From 2bcd27c1e0adcbed777e4ec0e748e8c9bebd9cd4 Mon Sep 17 00:00:00 2001
From: Rudy Celekli <rudy@gradiahq.com>
Date: Sat, 24 Oct 2026 00:23:44 -0400
Subject: [PATCH] fix(ais): keep a tracked vessel's motion when static data
 arrives

---
 server/providers/vessels/ais-store.js |  5 ++++
 src/data/aisStreamSentinels.test.mjs  | 36 +++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/server/providers/vessels/ais-store.js b/server/providers/vessels/ais-store.js
index 297081b38..a456e19f4 100644
--- a/server/providers/vessels/ais-store.js
+++ b/server/providers/vessels/ais-store.js
@@ -51,6 +51,11 @@ export function ingestAisStreamEnvelope(envelope) {
     };
     _aisStreamStatic.set(mmsi, staticData);
     mergeAisStaticIntoLiveVessel(mmsi, staticData);
+    // AISStream repeats the last known position in a static message's
+    // MetaData, but the message has no speed, course or heading and is not a
+    // new fix. Rebuilding a tracked vessel from it would blank its motion and
+    // restamp its position time, so only merge the static fields.
+    if (_aisStreamVessels.has(mmsi)) return true;
   }
 
   const lat = numberValue(
diff --git a/src/data/aisStreamSentinels.test.mjs b/src/data/aisStreamSentinels.test.mjs
index 6595b795e..49572c05d 100644
--- a/src/data/aisStreamSentinels.test.mjs
+++ b/src/data/aisStreamSentinels.test.mjs
@@ -63,3 +63,39 @@ test('the fastest encodable reading below the unavailable code is kept', () => {
   assert.equal(row.speed, 102.2);
   assert.equal(row.course, 359.9);
 });
+
+test('a static data report keeps the motion and fix time of a tracked vessel', () => {
+  ingestAisStreamEnvelope(
+    positionReport('244690000', { Sog: 12.4, Cog: 86.7, TrueHeading: 87 }),
+  );
+  // AISStream's MetaData carries the last known position on every message,
+  // static data included.
+  assert.equal(
+    ingestAisStreamEnvelope({
+      MessageType: 'ShipStaticData',
+      Message: {
+        ShipStaticData: {
+          UserID: 244690000,
+          Name: 'EXAMPLE VESSEL',
+          Type: 70,
+          Destination: 'ROTTERDAM',
+          ImoNumber: 9876543,
+        },
+      },
+      MetaData: {
+        MMSI: '244690000',
+        latitude: 53.40879,
+        longitude: 6.19741,
+        time_utc: '2026-09-17 20:05:00.000000000 +0000 UTC',
+      },
+    }),
+    true,
+  );
+  const row = rowFor('244690000');
+  assert.equal(row.speed, 12.4);
+  assert.equal(row.course, 86.7);
+  assert.equal(row.heading, 87);
+  assert.equal(row.last_position_UTC, '2026-09-17T20:00:00.000Z');
+  assert.equal(row.destination, 'ROTTERDAM');
+  assert.equal(row.imo, '9876543');
+});