Time between request and receive varies

Question

I got some Zigbees S2C and trying to do some distance measurements with it.
I have tried the RSSI option, and got some decent results (+- 2-5m) but not where i am aiming for (+- 0.5-1m).

Second option is Time of Flight. But i got variations within the request and receive time.
Examples of measured times:
11.15mS
10.91mS
10.79mS
10.29mS
10.50mS
10.79mS
10.15mS

So is varies with like 1mS. Any idea where this difference come from and if this can be reduced?

Answer 1

Most part of time taken between serial input on Tx module and serial output on Rx module is consumed in processing of data by modules.

Therefore, this makes distance calculation based on transmission time, not a practical application with these units.

Mathur
Digi Support

Answer 2

I understand that part.
But why does that time vary?

I dont mind that there is a delaying time. I can put that delaying time in my calculations. But why does it vary by 1mS?

Regards,
Nick