Oceanographic Tools: Navigation by Sound
Sound is the Answer to Underwater Navigation
Deep submergence vehicles like Alvin,
ROV Jason, the DSL-120 sonar and Argo II use acoustic navigation to determine
their positions as they travel over the seafloor, using instruments called
The technique is really quite similar to satellite navigation. Instead of orbiting
satellites transmitting radio signals, sound-transmitting transponders are sent
overboard and anchored to the seafloor. The positions of the transponders are
determined by using the GPS system on board the ship and ranging to them acoustically
while the ship circles where the transponder were dropped. The positions of the
transponders on the seafloor are known with an accuracy of about 10 meters.
|A transponder just after it is lowered into the
water. The ship slowly moves away from it as the tether line is paid
out. After all 185 meters of tether is paid out, the anchor is kicked
over the side and the transponder sinks to the seafloor to start its
Transponders have accurate clocks to measure time very precisely.
Each transponder is set to listen for sound signals transmitted either from the
deep submergence vehicle or the ship at a specific frequency, in our case the
frequency is 9 kiloHertz (kHz). The clocks on the vehicle and ship are synchronized.
When each transponder hears 9 kHz sound signals it is programmed to talk or
transmit a sound back to the vehicle and the ship. Each transponder talks at
a different frequency (between 8 kHz and 15 kHz) so when we receive the signals
at the vehicle or the ship, we can tell which transponder sent it.
To calculate the vehicles position, we use simple geometry and basic math.
We know the speed of sound in water (about 1,500 meters per second) and the time
it takes for signals from the transponders to reach the vehicle or the ship.
Multiplying the travel times by the speed of sound in water gives us the distances
between each transponder and the ship or sub. Using distance measurements from
the ships and the transponders (triangulation again), computers can calculate
the unique point in three-dimensional space where all distances measured from
all the transponders and the ship intersect. That is where the vehicle is!
All of this happens very quickly thanks to computers and software written by
oceanographic engineers and technicians so that we can constantly keep track
of the deep submergence vehicles position within about 5-10 meters during