W6: Add rov-altitude to cockpit_bridge for Ping2 downward sonar

- Add Range subscription on /ping2/altitude (sensor_msgs/Range)
- Publish rov-altitude to data lake (-1.0 sentinel = NO SONAR)
- Treats range_max (100m) as no-return, same as no-data sentinel

src/rov_mission/rov_mission/cockpit_bridge.py

@@ -16,6 +16,8 @@ Variables in the Cockpit data lake (all prefixed 'external/' by Cockpit):
   rov-ms         — mission state: 0=IDLE 1=RUNNING 2=PAUSED 3=COMPLETE 4=ABORTED
   rov-mp         — mission progress percent (int 0-100)
   rov-recording  — recorder active: 1=recording, 0=not recording (no-go indicator)
+  rov-altitude   — Ping2 downward sonar altitude in metres (float, 2 dp).
+                   -1.0 when no sonar data received (sentinel for W6 NO SONAR state).
 """
 
 import asyncio
@@ -24,7 +26,7 @@ import rclpy
 from rclpy.node import Node
 from rclpy.qos import QoSProfile, ReliabilityPolicy
 from std_msgs.msg import Bool, Float64
-from sensor_msgs.msg import BatteryState
+from sensor_msgs.msg import BatteryState, Range
 from rov_interfaces.msg import FailsafeStatus, MissionStatus
 import websockets
 
@@ -83,6 +85,12 @@ class CockpitBridge(Node):
             # and widgets can reflect recording health.
             # Default 0 (not recording) until recording_manager confirms active.
             'rov-recording': 0,
+
+            # Downward Ping2 sonar altitude in metres.
+            # -1.0 is the sentinel value for W6 — displayed as NO SONAR.
+            # Updated by _altitude_cb when /ping2/altitude messages arrive.
+            # Remains -1.0 until the Ping2 sonar is connected and publishing.
+            'rov-altitude':  -1.0,
         }
 
         # Set of currently connected Cockpit WebSocket clients.
@@ -132,6 +140,15 @@ class CockpitBridge(Node):
             Bool, '/rov/recording/active',
             self._recording_cb, 10)
 
+        # Downward Ping2 sonar altitude from the ping2 driver node.
+        # sensor_msgs/Range.range is the distance in metres to the nearest
+        # echo return (hull surface or seafloor when pointing downward).
+        # Topic will not publish until the Ping2 hardware is connected;
+        # rov-altitude stays at the -1.0 sentinel (W6 shows NO SONAR).
+        self.create_subscription(
+            Range, '/ping2/altitude',
+            self._altitude_cb, 10)
+
         # ------------------------------------------------------------------
         # Timers
         # ------------------------------------------------------------------
@@ -183,6 +200,32 @@ class CockpitBridge(Node):
         with self._lock:
             self._values['rov-recording'] = 1 if msg.data else 0
 
+    def _altitude_cb(self, msg: Range):
+        """
+        Update downward sonar altitude from Ping2 driver node.
+
+        sensor_msgs/Range.range is the measured distance in metres to the
+        nearest echo return. When the Ping2 is pointing downward, this is
+        the vehicle's altitude above the hull surface or seafloor.
+
+        A value of range_max (100m for the Ping2) indicates no echo was
+        received within the operating range — treated the same as no data
+        and published as the -1.0 sentinel so W6 shows NO SONAR.
+
+        The Ping2 range_max is 100.0m. A genuine 100m reading is extremely
+        unlikely in jacket inspection operations (max jacket height ~100m,
+        typical working depth 5-50m) so treating it as no-return is safe.
+        """
+        with self._lock:
+            raw = float(msg.range)
+
+            # Treat no-return (range_max) as sentinel — W6 shows NO SONAR
+            if raw >= msg.range_max:
+                self._values['rov-altitude'] = -1.0
+            else:
+                # Round to 2dp for display — Ping2 is ±1% accuracy at range
+                self._values['rov-altitude'] = round(raw, 2)
+
     # ------------------------------------------------------------------
     # WebSocket broadcast — called from ROS2 timer at 2 Hz
     # ------------------------------------------------------------------