Properly timestamping GNSS data

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Finally, I used the time_ref topic published by 'nmea_navsat_driver' package. It has the same timestamp as the fix messages (fix and time information are contained in the same NMEA line) but the time message contained within is slightly earlier and starts on the second and at intervals corresponding to the frequency at which the GPS is outputting data.

I wrote a node which matches fix messages with time_ref messages and outputs the fix messages over a new topic with the time given by the GPS module as the timestamp. This node also saves the GPS fixes.

This is the gist of it:

def matchGPStime(timeref, fixwithcov):
   assert timeref.header.stamp == fixwithcov.header.stamp
   #rospy.loginfo(rospy.get_caller_id())
   Fixsynch=NavSatFix()
   Fixsynch.latitude=fixwithcov.latitude
   Fixsynch.longitude=fixwithcov.longitude
   Fixsynch.altitude=fixwithcov.altitude    
   Fixsynch.position_covariance=fixwithcov.position_covariance
   Fixsynch.header.stamp=timeref.time_ref
   Fixsynch.header.frame_id=fixwithcov.header.frame_id
   publisher3.publish(Fixsynch)


time_sub = Subscriber("/time_reference", sensor_msgs.msg.TimeReference)
fix_sub = Subscriber("/fix", sensor_msgs.msg.NavSatFix)
publisher3 = rospy.Publisher("/fixsynch", NavSatFix,queue_size=10)

rospy.init_node('GPS_synchro', anonymous=False)
ats = ApproximateTimeSynchronizer([time_sub, fix_sub], queue_size=5, slop=0.01)
ats.registerCallback(matchGPStime)
rospy.spin()

So is your question answered at this point?

Yes, your first message answered my question. Thank you. I couldn't find an easy way to modify nmea_navsat_driver so I dealt with it as above. Moreover, it's nice to be able to tell the lag between my system clock and GPS time.