diff --git a/3-sentinel-data.py b/3-sentinel-data.py
index 4ea88bf..4ed3090 100644
--- a/3-sentinel-data.py
+++ b/3-sentinel-data.py
@@ -97,13 +97,15 @@ _SCL_ASSET = "scl"
 _MIN_BBOX_HALF_DEG = 0.008
 
 _GDAL_COG_ENV = {
-    "GDAL_HTTP_VERSION": "2",
+    # HTTP/1.1 avoids HTTP/2 multiplexing connection-reset cascades on S3.
+    "GDAL_HTTP_VERSION": "1.1",
     "GDAL_HTTP_MERGE_CONSECUTIVE_RANGES": "YES",
-    "GDAL_HTTP_MULTIPLEX": "YES",
     "GDAL_HTTP_TCP_KEEPALIVE": "YES",
     "GDAL_DISABLE_READDIR_ON_OPEN": "EMPTY_DIR",
     "CPL_VSIL_CURL_CACHE_SIZE": "200000000",
-    "GDAL_MAX_CONNECTIONS": "100",
+    # Built-in GDAL retries for 429/502/503/504 and transient resets.
+    "GDAL_HTTP_MAX_RETRY": "3",
+    "GDAL_HTTP_RETRY_DELAY": "0.5",
     "AWS_NO_SIGN_REQUEST": "YES",
 }
 
@@ -373,7 +375,7 @@ def download_s2_window(
     output_dir: Path,
     bands: list[str],
     ratio: int = RESOLUTION_RATIO,
-    max_workers: int = 32,
+    max_workers: int = 12,
 ) -> None:
     """Range-read S2 L2A COG windows and write masked REFL GeoTIFFs.
 
@@ -491,6 +493,39 @@ def _netcdf_to_geotiffs(nc_path: Path, output_dir: Path, epsg: int) -> int:
     return written
 
 
+_S3_DOWNLOAD_RETRIES = 4
+_S3_DOWNLOAD_BACKOFF = 30  # seconds; doubled on each retry
+
+
+def _download_with_retry(datacube: Any, nc_path: Path) -> None:
+    """Download an OpenEO datacube to *nc_path*, retrying on transient errors.
+
+    Retries up to ``_S3_DOWNLOAD_RETRIES`` times with exponential backoff
+    starting at ``_S3_DOWNLOAD_BACKOFF`` seconds.  Re-authenticates on each
+    attempt so an expired token never blocks a retry.
+    """
+    delay = _S3_DOWNLOAD_BACKOFF
+    last_exc: Exception | None = None
+    for attempt in range(1, _S3_DOWNLOAD_RETRIES + 1):
+        try:
+            if nc_path.exists():
+                nc_path.unlink()
+            datacube.download(str(nc_path), format="NetCDF")
+            return
+        except Exception as exc:
+            last_exc = exc
+            if attempt < _S3_DOWNLOAD_RETRIES:
+                print(
+                    f"[S3-OEO] Download attempt {attempt} failed ({exc}); "
+                    f"retrying in {delay}s..."
+                )
+                time.sleep(delay)
+                delay *= 2
+            else:
+                print(f"[S3-OEO] All {_S3_DOWNLOAD_RETRIES} download attempts failed")
+    raise RuntimeError(f"S3 download failed after {_S3_DOWNLOAD_RETRIES} attempts") from last_exc
+
+
 def download_s3_openeo(
     start_date: datetime,
     end_date: datetime,
@@ -537,7 +572,7 @@ def download_s3_openeo(
     nc_path = output_dir / "_s3_syn_l2.nc"
     print(f"[S3-OEO] Downloading NetCDF to {nc_path}...")
     t0 = time.time()
-    datacube.download(str(nc_path), format="NetCDF")
+    _download_with_retry(datacube, nc_path)
     print(f"[S3-OEO] Download completed in {time.time() - t0:.1f}s")
 
     print("[S3-OEO] Splitting into per-date GeoTIFFs...")
@@ -804,6 +839,11 @@ def main(argv: list[str] | None = None) -> int:
         default=None,
         help="Single sitename to process (default: all step-2 PASS sites)",
     )
+    parser.add_argument(
+        "--skip-downloaded",
+        action="store_true",
+        help="Skip sites whose directory already exists under data/sentinel_data/{year}/",
+    )
     args = parser.parse_args(argv)
     year = args.evaluation_year
 
@@ -821,16 +861,17 @@ def main(argv: list[str] | None = None) -> int:
     print(f"[Sentinel-3] Processing {len(pass_sites)} site(s)")
     for i, site in enumerate(pass_sites, 1):
         sitename = site["sitename"]
+        site_dir = DATA_DIR / "sentinel_data" / str(year) / sitename
+        if args.skip_downloaded and site_dir.exists():
+            print(f"[Sentinel-3] ({i}/{len(pass_sites)}) {sitename} — skipping (directory exists)")
+            continue
         print(f"[Sentinel-3] ({i}/{len(pass_sites)}) {sitename}")
-        try:
-            summary = process_site(sitename, site["lat"], site["lon"], year)
-            print(
-                f"[Sentinel-3] {sitename} done — "
-                f"{summary['s2_refl_count']} REFL, "
-                f"{summary['s3_composite_count']} composites"
-            )
-        except Exception as exc:
-            print(f"[Sentinel-3] {sitename} FAILED: {exc}")
+        summary = process_site(sitename, site["lat"], site["lon"], year)
+        print(
+            f"[Sentinel-3] {sitename} done — "
+            f"{summary['s2_refl_count']} REFL, "
+            f"{summary['s3_composite_count']} composites"
+        )
 
     return 0