YuKitsune · Oct 17, 2026 · Oct 13, 2026 · Oct 14, 2026 · Oct 14, 2026 · Oct 14, 2026
diff --git a/build/Build.cs b/build/Build.cs
@@ -176,9 +176,15 @@ class Build : FalloutBuild
         .Executes(() =>
         {
             var mainAssembly = PluginBuildOutputDirectory / PluginAssemblyFileName;
+            AbsolutePath[] nonMergedAssemblies =
+            [
+                // ILRepack cannot merge Harmony, so it stays as a separate file next to the plugin
+                PluginBuildOutputDirectory / "0Harmony.dll"
+            ];
+
             var assembliesToMerge = PluginBuildOutputDirectory
                 .GlobFiles("*.dll")
-                .Except([mainAssembly])
+                .Except([mainAssembly, ..nonMergedAssemblies])
                 .ToArray();
 
             if (!mainAssembly.FileExists())

diff --git a/docs/docs/concepts/index.md b/docs/docs/concepts/index.md
@@ -233,6 +233,75 @@ sequenceDiagram
     Note over YBBB,Pilot: Dialogue closes, connection terminates
 ```
 
+## ADS-C
+
+Automatic Dependent Surveillance - Contract (ADS-C) lets a ground station get position reports from an aircraft by data link.
+Controllers use ADS-C to see aircraft that are outside radar coverage, such as aircraft over the ocean.
+
+The aircraft sends reports automatically. The pilot does not have to do anything.
+Each report has the position, the altitude, and the time of the report.
+
+:::note
+ADS-C and CPDLC are separate systems. An aircraft can have an ADS-C contract with a ground station without a CPDLC connection.
+For convenience, this plugin connects ADS-C to CPDLC. The server requests an ADS-C contract when an aircraft logs on with CPDLC, and cancels the contract when the CPDLC connection ends.
+:::
+
+### Contracts
+
+The ground station requests a **contract** with the aircraft. The contract tells the aircraft when to send reports.
+The plugin uses these types of contract:
+
+- **Periodic contract:** The aircraft sends a report at a fixed interval, for example, each 10 minutes
+- **Demand contract:** The aircraft sends one report immediately
+
+An aircraft has only one periodic contract with each ground station. A new periodic request replaces the previous one.
+
+:::note
+Event contracts are not supported.
+:::
+
+### Default and Selected Contracts
+
+Each ATSU has a **default reporting interval**.
+When an aircraft logs on with CPDLC, the server automatically requests a periodic contract with the default reporting interval. This is the **default contract**.
+
+A controller can select a different reporting interval for an aircraft, for example, a shorter interval near a conflict. This is the **selected contract**.
+A selected contract ends after a fixed time. Then the server requests the default contract again.
+
+```mermaid
+sequenceDiagram
+    participant Pilot
+    participant ATSU
+
+    Pilot->>ATSU: REQUEST LOGON
+    ATSU->>Pilot: LOGON ACCEPTED
+    ATSU->>Pilot: Periodic contract (default interval)
+    Pilot->>ATSU: Report
+    Note over ATSU,Pilot: Default contract accepted
+    ATSU->>Pilot: Periodic contract (selected interval)
+    Pilot->>ATSU: Report
+    Note over ATSU,Pilot: Selected contract accepted
+    Note over ATSU,Pilot: Selected contract ends
+    ATSU->>Pilot: Periodic contract (default interval)
+```
+
+When the CPDLC connection ends, the server cancels the contract.
+
+### Contract State
+
+Each request has one of these states:
+
+| State | Description |
+| :---: | ----------- |
+| Sent | The request was sent to the aircraft. The aircraft has not answered. |
+| Accepted | The aircraft sent a report after the request. |
+| Rejected | The aircraft rejected the request. |
+
+The aircraft does not acknowledge a request. The first report after a request accepts that request.
+
+If the aircraft rejects a selected contract, the server requests the default contract again.
+If the aircraft rejects the default contract, the server does not request it again.
+
 ## CPDLC on VATSIM
 
 VATSIM does not have an official CPDLC solution, though one is in development. In the meantime, CPDLC for flight simulation networks typically uses Hoppie's ACARS network.
@@ -247,6 +316,9 @@ Stations connected to Hoppie's network poll for messages at regular intervals.
 Messages are not received instantly by the recipient.
 It may take up to one minute for a message to reach the pilot, and another minute for a response to return.
 
+This delay also applies to ADS-C. A report can arrive up to one minute after its report time, and a demand report can take up to two minutes to arrive.
+For this reason, the shortest reporting interval is one minute.
+
 ### Aggregation Server
 
 Hoppie's network does not allow multiple controllers to use a single ATSU callsign. The aggregation server solves this problem by acting as a relay between controllers and the ACARS network, allowing multiple controllers to share a single ATSU connection.
diff --git a/docs/docs/index.md b/docs/docs/index.md
@@ -19,6 +19,7 @@ The system consists of two components:
 Multiple controllers can share a single ATSU connection, with messages automatically routed to the appropriate controller.
 
 **[vatSys Plugin](/vatsys-plugin/installation)** provides the controller interface within vatSys for sending and receiving CPDLC messages with aircraft.
+It also shows ADS-C position reports on the ASD, and lets controllers change the ADS-C contract with an aircraft.
 
 ![Architecture Diagram](../static/diagram.png)
 

diff --git a/docs/docs/server/index.md b/docs/docs/server/index.md
@@ -46,6 +46,10 @@ Acars__0__FlightSimulationNetwork=VATSIM
 Acars__0__StationIdentifier=YBBB
 Acars__0__Url=http://www.hoppie.nl/acars/system/connect.html
 Acars__0__AuthenticationCode=your-hoppie-logon-code
+# Reporting interval in minutes of the ADS-C contract requested on CPDLC logon (default: 10)
+Acars__0__DefaultAdscReportingInterval=10
+# Minutes from the request of a selected ADS-C contract to its end. Then the default contract is requested again (default: 60)
+Acars__0__SelectedAdscContractDuration=60
 
 # Handoff Configuration
 # How many minutes before the expected transfer time to send the NEXT DATA AUTHORITY message (default: 10)
@@ -78,6 +82,19 @@ Acars__1__AuthenticationCode=your-ymmm-hoppie-code
 `Hoppie` is currently the only supported ACARS network type.
 :::
 
+### ADS-C
+
+When an aircraft logs on with CPDLC, the server requests the default ADS-C contract with the aircraft. When the CPDLC connection ends, the server cancels the contract.
+
+Each ATSU has its own default reporting interval, set with `Acars__<index>__DefaultAdscReportingInterval`. Controllers can read this value.
+
+A controller can select a different reporting interval for an aircraft. The selected contract ends `Acars__<index>__SelectedAdscContractDuration` minutes after the server sends the request. Then the server requests the default contract again. A controller can also go back to the default contract at any time.
+
+The aircraft does not acknowledge a request. The first report after a request accepts it. If the aircraft rejects a selected contract, the server requests the default contract again. If the aircraft rejects the default contract, the server does not send it again.
+
+The server keeps the latest report for each aircraft. When a controller connects, the plugin loads these reports, so that the controller sees the ADS-C tracks immediately.
+
+The ADS-C page of the dashboard (`/adsc`) shows all the contracts and their states, and a log of all the ADS-C messages that the server sent and received.
 ## Running
 
 ### Docker

diff --git a/docs/docs/vatsys-plugin/configuration.md b/docs/docs/vatsys-plugin/configuration.md
@@ -128,6 +128,17 @@ Default: `true`
 "ShowInstallationMenuItems": false
 ```
 
+### ADSCTrackExtrapolationThreshold
+
+The minimum time (in seconds) that an ADS-C track shows the reported position with the `ADSC` symbol. After this time, the next flight plan extrapolation moves the track along its route, and the track uses the `ADSCExtrapolated` symbol.
+
+Default: `10`
+
+**Example:**
+```json
+"ADSCTrackExtrapolationThreshold": 10
+```
+
 ## Uplink Messages
 
 The `UplinkMessages` object defines the available CPDLC messages for the controller to send.
@@ -224,6 +235,7 @@ Array of message groups for organizing messages in the editor. Each group has:
     { "AtsuCode": "YMMM", "Sectors": ["GUN", "BLA", "IND"] }
   ],
   "NdaRecalculationIntervalMinutes": 1,
+  "ADSCTrackExtrapolationThreshold": 10,
   "UplinkMessages": {
     "MasterMessages": [
       {

diff --git a/docs/docs/vatsys-plugin/plugin.md b/docs/docs/vatsys-plugin/plugin.md
@@ -5,7 +5,7 @@ sidebar_position: 2
 # Plugin Guide
 
 This guide covers the CPDLC Plugin for vatSys.
-It explains how to connect to the server, interpret track labels, and manage CPDLC messages with pilots.
+It explains how to connect to the server, interpret track labels, manage CPDLC messages with pilots, and use ADS-C.
 
 ## Connecting to the CPDLC Server
 
@@ -176,6 +176,72 @@ Enter a callsign in the ACID input to display that aircraft's message history. O
 
 Messages are displayed the same way as in the Current Messages Window. Messages prefixed with `M` were manually acknowledged by the controller.
 
+## ADS-C
+
+The plugin shows ADS-C position reports on the ASD, and lets you change the ADS-C contract with an aircraft.
+Refer to [ADS-C](/concepts#ads-c) for an explanation of contracts.
+
+You do not have to do anything to start ADS-C. When an aircraft logs on with CPDLC, the server automatically requests the default contract.
+
+### ADS-C Tracks
+
+![Screenshot of an ADS-C track on the ASD, next to the same track after extrapolation. Show the ADSC symbol and the ADSCExtrapolated symbol.](../../static/adsc_tracks.png)
+
+When a report arrives, the flight plan track of the aircraft moves to the reported position, and shows the `ADSC` symbol.
+The reported altitude becomes the pilot reported level (PRL) of the flight plan.
+
+After a short time, the track moves along the flight plan route from the reported position, and shows the `ADSCExtrapolated` symbol.
+The plugin uses the route estimates of the flight plan to calculate the speed of the track.
+You can change this time with the [`ADSCTrackExtrapolationThreshold`](configuration#adsctrackextrapolationthreshold) setting.
+
+The vatSys profile sets how the symbols look, so the exact symbols may vary.
+By default, the ADSC symbol is a filled triangle, and the ADSCExtrapolated is a hollow triangle.
+
+A radar track always has priority. When the flight plan is coupled to a radar track, the plugin does not move the track.
+
+### ADS Contract Window
+
+![Screenshot of the ADS Contract window with an aircraft loaded.](../../static/adsc_window.png)
+
+The ADS Contract Window shows the ADS-C contract that your ATSU has with an aircraft. You can use it to select a different reporting interval, or to request a demand report.
+
+To open the window, select a track, then open the CPDLC menu and select ADS Contract. The window shows the contract for the selected track.
+
+To show a different aircraft, click the ACID button, type the callsign, then press Enter.
+
+The window shows:
+
+- **Current Period:** The reporting interval of the latest periodic contract, in minutes
+- **Last Report:** The time of the latest report from the aircraft
+- **Default Period:** The default reporting interval of your ATSU, in minutes
+- **Selected Period:** The reporting interval that a controller selected, in minutes. `DEF` shows that the default period is selected.
+- **Selected Period End:** The time when the selected contract ends. Then the server requests the default contract again.
+
+A symbol next to each contract shows the state of the request:
+
+- `→`: The request was sent. The aircraft has not answered.
+- `✓`: The aircraft accepted the request.
+- `×`: The aircraft rejected the request.
+
+#### Selecting a Reporting Interval
+
+1. Click the Selected Period button. A list of periods opens.
+2. Click a period in the list, or type a period (in minutes) in the box at the bottom of the list, then press Enter.
+
+The server sends the request to the aircraft immediately.
+The selected contract ends after a time that the server sets (60 minutes by default).
+
+To go back to the default contract before the selected contract ends, click `DEF` in the list.
+
+#### Requesting a Demand Report
+
+Click Demand Contract to request one report from the aircraft immediately.
+A demand report does not change the periodic contract.
+
+:::tip
+Because of the delays on Hoppie's network, a demand report can take up to two minutes to arrive.
+:::
+
 ## Voice Capability Indicators
 
 ![Screenshot of voice capability label items](../../static/text_label_items.png)
@@ -220,7 +286,7 @@ This ensures pilots can acknowledge frequency change instructions before losing
 
 The plugin does not fully simulate all aspects of real-world CPDLC.
 
-**ADS-C:** ADS-C position reporting is not simulated due to limitations within the vatSys plugin API.
+**ADS-C:** Only periodic and demand contracts are supported. Event contracts are not supported.
 
 **Equipment codes:** Aircraft equipment codes are ignored. The `.` symbol indicates the aircraft is connected to an ACARS network, regardless of their filed equipment.
 

diff --git a/docs/static/adsc_tracks.png b/docs/static/adsc_tracks.png
diff --git a/docs/static/adsc_window.png b/docs/static/adsc_window.png
diff --git a/example.env b/example.env
@@ -13,6 +13,10 @@ Acars__0__FlightSimulationNetwork=VATSIM
 Acars__0__StationIdentifier=YBBB
 Acars__0__Url=http://www.hoppie.nl/acars/system/connect.html
 Acars__0__AuthenticationCode=your-authentication-code-here
+# Reporting interval in minutes of the ADS-C contract requested on CPDLC logon (default: 10)
+Acars__0__DefaultAdscReportingInterval=10
+# Minutes from the request of a selected ADS-C contract to its end. Then the default contract is requested again (default: 60)
+Acars__0__SelectedAdscContractDuration=60
 
 # Example: Second ACARS Configuration (uncomment to use)
 # Acars__1__Type=Hoppie

diff --git a/source/CPDLCPlugin/AdscContractStore.cs b/source/CPDLCPlugin/AdscContractStore.cs
@@ -0,0 +1,76 @@
+using CPDLCServer.Contracts;
+
+namespace CPDLCPlugin;
+
+public class AdscContractStore
+{
+    readonly List<AdscContractDto> _contracts = new();
+    readonly SemaphoreSlim _semaphore = new(1, 1);
+
+    public async Task Populate(AdscContractDto[] contracts, CancellationToken cancellationToken = default)
+    {
+        await _semaphore.WaitAsync(cancellationToken);
+        try
+        {
+            _contracts.Clear();
+            _contracts.AddRange(contracts);
+        }
+        finally
+        {
+            _semaphore.Release();
+        }
+    }
+
+    public async Task Upsert(AdscContractDto contractDto, CancellationToken cancellationToken = default)
+    {
+        await _semaphore.WaitAsync(cancellationToken);
+        try
+        {
+            _contracts.RemoveAll(c => c.Callsign == contractDto.Callsign && c.StationId == contractDto.StationId);
+            _contracts.Add(contractDto);
+        }
+        finally
+        {
+            _semaphore.Release();
+        }
+    }
+
+    public async Task<IReadOnlyCollection<AdscContractDto>> All(CancellationToken cancellationToken = default)
+    {
+        await _semaphore.WaitAsync(cancellationToken);
+        try
+        {
+            return _contracts.ToArray();
+        }
+        finally
+        {
+            _semaphore.Release();
+        }
+    }
+
+    public async Task<bool> Remove(string callsign, string stationId, CancellationToken cancellationToken = default)
+    {
+        await _semaphore.WaitAsync(cancellationToken);
+        try
+        {
+            return _contracts.RemoveAll(c => c.Callsign == callsign && c.StationId == stationId) > 0;
+        }
+        finally
+        {
+            _semaphore.Release();
+        }
+    }
+
+    public async Task Clear(CancellationToken cancellationToken = default)
+    {
+        await _semaphore.WaitAsync(cancellationToken);
+        try
+        {
+            _contracts.Clear();
+        }
+        finally
+        {
+            _semaphore.Release();
+        }
+    }
+}