[USRP-users] USRP X300/X310 samples received have the wrong GPS date

[CHARPENTIER Thomas via USRP-users]

Hi,

After synchronization to the GPS time, the USRP time does not match the real
GPS time (there is 1 second offset between the USRP time and the real GPS
time). I realized that 1 second offset by analyzing the signal content
produced by the USRP (which depends on the GPS time in my case).

Here the system configuration:
CentOs 7.2
UHD 3.9.6
USRP X300/X310
GCC 5.4


I also tried to run the application (with the exact same source code) on a
Windows based system. And there were no mismatch between the USRP time and the
real GPS time (the signal produced by the USRP was perfectly sync with the GPS
time).

Here the Windows based configuration:
Windows 7 Enterprise
UHD 3.9.4
USRP X300/X310
MSVC 2013


To synchronize the USRP time with the GPS time I used the code snippet from the
X300/X310 documentation, see below:

===============================================================================
while(! (usrp->get_mboard_sensor("gps_locked",0).to_bool()) ) {
   boost::this_thread::sleep(boost::posix_time::seconds(2));
}
usrp->set_time_source("gpsdo");

uhd::time_spec_t last = usrp->get_time_last_pps();
uhd::time_spec_t next = usrp->get_time_last_pps();
while(next == last) {
   boost::this_thread::sleep(boost::posix_time::seconds(0.05));
   last = next;
   next = usrp->get_time_last_pps();
}
boost::this_thread::sleep(boost::posix_time::seconds(0.2));

usrp->set_time_next_pps(uhd::time_spec_t(usrp->get_mboard_sensor("gps_time").to_int()+1));
===============================================================================


I was able to fix the issue by adding 1 second to the time when calling
set_time_next_pps, as follow:

===============================================================================
usrp->set_time_next_pps(uhd::time_spec_t(usrp->get_mboard_sensor("gps_time").to_int()+2));
===============================================================================

However I would like to understand why that additional second is needed (only
on CentOs) and get rid of it in order to keep the CentOs source code and the
Windows source code the same.


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