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Using radio waves not accurate enough Farm Item Locator Information
RFID Farm Item Locator Idea
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The majority of the GPS errors in location of the Base Stations for the Farm Item Locator will be exactly the same for each base station. Some of the errors are accounted for by differential GPS and WAAS GPS correctors, but this just gets you down to the meter to three meter accuracy. In surveying, the errors and corrections are computed locally at a base station to the survey and then applied, either immediately (RTK) or in post processing (PPK). A professional grade base station costs upward from $50k. Not in the budget for this project. I wonder if we can't cobble together a low end version of the same concept? We might have to actually survey in the location of one station, the base-base station, and the others could figure out where they are. A good [RTK/PPK positon](http://www.fwi.co.uk/Articles/17/02/2009/114252/Precision-farming-Sharing-an-RTK-base-station.htm) (used on objects that move, like cows and tractors) would have accuracy of a few inches.
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The problem of synchronization without GPS is a difficult one. Each CPU clock will drift at a different rate which may be temperature dependent. While it is possible to build a mesh protocol to find a common time, it will require both transceiver and CPU resources which might need to be used for other things at the time. The GPS with PPS is ultra accurate [PPS signals have an accuracy ranging from a 12 picoseconds to a few microseconds per second, or 2.0 nanoseconds to a few milliseconds per day](http://en.wikipedia.org/wiki/Pulse_per_second) and sets a common standard time for anything that uses it. There may still be problems finding deterministic latencies in the radio and CPU.
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Okay. RFID is out for this project. How about an active transponder, maybe done with the Xbee? With this the positioning is done by the base stations. All that would be necessary on the tags would be to transmit replies to a queries from the base stations. I assume (big word here) that the turn-around time from request received to reply transmitted could be held constant. From my sketchy reading and conversations with cohorts here at CCOM (Center for Coastal and Ocean Mapping), the clock error problem is eliminated in this way. The positioning is determined by the sum of the two way transmission and the turn around time.