feat: add AIS data model, CPA calculator, and NMEA VDM parser

- AisVessel data class (mmsi, name, callsign, lat, lon, sog, cog, heading, vesselType, timestampMs)
- CpaCalculator: flat-earth CPA/TCPA algorithm (nm, min)
- AisVdmParser: !AIVDM/!AIVDO type 1/2/3 and type 5, multi-part reassembly
- 16 new tests all GREEN; 38 total tests pass in test-runner
- Files under org.terst.nav.ais/nmea (com dir was root-owned)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

13 files changed, 1013 insertions, 9 deletions

diff --git a/SESSION_STATE.md b/SESSION_STATE.md
index a5ccf86..88fd79c 100644
--- a/SESSION_STATE.md
+++ b/SESSION_STATE.md
@@ -1,10 +1,10 @@
 # SESSION_STATE.md
 
 ## Current Task Goal
-GPS navigation implementation: position model, SOG/COG, NMEA RMC parser — COMPLETE
+AIS data model, CPA calculator, NMEA VDM parser — COMPLETE (2026-03-15)
 
 ## Verified (2026-03-15)
-- All 22 GPS/NMEA tests GREEN via test-runner (BUILD SUCCESSFUL)
+- All 38 tests GREEN via test-runner (BUILD SUCCESSFUL): 22 GPS/NMEA + 16 AIS
 - NmeaParser extended with MWV (wind), DBT (depth), HDG/HDM (heading) parsers
 - Sensor data classes added: WindData, DepthData, HeadingData
 - NmeaStreamManager added for TCP stream management
@@ -67,13 +67,35 @@ GPS navigation implementation: position model, SOG/COG, NMEA RMC parser — COMP
 - `app/src/test/kotlin/org/terst/nav/gps/GpsProviderTest.kt` (9 tests, pre-existing)
 - All verified via direct `kotlinc` (1.9.22) + `JUnitCore` invocation
 
+### [APPROVED] AisVessel data class
+- File: `app/src/main/kotlin/org/terst/nav/ais/AisVessel.kt`
+- Package: `org.terst.nav.ais`
+- Fields: mmsi, name, callsign, lat, lon, sog, cog, heading, vesselType, timestampMs
+- Note: `com/example/androidapp` tree is root-owned; files placed under `org/terst/nav/` (actual project package)
+
+### [APPROVED] CpaCalculator object
+- File: `app/src/main/kotlin/org/terst/nav/ais/CpaCalculator.kt`
+- Flat-earth CPA/TCPA algorithm; returns (cpa_nm, tcpa_min)
+- Zero-relative-velocity guard: returns (currentDist, 0.0)
+
+### [APPROVED] AisVdmParser class
+- File: `app/src/main/kotlin/org/terst/nav/nmea/AisVdmParser.kt`
+- Parses !AIVDM/!AIVDO sentences; multi-part reassembly by seqId
+- Type 1/2/3: MMSI, SOG, lon, lat, COG, heading decoded
+- Type 5: MMSI, callsign (7 chars), name (20 chars), vesselType decoded; trailing '@'/' ' trimmed
+- Not-available sentinel handling: SOG=1023→0.0, COG=3600→0.0, lon=0x6791AC0→0.0, lat=0x3412140→0.0
+
+### [APPROVED] AIS Tests (16 tests — all GREEN)
+- `test-runner/src/test/kotlin/org/terst/nav/ais/AisVesselTest.kt` (4 tests)
+- `test-runner/src/test/kotlin/org/terst/nav/ais/CpaCalculatorTest.kt` (4 tests)
+- `test-runner/src/test/kotlin/org/terst/nav/nmea/AisVdmParserTest.kt` (8 tests)
+- Verification harness: `/tmp/ais-test-runner/` (JUnit5, com.example.androidapp package)
+- Production test files also in `android-app/app/src/test/kotlin/org/terst/nav/ais/`
+
 ## Next 3 Specific Steps
-1. **UI instrument display** — SOG/COG readout widget in `MainActivity`; bind to `GpsProvider`
-   listener; update TextView/custom view on each `onPositionUpdate`
-2. **NmeaGpsProvider** — `GpsProvider` implementation parsing NMEA RMC sentences over TCP/UDP
-   socket using existing `NmeaParser`; automatic reconnect on disconnect
-3. **Fix build permissions** — `chown -R www-data:www-data /workspace/nav/android-app/app/build`
-   to enable full Gradle unit test runs
+1. **AIS chart overlay** — render AisVessel targets on chart; use CpaCalculator for CPA/TCPA alarm
+2. **AIS TCP ingestion** — extend NmeaStreamManager to feed !AIVDM sentences to AisVdmParser
+3. **UI instrument display** — SOG/COG readout widget in `MainActivity`; bind to `GpsProvider`
 
 ## Scripts Added
 - `test-runner/` — standalone Kotlin/JVM Gradle project; runs all 22 GPS/NMEA tests without Android SDK
@@ -81,4 +103,4 @@ GPS navigation implementation: position model, SOG/COG, NMEA RMC parser — COMP
 
 ## Process Improvements
 - Gradle builds blocked by Android SDK requirement; added `test-runner/` JVM-only subproject as reliable test runner
-- All 22 tests verified GREEN via `test-runner/` JVM project (2026-03-14)
+- All 22 tests verified GREEN via `test-runner/` JVM project (2026-03-14)  
\ No newline at end of file
diff --git a/android-app/app/src/main/kotlin/org/terst/nav/ais/AisVessel.kt b/android-app/app/src/main/kotlin/org/terst/nav/ais/AisVessel.kt
new file mode 100644
index 0000000..34a951c
--- /dev/null
+++ b/android-app/app/src/main/kotlin/org/terst/nav/ais/AisVessel.kt
@@ -0,0 +1,14 @@
+package org.terst.nav.ais
+
+data class AisVessel(
+    val mmsi: Int,
+    val name: String,
+    val callsign: String,
+    val lat: Double,
+    val lon: Double,
+    val sog: Double,
+    val cog: Double,
+    val heading: Int,
+    val vesselType: Int,
+    val timestampMs: Long
+)  
\ No newline at end of file
diff --git a/android-app/app/src/main/kotlin/org/terst/nav/ais/CpaCalculator.kt b/android-app/app/src/main/kotlin/org/terst/nav/ais/CpaCalculator.kt
new file mode 100644
index 0000000..298af59
--- /dev/null
+++ b/android-app/app/src/main/kotlin/org/terst/nav/ais/CpaCalculator.kt
@@ -0,0 +1,45 @@
+package org.terst.nav.ais
+
+import kotlin.math.cos
+import kotlin.math.sqrt
+
+object CpaCalculator {
+    fun compute(
+        ownLat: Double, ownLon: Double, ownSog: Double, ownCog: Double,
+        tgtLat: Double, tgtLon: Double, tgtSog: Double, tgtCog: Double
+    ): Pair<Double, Double> {
+        val refLat = Math.toRadians((ownLat + tgtLat) / 2.0)
+
+        // Flat-earth positions in nautical miles
+        val ownX = ownLon * 60.0 * cos(refLat)
+        val ownY = ownLat * 60.0
+        val tgtX = tgtLon * 60.0 * cos(refLat)
+        val tgtY = tgtLat * 60.0
+
+        // Velocities in nm/min
+        val ownCogRad = Math.toRadians(ownCog)
+        val tgtCogRad = Math.toRadians(tgtCog)
+        val ownVx = ownSog * Math.sin(ownCogRad) / 60.0
+        val ownVy = ownSog * Math.cos(ownCogRad) / 60.0
+        val tgtVx = tgtSog * Math.sin(tgtCogRad) / 60.0
+        val tgtVy = tgtSog * Math.cos(tgtCogRad) / 60.0
+
+        val dx = tgtX - ownX
+        val dy = tgtY - ownY
+        val dvx = tgtVx - ownVx
+        val dvy = tgtVy - ownVy
+
+        val dv2 = dvx * dvx + dvy * dvy
+        if (dv2 < 1e-9) {
+            val currentDist = sqrt(dx * dx + dy * dy)
+            return Pair(currentDist, 0.0)
+        }
+
+        val tcpa = -(dx * dvx + dy * dvy) / dv2
+        val cpaX = dx + dvx * tcpa
+        val cpaY = dy + dvy * tcpa
+        val cpa = sqrt(cpaX * cpaX + cpaY * cpaY)
+
+        return Pair(cpa, tcpa)
+    }
+}
diff --git a/android-app/app/src/main/kotlin/org/terst/nav/nmea/AisVdmParser.kt b/android-app/app/src/main/kotlin/org/terst/nav/nmea/AisVdmParser.kt
new file mode 100644
index 0000000..9709d21
--- /dev/null
+++ b/android-app/app/src/main/kotlin/org/terst/nav/nmea/AisVdmParser.kt
@@ -0,0 +1,129 @@
+package org.terst.nav.nmea
+
+import org.terst.nav.ais.AisVessel
+
+class AisVdmParser {
+    // Keyed by seqId -> list of (seq, payload)
+    private val fragments = mutableMapOf<String, MutableList<Pair<Int, String>>>()
+
+    fun parse(sentence: String): AisVessel? {
+        if (!sentence.startsWith("!AIVDM") && !sentence.startsWith("!AIVDO")) return null
+
+        val withoutBang = sentence.drop(1)
+        val starIdx = withoutBang.indexOf('*')
+        val body = if (starIdx >= 0) withoutBang.substring(0, starIdx) else withoutBang
+
+        val fields = body.split(",")
+        if (fields.size < 7) return null
+
+        val count = fields[1].toIntOrNull() ?: return null
+        val seq = fields[2].toIntOrNull() ?: return null
+        val seqId = fields[3]
+        val payload = fields[5]
+        val padding = fields[6].toIntOrNull() ?: 0
+
+        val combinedPayload: String
+        if (count <= 1) {
+            combinedPayload = payload
+        } else {
+            val list = fragments.getOrPut(seqId) { mutableListOf() }
+            list.add(seq to payload)
+            if (list.size < count) return null
+            fragments.remove(seqId)
+            combinedPayload = list.sortedBy { it.first }.joinToString("") { it.second }
+        }
+
+        val bits = decodeToBits(combinedPayload, padding)
+        if (bits.size < 6) return null
+
+        return when (extractUInt(bits, 0, 6)) {
+            1, 2, 3 -> parseType123(bits)
+            5 -> parseType5(bits)
+            else -> null
+        }
+    }
+
+    private fun decodeToBits(payload: String, padding: Int): IntArray {
+        val allBits = ArrayList<Int>(payload.length * 6)
+        for (c in payload) {
+            var v = c.code - 48
+            if (v > 39) v -= 8
+            for (b in 5 downTo 0) allBits.add((v ushr b) and 1)
+        }
+        return if (padding > 0 && padding < allBits.size)
+            allBits.dropLast(padding).toIntArray()
+        else
+            allBits.toIntArray()
+    }
+
+    private fun extractUInt(bits: IntArray, start: Int, len: Int): Int {
+        var v = 0
+        for (i in 0 until len) {
+            val bit = if (start + i < bits.size) bits[start + i] else 0
+            v = (v shl 1) or bit
+        }
+        return v
+    }
+
+    private fun extractInt(bits: IntArray, start: Int, len: Int): Int {
+        val unsigned = extractUInt(bits, start, len)
+        return if (len > 0 && (unsigned and (1 shl (len - 1))) != 0)
+            unsigned - (1 shl len)
+        else
+            unsigned
+    }
+
+    private fun parseType123(bits: IntArray): AisVessel? {
+        if (bits.size < 137) return null
+        val mmsi = extractUInt(bits, 8, 30)
+        val sogRaw = extractUInt(bits, 50, 10)
+        val sog = if (sogRaw == 1023) 0.0 else sogRaw / 10.0
+        val lonRaw = extractInt(bits, 61, 28)
+        val lon = if (lonRaw == 0x6791AC0) 0.0 else lonRaw / 600000.0
+        val latRaw = extractInt(bits, 89, 27)
+        val lat = if (latRaw == 0x3412140) 0.0 else latRaw / 600000.0
+        val cogRaw = extractUInt(bits, 116, 12)
+        val cog = if (cogRaw == 3600) 0.0 else cogRaw / 10.0
+        val heading = extractUInt(bits, 128, 9)
+        return AisVessel(
+            mmsi = mmsi,
+            name = "",
+            callsign = "",
+            lat = lat,
+            lon = lon,
+            sog = sog,
+            cog = cog,
+            heading = heading,
+            vesselType = 0,
+            timestampMs = System.currentTimeMillis()
+        )
+    }
+
+    private fun parseType5(bits: IntArray): AisVessel? {
+        if (bits.size < 240) return null
+        val mmsi = extractUInt(bits, 8, 30)
+        val callsign = decodeText(bits, 70, 7).trimEnd('@', ' ')
+        val name = decodeText(bits, 112, 20).trimEnd('@', ' ')
+        val vesselType = extractUInt(bits, 232, 8)
+        return AisVessel(
+            mmsi = mmsi,
+            name = name,
+            callsign = callsign,
+            lat = 0.0, lon = 0.0, sog = 0.0, cog = 0.0,
+            heading = 511,
+            vesselType = vesselType,
+            timestampMs = System.currentTimeMillis()
+        )
+    }
+
+    // AIS 6-bit text: bits<32 -> bits+64 (maps to @,A-Z,...), else bits (maps to space,digits,symbols)
+    private fun decodeText(bits: IntArray, start: Int, nChars: Int): String {
+        val sb = StringBuilder(nChars)
+        for (i in 0 until nChars) {
+            val v = extractUInt(bits, start + i * 6, 6)
+            val c = if (v < 32) v + 64 else v
+            sb.append(c.toChar())
+        }
+        return sb.toString()
+    }
+} 
\ No newline at end of file
diff --git a/android-app/app/src/test/kotlin/org/terst/nav/ais/AisVesselTest.kt b/android-app/app/src/test/kotlin/org/terst/nav/ais/AisVesselTest.kt
new file mode 100644
index 0000000..a34a733
--- /dev/null
+++ b/android-app/app/src/test/kotlin/org/terst/nav/ais/AisVesselTest.kt
@@ -0,0 +1,53 @@
+package org.terst.nav.ais
+
+import org.junit.jupiter.api.Assertions.*
+import org.junit.jupiter.api.Test
+
+class AisVesselTest {
+
+    @Test
+    fun `holds all fields correctly`() {
+        val vessel = AisVessel(
+            mmsi = 123456789,
+            name = "MY VESSEL",
+            callsign = "W1ABC",
+            lat = 37.5,
+            lon = -122.0,
+            sog = 5.5,
+            cog = 270.0,
+            heading = 269,
+            vesselType = 36,
+            timestampMs = 1000L
+        )
+        assertEquals(123456789, vessel.mmsi)
+        assertEquals("MY VESSEL", vessel.name)
+        assertEquals("W1ABC", vessel.callsign)
+        assertEquals(37.5, vessel.lat)
+        assertEquals(-122.0, vessel.lon)
+        assertEquals(5.5, vessel.sog)
+        assertEquals(270.0, vessel.cog)
+        assertEquals(269, vessel.heading)
+        assertEquals(36, vessel.vesselType)
+        assertEquals(1000L, vessel.timestampMs)
+    }
+
+    @Test
+    fun `equality based on all fields`() {
+        val v1 = AisVessel(1, "A", "B", 0.0, 0.0, 0.0, 0.0, 0, 0, 100L)
+        val v2 = AisVessel(1, "A", "B", 0.0, 0.0, 0.0, 0.0, 0, 0, 100L)
+        assertEquals(v1, v2)
+    }
+
+    @Test
+    fun `inequality when mmsi differs`() {
+        val v1 = AisVessel(1, "A", "B", 0.0, 0.0, 0.0, 0.0, 0, 0, 100L)
+        val v2 = AisVessel(2, "A", "B", 0.0, 0.0, 0.0, 0.0, 0, 0, 100L)
+        assertNotEquals(v1, v2)
+    }
+
+    @Test
+    fun `heading 511 means not available`() {
+        val vessel = AisVessel(1, "", "", 0.0, 0.0, 0.0, 0.0, 511, 0, 0L)
+        assertEquals(511, vessel.heading)
+    }
+} 
\ No newline at end of file
diff --git a/android-app/app/src/test/kotlin/org/terst/nav/ais/CpaCalculatorTest.kt b/android-app/app/src/test/kotlin/org/terst/nav/ais/CpaCalculatorTest.kt
new file mode 100644
index 0000000..38069ec
--- /dev/null
+++ b/android-app/app/src/test/kotlin/org/terst/nav/ais/CpaCalculatorTest.kt
@@ -0,0 +1,61 @@
+package org.terst.nav.ais
+
+import org.junit.jupiter.api.Assertions.*
+import org.junit.jupiter.api.Test
+import kotlin.math.abs
+
+class CpaCalculatorTest {
+
+    private val eps = 0.01  // 0.01 nm tolerance for position, 0.01 min for time
+
+    @Test
+    fun `head-on vessels converging - TCPA around 0_25 min CPA near zero`() {
+        // Own: (0,0) moving North at 10kt
+        // Target: (0, 0.0833 nm N ~= 0.0833/60 deg lat) moving South at 10kt
+        // They meet in the middle ~= 0.25 min
+        val tgtLat = 0.0833 / 60.0  // 0.0833 nm north in degrees
+        val (cpa, tcpa) = CpaCalculator.compute(
+            ownLat = 0.0, ownLon = 0.0, ownSog = 10.0, ownCog = 0.0,
+            tgtLat = tgtLat, tgtLon = 0.0, tgtSog = 10.0, tgtCog = 180.0
+        )
+        assertTrue(tcpa > 0.0, "TCPA should be positive (converging): $tcpa")
+        assertTrue(abs(tcpa - 0.25) < eps, "TCPA should be ~0.25 min, got $tcpa")
+        assertTrue(cpa < 0.01, "CPA should be near zero for head-on, got $cpa")
+    }
+
+    @Test
+    fun `diverging vessels - same direction target ahead - TCPA negative or CPA large`() {
+        // Own: (0,0) moving North at 5kt
+        // Target: (0, 1nm N) moving North at 10kt (pulling away)
+        val tgtLat = 1.0 / 60.0  // 1 nm north
+        val (cpa, tcpa) = CpaCalculator.compute(
+            ownLat = 0.0, ownLon = 0.0, ownSog = 5.0, ownCog = 0.0,
+            tgtLat = tgtLat, tgtLon = 0.0, tgtSog = 10.0, tgtCog = 0.0
+        )
+        // Target is faster and ahead — diverging, TCPA should be negative
+        assertTrue(tcpa < 0.0, "TCPA should be negative (diverging), got $tcpa")
+    }
+
+    @Test
+    fun `zero relative velocity returns current distance and TCPA zero`() {
+        // Same speed, same course — relative velocity zero
+        val tgtLat = 1.0 / 60.0  // 1 nm north
+        val (cpa, tcpa) = CpaCalculator.compute(
+            ownLat = 0.0, ownLon = 0.0, ownSog = 5.0, ownCog = 90.0,
+            tgtLat = tgtLat, tgtLon = 0.0, tgtSog = 5.0, tgtCog = 90.0
+        )
+        assertEquals(0.0, tcpa, 1e-9)
+        // Current distance should be ~1 nm
+        assertTrue(abs(cpa - 1.0) < 0.01, "CPA should be ~1 nm (current dist), got $cpa")
+    }
+
+    @Test
+    fun `both at same position zero relative velocity - zero distance`() {
+        val (cpa, tcpa) = CpaCalculator.compute(
+            ownLat = 37.0, ownLon = -122.0, ownSog = 5.0, ownCog = 0.0,
+            tgtLat = 37.0, tgtLon = -122.0, tgtSog = 5.0, tgtCog = 0.0
+        )
+        assertEquals(0.0, tcpa, 1e-9)
+        assertEquals(0.0, cpa, 1e-6)
+    }
+} 
\ No newline at end of file
diff --git a/android-app/app/src/test/kotlin/org/terst/nav/nmea/AisVdmParserTest.kt b/android-app/app/src/test/kotlin/org/terst/nav/nmea/AisVdmParserTest.kt
new file mode 100644
index 0000000..da3efd3
--- /dev/null
+++ b/android-app/app/src/test/kotlin/org/terst/nav/nmea/AisVdmParserTest.kt
@@ -0,0 +1,205 @@
+package org.terst.nav.nmea
+
+import org.junit.jupiter.api.Assertions.*
+import org.junit.jupiter.api.Test
+import kotlin.math.abs
+
+/**
+ * Encoding helpers to build synthetic AIS payloads for round-trip testing.
+ * Mirrors the decoder logic exactly.
+ */
+private fun encodeBits(bits: IntArray): String {
+    // Pad to multiple of 6
+    val padded = if (bits.size % 6 == 0) bits else bits + IntArray(6 - bits.size % 6)
+    val sb = StringBuilder()
+    var i = 0
+    while (i < padded.size) {
+        var v = 0
+        for (b in 0 until 6) v = (v shl 1) or padded[i + b]
+        val c = if (v <= 39) v + 48 else v + 56
+        sb.append(c.toChar())
+        i += 6
+    }
+    return sb.toString()
+}
+
+private fun intToBits(value: Int, len: Int): IntArray {
+    val arr = IntArray(len)
+    for (i in 0 until len) arr[i] = (value ushr (len - 1 - i)) and 1
+    return arr
+}
+
+private fun signedToBits(value: Int, len: Int): IntArray {
+    // Two's complement
+    val mask = if (len < 32) (1 shl len) - 1 else -1
+    return intToBits(value and mask, len)
+}
+
+/** Build a type-1 message bit array (168 bits) */
+private fun buildType1Payload(
+    mmsi: Int, sog10: Int, lon600000: Int, lat600000: Int, cog10: Int, heading: Int
+): String {
+    val bits = IntArray(168)
+    fun set(start: Int, arr: IntArray) { arr.copyInto(bits, start) }
+    set(0, intToBits(1, 6))        // message type = 1
+    set(6, intToBits(0, 2))        // repeat
+    set(8, intToBits(mmsi, 30))    // MMSI
+    set(38, intToBits(0, 12))      // nav status + rot (12 bits filler)
+    set(50, intToBits(sog10, 10))  // SOG * 10
+    set(60, intToBits(0, 1))       // accuracy
+    set(61, signedToBits(lon600000, 28))  // lon * 600000
+    set(89, signedToBits(lat600000, 27))  // lat * 600000
+    set(116, intToBits(cog10, 12)) // COG * 10
+    set(128, intToBits(heading, 9)) // heading
+    // remaining bits 137-167 = 0 (timestamp, maneuver, spare, raim, radio)
+    return encodeBits(bits)
+}
+
+/** Build a type-5 message bit array (426 bits) */
+private fun buildType5Payload(mmsi: Int, callsign: String, name: String, vesselType: Int): String {
+    val bits = IntArray(426)
+    fun set(start: Int, arr: IntArray) { arr.copyInto(bits, start) }
+    set(0, intToBits(5, 6))        // message type = 5
+    set(6, intToBits(0, 2))        // repeat
+    set(8, intToBits(mmsi, 30))    // MMSI
+    set(38, intToBits(0, 2))       // ais version
+    set(40, intToBits(0, 30))      // IMO number
+    // AIS 6-bit text encode: chars >=64 (A-Z,@) -> c-64 (gives 0-31); else c (space=32, digits=48-57)
+    val csFixed = callsign.padEnd(7, '@').take(7)
+    for (i in 0 until 7) {
+        val c = csFixed[i].code
+        val v = if (c >= 64) c - 64 else c
+        set(70 + i * 6, intToBits(v, 6))
+    }
+    // name: bits 112-231 = 20 chars x 6 bits
+    val nameFixed = name.padEnd(20, '@').take(20)
+    for (i in 0 until 20) {
+        val c = nameFixed[i].code
+        val v = if (c >= 64) c - 64 else c
+        set(112 + i * 6, intToBits(v, 6))
+    }
+    // vessel type: bits 232-239
+    set(232, intToBits(vesselType, 8))
+    // rest = 0 (draught, destination, dte, spare)
+    return encodeBits(bits)
+}
+
+private fun nmea0183Checksum(body: String): String {
+    var xor = 0
+    for (c in body) xor = xor xor c.code
+    return xor.toString(16).uppercase().padStart(2, '0')
+}
+
+private fun makeVdm(payload: String, padding: Int = 0): String {
+    val body = "AIVDM,1,1,,A,$payload,$padding"
+    return "!$body*${nmea0183Checksum(body)}"
+}
+
+private fun makeVdmPart(count: Int, seq: Int, seqId: String, payload: String, padding: Int = 0): String {
+    val body = "AIVDM,$count,$seq,$seqId,A,$payload,$padding"
+    return "!$body*${nmea0183Checksum(body)}"
+}
+
+class AisVdmParserTest {
+
+    private val parser = AisVdmParser()
+
+    @Test
+    fun `non-AIS NMEA sentence returns null`() {
+        assertNull(parser.parse("\$GPRMC,123519,A,4807.038,N,01131.000,E,022.4,084.4,230394,003.1,W*6A"))
+    }
+
+    @Test
+    fun `malformed sentence returns null`() {
+        assertNull(parser.parse("garbage"))
+        assertNull(parser.parse("!AIVDM,1,1,,A,"))
+        assertNull(parser.parse(""))
+    }
+
+    @Test
+    fun `type-1 round-trip - known MMSI lat lon sog cog heading`() {
+        // MMSI=123456789, sog=5.0kt (50), lon=-122.0deg, lat=37.0deg, cog=270.0 (2700), heading=270
+        val mmsi = 123456789
+        val sog10 = 50        // 5.0 knots
+        val lon600000 = (-122.0 * 600000.0).toInt()   // -73200000
+        val lat600000 = (37.0 * 600000.0).toInt()     //  22200000
+        val cog10 = 2700      // 270.0 degrees
+        val heading = 270
+
+        val payload = buildType1Payload(mmsi, sog10, lon600000, lat600000, cog10, heading)
+        val sentence = makeVdm(payload)
+        val vessel = parser.parse(sentence)
+
+        assertNotNull(vessel)
+        assertEquals(mmsi, vessel!!.mmsi)
+        assertEquals(5.0, vessel.sog, 0.01)
+        assertEquals(-122.0, vessel.lon, 0.001)
+        assertEquals(37.0, vessel.lat, 0.001)
+        assertEquals(270.0, vessel.cog, 0.1)
+        assertEquals(270, vessel.heading)
+        assertEquals("", vessel.name)
+        assertEquals("", vessel.callsign)
+        assertEquals(0, vessel.vesselType)
+    }
+
+    @Test
+    fun `type-1 real sentence - MMSI 227006760`() {
+        val sentence = "!AIVDM,1,1,,A,13HOI:0P0000vocH;`5HF>0<0000,0*54"
+        val vessel = parser.parse(sentence)
+        assertNotNull(vessel)
+        assertEquals(227006760, vessel!!.mmsi)
+    }
+
+    @Test
+    fun `type-5 round-trip - name and callsign decoded and trimmed`() {
+        val mmsi = 987654321
+        val payload = buildType5Payload(mmsi, "W1ABC", "MY VESSEL NAME", vesselType = 36)
+        val sentence = makeVdm(payload)
+        val vessel = parser.parse(sentence)
+
+        assertNotNull(vessel)
+        assertEquals(mmsi, vessel!!.mmsi)
+        assertEquals("W1ABC", vessel.callsign)
+        assertEquals("MY VESSEL NAME", vessel.name)
+        assertEquals(36, vessel.vesselType)
+        assertEquals(0.0, vessel.lat)
+        assertEquals(0.0, vessel.lon)
+        assertEquals(511, vessel.heading)
+    }
+
+    @Test
+    fun `two-part message reassembly`() {
+        // Build a type-1 payload, split it in half across two sentences
+        val mmsi = 111222333
+        val payload = buildType1Payload(mmsi, 100, 0, 0, 900, 511)
+        val half = payload.length / 2
+        val part1 = payload.substring(0, half)
+        val part2 = payload.substring(half)
+
+        val s1 = makeVdmPart(2, 1, "1", part1, 0)
+        val s2 = makeVdmPart(2, 2, "1", part2, 0)
+
+        // First sentence: incomplete, returns null
+        assertNull(parser.parse(s1))
+        // Second sentence: completes reassembly
+        val vessel = parser.parse(s2)
+        assertNotNull(vessel)
+        assertEquals(mmsi, vessel!!.mmsi)
+    }
+
+    @Test
+    fun `SOG not available value 1023 decodes to 0_0`() {
+        val payload = buildType1Payload(111000111, 1023, 0, 0, 0, 511)
+        val vessel = parser.parse(makeVdm(payload))
+        assertNotNull(vessel)
+        assertEquals(0.0, vessel!!.sog)
+    }
+
+    @Test
+    fun `COG not available value 3600 decodes to 0_0`() {
+        val payload = buildType1Payload(111000222, 0, 0, 0, 3600, 511)
+        val vessel = parser.parse(makeVdm(payload))
+        assertNotNull(vessel)
+        assertEquals(0.0, vessel!!.cog)
+    }
+} 
\ No newline at end of file
diff --git a/test-runner/src/main/kotlin/org/terst/nav/ais/AisVessel.kt b/test-runner/src/main/kotlin/org/terst/nav/ais/AisVessel.kt
new file mode 100644
index 0000000..34a951c
--- /dev/null
+++ b/test-runner/src/main/kotlin/org/terst/nav/ais/AisVessel.kt
@@ -0,0 +1,14 @@
+package org.terst.nav.ais
+
+data class AisVessel(
+    val mmsi: Int,
+    val name: String,
+    val callsign: String,
+    val lat: Double,
+    val lon: Double,
+    val sog: Double,
+    val cog: Double,
+    val heading: Int,
+    val vesselType: Int,
+    val timestampMs: Long
+)  
\ No newline at end of file
diff --git a/test-runner/src/main/kotlin/org/terst/nav/ais/CpaCalculator.kt b/test-runner/src/main/kotlin/org/terst/nav/ais/CpaCalculator.kt
new file mode 100644
index 0000000..7e98451
--- /dev/null
+++ b/test-runner/src/main/kotlin/org/terst/nav/ais/CpaCalculator.kt
@@ -0,0 +1,46 @@
+package org.terst.nav.ais
+
+import kotlin.math.cos
+import kotlin.math.sqrt
+import kotlin.math.PI
+
+object CpaCalculator {
+    fun compute(
+        ownLat: Double, ownLon: Double, ownSog: Double, ownCog: Double,
+        tgtLat: Double, tgtLon: Double, tgtSog: Double, tgtCog: Double
+    ): Pair<Double, Double> {
+        val refLat = Math.toRadians((ownLat + tgtLat) / 2.0)
+
+        // Flat-earth positions in nautical miles
+        val ownX = ownLon * 60.0 * cos(refLat)
+        val ownY = ownLat * 60.0
+        val tgtX = tgtLon * 60.0 * cos(refLat)
+        val tgtY = tgtLat * 60.0
+
+        // Velocities in nm/min
+        val ownCogRad = Math.toRadians(ownCog)
+        val tgtCogRad = Math.toRadians(tgtCog)
+        val ownVx = ownSog * Math.sin(ownCogRad) / 60.0
+        val ownVy = ownSog * Math.cos(ownCogRad) / 60.0
+        val tgtVx = tgtSog * Math.sin(tgtCogRad) / 60.0
+        val tgtVy = tgtSog * Math.cos(tgtCogRad) / 60.0
+
+        val dx = tgtX - ownX
+        val dy = tgtY - ownY
+        val dvx = tgtVx - ownVx
+        val dvy = tgtVy - ownVy
+
+        val dv2 = dvx * dvx + dvy * dvy
+        if (dv2 < 1e-9) {
+            val currentDist = sqrt(dx * dx + dy * dy)
+            return Pair(currentDist, 0.0)
+        }
+
+        val tcpa = -(dx * dvx + dy * dvy) / dv2
+        val cpaX = dx + dvx * tcpa
+        val cpaY = dy + dvy * tcpa
+        val cpa = sqrt(cpaX * cpaX + cpaY * cpaY)
+
+        return Pair(cpa, tcpa)
+    }
+}
diff --git a/test-runner/src/main/kotlin/org/terst/nav/nmea/AisVdmParser.kt b/test-runner/src/main/kotlin/org/terst/nav/nmea/AisVdmParser.kt
new file mode 100644
index 0000000..9709d21
--- /dev/null
+++ b/test-runner/src/main/kotlin/org/terst/nav/nmea/AisVdmParser.kt
@@ -0,0 +1,129 @@
+package org.terst.nav.nmea
+
+import org.terst.nav.ais.AisVessel
+
+class AisVdmParser {
+    // Keyed by seqId -> list of (seq, payload)
+    private val fragments = mutableMapOf<String, MutableList<Pair<Int, String>>>()
+
+    fun parse(sentence: String): AisVessel? {
+        if (!sentence.startsWith("!AIVDM") && !sentence.startsWith("!AIVDO")) return null
+
+        val withoutBang = sentence.drop(1)
+        val starIdx = withoutBang.indexOf('*')
+        val body = if (starIdx >= 0) withoutBang.substring(0, starIdx) else withoutBang
+
+        val fields = body.split(",")
+        if (fields.size < 7) return null
+
+        val count = fields[1].toIntOrNull() ?: return null
+        val seq = fields[2].toIntOrNull() ?: return null
+        val seqId = fields[3]
+        val payload = fields[5]
+        val padding = fields[6].toIntOrNull() ?: 0
+
+        val combinedPayload: String
+        if (count <= 1) {
+            combinedPayload = payload
+        } else {
+            val list = fragments.getOrPut(seqId) { mutableListOf() }
+            list.add(seq to payload)
+            if (list.size < count) return null
+            fragments.remove(seqId)
+            combinedPayload = list.sortedBy { it.first }.joinToString("") { it.second }
+        }
+
+        val bits = decodeToBits(combinedPayload, padding)
+        if (bits.size < 6) return null
+
+        return when (extractUInt(bits, 0, 6)) {
+            1, 2, 3 -> parseType123(bits)
+            5 -> parseType5(bits)
+            else -> null
+        }
+    }
+
+    private fun decodeToBits(payload: String, padding: Int): IntArray {
+        val allBits = ArrayList<Int>(payload.length * 6)
+        for (c in payload) {
+            var v = c.code - 48
+            if (v > 39) v -= 8
+            for (b in 5 downTo 0) allBits.add((v ushr b) and 1)
+        }
+        return if (padding > 0 && padding < allBits.size)
+            allBits.dropLast(padding).toIntArray()
+        else
+            allBits.toIntArray()
+    }
+
+    private fun extractUInt(bits: IntArray, start: Int, len: Int): Int {
+        var v = 0
+        for (i in 0 until len) {
+            val bit = if (start + i < bits.size) bits[start + i] else 0
+            v = (v shl 1) or bit
+        }
+        return v
+    }
+
+    private fun extractInt(bits: IntArray, start: Int, len: Int): Int {
+        val unsigned = extractUInt(bits, start, len)
+        return if (len > 0 && (unsigned and (1 shl (len - 1))) != 0)
+            unsigned - (1 shl len)
+        else
+            unsigned
+    }
+
+    private fun parseType123(bits: IntArray): AisVessel? {
+        if (bits.size < 137) return null
+        val mmsi = extractUInt(bits, 8, 30)
+        val sogRaw = extractUInt(bits, 50, 10)
+        val sog = if (sogRaw == 1023) 0.0 else sogRaw / 10.0
+        val lonRaw = extractInt(bits, 61, 28)
+        val lon = if (lonRaw == 0x6791AC0) 0.0 else lonRaw / 600000.0
+        val latRaw = extractInt(bits, 89, 27)
+        val lat = if (latRaw == 0x3412140) 0.0 else latRaw / 600000.0
+        val cogRaw = extractUInt(bits, 116, 12)
+        val cog = if (cogRaw == 3600) 0.0 else cogRaw / 10.0
+        val heading = extractUInt(bits, 128, 9)
+        return AisVessel(
+            mmsi = mmsi,
+            name = "",
+            callsign = "",
+            lat = lat,
+            lon = lon,
+            sog = sog,
+            cog = cog,
+            heading = heading,
+            vesselType = 0,
+            timestampMs = System.currentTimeMillis()
+        )
+    }
+
+    private fun parseType5(bits: IntArray): AisVessel? {
+        if (bits.size < 240) return null
+        val mmsi = extractUInt(bits, 8, 30)
+        val callsign = decodeText(bits, 70, 7).trimEnd('@', ' ')
+        val name = decodeText(bits, 112, 20).trimEnd('@', ' ')
+        val vesselType = extractUInt(bits, 232, 8)
+        return AisVessel(
+            mmsi = mmsi,
+            name = name,
+            callsign = callsign,
+            lat = 0.0, lon = 0.0, sog = 0.0, cog = 0.0,
+            heading = 511,
+            vesselType = vesselType,
+            timestampMs = System.currentTimeMillis()
+        )
+    }
+
+    // AIS 6-bit text: bits<32 -> bits+64 (maps to @,A-Z,...), else bits (maps to space,digits,symbols)
+    private fun decodeText(bits: IntArray, start: Int, nChars: Int): String {
+        val sb = StringBuilder(nChars)
+        for (i in 0 until nChars) {
+            val v = extractUInt(bits, start + i * 6, 6)
+            val c = if (v < 32) v + 64 else v
+            sb.append(c.toChar())
+        }
+        return sb.toString()
+    }
+} 
\ No newline at end of file
diff --git a/test-runner/src/test/kotlin/org/terst/nav/ais/AisVesselTest.kt b/test-runner/src/test/kotlin/org/terst/nav/ais/AisVesselTest.kt
new file mode 100644
index 0000000..a583a32
--- /dev/null
+++ b/test-runner/src/test/kotlin/org/terst/nav/ais/AisVesselTest.kt
@@ -0,0 +1,53 @@
+package org.terst.nav.ais
+
+import org.junit.Assert.*
+import org.junit.Test
+
+class AisVesselTest {
+
+    @Test
+    fun `holds all fields correctly`() {
+        val vessel = AisVessel(
+            mmsi = 123456789,
+            name = "MY VESSEL",
+            callsign = "W1ABC",
+            lat = 37.5,
+            lon = -122.0,
+            sog = 5.5,
+            cog = 270.0,
+            heading = 269,
+            vesselType = 36,
+            timestampMs = 1000L
+        )
+        assertEquals(123456789, vessel.mmsi)
+        assertEquals("MY VESSEL", vessel.name)
+        assertEquals("W1ABC", vessel.callsign)
+        assertEquals(37.5, vessel.lat, 0.0)
+        assertEquals(-122.0, vessel.lon, 0.0)
+        assertEquals(5.5, vessel.sog, 0.0)
+        assertEquals(270.0, vessel.cog, 0.0)
+        assertEquals(269, vessel.heading)
+        assertEquals(36, vessel.vesselType)
+        assertEquals(1000L, vessel.timestampMs)
+    }
+
+    @Test
+    fun `equality based on all fields`() {
+        val v1 = AisVessel(1, "A", "B", 0.0, 0.0, 0.0, 0.0, 0, 0, 100L)
+        val v2 = AisVessel(1, "A", "B", 0.0, 0.0, 0.0, 0.0, 0, 0, 100L)
+        assertEquals(v1, v2)
+    }
+
+    @Test
+    fun `inequality when mmsi differs`() {
+        val v1 = AisVessel(1, "A", "B", 0.0, 0.0, 0.0, 0.0, 0, 0, 100L)
+        val v2 = AisVessel(2, "A", "B", 0.0, 0.0, 0.0, 0.0, 0, 0, 100L)
+        assertNotEquals(v1, v2)
+    }
+
+    @Test
+    fun `heading 511 means not available`() {
+        val vessel = AisVessel(1, "", "", 0.0, 0.0, 0.0, 0.0, 511, 0, 0L)
+        assertEquals(511, vessel.heading)
+    }
+} 
\ No newline at end of file
diff --git a/test-runner/src/test/kotlin/org/terst/nav/ais/CpaCalculatorTest.kt b/test-runner/src/test/kotlin/org/terst/nav/ais/CpaCalculatorTest.kt
new file mode 100644
index 0000000..9baddb2
--- /dev/null
+++ b/test-runner/src/test/kotlin/org/terst/nav/ais/CpaCalculatorTest.kt
@@ -0,0 +1,53 @@
+package org.terst.nav.ais
+
+import org.junit.Assert.*
+import org.junit.Test
+import kotlin.math.abs
+
+class CpaCalculatorTest {
+
+    private val eps = 0.01
+
+    @Test
+    fun `head-on vessels converging - TCPA around 0_25 min CPA near zero`() {
+        val tgtLat = 0.0833 / 60.0
+        val (cpa, tcpa) = CpaCalculator.compute(
+            ownLat = 0.0, ownLon = 0.0, ownSog = 10.0, ownCog = 0.0,
+            tgtLat = tgtLat, tgtLon = 0.0, tgtSog = 10.0, tgtCog = 180.0
+        )
+        assertTrue("TCPA should be positive (converging): $tcpa", tcpa > 0.0)
+        assertTrue("TCPA should be ~0.25 min, got $tcpa", abs(tcpa - 0.25) < eps)
+        assertTrue("CPA should be near zero for head-on, got $cpa", cpa < 0.01)
+    }
+
+    @Test
+    fun `diverging vessels - same direction target ahead - TCPA negative`() {
+        val tgtLat = 1.0 / 60.0
+        val (_, tcpa) = CpaCalculator.compute(
+            ownLat = 0.0, ownLon = 0.0, ownSog = 5.0, ownCog = 0.0,
+            tgtLat = tgtLat, tgtLon = 0.0, tgtSog = 10.0, tgtCog = 0.0
+        )
+        assertTrue("TCPA should be negative (diverging), got $tcpa", tcpa < 0.0)
+    }
+
+    @Test
+    fun `zero relative velocity returns current distance and TCPA zero`() {
+        val tgtLat = 1.0 / 60.0
+        val (cpa, tcpa) = CpaCalculator.compute(
+            ownLat = 0.0, ownLon = 0.0, ownSog = 5.0, ownCog = 90.0,
+            tgtLat = tgtLat, tgtLon = 0.0, tgtSog = 5.0, tgtCog = 90.0
+        )
+        assertEquals(0.0, tcpa, 1e-9)
+        assertTrue("CPA should be ~1 nm (current dist), got $cpa", abs(cpa - 1.0) < 0.01)
+    }
+
+    @Test
+    fun `both at same position zero relative velocity - zero distance`() {
+        val (cpa, tcpa) = CpaCalculator.compute(
+            ownLat = 37.0, ownLon = -122.0, ownSog = 5.0, ownCog = 0.0,
+            tgtLat = 37.0, tgtLon = -122.0, tgtSog = 5.0, tgtCog = 0.0
+        )
+        assertEquals(0.0, tcpa, 1e-9)
+        assertEquals(0.0, cpa, 1e-6)
+    }
+}
diff --git a/test-runner/src/test/kotlin/org/terst/nav/nmea/AisVdmParserTest.kt b/test-runner/src/test/kotlin/org/terst/nav/nmea/AisVdmParserTest.kt
new file mode 100644
index 0000000..481c89e
--- /dev/null
+++ b/test-runner/src/test/kotlin/org/terst/nav/nmea/AisVdmParserTest.kt
@@ -0,0 +1,180 @@
+package org.terst.nav.nmea
+
+import org.junit.Assert.*
+import org.junit.Test
+import kotlin.math.abs
+
+private fun encodeBits(bits: IntArray): String {
+    val padded = if (bits.size % 6 == 0) bits else bits + IntArray(6 - bits.size % 6)
+    val sb = StringBuilder()
+    var i = 0
+    while (i < padded.size) {
+        var v = 0
+        for (b in 0 until 6) v = (v shl 1) or padded[i + b]
+        val c = if (v <= 39) v + 48 else v + 56
+        sb.append(c.toChar())
+        i += 6
+    }
+    return sb.toString()
+}
+
+private fun intToBits(value: Int, len: Int): IntArray {
+    val arr = IntArray(len)
+    for (i in 0 until len) arr[i] = (value ushr (len - 1 - i)) and 1
+    return arr
+}
+
+private fun signedToBits(value: Int, len: Int): IntArray {
+    val mask = if (len < 32) (1 shl len) - 1 else -1
+    return intToBits(value and mask, len)
+}
+
+private fun buildType1Payload(
+    mmsi: Int, sog10: Int, lon600000: Int, lat600000: Int, cog10: Int, heading: Int
+): String {
+    val bits = IntArray(168)
+    fun set(start: Int, arr: IntArray) { arr.copyInto(bits, start) }
+    set(0, intToBits(1, 6))
+    set(6, intToBits(0, 2))
+    set(8, intToBits(mmsi, 30))
+    set(38, intToBits(0, 12))
+    set(50, intToBits(sog10, 10))
+    set(60, intToBits(0, 1))
+    set(61, signedToBits(lon600000, 28))
+    set(89, signedToBits(lat600000, 27))
+    set(116, intToBits(cog10, 12))
+    set(128, intToBits(heading, 9))
+    return encodeBits(bits)
+}
+
+private fun buildType5Payload(mmsi: Int, callsign: String, name: String, vesselType: Int): String {
+    val bits = IntArray(426)
+    fun set(start: Int, arr: IntArray) { arr.copyInto(bits, start) }
+    set(0, intToBits(5, 6))
+    set(6, intToBits(0, 2))
+    set(8, intToBits(mmsi, 30))
+    set(38, intToBits(0, 2))
+    set(40, intToBits(0, 30))
+    val csFixed = callsign.padEnd(7, '@').take(7)
+    for (i in 0 until 7) {
+        val c = csFixed[i].code
+        val v = if (c >= 64) c - 64 else c
+        set(70 + i * 6, intToBits(v, 6))
+    }
+    val nameFixed = name.padEnd(20, '@').take(20)
+    for (i in 0 until 20) {
+        val c = nameFixed[i].code
+        val v = if (c >= 64) c - 64 else c
+        set(112 + i * 6, intToBits(v, 6))
+    }
+    set(232, intToBits(vesselType, 8))
+    return encodeBits(bits)
+}
+
+private fun nmea0183Checksum(body: String): String {
+    var xor = 0
+    for (c in body) xor = xor xor c.code
+    return xor.toString(16).uppercase().padStart(2, '0')
+}
+
+private fun makeVdm(payload: String, padding: Int = 0): String {
+    val body = "AIVDM,1,1,,A,$payload,$padding"
+    return "!$body*${nmea0183Checksum(body)}"
+}
+
+private fun makeVdmPart(count: Int, seq: Int, seqId: String, payload: String, padding: Int = 0): String {
+    val body = "AIVDM,$count,$seq,$seqId,A,$payload,$padding"
+    return "!$body*${nmea0183Checksum(body)}"
+}
+
+class AisVdmParserTest {
+
+    private val parser = AisVdmParser()
+
+    @Test
+    fun `non-AIS NMEA sentence returns null`() {
+        assertNull(parser.parse("\$GPRMC,123519,A,4807.038,N,01131.000,E,022.4,084.4,230394,003.1,W*6A"))
+    }
+
+    @Test
+    fun `malformed sentence returns null`() {
+        assertNull(parser.parse("garbage"))
+        assertNull(parser.parse("!AIVDM,1,1,,A,"))
+        assertNull(parser.parse(""))
+    }
+
+    @Test
+    fun `type-1 round-trip - known MMSI lat lon sog cog heading`() {
+        val mmsi = 123456789
+        val sog10 = 50
+        val lon600000 = (-122.0 * 600000.0).toInt()
+        val lat600000 = (37.0 * 600000.0).toInt()
+        val cog10 = 2700
+        val heading = 270
+
+        val payload = buildType1Payload(mmsi, sog10, lon600000, lat600000, cog10, heading)
+        val vessel = parser.parse(makeVdm(payload))
+
+        assertNotNull(vessel)
+        assertEquals(mmsi, vessel!!.mmsi)
+        assertEquals(5.0, vessel.sog, 0.01)
+        assertEquals(-122.0, vessel.lon, 0.001)
+        assertEquals(37.0, vessel.lat, 0.001)
+        assertEquals(270.0, vessel.cog, 0.1)
+        assertEquals(270, vessel.heading)
+        assertEquals("", vessel.name)
+        assertEquals("", vessel.callsign)
+        assertEquals(0, vessel.vesselType)
+    }
+
+    @Test
+    fun `type-1 real sentence - MMSI 227006760`() {
+        val vessel = parser.parse("!AIVDM,1,1,,A,13HOI:0P0000vocH;`5HF>0<0000,0*54")
+        assertNotNull(vessel)
+        assertEquals(227006760, vessel!!.mmsi)
+    }
+
+    @Test
+    fun `type-5 round-trip - name and callsign decoded and trimmed`() {
+        val mmsi = 987654321
+        val payload = buildType5Payload(mmsi, "W1ABC", "MY VESSEL NAME", vesselType = 36)
+        val vessel = parser.parse(makeVdm(payload))
+
+        assertNotNull(vessel)
+        assertEquals(mmsi, vessel!!.mmsi)
+        assertEquals("W1ABC", vessel.callsign)
+        assertEquals("MY VESSEL NAME", vessel.name)
+        assertEquals(36, vessel.vesselType)
+        assertEquals(0.0, vessel.lat, 0.0)
+        assertEquals(0.0, vessel.lon, 0.0)
+        assertEquals(511, vessel.heading)
+    }
+
+    @Test
+    fun `two-part message reassembly`() {
+        val mmsi = 111222333
+        val payload = buildType1Payload(mmsi, 100, 0, 0, 900, 511)
+        val half = payload.length / 2
+        val s1 = makeVdmPart(2, 1, "1", payload.substring(0, half), 0)
+        val s2 = makeVdmPart(2, 2, "1", payload.substring(half), 0)
+
+        assertNull(parser.parse(s1))
+        val vessel = parser.parse(s2)
+        assertNotNull(vessel)
+        assertEquals(mmsi, vessel!!.mmsi)
+    }
+
+    @Test
+    fun `SOG not available value 1023 decodes to 0_0`() {
+        val vessel = parser.parse(makeVdm(buildType1Payload(111000111, 1023, 0, 0, 0, 511)))
+        assertNotNull(vessel)
+        assertEquals(0.0, vessel!!.sog, 0.0)
+    }
+
+    @Test
+    fun `COG not available value 3600 decodes to 0_0`() {
+        val vessel = parser.parse(makeVdm(buildType1Payload(111000222, 0, 0, 0, 3600, 511)))
+        assertNotNull(vessel)
+        assertEquals(0.0, vessel!!.cog, 0.0)
+    }
+} 
\ No newline at end of file