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Researchers in aviation usually test systems in simulated environments to find out the pros, cons, challenges, etc. Now ADS-B is a key component of NextGen and its European counterpart. And I've seen papers/researches on applications than utilize ADS-B.

For the US, even shortly before the current pandemic, the Office of Inspector General was not too thrilled with NextGen, with the latest in a series of audits being announced in Oct of 2019. Based on earlier audits, the benefits will not be realized before 2025, and that was before the pandemic, and the latest audit is currently questioning the benefits:

Given concerns over the progress and benefits of NextGen, the FAA Reauthorization Act of 2018 mandated that our office study the potential impacts of a significantly delayed, diminished, or completely failed delivery of FAA's NextGen initiative.

Two years ago I asked, What benefits does Automatic Dependent Surveillance bring to continental ATC over Mode S? Of the pros noted, there were these two relevant to my question here:

  1. ADS-B updates faster
  2. ADS-B is cheaper

Since one of NextGen's big aims is boosting the traffic capacity, which is mostly limited by separation in the terminal environment, were alternatives tested? For example:

  1. Sector limited radars (one per approach end) that would provide the faster update rates. (Radars that don't make complete revolutions.)
  2. Its relative cost, i.e. equipping thousands of planes versus tens of super busy airports.

If no, were there [then-valid] reasons?

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NextGen is an FAA programme that strives to modernise America's air transportation system to make flying safer, more efficient, and more predictable.

NextGen is not one technology, product, or goal. As such, you can't test NextGen against an alternative.

Also the major benefit of ADS-B is not that it providing faster updates or that it is cheaper. It allows for a paradigm shift in ATC.

Separation minima in the TMA are not primarily driven by surveillance performance. They are driven by the latency in the chain from surveillance -> interpretation of situation by controller -> deciding resolution of situation by controller -> communicating instructions to affected aircraft -> pilots implementing the instructions -> aircraft reacting to pilot commands. Wake turbulence is another driving factor. Surveillance update rate is a minor factor in all of this.

The more aircraft there are in an area, the more the controller needs to monitor and to communicate, the more critical the separation becomes. We are reaching a fundamental barrier which can't be solved by adding more air traffic controllers. The only way to break that barrier is to automate some of the functions the air traffic controller performs in the Air Traffic Control System, or to offload them to the cockpit. ADS-B is an enabler for that.

ADS-B allows for the development of a Cockpit Display of Traffic Information (CDTI), increasing the situation awareness in the cockpit and thereby improving safety.

ADS-B enables applications like Cockpit based Interval Management, where the pilot receives speed and heading instructions from a system based on ADS-B in order to achieve a spacing distance or spacing time with respect to another aircraft, taking workload of the air traffic controller while achieving a higher arrival throughput.

ADS-B enables CDTI Assisted Visual Separation (CAVS), which will allow airport to achieve visual arrival rates in marginal visual meteorological conditions.

ADS-B allows for new types of TCAS, reducing the RF overload on the radar frequencies that are congested in some area. (Your suggestion of just adding radars on every runway end is probably achieving the opposite of what you want. It causes transponder overload and a very poor update rate as a result)

ADS-B allows for space based surveillance, providing global coverage and thereby improving safety and efficiency on oceanic flights.

I am sure alternatives were discussed. Some applications did not make it through the whole process, others did. But ADS-B, conceptually, is fundamental to the many of the changes in NextGen. That is not to say that ADS-B is perfect, or even good. On the technology level it could be a lot better and for example a lot more secure. But it would have costed a lot more too.

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