Three signals from three different sources (satellite, satellite, and terrestrial) are therefore combined in the receiver as radio signals, (not as audio signals). The three signals need to be combined constructively (avoiding situations where bad signals pollute good signals) in the receiver before being decoded. Heavy error correction is applied to the signals. All three signals contain the same audio content on all the channels that the receiver can receive, with the exception of one audio program waveform being transmitted ahead of the other two by approximately four seconds. With this time skew, the signals, once realigned, need to see an 8-second obstruction of overpass fade in order to lose audio content. This increases the robustness of the signal delivery in most driving conditions.