I've been reading the Zigbee spec sheets and it looks like the mesh networking will be introducing too much latency for accurate speed-of-light measurements between the anchor points and the ear tags. For now, we'll "turn it off" by simply not having any router nodes in the network. For security reasons, we need to be able to turn off mesh networking entirely.
Unfortunately, this means we'll be limited to 400' line-of-site with the current XBee's. So we'll probably want to upgrade to XBee Pro's to get the 1.6 km; this will also have power consumption consequences. But the software should still work regardless if we use the Pro or non-Pro, Series II devices. In other words, we should be able to continue development (especially software) with the existing non-Pro devices we currently have.
I believe we need to be prepared to put anchor points at locations that are less than 400' away from other anchor points. Depending on the size of the field, this could end up being a lot more than 3 anchor points! (Of course, if we're able to use the Pro versions and still stay within our power consumption requirements, then that will substantially reduce the number of anchor points required.)