ADS-C over ACARS

ACARS is a complex system which consists of several software modules which cooperate. Depending on whether you want to simulate an airplane or a dispatch/ATC station, you need another module.

Project Progress

2024-04-21
Those changes seem to have worked wonders. The system now pulls over 1000 parallel users without blinking. I've also extended the maximum length of names and e-mail addresses.

2024-04-20
I'm disabling various noncritical interfaces to reduce the system load. There are software clients out there that respond in a bad way to increased waiting times for connections. Any client should be able to cope with 20-30 seconds of waiting time without doing something silly, like firing dozens of similar requests per second. This makes the matter only worse! Please, if you program an ACARS client and you detect a long latency or other problem that probably is a performance issue, back off and reduce your polling rate. Go from once per minute down to once per two minutes. No it ain't great, but that keeps the system working properly.

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Automatic Dependent Surveillance -- Contract

ADS (Automatic Dependent Surveillance) is a specific application on top of ACARS. It is essentially position reporting, either automatic or on request.

The application included in the system described here is "addressed" or "contract" ADS-C. The position reports are sent to a specific station which needs to ask for reports. There is another system, ADS-B for "broadcast", which uses Mode-S transponder signals much like TCAS to broadcast your position to anybody nearby who may be interested. This ADS-B is not based on ACARS and therefore has nothing to do with the ADS-C system, despite their names.

To add to the confusion, you can also transmit formal position reports as plain ACARS messages. This is yet another system. These formal position reports are the telex equivalent of the voice position reports to air traffic control. They are always sent explicitly by the pilot in real life, but this simulated ACARS system allows an ATC or Dispatch station to request such a report as well. However this is not ADS-C.

How to Enable ADS-C Position Reports

It is important to realise that ADS-C is by default enabled for most aircraft that have ACARS running. If your ACARS client program supports it, your plane is ready, the pilot does not need to do anything. It is the ground station that needs to initiate the reporting: it needs to send a report request (contract) to the aircraft.

In order to offer an ADS contract to an aircraft, call up the technical ADS-C editor page. It may look daunting, but you just need to:

1. Fill in your own call sign in From (once, cookie).
2. Fill in the aircraft call sign in To (once, cookie).
3. Fill in your ACARS logon code in Logon Code (once, cookie).
4. Press Submit ADS-C Message.

This will offer the ADS-C contract to the aircraft. If it is technically capable of accepting the contract, it will start emitting ADS-C position reports from now on until it goes offline. Should you want to always track this callsign, check the Make Persistant box. NOTE: this is a feature that does not exist in real life. Do not depend on this feature to remain in the system. In real life, a new contract needs to be established between an ATC station and the aircraft every time it logs on to CPDLC. The feature has only been added to the system because the online presence of simulated aircraft is quite unstable.

If you send an aircraft an ADS-C contract request and nothing happens after a minute or two (the plane does not appear on a map), just check the message log. You should see the message you sent to the plane ("Received" time), and when the plane picked it up ("Relayed" time). Only if the plane has picked up the request, there is any chance of getting it to respond.

If an aircraft does not respond to an ADS-C contract request, it usually means that the airborne client does not have the required minimum data or simply did not implement ADS-C.

Here is the last known position of all ADS-C reporting aircraft:

Like this map? You can have it on your own web page!