Dwm1000 receive time variation


I have made a system that timestamps the time of arrival on anchors from the signal of a tag.
when i put a tag in the middle of the four anchors i would expect that all anchors would receive the signal at the same time. Although i notice that there is up to 3 ns of diffrence between the arrival time at the anchors.

Please note that i have accounted for the lack of synchronisation.

any clue where this could come from?
I am using the dwm1000 module.

Kind regards,

Are they all own developed boards with a DW1000 or DWM1000 and how far are the nodes apart?
Is the 3ns (=/- 1m) difference with all anchors, or just the ones not facing the front of the antenna in the middle?
If with all anchors , then maybe calibration the nodes should help. If the TAG’s antenna is not facing the anchors then there could be reflection causing the inaccuracy.
See the section 8 on calibration in our IC user manual.
Recommended reading: APS011 (sources of error) and APS006 (part 1) on time stamp inaccuracy .