ACAS is the system specification for equipment designed to provide a last resort method of avoiding mid air collisions.
TCAS is the only commercially available equipment which currently meets the specification.
You can get a lot of info on ACAS (from the European point of view) here:
http://www.eurocontrol.be/projects/eatchip/acas/
FANS is a datalink application which provides Automatic Dependent Surveillance (e.g POS reports) and/or Controller/Pilot Datalink Clearances (CPDLC). To do this it either uses Satellite Aircom or VHF Aircom links. ACARS is one method of sending messages via the VHF Aircom route.
The ACARS messages are sent to/from the aircraft and the ATC unit via a service provider, e.g. ARINC or SITA. You shoud be able to find more on those companies websites.
http://www.arinc.com/Products_Servic...Link/cads.html
http://www.sita.com/Default.htm?/air...ag_fans1a.html
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10 West
UK ATC'er
[email protected]