[USRP-users] Sychnronize two USRP X310 with GPS

Simon Hartmann uqdqj at student.kit.edu
Fri Dec 19 08:10:19 EST 2014


Hi all,

the last weeks we tried to synchronize our two usrps X310 between two 
stations with two seperate GPS antenna, but the results weren't as good 
as we hoped. 
For your information, every usrp is connected to his own pc on which 
the same code is running.

Our first intent was to check if the gps antenna is locked 
(get_mboard_sensor('gps_locked')) and then start to receive samples 
with the same time given 
(set_start_time(uhd.time_spec_t(self.start_time))). We only want to 
receive samples, therefore we used set_start_time() instead of 
set_command_time().

When we checked the sync, there was a difference about 20 - 100 us. 
According to the documentation only ~100 ns should be allowed.
We also read out the rx-time stream tags for both measurements and 
compared them. However, they were identically.

In the documentation of Ettus it's explained, if the GPS is locked to 
the satellites the internal VITA timestamp will be initialized to the 
GPS time, and the internal oscillator will be phase-locked to the 10MHz 
GPSDO reference. 

So if their both locked they should have the same timestamp, or? Or do 
we need some further code to initialize the gps time to the usrp?
What else could be the problem?

Thanks,

Simon
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