Using multiple Active RFID transponders to pinpoint the location of the reader

I've had this idea for a GPS enabled lawnmower that would use GPS coordinates to tell the mower if it was on the right track or not. The problem of course is the inaccuracy of GPS. So a fellow student suggested creating towers to track the location.

So the idea is to create a RFID transponder that contains a solar panel and battery for power and affix these transponders around the yard and on trees and the like. Then the more transponders that are in range, the more precise the location.

My problem though is that I'm not sure what information the transponders send. I'm sure its just a string of digits or something. I kinda wish they sent a signal that was synced with the receiver to tell how long (in time) the signal was sent. The longer the time the further away the receiver is from the tower.

Oh, and when I say tower I mean anything from 4 or 6 feet. Anything taller would cost more than I'd like (not saying I'm closed to the idea).

What sort of area and resolution are you hoping for ?
This is critical.
The solution you're looking for is 'trilateration', not easy to implement - and lots of maths!

With multiple known base positions over a 100m squared field, we could locate 'active' RFID tags down to about 3-4 meters within that field (open spaces, and solid strutures with internal walls).

M_Eads:
I've had this idea for a GPS enabled lawnmower that would use GPS coordinates to tell the mower if it was on the right track or not. The problem of course is the inaccuracy of GPS. So a fellow student suggested creating towers to track the location.

So the idea is to create a RFID transponder that contains a solar panel and battery for power and affix these transponders around the yard and on trees and the like. Then the more transponders that are in range, the more precise the location.

Zillions of people would by now have had the idea that it would be a great to have a lawnmower
automatically do its job. Its mentioned on these forums on a regular basis.

So what systems have you been able to find that have actually been implemented, and how do they work ?

Mentioned often here, meant for indoor but almost certainly applicable outdoor as well: Pozyx. No, it's not cheap.

lastchancename:
What sort of area and resolution are you hoping for ?
This is critical.
The solution you're looking for is 'trilateration', not easy to implement - and lots of maths!

With multiple known base positions over a 100m squared field, we could locate 'active' RFID tags down to about 3-4 meters within that field (open spaces, and solid strutures with internal walls).

The area is around 3 acres and as for resolution (I'm assuming you mean accuracy), it would be ideal if down to the nearest inch, but I could live with 24 -48 inches.

Pozyx claims cm accuracy (I don't know about their resolution - that are two different things). So that'd work for you.

And there is of course differential GPS, the sort of GPS that farmers use to guide tractors around.

wvmarle:
Pozyx claims cm accuracy (I don't know about their resolution - that are two different things). So that'd work for you.

That is genius! The price is a killer, but maybe I can get some donations from somewhere.