Accuracy of rx_time in UHD API stream receive command?

[Mike]

Hello,

I apologize if this is covered in a document somewhere, but from exactly
where is the rx_time value in the UHD API stream receive command derived?

I'd like to determine how close that value is to the actual arrival time
of the signal at the antenna (in nanoseconds) and the variance I might
expect over time.  I'm using a B200 and UHD 3.15.0 under Ubuntu 20.04. 
The intent is to use rx_time in a ranging application and I'd like to
work out the timing error budget.

Thanks!