1 m² opening
(Top) Time-of-fight measurements between two nodes.
(Bottom) Water temperature probe measurements.
time-stamp acoustic signals precisely.
These features are required to properly
time-stamp collected environmental
data and perform acoustic tomography.
Acoustic tomography is possible
in water because the speed of sound
for an acoustic wave in water is approximately proportional to the water
temperature. Thus, the precision with
which we can measure the speed at
which an acoustic signal travels directly relates to the precision to which we
can measure the water temperature.
The question then is: how precise do
we need to be?
Using the MacKenzie equation to
approximate the speed of sound in
water, we can work out the precision
attainable by our acoustic tomography
system. At a nominal speed of sound
of 1,500 meters per second, the travel time between two nodes spaced
50 meters apart is 33 milliseconds. If
the water temperature changes by 1o
C, the travel time will change by ap-
proximately 100 microseconds. A temperature precision of 0.1o C will thus
require time-of-fight measurements
precise to 10 microseconds.
Obtaining such precision is nontrivial, and a naive implementation
would use the system clock and timing facilities provided by the operating
system running on an underwater sensor node. Such timing techniques are
known to suffer large errors (up to milliseconds) due to the nondeterministic
nature of interrupts and process scheduling on commodity hardware.
To overcome this challenge, we
developed a novel self-listening technique that utilizes the simultaneous
send and receive capabilities provided
by having both transmit and receive
hydrophones available. We sample the
GPS pulse-per-second signal simultaneously with the signal from the receive hydrophone. Doing so allows us
to assign a globally accurate (to 1 microsecond) time stamp to each audio
sample recorded by the sound card.
Since we loop back the local channel,
See us at
ooth C 05
July 2013 / st