[time-nuts] GPS dropouts and crystals

Randy Warner Randy at synergy-gps.com
Thu Sep 7 14:20:14 EDT 2006


I would tend to believe the problem you are having is more receiver
related. If your entire satellite display is going red it means you are
losing all the SV's at once. While it's true that there are some pretty
big holes in the system right now, you shouldn't just lose everything at

The problem could be anything from a bad antenna to a receiver problem.
Working with GPS receivers for the last 10 years I have seen many cases
of the receiver just dumping everything. It usually comes down to an
antenna connection problem or a crystal/TCXO problem in the receiver. 

About 4 years ago we went through several thousand UT+ and GT+ receivers
that would come up, track for awhile, and then drop all channels. They
would cycle like this every couple of minutes, with the time between
drops getting longer as the receiver warmed up. The culprit was poor
quality crystals that were experiencing "micro-jumps", meaning that they
were shifting back and forth slightly in frequency.

This doesn't affect just GPS receivers. Crystals are still somewhat of a
black art. During this period of bad UT+'s I went down to an assembly
house in North Carolina that was having horrible thru-put problems
because of the jumping UT+'s. Once we figured out what was going on the
QA manager related that their Motorola cell phone line had been down for
several weeks about a year back because of the very same problem. Turned
out to be the same supplier too. Moto doesn't buy crystals from this
vendor anymore.......


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Neon John
Sent: Thursday, September 07, 2006 4:33 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Health of GPS constellation questioned

On Thu, 7 Sep 2006 13:12:07 +0200 (CEST), "Bart Smit" <bit at pipe.nl>

>On Wed, September 6, 2006 5:22, Christopher Hoover said:
>> [ the story seems overblown to me, but still worth sharing.  follow 
>> the last link for some real data. - ch ]
>The last link is interesting indeed. Seems that the problems are more 
>perceived than real, with the GPS constellation operating well within 
>spec, but users wanting more.

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

I've gotten curious enough about this to bring along another receiver
but so far nothing has happened to give me an opportunity to check with
the second receiver.

The system is Street Atlas with the Delorme Earthmate hockey-puck
receiver.  I've had this system about 4 years and it is only in the last
few months that this has started happening.  I'd attribute this to
something in my receiver except that I recall also having occasional
loss of signal on my other receiver, a Magellan Meridian Platinum, which
I just attributed to "stuff happens".

John De Armond
See my website for my current email address http://www.neon-john.com
Cleveland, Occupied TN Don't let your schooling interfere with your
education-Mark Twain

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