I think the propagation time is to small to be measured using a PC.
I guess wave propagation is 3*10+9m/s this means 150km for 50 us
The difference beetween the sending timestamp and the ack time stamp
include a lot of processing and the time to code the symbols which are
far longer than the propapagation delay

One warning. The resolution of the timestamps is going to be way too coarse to measure distance accurately. With one microsecond resolution the signal traverses 300m through the air. We're playing at this using a GNURadio/USRP combination which allows us more precise measurement but alas not got to the practicals yet. 

The research paper I'm using as a basis for my program assumes that the smallest time resolution possible is 1us.  One thing I missed before is that the timestamps have to be created by the wireless card so that interrupt latencies are avoided;  it doesn't matter when the application receives the data.  The most important part is that I have a timestamp on each sent or received 802.11 frame.