“Motivated by success on other acoustic projects and by this brain transplant blog, a sonar project sprang to mind. A phased array sonar works in the same way as a phased array radar. Rather rotating the sensor to look direction, this is achieved by combining the signals from an array of fixed sensors each with different phases. Normally a large number (100s) of sensors are used. In astrophysics they are randomly sparsely populated. In this project only two ultrasonic receivers are used. Only one transmitter is used. The existing HC-SR04 sends out 8 40kHz pulses. In this hack, I’m replacing it with binary phased shifted key (BPSK) m-sequence with each key consisting of 5 pulses. This has better signal properties.”