Louis

But turn-around does eliminate the need for synchronization at least... Synchronization won't be too bad since distance will be easier to calculate (even without GPS) since both sides have good & identical clocks. Actually, it would be neat to do this without GPS which makes this system entirely relative until you take some global references from GPS!

Hey guys, Sorry I'm probably late to the game but I have been tied up moving into a new apartment and office last week. This discussion has made great reading material though! Just to make sure I've understood, it's settled that a good clock is only needed on the anchor station? Seems like this turn-around technique is a nice solution and will yield an absolute answer for each individual anchor (ie: it will tell you node 32 is 50m away). I just wanted to offer another idea which I thought came up in one of the discussion threads but I can't seem to find and that's to use ratios: If you have 3 anchors and they all receive the SAME signal from a node, they will have three different times when they clocked in the message (let's say, 00:01, 00:02, 00:03). The differential in time received will help determine the relative difference of the node (that is, if the node is distance X away from node 3, then it is 2x/3 from node 2, and x/3 from node 1 - a unique point satisfies that relation). I like the turn-around method better since it immediately yields absolute distance and doesn't require synchronization but if that becomes a problem for technical reasons, this could be another approach.

Look forward to those. I might have interest in that rain barrel supplied automatic irrigation system for my home!

The datalogging shield from Adafruit includes an integrated circuit that provides a Real-Time Clock (RTC). It gets the current time when you upload code. It will use the coin battery to maintain accurate time when the shield is not getting powered. If you notice that the time isn't being kept properly, it may be the coin battery that doesn't have a decent contact. I had that problem because I didn't put a significant enough blob on the circuit board. Let me know if you are having any issues and I can help you troubleshoot!