Cause of time drift in received signal using USRP2955

[BabarAly1]

• I am simulating a burst signal with 96% on time and 4% off time and transmitting it using USRPx310 at center frequency of 360MHz. Period of the signal is 5ms and BW of the signal itself is 25KHz while IQ rate of transmitter is 5MS/s. 
• Then, I'm recording the signal using USRP2955 at IQ Rate of 5MS/s. After performing analysis on recorded signal frame to frame basis, a drift in time is observed?
• What are the possible causes of this time drift in recorded signal. However, there is no such drift is present in the simulated signal??

P.S. First snap is at starting frame and second snap is at 1000th frame. 

[BabarAly1 · Accepted solution]

The issue has amicably resolved. There were two solutions: 

(1) Synchronize TX and RX using REF Out port from TX to REF IN port.

(2) Synchronize both TX and RX to GPS signal.

Thanks @nestorg780 for kind suggestions.