[Discuss-gnuradio] USRP for two way ranging

Hello.

I need to implent a two way ranging. A device send an "echo", a second device discover the "echo" and send a reply, then the first device can calculate the distance. For a correct distance evalutation, the second device must calculate the time elapsed between the discover of the first echo and the reply response.

Let's assume that the second device is a SDR with gnu radio & USRP1 device.

The problem is: how to time-correlate the streaming sample received from the USRP and the streaming sample transmitted to the usrp.

If i pre-fill the usrp transmit buffer with dummy data (preventing underflows) before starting acquisition&transmission, then i can correlate the two streams, right? (Yes, with the hypothesis that i don't miss data from usb).

Thank you