I've finally managed to rework the Atmel RTB Evaluation Application (http://www.atmel.com/tools/REB233SMAD-EK.aspx?tab=documents) to run on the ATxmega256A3U and AT86RF233 Zigbit Module (~$30), rather than having to purchase the expensive REB233SMAD-EK.
With this rework I can now start developing an extensible addon for SubPos to get additional, more accurate (+-10cm) ranging for better distance calculation.
This addon will not work with existing infrastructure or smartphones and will require a specialised receiver. However, I also have SubPos and trilateration code working on a Teensy 3.2 that this ranging module will feed into to allow it to be attached to autonomous devices for navigation indoors.
Actual output from two Zigbit modules with a single antenna:
I'll have a demo of the ranging operating shortly.