Bandwidth
Some interesting posts over the past few days on having aircraft sendind more regular location updates and the impact of that on available satellite capacity.
In the maritime world the AIS (ships Automatic Identification System) transmits according to the rate of change of course. A ship stationary or travelling slowly will transmit its position much less often than one travelling faster. A ship changing course will transmit more often than one travelling in a straight line. For anyone not familiar with AIS 'marinetraffic' is worth googling.
I'm sure something similar could be implemented for aircraft to include the gaining or losing height as additional criteria for increased frequency of reporting