Signal processing of acoustic ranging in GPS/Acoustic seafloor geodesy

Acoustic signal processing is crucial for precise positioning of a seafloor transponder in GPS/Acoustic seafloor geodesy, which aims at 1 cm-precision. For this purpose, pseudorandom signals are usually employed for acoustic ranging. Considering the characteristics of the signal and the typical frequency of 10 kHz in carrier wave, detecting an arrival-time with 0.01 ms (~0.75 cm) resolution in round trip can be readily achieved if the correct peak in correlogram is identified. However, due to sidelobes around the peak and pseudo peaks associated with possible multipath, a simple algorithm detecting the maximum peak often results in a wrong peak, which has a offset with a wavelength or larger from the correct peak. In our numerical study, behavior of the pseudo peak is found being quite complex having a no-integer wavelength offset when a short-range multipath is superimposed. Our observed data clearly show nadir-angle dependence in the appearance of received signals. This is attributed to significant change in the short-range multipath with angle within a sound-hood of a transducer as well as transducer characteristics itself. In this study, firstly we demonstrate the complexity in the behavior of the sidelobes with varying offset of multipath. Then, we analyze observed signal to reveal the angle-dependency of our transducer with different types of sound-hoods.