Ultrasound RTLS

Why ultrasound

The most accurate positioning systems are using trilateration:

  • GPS
  • UWB
  • Marvelmind Indoor “GPS”

Trilateration (or multilateration) requires precise knowledge of distances to multiple reference points: stationary beacons in ultrasound, anchors in UWB, satellites in GPS.

Measuring distance relies on two things:

  1. Exact knowledge of propagation speed (speed of light/radio, speed of ultrasound)
  2. Precise measurement of time
Therefore, with other things (measurement of time, for example) equal, the slower the propagation speed the more precise distance measurement can be.

Ultrasound propagates ~1,000,000 times slower than the electromagnetic waves. Thus, it is, theoretically, 1,000,000 easier to achieve the higher accuracy with ultrasound based RTLS than with UWB based RTLS. Or you can put it differently: ultrasound RTLS can be 1,000,000 more accurate than UWB systems (in theory).

Of course, there are many other factors affecting the accuracy, for example, uncertainty of ionosphere propagation delay for the GPS signal or uncertainty due to size of ultrasound transducers where exactly the ultrasound was emitted or where exactly was the UWB signal transmitted, because UWB antennas are not infinitely small either. Those factors prevail in the basic 1,000,000 ultrasound vs. UWB comparison. As results ultrasound RTLS is not 1,000,000 more accurate than UWB RTLS. However, the fundamentals are there and ultrasound systems are easily 10 times more accurate than UWB in typical industrial applications. See examples:

Other examples of how accurate ultrasound RTLS actually is:

Why trilateration but not triangulation

Triangulation based systems (angular laser systems, optical systems) become less accurate with the distance.

Trilateration-based system are not becoming less accurate. Yes, the results can be noisier, confidence of measurements lower, etc., but they provide the same theoretical accuracy, because the uncertainty of measurement doesn’t directly depend on the distance. Accuracy is a very complex term and we won’t go into definitions here, but, as a practical example, the spot of location for 3m distance and for 20m is effectively the same for the trilateration system, whereas for triangulation system it will clearly decreases with 20/3 ratio.

Thus, trilateration systems are inherently more accurate than triangulation based systems.

Of course, there are practical, engineering, and commercial factors. And very often they prevail. Thus, theoretically more accurate systems may lose to practically more implementable systems, etc. But it is not the case for this example. Trilateration based systems are indeed more accurate for industrial systems than triangulation systems for the majority of applications because of theoretical and practical aspects.

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