[USRP-users] GPSDO Time Synchronization

Josh Blum josh at ettus.com
Wed Feb 20 13:52:34 EST 2013

On 02/19/2013 01:17 PM, Scott Johnston wrote:
> Hello,
> I am having trouble with GPSDO Time Synchronization across multiple
> USRPs that are connected to separate computers and each one has its own
> I am observing that about half the time, two or more USRPs will disagree
> about what time it is. I'm testing this by transmitting from one USRP
> and receiving that signal split into two USRPs. I then compare the
> difference in time in the stream tags of the two receiving USRPs to the
> difference in time when they start receiving the common transmitted signal.
> Sometimes its the same and sometimes it is exactly 1 second off.
> The GRC file I used on the receive side is attached. The transmit side
> is irrelevant, as long as the signal is split into receivers.
> I have ensured that the GPSDO units are locked to GPS.
> Can someone please explain how to fix this 1 second ambiguity?

It might be the same race condition as the query gpsdo sensors app. The
USRP is told to sync at the next PPS edge, but things may be done before
this event occurs.

Can you try adding a time.sleep(1) after the usrp source object is
created in the flow graph? Let me know if that works and we can find a
more polished way to handle this.


> Thanks
> Scott Johnston
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