moz-signature" cols="72">--
Jay Lozier
<a class="moz-txt-link-abbreviated" href="mailto:jslozier@gmail.com">jslozier@gmail.com</a></pre>
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