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SciAm has an article (A Positioning System That Goes Where GPS Can't) on the topic, and Sonitor Technologies (based out of Norway) appears to be a forerunner in the field.

Notes on ultrasound, as well as a survey of all other positioning technologies, is in Global Positioning: Technologies and Performance (covers indoor, despite the title).

Advantages:

  • Significantly higher resolution than radio-based location technologies (room-level accuracy or better, according to Sonitor Technologies)
  • Avoids problems associated with radio interference (metal objects bouncing the signal, other radio signals causing interference, etc.)
  • Device emitting the ultrasound is very small (~1"x0.5") and light (~4g)
  • Ultrasound devices themselves are relatively cheap

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