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

Randy Warner randy at synergy-gps.com
Thu Jul 7 12:58:49 EDT 2005


Basically, all of the responses I have seen are correct. 1PPS availability on the M12+T is controlled by the @@Gc message. This was handled by the @@En command in the older 8 channel receivers.

Setting up the 1PPS and TRAIM is a little more involved in the M12+T than it was in the VP and UT+ as you have to work with several messages in the M12+T whereas @@En controlled everything in the 8 channel units. With the M12+ you have to set the @@Gc, @@Ge, and @@Gf messages to the options you want. It does take more messages, but it gives you three short messages with fewer variables than the old @@En.

Having the 1PPS on all the time has been the default condition for all Motorola receivers. To prevent unwanted pulses from being generated, choose option 0x02 or 0x03 in the @@Gc message when you start up.

Others who want to have the 1PPS active all the time (maybe they are using the 1PPS to trigger another event) leave the 1PPS on all the time but watch the TRAIM message for an error indication, and/or the @@Ha message to make sure at least one satellite is being tracked. This way they can continue to log data, but they also record the fact that the 1PPS was running asynchronously.

Best Regards,

Randy Warner
Senior Applications Engineer
Synergy Systems, LLC

 -----Original Message-----
From: 	time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]  On Behalf Of David Kirkby
Sent:	Thursday, July 07, 2005 2:37 AM
To:	Discussion of precise time and frequency measurement
Subject:	[time-nuts] 1pps signal presence with no GPS signal.

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 

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.

David Kirkby,

Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/

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