GOAT logo

Join the conversation! The forum activity is now at GOATeach.org!  We are working to cross pollinate our conversations. Document and share tools at farm hack and talk at GOAT!  Also join GOAT riot and introduce yourself and your projects!

Determining Distance

Topic Type: 

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?

jbd's picture

The original plan has us capturing the time the packet arrives (which we can do on the beaglebone because the clock is 700MHz), but we can't capture the time of departure from the tag.

Hmmm. Do we really need the departure time?