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I'm trying to read up on some of the new developments in ATC (ATN, FANS etc.) and I've come across CPDLC: Controller–Pilot Data Link Communications. In particular, I'm confused of how the VHF messages travel to and from Air Traffic Control and the aircraft.

For the oceanic/remote flying, transmitting messages over SatCom sounds entirely reasonable. But everything I can find on VHF seems to imply that they use a separate Datalink Service Provider, similar how ACARS functions.

I always thought ATC had a bunch of antennas on the tower roof to do voice VHF. If this is the case, what prevents ATC from having their own datalink transceiver and doing all of this stuff 'in-house', avoiding more parties and infrastructure?

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  • $\begingroup$ It's not impossible the VHF equipment on the tower is part of the GCN visible in the picture, but it seems there is this requirement: "the communication network must “always” be available and interruptions are “not” acceptable" which makes a single receiver solution unusable and a network mandatory (source). On the other hand ATC voice channels are also going digital, which makes a new (digital) solution potentially more valuable. $\endgroup$
    – mins
    Oct 6, 2016 at 10:32

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For non-oceanic operations, aside from the tower radios, most ATC communications go through remote communications outlets (RCO) scattered around to get the necessary coverage. most are co-located with VORs or other navigational aids. The controllers are located in either a TRACON or Center facility that's often a long distance to the RCOs.

The problem is this equipment is these systems were designed for voice comm (double-side band AM radios). For data link, the radios operate as a radio modem using VHF data link (VDL) Mode 2. So three things have to happen: 1. the controller needs a data link enabled workstation, 2. there has to be a network connection to the RCOs, and 3. the radios need to support VDL mode 2.

So the real issue boils down to the two primary factors; time and money. The implementation in the US is part of Nexgen. To speed up the implementation, there is the alternative of leasing services of network providers such as ARINC and SITA.

BTW, the most comprehensive reference on data link is the Global Operational Data Link Document.

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