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What is the difference between ACARS and ADS-B?

For weather information in-flight or pre-flight which one is used?

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    $\begingroup$ They're two separate protocols. ADS-B is used for transponder reporting and TCAS; ACARS is for relaying information from the company (airline) to pilots. $\endgroup$ – verandaguy Nov 18 '19 at 16:01
  • $\begingroup$ Are you referring to FIS-B, rather than ADS-B? $\endgroup$ – StephenS Nov 18 '19 at 16:41
  • $\begingroup$ You may add your current understanding of both systems so that we can adapt the answer to address the points unclear to you. $\endgroup$ – Manu H Nov 19 '19 at 7:50
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One can easily get lost in the alphabet soup that is datalink.

Both ACARS and ADS-B provide datalink applications – which can be broken down in terms of CNS: Communication, Navigation, and Surveillance.

  • ACARS is used for:

    • Communication: includes uplinking (ground → aircraft) weather data upon a downlink request to the company/service provider – not a public service
    • Navigation: if you consider flex-routing
    • Surveillance: position reports in the North Atlantic for example (CPDLC over ACARS).
  • ADS-B is used solely for Surveillance; broadcasting (OUT) own location – and can include ownship surveillance by receiving (IN) traffic information.

    • If this IN setup is via the UAT frequency, then the aircraft can also receive weather information using FIS-B (free service). Its only relation to ADS-B in this particular instance is the UAT frequency, and it is more of a general aviation (and low-flying commercial) service.

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Source: https://www.faa.gov/nextgen/programs/adsb/pilot/#fisb

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