Hi Tor,
TCAS provides a system to meet ACAS requirements. So could something else if it was invented and met the specs. It's just there's nothing else around at the moment.
Having said that, US cargo operators are trialling an Automatic Dependent Surveillance - Broadcast (ADS-B) system with a view to providing an alternative to TCAS.
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10 West
UK ATC'er
[email protected]