diff --git a/extract/inspect_usv_csv.py b/extract/inspect_usv_csv.py
new file mode 100644
index 0000000..38e5236
--- /dev/null
+++ b/extract/inspect_usv_csv.py
@@ -0,0 +1,56 @@
+import sys
+import csv
+import ast
+import glob
+from collections import Counter
+
+def inspect_nav(csv_path: str, n=500):
+    data_vals = Counter()
+    rows = 0
+    with open(csv_path, newline="", encoding="utf-8", errors="replace") as f:
+        reader = csv.DictReader(f)
+        print(f"Columns: {reader.fieldnames}")
+        for row in reader:
+            rows += 1
+            if rows <= n:
+                data_vals[row.get("data", "?")] += 1
+    print(f"\nTotal rows: {rows}")
+    print("\nUnique 'data' field values (first 500 rows):")
+    for k, v in data_vals.most_common(30):
+        print(f"  {k}: {v}")
+
+def inspect_usbl(csv_path: str, n=10):
+    with open(csv_path, newline="", encoding="utf-8", errors="replace") as f:
+        reader = csv.reader(f)
+        print(f"Columns: [timestamp, direction, message]")
+        received = 0
+        for row in reader:
+            if len(row) < 3:
+                continue
+            direction = row[1].strip().upper()
+            if direction != "RECEIVED":
+                continue
+            received += 1
+            if received > n:
+                break
+            try:
+                raw = ast.literal_eval(row[2].strip())
+                if isinstance(raw, bytes):
+                    print(f"  ts={row[0]} len={len(raw)} hex_start={raw[:16].hex()}")
+            except Exception as e:
+                print(f"  ts={row[0]} parse_error={e}")
+
+if __name__ == "__main__":
+    ship_dir = sys.argv[1]
+    nav_files = glob.glob(f"{ship_dir}/*_navigation_log.csv")
+    usbl_files = glob.glob(f"{ship_dir}/*_usbl.csv")
+    if nav_files:
+        print(f"\n=== NAV LOG: {nav_files[0]} ===")
+        inspect_nav(nav_files[0])
+    if usbl_files:
+        print(f"\n=== USBL (first 10 RECEIVED): {usbl_files[0]} ===")
+        inspect_usbl(usbl_files[0])
+    if not nav_files and not usbl_files:
+        print(f"No CSV files found in {ship_dir}")
+        import os
+        print("Files found:", os.listdir(ship_dir))