[time-nuts] 1pps signal presence with no GPS signal.

Brooke Clarke brooke at pacific.net
Thu Jul 7 12:26:34 EDT 2005

Hi David:

The older Motorola GPS receivers did not implement the Time RAIM 
function and so had the 1 PPS output on all the time so I expect that's 
the default for the M12T+.

I tried using Time RAIM to turn off the 1 PPS output and it does work, 
but it will not work with just one satellite, there is a trade off 
between how big a Time RAIM error you allow and the number of satellites 
being tracked.

My thought was that if the GPS 1 PPS is used as the start signal to a 
time interval counter then when it gets turned off the counter just will 
not start.  If the local oscillator is the counter start then when the 1 
PPS is off the counter will count for more than 1 second and the final 
output when the 1 PPS starts again will be in error.

When I first tried it the 1 PPS stopped completely since my elevation 
mask angle is set to 50 degrees to get rid of multipath problems.  So it 
does not work for me, but would for someone who can use a lower 
elevation mask angle.


Brooke Clarke, N6GCE
w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml

David Kirkby wrote:
> I have a Synergy GPS unit, which is fitted with a Motorola M12+ 
> receiver. Even if there is no signal (even no antenna connected) the 
> 1pps light flashes on the front and there is a 1pps signal at the output 
> connector.
> It would seem to me that if you lock a crystal/rubidium or whatever to a 
> GPS system, the last thing you want to do is to steer your source from a 
> 1pps signal that must be pretty inaccurate. I assume this gets derived 
> from the uncompensated crystal oscillator on the Motorola board, and so 
> is far less accurate than the source you are phase locking to it.
> I would have thought it more sensible to cease the 1pps signal as soon 
> as a GPS is not locked to at least one satellite.
> Comments?

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