Multi-rotors use multiple electric motors to drive propellers that allow the multi-rotor to fly, turn, bank, etc. The multiple motors and the associated control device produce EM signals that can be used to detect multi-rotors and distinguish them from other devices with electric motors. To drive a brushless motor, the ESC takes DC from the battery and turns it into three phase AC (sinusoidal or trapezoidal wave), and then measures back EMF pulses (sensorless). This allows one to ensure that the three phase AC is being generated at the proper frequency to turn the motor (timing). For each propeller in the multi-rotor there are usually four different correlated signals at multiple frequencies that are used for detection and false alarm rejection.