We've run into a problem in the design.
We originally thought we could measure the distance by measuring the "time of flight" between the tag and the anchor points. However, this requires the XBee radios to either give us an exact time stamp (to 300+ millionths of a second) of when messages are sent and when they arrive, or by having a very high resolution clock on both the tags and the Beaglebones (and then having the clocks calibrated when the system is deployed).
Currently, we have not figured out how to get the "time of flight" information from the XBees, and it doesn't appear to be documented.
The fallback was to add a clock to all the tags and to each anchor point, but I can't seem to find a clock that will operate reliably in the conditions of the field, and is still cheap and doesn't consume "too much" power (the tags need to operate on a watch battery for very long times - preferably years).
Any ideas?
Can get time packets arrive, but not when they depart.