 SPHERES satellites determine their position and attitude by using 23 on-board ultrasonic receivers (Murata MA40S4R) and 5 external ultrasonic reference beacons.The ultrasonic time-of-flight measurements from the external beacons to the on-board receivers are used to calculate the satellite's position with respect to the external reference frame.For rapid determination of position, the ultrasonic time-of-flight information is supplemented with the data from on-board accelerometers (3x Honeywell QA-750 single-axis accelerometers) and gyroscopes (3x Systron Donner QRS14 single-axis rate gyroscopes).