Just to be extra clear, this is the topology of our little navigation system:

The tag measures distances to anchors in sight and reports to an external CPU/MCU, who in turn does localisation. The ATmega328P of the tag is not powerful enough to do that job by itself while simultaneously measuring distances to anchors. Data can travel either via UART, I2C or radio between the tag and the external CPU/MCU. At least one tag and and one anchor is needed. A standard setup got at least one tag and three or four anchors.

In our specific case (http://www.loligo.se/roaming_beds/) we use two tags per moving object in order to keep track of what is front and back. Twelve anchors are located in the ceiling to give redundancy and good coverage.