diff --git a/app/build.gradle.kts b/app/build.gradle.kts
index 9c4b80c..8b6a946 100644
--- a/app/build.gradle.kts
+++ b/app/build.gradle.kts
@@ -41,6 +41,7 @@ android {
 dependencies {
     implementation(project(":llm"))
     implementation(project(":tak-plugin"))
+    implementation(project(":core"))
 
     implementation("org.jetbrains.kotlin:kotlin-stdlib:1.9.24")
     implementation("androidx.appcompat:appcompat:1.7.0")
diff --git a/app/src/main/AndroidManifest.xml b/app/src/main/AndroidManifest.xml
index 0c18b8a..c6d9bb4 100644
--- a/app/src/main/AndroidManifest.xml
+++ b/app/src/main/AndroidManifest.xml
@@ -1,6 +1,8 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="com.example.tacticalapp">
 
+    <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
+
     <application
         android:allowBackup="true"
         android:label="@string/app_name"
@@ -15,6 +17,13 @@
             </intent-filter>
         </activity>
         <activity android:name=".LocalChatActivity" />
+        <activity android:name=".InteropSettingsActivity" />
+
+        <receiver android:name=".CotMarkerReceiver" android:exported="true">
+            <intent-filter>
+                <action android:name="com.tacticalapp.SEND_MARKER" />
+            </intent-filter>
+        </receiver>
     </application>
 
 </manifest>
diff --git a/app/src/main/java/com/example/tacticalapp/CotMarkerReceiver.kt b/app/src/main/java/com/example/tacticalapp/CotMarkerReceiver.kt
new file mode 100644
index 0000000..acd3722
--- /dev/null
+++ b/app/src/main/java/com/example/tacticalapp/CotMarkerReceiver.kt
@@ -0,0 +1,14 @@
+package com.example.tacticalapp
+
+import android.content.BroadcastReceiver
+import android.content.Context
+import android.content.Intent
+import com.example.core.interop.Interop
+
+/** Receives external marker intents and publishes them as CoT. */
+class CotMarkerReceiver : BroadcastReceiver() {
+    override fun onReceive(context: Context, intent: Intent) {
+        val json = intent.getStringExtra("marker") ?: return
+        Interop.publishMarkerJson(json)
+    }
+}
diff --git a/app/src/main/java/com/example/tacticalapp/InteropSettingsActivity.kt b/app/src/main/java/com/example/tacticalapp/InteropSettingsActivity.kt
new file mode 100644
index 0000000..3c94abf
--- /dev/null
+++ b/app/src/main/java/com/example/tacticalapp/InteropSettingsActivity.kt
@@ -0,0 +1,31 @@
+package com.example.tacticalapp
+
+import android.os.Bundle
+import androidx.appcompat.app.AppCompatActivity
+import com.example.core.interop.Interop
+import com.example.tacticalapp.databinding.ActivityInteropSettingsBinding
+
+class InteropSettingsActivity : AppCompatActivity() {
+    private lateinit var binding: ActivityInteropSettingsBinding
+
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+        binding = ActivityInteropSettingsBinding.inflate(layoutInflater)
+        setContentView(binding.root)
+
+        // Load current settings
+        binding.switchEnabled.isChecked = Interop.enabled
+        binding.editHost.setText(Interop.host)
+        binding.editPort.setText(Interop.port.toString())
+
+        binding.btnSave.setOnClickListener {
+            Interop.enabled = binding.switchEnabled.isChecked
+            Interop.host = binding.editHost.text.toString()
+            Interop.port = binding.editPort.text.toString().toIntOrNull() ?: Interop.port
+            if (Interop.enabled) {
+                Interop.startSelfBeacon(this)
+            }
+            finish()
+        }
+    }
+}
diff --git a/app/src/main/java/com/example/tacticalapp/MainActivity.kt b/app/src/main/java/com/example/tacticalapp/MainActivity.kt
index d5dd6c8..0742377 100644
--- a/app/src/main/java/com/example/tacticalapp/MainActivity.kt
+++ b/app/src/main/java/com/example/tacticalapp/MainActivity.kt
@@ -20,5 +20,19 @@ class MainActivity : AppCompatActivity() {
         binding.btnTakStatus.setOnClickListener {
             Toast.makeText(this, "TAK plugin status: OK", Toast.LENGTH_SHORT).show()
         }
+
+        binding.btnShareCot.setOnClickListener {
+            val marker = com.example.core.interop.MarkerEntity(
+                uid = "share-${System.currentTimeMillis()}",
+                lat = 0.0,
+                lon = 0.0,
+            )
+            com.example.core.interop.Interop.publishMarker(marker)
+            Toast.makeText(this, "CoT sent", Toast.LENGTH_SHORT).show()
+        }
+
+        binding.btnInteropSettings.setOnClickListener {
+            startActivity(Intent(this, InteropSettingsActivity::class.java))
+        }
     }
 }
diff --git a/app/src/main/res/layout/activity_interop_settings.xml b/app/src/main/res/layout/activity_interop_settings.xml
new file mode 100644
index 0000000..51b045a
--- /dev/null
+++ b/app/src/main/res/layout/activity_interop_settings.xml
@@ -0,0 +1,32 @@
+<?xml version="1.0" encoding="utf-8"?>
+<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    android:layout_width="match_parent"
+    android:layout_height="match_parent"
+    android:orientation="vertical"
+    android:padding="16dp">
+
+    <Switch
+        android:id="@+id/switchEnabled"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:text="Enable CoT" />
+
+    <EditText
+        android:id="@+id/editHost"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:hint="Host" />
+
+    <EditText
+        android:id="@+id/editPort"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:inputType="number"
+        android:hint="Port" />
+
+    <Button
+        android:id="@+id/btnSave"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:text="Save" />
+</LinearLayout>
diff --git a/app/src/main/res/layout/activity_main.xml b/app/src/main/res/layout/activity_main.xml
index 57604eb..1a1df9b 100644
--- a/app/src/main/res/layout/activity_main.xml
+++ b/app/src/main/res/layout/activity_main.xml
@@ -18,4 +18,18 @@
         android:layout_marginTop="16dp"
         android:text="TAK Plugin Status" />
 
+    <Button
+        android:id="@+id/btnShareCot"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:layout_marginTop="16dp"
+        android:text="Share as CoT" />
+
+    <Button
+        android:id="@+id/btnInteropSettings"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:layout_marginTop="16dp"
+        android:text="CoT Settings" />
+
 </LinearLayout>
diff --git a/core/build.gradle.kts b/core/build.gradle.kts
new file mode 100644
index 0000000..8a164f6
--- /dev/null
+++ b/core/build.gradle.kts
@@ -0,0 +1,26 @@
+plugins {
+    id("com.android.library")
+    id("org.jetbrains.kotlin.android")
+}
+
+android {
+    namespace = "com.example.core"
+    compileSdk = 34
+
+    defaultConfig {
+        minSdk = 26
+    }
+
+    compileOptions {
+        sourceCompatibility = JavaVersion.VERSION_17
+        targetCompatibility = JavaVersion.VERSION_17
+    }
+    kotlinOptions {
+        jvmTarget = "17"
+    }
+}
+
+dependencies {
+    implementation("androidx.work:work-runtime-ktx:2.9.0")
+    implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.3")
+}
diff --git a/core/src/main/java/com/example/core/interop/CotEvent.kt b/core/src/main/java/com/example/core/interop/CotEvent.kt
new file mode 100644
index 0000000..660aa18
--- /dev/null
+++ b/core/src/main/java/com/example/core/interop/CotEvent.kt
@@ -0,0 +1,16 @@
+package com.example.core.interop
+
+import java.time.Instant
+
+/**
+ * Minimal representation of a Cursor-on-Target event.
+ */
+data class CotEvent(
+    val uid: String,
+    val type: String,
+    val how: String,
+    val time: Instant,
+    val stale: Instant,
+    val lat: Double,
+    val lon: Double,
+)
diff --git a/core/src/main/java/com/example/core/interop/CotSender.kt b/core/src/main/java/com/example/core/interop/CotSender.kt
new file mode 100644
index 0000000..0157f02
--- /dev/null
+++ b/core/src/main/java/com/example/core/interop/CotSender.kt
@@ -0,0 +1,27 @@
+package com.example.core.interop
+
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.withContext
+import java.net.DatagramPacket
+import java.net.DatagramSocket
+import java.net.InetAddress
+
+/** Sends CoT XML payloads over UDP. */
+class CotSender(
+    private var host: String = DEFAULT_HOST,
+    private var port: Int = DEFAULT_PORT,
+) {
+    suspend fun send(xml: String) = withContext(Dispatchers.IO) {
+        val data = xml.toByteArray()
+        val packet = DatagramPacket(data, data.size, InetAddress.getByName(host), port)
+        DatagramSocket().use { socket ->
+            socket.broadcast = true
+            socket.send(packet)
+        }
+    }
+
+    companion object {
+        const val DEFAULT_HOST = "239.2.3.1"
+        const val DEFAULT_PORT = 6969
+    }
+}
diff --git a/core/src/main/java/com/example/core/interop/CotXml.kt b/core/src/main/java/com/example/core/interop/CotXml.kt
new file mode 100644
index 0000000..6d874f2
--- /dev/null
+++ b/core/src/main/java/com/example/core/interop/CotXml.kt
@@ -0,0 +1,57 @@
+package com.example.core.interop
+
+import org.w3c.dom.Element
+import java.io.StringWriter
+import java.time.format.DateTimeFormatter
+import javax.xml.parsers.DocumentBuilderFactory
+import javax.xml.transform.OutputKeys
+import javax.xml.transform.TransformerFactory
+import javax.xml.transform.dom.DOMSource
+import javax.xml.transform.stream.StreamResult
+
+/** Utilities for serializing and deserializing CoT events to XML. */
+object CotXml {
+    private val df: DateTimeFormatter = DateTimeFormatter.ISO_INSTANT
+
+    fun toXml(event: CotEvent): String {
+        val doc = DocumentBuilderFactory.newInstance().newDocumentBuilder().newDocument()
+        val root = doc.createElement("event")
+        root.setAttribute("version", "2.0")
+        root.setAttribute("uid", event.uid)
+        root.setAttribute("type", event.type)
+        root.setAttribute("how", event.how)
+        root.setAttribute("time", df.format(event.time))
+        root.setAttribute("start", df.format(event.time))
+        root.setAttribute("stale", df.format(event.stale))
+
+        val point: Element = doc.createElement("point")
+        point.setAttribute("lat", event.lat.toString())
+        point.setAttribute("lon", event.lon.toString())
+        point.setAttribute("hae", "0")
+        point.setAttribute("ce", "9999999")
+        point.setAttribute("le", "9999999")
+        root.appendChild(point)
+        doc.appendChild(root)
+
+        val tf = TransformerFactory.newInstance().newTransformer()
+        tf.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes")
+        val writer = StringWriter()
+        tf.transform(DOMSource(doc), StreamResult(writer))
+        return writer.toString()
+    }
+
+    fun fromXml(xml: String): CotEvent {
+        val doc = DocumentBuilderFactory.newInstance().newDocumentBuilder()
+            .parse(xml.byteInputStream())
+        val root = doc.documentElement
+        val uid = root.getAttribute("uid")
+        val type = root.getAttribute("type")
+        val how = root.getAttribute("how")
+        val time = df.parse(root.getAttribute("time"), java.time.Instant::from)
+        val stale = df.parse(root.getAttribute("stale"), java.time.Instant::from)
+        val point = root.getElementsByTagName("point").item(0) as Element
+        val lat = point.getAttribute("lat").toDouble()
+        val lon = point.getAttribute("lon").toDouble()
+        return CotEvent(uid, type, how, time, stale, lat, lon)
+    }
+}
diff --git a/core/src/main/java/com/example/core/interop/Interop.kt b/core/src/main/java/com/example/core/interop/Interop.kt
new file mode 100644
index 0000000..75fdb16
--- /dev/null
+++ b/core/src/main/java/com/example/core/interop/Interop.kt
@@ -0,0 +1,52 @@
+package com.example.core.interop
+
+import android.content.Context
+import androidx.work.ExistingPeriodicWorkPolicy
+import androidx.work.PeriodicWorkRequestBuilder
+import androidx.work.WorkManager
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.launch
+import kotlinx.serialization.json.Json
+import java.time.Instant
+import java.util.concurrent.TimeUnit
+
+/** Public API for CoT interoperability. */
+object Interop {
+    var enabled: Boolean = false
+    var host: String = CotSender.DEFAULT_HOST
+    var port: Int = CotSender.DEFAULT_PORT
+
+    fun sendCoT(event: CotEvent) {
+        if (!enabled) return
+        val xml = CotXml.toXml(event)
+        CoroutineScope(Dispatchers.IO).launch {
+            CotSender(host, port).send(xml)
+        }
+    }
+
+    fun publishMarker(marker: MarkerEntity) {
+        val now = Instant.now()
+        val evt = CotEvent(
+            uid = marker.uid,
+            type = marker.type,
+            how = "m-g",
+            time = now,
+            stale = now.plusSeconds(60),
+            lat = marker.lat,
+            lon = marker.lon,
+        )
+        sendCoT(evt)
+    }
+
+    fun publishMarkerJson(json: String) {
+        val marker = Json.decodeFromString(MarkerEntity.serializer(), json)
+        publishMarker(marker)
+    }
+
+    fun startSelfBeacon(context: Context) {
+        val request = PeriodicWorkRequestBuilder<SelfBeaconWorker>(5, TimeUnit.SECONDS).build()
+        WorkManager.getInstance(context)
+            .enqueueUniquePeriodicWork("cot-beacon", ExistingPeriodicWorkPolicy.UPDATE, request)
+    }
+}
diff --git a/core/src/main/java/com/example/core/interop/MarkerEntity.kt b/core/src/main/java/com/example/core/interop/MarkerEntity.kt
new file mode 100644
index 0000000..9ec96d1
--- /dev/null
+++ b/core/src/main/java/com/example/core/interop/MarkerEntity.kt
@@ -0,0 +1,11 @@
+package com.example.core.interop
+
+import kotlinx.serialization.Serializable
+
+@Serializable
+data class MarkerEntity(
+    val uid: String,
+    val lat: Double,
+    val lon: Double,
+    val type: String = "b-m-p",
+)
diff --git a/core/src/main/java/com/example/core/interop/SelfBeaconWorker.kt b/core/src/main/java/com/example/core/interop/SelfBeaconWorker.kt
new file mode 100644
index 0000000..edc37d1
--- /dev/null
+++ b/core/src/main/java/com/example/core/interop/SelfBeaconWorker.kt
@@ -0,0 +1,30 @@
+package com.example.core.interop
+
+import android.content.Context
+import android.location.Location
+import android.location.LocationManager
+import androidx.work.CoroutineWorker
+import androidx.work.WorkerParameters
+import java.time.Instant
+
+/** Periodically broadcasts device position as a FRIENDLY CoT marker. */
+class SelfBeaconWorker(ctx: Context, params: WorkerParameters) : CoroutineWorker(ctx, params) {
+    override suspend fun doWork(): Result {
+        if (!Interop.enabled) return Result.success()
+        val lm = applicationContext.getSystemService(Context.LOCATION_SERVICE) as LocationManager
+        val loc: Location? = lm.getLastKnownLocation(LocationManager.GPS_PROVIDER)
+        loc ?: return Result.success()
+        val now = Instant.now()
+        val evt = CotEvent(
+            uid = "SELF",
+            type = "a-f-G-U-C-I",
+            how = "m-g",
+            time = now,
+            stale = now.plusSeconds(15),
+            lat = loc.latitude,
+            lon = loc.longitude,
+        )
+        Interop.sendCoT(evt)
+        return Result.success()
+    }
+}
diff --git a/settings.gradle.kts b/settings.gradle.kts
index 2b303c2..282def7 100644
--- a/settings.gradle.kts
+++ b/settings.gradle.kts
@@ -14,4 +14,4 @@ dependencyResolutionManagement {
 }
 
 rootProject.name = "TacticalApp"
-include(":app", ":llm", ":tak-plugin")
+include(":app", ":llm", ":tak-plugin", ":core")