How to set usrp transmitting time and receiving time in GNU radio

Question

I'm currently using two USRP X310, one for transmitting and another one for receiving. Programming is done using GNU Radio. I want to transmit a chirp signal to find the distance between both USRPs.

However, before I can do it, I need to find out the receiving time of the signal. Anyone know how to do it using GNU radio?

Answer 1

The GNU Radio UHD source will produce rx_time stream tags on streaming start and after every stream disruption, which will give you the device time for the first sample. After that, it's that time + n_samples_since_tag / sampling_rate to get the current time.

See

• details on stream tags
• documentation of the usrp_source

for details, which are not easy to explain here, but nicely covered in the official documentation.

Answer 2

rx_time tag may give you the time when the USRP receives noise, not the time of reception of desired signal. The only way i can think of is calculate the FFT of the signal and get the time when the PSD of the FFT goes above certain level