Precise location sensor

Does anyone know of a technology, module, chip that can do precise location determination...
I need something that can get 4cm (centimeters) or better repeatable (not absolute) location accuracy, in an area approx 100m x 100m (or so)... 1 update per second is fine.
I am pretty sure that this is beyond the capability of GPS.
I can envision a "local LORAN-like" solution consisting of: 3 fixed stations, 1 which is a master transmitter, and the other two of which transmit a different frequency, but with a phase-locking arrangement. Then, the sensor would need to sense these 3 frequencies, do a phase difference determination, and then use this to augment a GPS coordinate with a hyperbolic line of transmission.
Just as a reasonability check, 10degrees of phase at 150MHZ works out to something like 5cm or so. Higher frequencies being even more precise.
Thanks for any ideas, before I go about building something like this myself ! Tman
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I meant "line of location"...
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You need DME, distance measuring equipment. It has replaced almost all surveyor chaining. I do not know who makes these now. HP maid one using IR LED's which got rid of some problems with laser coherence. Microwave versions under the name telurometer were manufactured. A bicycle retroreflector with work well with optical DME. I believe Zeiss mad DME that fitted onto a theodolite for use at Olympic Games. That can be used to get position rather than just distance.
Let us know what is currently available.
--

Sam

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Doesn't that Roomba self-driving vacuum do that?
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