Tech Briefs Magazine - April 2021 - 35

In principle, the same technology could
provide location information. An observation unit could emit a soundwave, then
clock how long it takes that soundwave to
reflect off the piezoelectric sensor and
return to the observation unit. The elapsed
time could be used to calculate the distance between the observer and the piezoelectric sensor. But in practice, timing such
backscatter is complicated because the
ocean can be an echo chamber.
The sound waves don't just travel directly between the observation unit and sensor. They also careen between the surface
and seabed, returning to the unit at different times. Accounting for reflections is an
even greater challenge in shallow water.
The short distance between seabed and
surface means the confounding rebound
signals are stronger.
The researchers overcame the reflection
issue with frequency hopping. Rather than
sending acoustic signals at a single frequency, the observation unit sends a sequence
of signals across a range of frequencies.
Each frequency has a different wavelength,
so the reflected sound waves return to the
observation unit at different phases. By
combining information about timing and
phase, the observer can pinpoint the distance to the tracking device.
Where echoes run rampant between
the surface and seabed, the team had to
slow the flow of information. They
reduced the bit rate, essentially waiting
longer between each signal sent out by
the observation unit. That allowed the
echoes of each bit to die down before
potentially interfering with the next bit.
Whereas a bit rate of 2,000 bits/second
sufficed in simulations of deep water, the
researchers had to dial it down to 100

bits/second in shallow water to obtain a
clear signal reflection from the tracker.
But a slow bit rate didn't solve everything.
To track moving objects, the researchers
actually had to boost the bitrate. One thousand bits/second was too slow to pinpoint
a simulated object moving through deep
water at 30 centimeters/second. At 10,000
bits/second, they were able to track the
object through deep water.
The proof-of-concept was tested in a
shallow-water environment. UBL esti-

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mated the distance between a transmitter and backscatter node at various distances up to nearly half a meter.
UBL could one day help autonomous
vehicles stay found underwater, without
spending battery power. The technology could also help subsea robots work
more precisely and provide information about climate change impacts in
the ocean.
For more information, contact Abby Abazorius at abbya@mit.edu; 617-253-2709.

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The battery-free Underwater Backscatter Localization (UBL) system. This photo shows the
battery-free sensor encapsulated in a polymer
before it is dipped into a river. (Image: Reza
Ghaffarivardavagh)

Tech Briefs, April 2021

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Tech Briefs Magazine - April 2021

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