diff --git a/core/domain/src/main/java/com/rtbishop/look4sat/core/domain/aprs/AprsPacket.kt b/core/domain/src/main/java/com/rtbishop/look4sat/core/domain/aprs/AprsPacket.kt
index d0b5a367..37e84237 100644
--- a/core/domain/src/main/java/com/rtbishop/look4sat/core/domain/aprs/AprsPacket.kt
+++ b/core/domain/src/main/java/com/rtbishop/look4sat/core/domain/aprs/AprsPacket.kt
@@ -2,6 +2,7 @@ package com.rtbishop.look4sat.core.domain.aprs
 
 import kotlin.math.abs
 import kotlin.math.round
+import java.util.Locale
 
 /**
  * APRS-IS protocol core (pure Kotlin, no Android dependencies).
@@ -35,20 +36,20 @@ object AprsPacket {
 
     /** Optional distance filter: filter r/lat/lon/dist */
     fun formatRangeFilter(latitude: Double, longitude: Double, distKm: Int): String {
-        return String.format("r/%.3f/%.3f/%d", latitude, longitude, distKm)
+        return String.format(Locale.ROOT, "r/%.3f/%.3f/%d", latitude, longitude, distKm)
     }
 
     /** Altitude extension /A=00000 (feet) */
     fun formatAltitude(altitudeMeters: Double?): String {
         if (altitudeMeters == null) return ""
-        return String.format("/A=%06d", (altitudeMeters * 3.2808399).toInt())
+        return String.format(Locale.ROOT, "/A=%06d", (altitudeMeters * 3.2808399).toInt())
     }
 
     /** Speed/course extension (knots/degrees) */
     fun formatCourseSpeed(speedMps: Double?, bearing: Float?): String {
         if (speedMps == null || bearing == null) return ""
         val knots = (speedMps * 1.94384449).toInt()
-        return String.format("/%03d/%03d", bearing.toInt(), knots)
+        return String.format(Locale.ROOT, "/%03d/%03d", bearing.toInt(), knots)
     }
 }
 
@@ -100,17 +101,17 @@ class AprsPosition(
         val hundredths = iRound % 100
         val frac = when (positionAmbiguity) {
             1 -> "  .  "
-            2 -> String.format("%d .  ", minutes / 10)
-            3 -> String.format("%02d.  ", minutes)
-            4 -> String.format("%02d.%d ", minutes, hundredths / 10)
-            else -> String.format("%02d.%02d", minutes, hundredths)
+            2 -> String.format(Locale.ROOT, "%d .  ", minutes / 10)
+            3 -> String.format(Locale.ROOT, "%02d.  ", minutes)
+            4 -> String.format(Locale.ROOT, "%02d.%d ", minutes, hundredths / 10)
+            else -> String.format(Locale.ROOT, "%02d.%02d", minutes, hundredths)
         }
         return if (isLat) {
             val ns = if (value >= 0) 'N' else 'S'
-            String.format("%02d%s%c", degrees, frac, ns)
+            String.format(Locale.ROOT, "%02d%s%c", degrees, frac, ns)
         } else {
             val ew = if (value >= 0) 'E' else 'W'
-            String.format("%03d%s%c", degrees, frac, ew)
+            String.format(Locale.ROOT, "%03d%s%c", degrees, frac, ew)
         }
     }
 }
diff --git a/core/domain/src/test/java/com/rtbishop/look4sat/core/domain/aprs/AprsPacketLocaleTest.kt b/core/domain/src/test/java/com/rtbishop/look4sat/core/domain/aprs/AprsPacketLocaleTest.kt
new file mode 100644
index 00000000..6ed412f4
--- /dev/null
+++ b/core/domain/src/test/java/com/rtbishop/look4sat/core/domain/aprs/AprsPacketLocaleTest.kt
@@ -0,0 +1,75 @@
+package com.rtbishop.look4sat.core.domain.aprs
+
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import java.util.Locale
+
+/**
+ * APRS-IS is an ASCII line protocol. Formatting the position, altitude and
+ * course/speed extensions with the JVM default locale produced Eastern Arabic
+ * or Bengali digits on devices set to ar/fa/bn, and the server rejects those
+ * packets.
+ *
+ * Regression guard: every formatted field must stay ASCII regardless of the
+ * default locale.
+ */
+class AprsPacketLocaleTest {
+
+    private val original: Locale = Locale.getDefault()
+
+    @After
+    fun restoreLocale() {
+        Locale.setDefault(original)
+    }
+
+    private val asciiPacket = Regex("^[\\x20-\\x7E]*$")
+
+    @Test
+    fun position_staysAsciiUnderArabicLocale() {
+        Locale.setDefault(Locale.forLanguageTag("ar-EG"))
+
+        val encoded = AprsPosition(39.9042, 116.4074, '/', '>').toUncompressedString()
+
+        assertTrue("not ASCII: $encoded", asciiPacket.matches(encoded))
+        assertEquals("3954.25N/11624.44E>", encoded)
+    }
+
+    @Test
+    fun position_staysAsciiUnderBengaliLocale() {
+        Locale.setDefault(Locale.forLanguageTag("bn-BD"))
+
+        val encoded = AprsPosition(-33.8688, 151.2093, '/', '>').toUncompressedString()
+
+        assertTrue("not ASCII: $encoded", asciiPacket.matches(encoded))
+        assertEquals("3352.13S/15112.56E>", encoded)
+    }
+
+    @Test
+    fun altitudeAndCourseSpeed_stayAsciiUnderPersianLocale() {
+        Locale.setDefault(Locale.forLanguageTag("fa-IR"))
+
+        val altitude = AprsPacket.formatAltitude(100.0)
+        val courseSpeed = AprsPacket.formatCourseSpeed(10.0, 90f)
+        val filter = AprsPacket.formatRangeFilter(39.9042, 116.4074, 100)
+
+        assertTrue("not ASCII: $altitude", asciiPacket.matches(altitude))
+        assertTrue("not ASCII: $courseSpeed", asciiPacket.matches(courseSpeed))
+        assertTrue("not ASCII: $filter", asciiPacket.matches(filter))
+        assertEquals("/A=000328", altitude)
+        assertEquals("/090/019", courseSpeed)
+        assertEquals("r/39.904/116.407/100", filter)
+    }
+
+    @Test
+    fun ambiguousPosition_staysAsciiUnderArabicLocale() {
+        Locale.setDefault(Locale.forLanguageTag("ar-EG"))
+
+        for (ambiguity in 1..4) {
+            val encoded = AprsPosition(39.9042, 116.4074, '/', '>', ambiguity)
+                .toUncompressedString()
+            assertTrue("ambiguity=$ambiguity not ASCII: $encoded", asciiPacket.matches(encoded))
+        }
+    }
+}