literally I am talking about a transmitter within a cuboid shaped
enclosure around 10m maximum and being able to pinpoint that transmitter
within that enclosure accurately, to around 1cm, perhaps 2cm.

This is a typical application of an acoustic positioning system. With a
speed of sound of about 340 m/s, this is feasible. I have made such a system
with acoustic tags on 40 kHz using 8 receiver nodes in the room. That's more
than required, but it gives enough redundancy to be robust against shadowing
as things move around in the room. Accuracy is in the 1-2 cm range.

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Sverre Holm, LA3ZA
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www.qsl.net/la3za