Wikipedia [1] says that there are two challenges: - ADS-B can be broadcast on two frequencies (978 MHz and 1090 MHz). FAA requires aircraft operating above 180

Wikipedia [1] says that there are two challenges: - ADS-B can be broadcast on two frequencies (978 MHz and 1090 MHz). FAA requires aircraft operating above 18000 feet to use 1090, while either may be used by craft flying lower. (It seems like the equipment is less expensive). This means 1090 is used more heavily by large commercial craft and so 1090 is the only band the satellites currently listen to. - Smaller private craft sometimes use belly-mounted antennas and so their hull can block signal propagation upwards [1] https://en.wikipedia.org/wiki/Automatic_Dependent_Surveillance%E2%80%93Broadcast#Satellite_(space_based)_ADS-B_Collection https://en.wikipedia.org/wiki/Automatic_Dependent_Surveillan...