GPS time in tlog

After comparing the time of the tlog and the time of the log of another system, I recogniced more then 3 minutes time difference.
All the two GPS receivers are on board of my Hexacopter, means, GPS receiver pixhawk and GPS receiver spectrum analyzer.
Which time is used for the time stamp of the tlock, is it the time of the pixhawks GPS receiver, or is it the time of the PC, where the datas are stored, after receiving it from the pixhawk ?
If the time of the PC is used, how can I do it, that the time of the pixhawks GPS receiver is used ?

greatings
Martin