Timer sync over the air

I spent 5 days trying to figure this one out, but nothing so far. I am using two EVK1000.

One of my objectives is to sync the DW1000 timers (dwt_readsystimestamphi32() or SYS_TIME) over the air to a maximum error of 10 microseconds. Assuming board A is the timer reference, I thought I could sync the B timer like this:

T_a = T_b + Sync

So the problem remaining is to estimate Sync.

In board A, I am using a delayed transmission with the pre-calcualted Tx_time embedded in a message. In board B, I am reading the Rx_time (dwt_readrxtimestamphi32() or RX_TIME) and so I can estimate Sync in board B with

Sync = Tx_time – Rx_time

Now I thought the estimate could easily be done within a maximum error of 10 microsecond since the antenna delays and the transmission will not ever be larger than 1 microsecond, but the errors I am getting from a handful of experiments are withing (plus/minus) 130 milliseconds.

The way I am checking the timing is using GPIO pin of both boards connected to the oscilloscope. On board A, I trigger the pin by polling T_a in an endless while loop and discount the difference between poll and timer threshold. On board B, I do the same by polling T_b + Sync.

I read and reread the manuals, checked my code several times. Pulling my hair out at the moment. Any help ot tips would be much appreciated.