We are now doing that. Planes with ADS-B send out their position, altitude, direction and speed every eight seconds. That's how flightradar24 tracks most airplanes. ADS-B has been introduced sometime in the mid-2000s, and became mandatory in many countries in 2020. It originally depended on ground receivers or other aircraft picking up the transmission though, which is an issue if a plane goes missing in the middle of an ocean. This seems to be mostly solved now by satellites equiped with receivers.
Just FYI that not all countries require equipment for space based ADSB ( like the USA ). Many of our commercial airliners have it but mostly in the name of travel abroad.
- 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
historically, aircraft have those antennas mounted on their belly and with low power they blast them at the ground for powerful receivers to catch. For "ADS-B Diversity", which is the space based version, on a generic Cessna 172 that meets current ADS-B requirements, I would need to rip out my current ADS-B system and replace it with a $9,000 (plus labor) ADS-B system that has dual broadcasting antennas. I think the top mount antenna is also more powerful but I honestly do not know.
Space based ADSB only improves coverage for mountain ranges and oceans. It's mission in a mountainous situation is to fill the gap when radar coverage is lost but you have low altitude (in relation to ground, not sea level) traffic, such as arriving and departing traffic.
assuming this map is accurate, you can appreciate how aircraft below 15,000 ft would be constantly falling in and out of ADS-B coverage if they themselves were changing altitude.
But the ocean is an other matter entirely, water is great at absorbing radio waves, and when a plane crashes and breaks up over deep waters the currents can carry far and disperse wide.
