I have been playing with a SDR dongle. I have been able to decode some ACARS signals using SDR# and acarsd. I have noticed in my area (35 miles west of DFW airport) there are a number of frequencies used. I have been able to decode transmissions on 130.450, 131.125, and 131.475. Are there other common frequencies? What determines the frequency used by a particular plane? I find it odd that I can pick up an AA flight 30 miles away and yet when a similar flight passes directly overhead, I am unable to find their ACARS on any of those frequencies.
According to the specs ACARS supports direct to satellite linking so planes flying over you may not be broadcasting on one of those frequencies. Similarly, if broadcasting over VHF you are line of sight limited. At high altitudes you may see ranges up to 200 Nautical Miles however if the planes are low overhead they may not be broadcasting as they are out of sight of the possible ground station. Your dongle may also filter/ignore low power signals or it simply cant pick up certain signals due to their strength.
Here is a list of common ACARS Frequencies.