Question about SELCAL and how it works...
Two pairs of frequencies, so say 4 frequencies in use... how does it work? When a controller wants to reach aircraft with SELCAL AB-CD, does the transmitter transmit on all four frequencies? If so, does this act like a "key" to the aircraft's system? (In other words, the aircraft expects a signal on each pair of frequencies (2x2=4) in order to respond and alert the pilots?)