[time-nuts] time-nuts Digest, Vol 26, Issue 5
Tom Clark, K3IO
K3IO at verizon.net
Mon Sep 11 20:09:11 EDT 2006
The question was asked::
A more basic question. Do these degradations affect ordinary users?
The reason I ask is that I have a GPS navigation system on in my truck
24/7 and use it every waking hour (I'm a retired engineer who's living a
childhood fantasy - driving a long-haul 18-wheeler :-) Fairly
frequently I experience fairly long intervals where the system goes
completely dead. The satellite screen is all red - the system knows
where the satellites are but no signal is received. This can last for
miles. I've relocated the receiver, even stopping and removing the
receiver from the truck just in case EMI was the culprit. Then just as
quickly, the receiver gets lock and everything is fine.
The distance traveled - sometimes 10 miles or more - during an outage is
such that ground interference can fairly confidently be eliminated.
Almost all my running is on open interstates so ground factors can be
This is almost certainly a case of RFI from some nearby high-powered
As an example, if you are in northwest Baltimore (around Owings Mill)
you will find some outages due to Maryland's PBS transmitter WMPT on
Channel 67. Channel 67 has a video carrier @ 789.25 MHz. The second
harmonic of these signals is 789.25*2 = 1578.5 MHz. The GPS L1
frequency which is used by all GPS receivers spans L1 = 1575.42 ±
1.023 MHz (to the first null of the CDMA spectrum. Receivers are
typically ~1.8 MHz wide). It only takes a corroded telephone pole
crossbar or a rusty barbed wire fence to act as a non-linear diode to
make RFI that can wipe out GPS.
The "bad" TV channels are 66 & 67. You can see a listing of channel 66
& 67 stations at http://www.w9wi.com/tvdb/channels/66.htm &
Other bad combinations can result when a VHF & UHF TV station mix to
make a signal near the L1 frequency. If you want to play with the
numbers, a listing TV frequencies and be found at
http://www.arrl.org/tis/info/catv-ch.html where the "AIR" channels
at the top of the list are the ones of concern.
Also note that the Channel 60-70 TV stations will be the "first to go"
as broadcasters meet the mandatory 2009 HDTV rules. Channel 60-70
spectrum is supposed to be auctioned off to be used by police, fire
etc emergency services.
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