diff --git a/pipeline_runner/main.py b/pipeline_runner/main.py
index a78905d..dafbed0 100644
--- a/pipeline_runner/main.py
+++ b/pipeline_runner/main.py
@@ -1,7 +1,9 @@
 import asyncio
+import csv
 import gzip
 import json
 import time
+from datetime import datetime, timezone
 from pathlib import Path
 from typing import AsyncGenerator
 
@@ -138,3 +140,76 @@ async def get_tracks(sortie_id: str):
         raise HTTPException(404, "tracks.geojson not found")
     with open(p) as f:
         return JSONResponse(json.load(f))
+
+
+def _ts_nav(ts_str: str) -> float:
+    for fmt in ("%Y-%m-%d %H:%M:%S.%f", "%Y-%m-%d %H:%M:%S"):
+        try:
+            return datetime.strptime(ts_str.strip(), fmt).replace(tzinfo=timezone.utc).timestamp()
+        except ValueError:
+            continue
+    return 0.0
+
+
+def _read_usv_track(nav_log_path: Path, max_pts: int = 2000) -> list[dict]:
+    """Read navigation_log.csv → [{t_ms, lat, lon, heading, source}] downsampled."""
+    pts: list[dict] = []
+    lat_map: dict[float, float] = {}
+    lon_map: dict[float, float] = {}
+    heading_map: dict[float, float] = {}
+    with open(nav_log_path, newline="", encoding="utf-8") as f:
+        reader = csv.reader(f)
+        next(reader, None)  # skip header
+        for row in reader:
+            if len(row) < 3:
+                continue
+            ts_str, field, val = row[0], row[1], row[2]
+            if field not in ("Lat", "Lon", "Heading"):
+                continue
+            t = _ts_nav(ts_str)
+            try:
+                v = float(val)
+            except ValueError:
+                continue
+            if field == "Lat":
+                lat_map[t] = v
+            elif field == "Lon":
+                lon_map[t] = v
+            else:
+                heading_map[t] = v
+    # Join on lat timestamps (master)
+    source = nav_log_path.parent.name
+    for t, lat in sorted(lat_map.items()):
+        lon = lon_map.get(t)
+        if lon is None:
+            # nearest lon within 1s
+            near = min(lon_map.keys(), key=lambda x: abs(x - t), default=None)
+            if near is None or abs(near - t) > 1.0:
+                continue
+            lon = lon_map[near]
+        pts.append({
+            "t_ms": int(t * 1000),
+            "lat": lat,
+            "lon": lon,
+            "heading": heading_map.get(t),
+            "source": source,
+        })
+    # Simple stride downsampling
+    if len(pts) > max_pts:
+        step = len(pts) // max_pts
+        pts = pts[::step]
+    return pts
+
+
+@app.get("/sorties/{sortie_id:path}/usv_track")
+async def get_usv_track(sortie_id: str):
+    """Return USV GPS track [{t_ms, lat, lon, heading, source}] for map polylines."""
+    raw_dir = OUTPUT_DIR / sortie_id / "raw"
+    nav_logs = list(raw_dir.rglob("*_navigation_log.csv")) if raw_dir.exists() else []
+    if not nav_logs:
+        raise HTTPException(404, "No navigation_log.csv found — run pipeline first")
+    pts: list[dict] = []
+    for nav_log in nav_logs:
+        pts.extend(await asyncio.to_thread(_read_usv_track, nav_log))
+    pts.sort(key=lambda p: p["t_ms"])
+    return JSONResponse(pts)
diff --git a/viewer/index.html b/viewer/index.html
index 57f8fd3..a1dbaa8 100644
--- a/viewer/index.html
+++ b/viewer/index.html
@@ -1280,8 +1280,42 @@ function setupSyncedZoom(chartIds) {
 async function loadSortieData(sortieId) {
   const prog = document.getElementById('sync-progress');
   try {
-    prog.textContent = 'Chargement USV…';
-    const usvResp = await fetch(`${API2}/sorties/${encodeURIComponent(sortieId)}/usv`);
+    // Reset state for sortie mode (independent from datebar)
+    clearMapLayers();
+    allPoints = [];
+    usblPoints = [];
+
+    // Phase 1: USV GPS track → polylines on map immediately (<3s target)
+    prog.textContent = 'Chargement track USV…';
+    const trackResp = await fetch(`${API2}/sorties/${encodeURIComponent(sortieId)}/usv_track`,
+      { signal: AbortSignal.timeout(15000) });
+    if (trackResp.ok) {
+      const trackPts = await trackResp.json();
+      allPoints.push(...trackPts);
+      allPoints.sort((a, b) => a.t_ms - b.t_ms);
+      if (allPoints.length > 0) {
+        const times = allPoints.map(p => p.t_ms);
+        tMin = Math.min(...times);
+        tMax = Math.max(...times);
+        tNow = tMax;
+        // Fit map bounds
+        const lats = allPoints.map(p => p.lat).filter(Boolean);
+        const lons = allPoints.map(p => p.lon).filter(Boolean);
+        if (lats.length) {
+          map.fitBounds([
+            [Math.min(...lats), Math.min(...lons)],
+            [Math.max(...lats), Math.max(...lons)],
+          ], { padding: [40, 40] });
+        }
+        showNoDataOverlay(false);
+        applyTrailAndCursor(); // PERF FIX: polylines visible NOW, before charts
+        prog.textContent = `Track USV ${allPoints.length} pts — chargement charts…`;
+      }
+    }
+
+    // Phase 2: series + AUV (charts populate progressively)
+    const usvResp = await fetch(`${API2}/sorties/${encodeURIComponent(sortieId)}/usv`,
+      { signal: AbortSignal.timeout(20000) });
     if (usvResp.ok) {
       const usvData = await usvResp.json();
       switchTab('usv');  // BUG1d FIX: switch BEFORE renderUSV so Plotly renders on visible divs
@@ -1291,7 +1325,7 @@ async function loadSortieData(sortieId) {
     prog.textContent = 'Chargement AUV…';
     await loadAuvTabs(sortieId);
     prog.textContent = `${sortieId} chargé`;
-    // POLYLINE FIX: ensure map trail is drawn if allPoints already loaded from datebar
+    // Re-apply after AUV usbl_track loaded (adds AUV trail if available)
     if (allPoints.length > 0) applyTrailAndCursor();
   } catch(e) {
     prog.textContent = `Erreur: ${e.message}`;