The radio-frequency signal that is transmitted from the ground is received by the command receiver and is amplified and demodulated. Amplification is necessary because the signal is very weak after traveling the long distance. Next in the command system is the command decoder. This device examines the subcarrier signal and detects the command message that it is carrying. The output for the decoder is normally non-return-to-zero data. The command decoder also provides clock information to the command logic and this tells the command logic when a bit is valid on the serial data line. The command bit stream that is sent to the command processor has a unique feature for spacecraft. Among the different types of bits sent, the first are spacecraft address bits. These carry a specific identification code for a particular spacecraft and prevent the intended command from being performed by another spacecraft. This is necessary because there are many satellites using the same frequency and modulation type.