I noticed that no planes heading to FMMI are plotted on all the prominent online radars despite the fact that most of them are likely equipped with ADS-B.
What does it take to remedy that lack of coverage?
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For services like FlightRadar24 they typically get data in two ways: via volunteers hosting ADS-B receiver stations on the ground, and directly from the ATC radar system in places where that data is readily available.
For the ADS-B equipped aircraft these services would need volunteers in the area of Ivato Airport (FMMI) to host ADS-B receivers. They would almost certainly be happy to have more receivers, and they even offer free "premium" services (and occasionally free receivers) to people who help provide data. If it's legal in your area, and you are willing to run a receiver station you may want to look into it, and this will get you aircraft which are broadcasting ADS-B data (Mode S+ES with GPS position information).
For the aircraft which are not broadcasting ADS-B information (because they lack an appropriate transponder or GPS system) the problem is a little more complicated - the organization that runs ATC in Madagascar would need to set up radar sites (which presumably exist), and make that information publicly available (which may or may not be the case). A system would then need to be developed to allow the online radar services to process that data and make the flight tracks available (subject to whatever delays may be imposed in that process).
As I don't know much (anything really) about the ATC system in Madagascar, so I'm not sure how robust the radar infrastructure is or how open they might be with their data, so I can't really comment on that half of the system.