A transponder is an instrument anchored to the seafloor on a tether that receives an acoustic signal at a certain frequency (frequency is number of wavelengths that pass a fixed point per second) and transmits a response at a different frequency. In the illustration, transponders are anchored along the top of the ridge and spaced at even intervals.
Each transponder is dropped over the side of the ship. The transponder is attached to a
weight (the anchor) by a wire. After the weight reaches the sea floor, the
transponder floats above it. Sixteen transponders will be deployed at the top of the Puna
Ridge; they are spaced 4 km apart.
To accurately locate each transponder, the ship sails around it transmitting to the transponder and receiving a response from it as illustrated. Distance to the transponder is calculated by multiplying the speed of sound in water by half the time that it takes for the signal to travel to the transponder and back (to account for 2-way travel time). Distances to the transponder from several ship locations are obtained and combined to locate the seafloor transponder.
The next step is to locate the DSL-120 vehicle within the network of transponders. In some places the vehicle may be several kilometers behind the ship and several kilometers below the sea surface. A transponder is attached to the DSL-120 vehicle. The ship transmits to the DSL-120 transponder which in turn sends an acoustic signal back to the ship and to nearby seafloor transponders. When they receive the transmission, the seafloor transponders send an acoustic signal to the ship. Although it is somewhat complicated, all of the travel times can be combined to determine the distances between the DSL-120 vehicle, the ship, and the seafloor transponders!
How far from the ship is a seafloor transponder if an acoustic signal takes 4 seconds to travel from the ship to the seafloor transponder and back to the ship? Use 1500 m/s for the speed of sound in the ocean.
Check your answer below to see if you are right.