Swarm: update notes

Signed-off-by: Rhys Mainwaring <rhys.mainwaring@me.com>

Author: srmainwaring

scripts/CopterSwarm.md

@@ -142,7 +142,172 @@ python ./src/ardupilot_gazebo/scripts/copter_swarm.py --world runway --model iri
 Statically defined models
 
 - IMU sensor causing segmentation fault
+- Fix - add check before dereferencing pointer Imu.cc, ImuPrivate::AddSensor, L:233
+- Why is this not raising an exception in the non-distributed case?
+- 
 
 Dynamically defined models
 
 - Not found on secondary
+
+SITL Swarm
+
+- Not working for `--count 1` - error loading params?
+
+Gazebo
+
+- What happens to the socket when a level is unloaded?
+
+Levels
+
+- Don't work well if non-performer entities are loaded dynamically
+- GUI crashed as levels loaded / unloaded (repeatable?)
+- Level tiles loading and unloading correctly if defined statically
+
+Test 1:
+
+```bash
+ros2-ardupilot % gz sim -v4 -s -r --levels runway.sdf
+```
+
+- number of copters: 1
+- dynamically load copters: no
+- dynamically load performers: no
+- number of levels: 25
+- dynamically load level visuals: no
+- distributed: no
+- number of secondaries: 0
+- status: ok
+
+Test 2:
+
+```bash
+gz sim -v4 -s -r --levels --network-role primary --network-secondaries 0 runway.sdf
+```
+
+- number of copters: 1
+- dynamically load copters: no
+- dynamically load performers: no
+- number of levels: 25
+- dynamically load level visuals: no
+- distributed: yes
+- number of secondaries: 0
+- status: ok
+
+Test 3:
+
+```bash
+gz sim -v4 -s -r --levels --network-role primary --network-secondaries 1 runway.sdf
+```
+
+```bash
+gz sim -v4 -s -r --levels --network-role secondary runway.sdf
+```
+
+- number of copters: 1
+- dynamically load copters: no
+- dynamically load performers: no
+- number of levels: 25
+- dynamically load level visuals: no
+- distributed: yes
+- number of secondaries: 1
+- status: fail
+
+primary
+
+```bash
+(2025-11-04 19:53:15.353) [debug] [SystemManager.cc:80] Loaded system [ArduPilotPlugin] for entity [43]
+...
+(2025-11-04 19:53:15.980) [info] [ArduPilotPlugin.cc:1147] Found IMU sensor with name [iris_with_standoffs::imu_link::imu_sensor]
+(2025-11-04 19:53:15.980) [debug] [ArduPilotPlugin.cc:1161] Computed IMU topic to be: world/runway/model/iris_with_ardupilot_1/model/iris_with_standoffs/link/imu_link/sensor/imu_sensor/imu
+```
+
+secondary
+
+```bash
+SocketUDP Bind failed: Address already in use
+(2025-11-04 19:53:15.877) [error] [ArduPilotPlugin.cc:1282] [iris_with_ardupilot_1] failed to bind with 127.0.0.1:9002 aborting plugin.
+...
+(2025-11-04 19:53:16.598) [warning] [ArduPilotPlugin.cc:1568] Incorrect protocol magic 255 should be 18458
+```
+
+Sockets are going to present a problem - we cannot load a model twice using the same port. The binding occurs at model load, even though the entity is not loaded, the plugin is bound.
+
+Need to seek an alternative approach...
+
+- Use a proxy model to load the plugin, but assign outputs to the actual model.
+- Still have the same problem - the model is loaded into all `gz sim instances`
+  - unless we also have two separate worlds, one with the proxy, and one without
+  - issue is that the entity ids will then be different - so inconsistent.
+
+- Alternative settings for UDP sockets?
+  - Can use `SO_REUSEPORT` and allow multiple sockets to bind to the same port, however this causes other errors at runtime, as processes where the entity is unloaded continue to send (invalid) data.
+  - Can we check if an entity is loaded, and disable the plugin if that is the case?
+  - Or use an entirely gz-transport based approach (c.f. PX4).
+
+
+
+How does PX4 manage this?
+
+- PX4-Autopilot/src/modules/simulation/gz_bridge/GZBridge.hpp
+
+GZBridge: 
+
+Interface
+
+- Class `GZBridge` constructed with world and model names.
+- Subscriptions to topics
+  - Clock
+  - PoseIno
+  - Imu
+  - Mag
+  - Odometry
+  - LaserScan
+  - DistanceSensor
+  - AirSpeed
+  - AirPressure
+  - NavSat
+  - OpticalFlow
+- Mixing interface objects are used for
+  - Motors
+  - Servos
+  - Wheels (why different to motors?)
+  - Gimbal
+- Facility for adding noise (why not just use physics engine?)
+- Separate object to handle one gimbal
+
+Implementation
+
+- init
+  - subscribe to all topics
+- Run
+  - updates parameters
+- subscribeXXX
+  - Only support one IMU and must be on /link/base_link/sensor/imu_sensor/imu
+  - ditto magnetometer
+  - similarly for other sensors which have hardcoded link and sensor names
+- clockCallback
+  - keeps PX4 RT clock synchronised with the sim time
+- sensor callbacks
+  - Read messages, update devices directly
+- Can't see any mechanism for maintaining lockstep?
+
+MixingInterfaceESC : OutputModuleInterface
+
+Interface
+
+- MixingOutput _mixing_output
+
+- init
+  - fixed topic name /{model_name}/command/motor_speed
+- updateOutputs
+  - publish gz::msg::Actuators
+
+## Notes
+
+- Change iris_with_ardupilot to use JointController
+- Publish commands from the ArduPilotPlugin
+- Topic name /model/{model}/joint/{joint_name}/cmd_vel
+- Consider using /model/{model}/joint/{joint_name}/command/motor_speed to standardise (PX4)?
+- A
+