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 37e84237..3b1d4c0f 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
@@ -39,17 +39,28 @@ object AprsPacket {
         return String.format(Locale.ROOT, "r/%.3f/%.3f/%d", latitude, longitude, distKm)
     }
 
-    /** Altitude extension /A=00000 (feet) */
+    /**
+     * Altitude extension /A=000000 (feet). The field is a fixed six-digit
+     * decimal, so a negative altitude (below sea level, or a bad GPS fix) must
+     * be clamped: "%06d" of -164 yields "/A=-00164", which is not a valid
+     * extension and corrupts the rest of the comment field.
+     */
     fun formatAltitude(altitudeMeters: Double?): String {
         if (altitudeMeters == null) return ""
-        return String.format(Locale.ROOT, "/A=%06d", (altitudeMeters * 3.2808399).toInt())
+        val feet = (altitudeMeters * 3.2808399).toInt().coerceIn(0, 999999)
+        return String.format(Locale.ROOT, "/A=%06d", feet)
     }
 
-    /** Speed/course extension (knots/degrees) */
+    /**
+     * Speed/course extension /CCC/SSS (degrees/knots). Course wraps into
+     * 0..359 and speed is clamped to three digits, because "%03d" of an
+     * out-of-range value widens the field and breaks the fixed-width format.
+     */
     fun formatCourseSpeed(speedMps: Double?, bearing: Float?): String {
         if (speedMps == null || bearing == null) return ""
-        val knots = (speedMps * 1.94384449).toInt()
-        return String.format(Locale.ROOT, "/%03d/%03d", bearing.toInt(), knots)
+        val knots = (speedMps * 1.94384449).toInt().coerceIn(0, 999)
+        val course = ((bearing.toInt() % 360) + 360) % 360
+        return String.format(Locale.ROOT, "/%03d/%03d", course, knots)
     }
 }
 
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
index 6ed412f4..c1843575 100644
--- 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
@@ -62,6 +62,22 @@ class AprsPacketLocaleTest {
         assertEquals("r/39.904/116.407/100", filter)
     }
 
+    @Test
+    fun altitude_clampsNegativeToKeepSixDigitField() {
+        // "%06d" of a negative value yields "/A=-00164": the '-' takes a digit
+        // slot, so the extension is no longer a valid fixed-width field.
+        assertEquals("/A=000000", AprsPacket.formatAltitude(-50.0))
+        assertEquals("/A=000000", AprsPacket.formatAltitude(-1.0))
+        assertEquals("/A=000328", AprsPacket.formatAltitude(100.0))
+    }
+
+    @Test
+    fun courseSpeed_wrapsCourseIntoValidRange() {
+        assertEquals("/000/019", AprsPacket.formatCourseSpeed(10.0, 360f))
+        assertEquals("/359/019", AprsPacket.formatCourseSpeed(10.0, -1f))
+        assertEquals("/090/019", AprsPacket.formatCourseSpeed(10.0, 90f))
+    }
+
     @Test
     fun ambiguousPosition_staysAsciiUnderArabicLocale() {
         Locale.setDefault(Locale.forLanguageTag("ar-EG"))