The modulation signal is applied to a radio transmitter. Each spatial stream requires a discrete antenna at both the transmitter and the receiver. Frequency determines the line of communication between a receiver and transmitter. Active sonar uses a sound transmitter and a receiver. A radar set consists of a transmitter and receiver. When the transmitter and receiver are separated it is bistatic operation. This protects the receiver from the high power of the transmitter. The changes are controlled by a code known to both transmitter and receiver. Also, alignment requirements between the transmitter and receiver are less critical. This greatly simplifies the design of both the transmitter and the receiver. Coded bursts of data sent by a shortwave radio transmitter were also used. YIG is also very efficient as an acoustic energy transmitter and transducer. These will be ellipsoids with foci at the transmitter and receiver. Their basic tools were a HT radio transmitter and a whistle. This occurred whenever an aircraft passed over the line between the transmitter and receiving meter.