Car sat nav systems don't send, they only receive. The GP Staellites send out data pulses which are received by the onboard system and used to calculate the position of the receiver - the satnav uses a sophisticated form of triangulation to calculate where the vehicle is, relative to the satellites.
It works with an almost infinite number of vehicles because they're all receiving - the satellite doesn't have to cope with two-way traffic.
If aircraft constantly transmitted data-bursts to a satellite the data would have to be sent down to a ground station, and with aircraft moving at high speeds there would be more than one ground station involved - there might be dozens. The data would then have to be collated into a single file and stored somewhere until/unless needed. The potential for data corruption and loss is considerable. Flight recorders and cockpit voice recorders do the job, provided they can be recovered, and in all but a very few cases they can.