What benefits does Automatic Dependent Surveillance bring to continental ATC over Mode S? In particular, to the extent that there are mandates for it in USA and Europe?

I checked various posts here and official descriptions (e.g., faa.gov), but I can't figure out the following:


For continental (non-oceanic) usage, what benefits does ADS(-B) provide to ATC that is not possible with Mode S SSR?


I came to a dead end at How can ADS-B replace primary radar when FlightRadar24, using ADS-B, is so inaccurate? In particular:


  ADS-B can and will replace a number of secondary radars, but not all of them.


By not all of them, I reckon those in congested areas will not be replaced, say the New York/New Jersey/Philadelphia Airspace. One could say to provide cheap coverage elsewhere where radars can't reach (too low targets), or not enough traffic to sustain the cost, but then for the FAA mandate, for example, outside Class A, B, C, and E (above 10,000 ft), one is not required to have ADS-B. (What is ADS-B and who needs it?)

I understand that there are plenty of beneficial uses for ADS: [Cheap] online tracking that benefits the operators and general public alike, and situational awareness for any GA pilot willing to spend money on ADS-B IN. But my question is about the benefits to ATC, that Mode S can't provide.



Edit: I found one usage (benefit?) in the post Is ADS-B intended to replace other traffic management systems?:


  A more advanced usage of ADS-B will be flight deck based interval management (FIM) where ATC will be able to instruct aircraft to 'follow that plane XX seconds behind for landing on runway YY'.


Technologically, this is cool. But why the complication? Terminal airspace design that utilizes RNP routes, combined with requested times of arrival relayed to each aircraft, can accomplish the same, and is already a thing. Why let the separation be the responsibility of many, versus a few, i.e., bigger chance of something going wrong.

I'm thinking there must be more to it, even if not yet implemented.
ADS-B is more accurate.  SSR calculates the distance to a target based on when it receives a reply from the transponder, which is inherently imprecise, and calculates the azimuth by the sweep, which is also inherently imprecise.  A blip on a radar scope isn't exactly where the plane is but rather the center of an area where the plane is most likely to be.  ADS-B means ATC knows exactly where the plane is, which means they can (eventually) use tighter spacing if needed.

ADS-B is more reliable.  If two aircraft are near each other, their transponder replies may overlap, making both unreadable--and at the time it is most critical for ATC to be able to see both aircraft for separation.  ADS-B messages are transmitted randomly, which does result in occasional errors but means that two aircraft near each other aren't any more likely to step on each other than two far away, which is a substantial reduction in risk.

ADS-B can improve TCAS by not requiring air-to-air interrogations, reducing the total number of mod eS messages being broadcast and therefore the odds of aircraft stepping on each other.

ADS-B updates faster.  SSR relies on a radar slowly sweeping around, which means targets are only updated every 5-10 seconds, and this magnifies the effect of message errors.  This is "solved" in busy terminal areas by having several overlapping SSRs, but that also increases the error rate so it's of limited use.  ADS-B transmits position randomly every second, and even if some overlap, the readable update rate is still far higher than SSR.

ADS-B is cheaper.  You can put simple ADS-B receivers in many places where expensive SSR wouldn't be economically justified, giving ATC much improved coverage, particularly close to the ground and in mountainous areas.  You can also reduce SSR overlap, even if SSR doesn't go away entirely, which also saves money.

ADS-B provides improved situational awareness to pilots.  SSR doesn't provide any traffic data to pilots, and even TCAS isn't all that accurate (which means lots of false alarms) despite being extremely expensive.  ADS-B cheaply shows pilots exactly where the other traffic around them is--even non-participating traffic in an area with TIS-B.

Future ADS-B enhancements will also show aircraft intent.  SSR (and current ADS-B, to be fair) only show a plane's current position, often with a line projecting where it will be in N seconds/minutes if it continues along the same track since the last position.  ADS-B allows an aircraft to indicate what it will do in the (near) future, which is very useful when you have two aircraft near each other and one or both are turning and thus the straight-line projections aren't accurate.

The inaccurate flight paths you see on FR24 et al are due to aircraft not having ADS-B; the jagged tracks are generally from mode S-only aircraft, and the receivers have software that uses multilateration (MLAT) to cooperatively estimate where non-ADSB messages came from.  The FAA has experimented with this tech themselves in mountainous areas, and it's better than having nothing at all, but it's still far less accurate than SSR, much less ADS-B.