GPS interference
robert....@channel1.com
A private message of general interest, reposted with permission:
***
>
> Ira,
>
> Thanks for your reply about weekend GPS problems.
>
> Since you are well versed in matters of signal interference, perhaps you
> could mention in the sc.geo.satellite-nav conference an outline of this
> problem.
Hello Bob,
My ISP does not provide good newsgroup connectivity, especially for
posting. If you wish to repost this on the conference, please do.
> Backing up, I am new to GPS. I am not new to electronic navigation,
> since I owned a well equipped private plane. I had assumed there would
> be no electronic interference.
>
> What is the frequency of transmission of GPS, 1.5 ghz?
I forgot the exact frequencies, there are two in the 1.4-1.5 gig range.
One is the civilian freq. the other military with encryption.
> Why is there interference? From what sources?
Any terrestrial signal that is very strong can overload the receiver
front end or other electronics in a GPS sensor. The expensive
professional surveyor grade units have extensive filtering in their
antennas, LNA's etc., to mitigate those problems.
That form of interference causes the GPS to not be able to receive or
lock to any satellites.
The issue I was posting about [June 9 in this conference] is intentional
interference that disguises itself as legitimate satellite signals but
with erroneous data, jamming if you will. The DoD is working on being
able to do so in limited locations as part of their plans to turn off SA
globally. If they can, at will, degrade performance in localized areas
when required, they can live with SA turned off.
> I recall someone mentioning in the conference, multipath interference.
> I assume 1.5 ghz is very much line of sight. Would a cone shield, say
> made of aluminum foil, perhaps grounded, eliminate ground based
> interference? I am assuming a cone shielding the bottom and the low
> sides, since what we want are the signals from the sky, not the ground.
> Or would this be useless?
Any ground plane below the antenna would eliminate bounces from directly
under the antenna. But these multipath reflections are short. Their
errors would be of significance to a surveyor making centimeter
resolution measurements but of no consequence to most navigational users
who can only get 10-100 meter accuracy anyhow.
The multipath that can affect the casual user are bounces off of
mountainsides, tall buildings, etc. That could add several hundred
meters to your position error. My problem in the Natick MA area was a
sudden solution shift of approximately 200 miles! Certainly not
multipath.
Surveyor grade systems utilize a ground plane with concentric ring
ridges that surround a patch antenna element. These choke rings absorb
or cancel out RF coming in at low angles just above the horizon. In so
doing they eliminate some reflections from nearby objects, manmade or
natural.
Cockpit GPS onboard aircraft must be FAA approved avionic grade. Many
of the hand held and fixed mounted marine units emit too much RF noise
from their own power supplies, computer chips and displays! Thus they
have to be located a minimum distance away from flux gate compasses,
other radios, etc. They'll work fine in the passenger cabin of an
airliner as the distance to avionic equipment is great enough.
--Ira Wilner
* *
/|-------------------------------------------|\
/ | Ira A. Wilner | \
/ | ** WILNER ASSOCIATES ** | \
/ | Broadcast Engineering Services | \
/ | Email: bd...@vermontel.com | \
/ | URL: http://www.vermontel.com/~bdcst | \
/ |-------------------------------------------| \
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