Comparison of the Ranging Function of Three Types of Underwater Acoustic Modems

Finding the position of a device within an underwater network, known as localization, has been critical for the advancement of autonomous underwater vehicle (AUV) research. Many of the current methods of underwater localization rely on a modem's ability to accurately estimate the time of flight for a given message transmission. This ability is also utilized by a modem's built-in ranging function. To advance the research in underwater localization, a comparison study was done on the ranging function of three commercially available underwater acoustic modems, all of which utilize different modulation schemes. A pair of AquaSeNT (OFDM), Benthos (MFSK), and LinkQuest (DSSS) acoustic modems were tested at several positions in both a pool and lake environment. It was found that the AquaSeNT was the most robust, as in it always calculated a distance, at the expense of precision. The precision of the AquaSeNT was effected by a consistent 8-m bias. The LinkQuest was found to be the most accurate and precise, but had issues communicating in short channels (large delay spread), and would occasionally produce erroneous measurements. The Benthos had a balance between communication success as well as precision and accuracy.