jbd

RFID requires close proximity to read the tags (the military has a system that can read tags up to 300' away - but it requires a comparatively large tag and a high powered reader - neither of which are suitable for our application.) EZ-Pass uses a high powered reader, but even its range is about 30'. Most marathon RFID systems need a proximity of 7' or less. Our application needs a "proximity" that's a MINIMUM of 300' (outdoor; line-of-site), which our prototype XBee's meet - and are a lot cheaper than RFID tags with equivalent range; better proximity (for farms) would be in terms of thousands of feet or miles, which the XBee Pros give - but they consume more power. Rangeland would require ranges in miles to tens of miles. But simply adding anchor points may accomplish the same purpose. Unfortunately, cell phones don't provide sub-meter accuracy. There are several systems in use by the cell manufacturers, [Wikipedia](http://en.wikipedia.org/wiki/Mobile_phone_tracking) has a good description of the various techniques. Of course, most farms are located in rural areas - notorious for lack of cell towers. I originally wanted to have "inch" accuracy, but I'm willing to be a little sloppy on that. I do need identification of the tag, and get as close as possible; which is why I wanted a very high resolution clock on the radios. With synchronized high resolution clocks on both sides of the signal, we can calculate a rather precise distance between the endpoints, and not have to worry about software/interfacing latencies. (Although I believe the XBee's mesh networking may complicate or render that approach impossible. Without further testing, we can't be sure.) I should point out that the XBee's provide a received signal strength metric, but its only accurate to 2 decimal places (-40db to current gain setting). But I don't believe this will provide sufficient accuracy in determining distance.

You'll need something to convert the sensor signal to serial data so it could be sent over the XBee's modem. Unless you're just sending 1 bit of info, in which case you can use the DTR pin on the XBee. This would probably work for preset alarm info.

I really like the idea of having classifieds. Although I think it would be best to exclude companies from submitting things for sell, just let it be individuals.

Finally found some specs, and it looks like the oscillator on the Chronodot is only 23.768KHz. I believe we'll need at least 300MHz for 1 meter accuracy. So, even if the SQW on the Chronodot (a.k.a. Maxim DS3232) rises at the start of a second, it won't be accurate enough for us. Sigh.