Separation of overlapping signal in 1999 tomography experiments.

A tomography array which consists of five 200 Hz transceivers were deployed in the Central Equatorial Pacific in December 1998 and measurements for two years were conducted. In our previous experiments, each transceiver transmitted sequentially every 30 minutes, and the total turn-around time was 2.5 hours. In this experiment, we intend to shorten a measurement time by simultaneous transmissions of five transceivers, which improves an accuracy of measurement for water current velocity due to reduce the effects of internal waves. In this case, however, each transceiver receives an overlapping signal of other transceivers. Multiple M-sequence codes are needed to separate mutual signals among transceivers. Simulation for separation of overlapping signals was conducted according to the arrangement of five transceivers and then five sets of M-sequences were selected carefully and written in the memory of the five transceivers. After recovery of all the equipment in August 1999, the observed data were read out from the transceivers and processed to separate overlapping signals successfully.