The cost. This affects general aviation as older planes will require avionics upgrades. Otherwise I’m excited for the sheer amount of data that will be made available to the public. Now you will be able to track all FAA registered planes
Almost all.
If you're flying VFR you can put your transponder into an anonymous / incognito mode (if it supports it):
* https://www.avweb.com/aviation-news/faa-acts-to-preserve-ads...
If you want IFR or flight following you'll have to de-anonymized, which is no worse than it was before ADS-B.
Does incognito mode in your browser stop you from being tracked if e.g Google or facebook can see every or almost all of your requests?
So people can see that someone is in a particular place, but not who that person/plane is.
I very much doubt that we'll get to even 80% of the fleet of FAA registered airplanes having ADS-B out by 2030. We're at about 50% right now.
https://www.faa.gov/nextgen/equipadsb/installation/current_e...
I feel like, for example, I'd be worried if fine-detail GPS data was being published live for example, or live enough to predict where the plane is "right" now...
Any time my dad takes his personal plane up for an hour's flight, I get a text message and can watch his track (and altitude/speed) in real-time.
The FAA in the US does publish a 'privacy list' of aircraft whose owners have requested that they not be included in services like this, and most commercial providers comply with that list, but enforcement is sparse and several hobbyist projects (e.g. ADS-B Exchange) do not and even emphasize aircraft that have put in such a request, since the tend to be a bit more interesting. A bit of a Streisand effect.
The same situation exists with the similar AIS system used by ships at sea, and has produced concerns over malicious use, but there continue to be plenty of websites that will show you maps of ship locations picked up by their contributing AIS receivers.
https://en.m.wikipedia.org/wiki/Tethered_Aerostat_Radar_Syst...
As you'll see from the various documentaries, historically planes operating illegally and/or doing illegal things will fly "under the radar" and keep their transponder turned off to evade the authorities.
They'll do the same thing with GPS.