message_filters and Time-Synchronized Sensors

Source: ros2-copilot-skills message_filters skill

Why This Matters

Many robot perception and fusion pipelines need data that is merely “close in time,” not just the latest message on each topic. message_filters exists for that problem, but it only works well when headers, timestamps, QoS, and queue settings are all aligned.

Distilled Takeaways

• Use exact synchronization only when the publisher timestamps really match exactly.
• Approximate synchronization is usually the practical choice for heterogeneous sensors.
• slop and queue_size are not cosmetic. They define how much timing mismatch the system tolerates and how much history it can use to find pairs.
• Header timestamps are mandatory for meaningful time synchronization.
• A sync callback that never fires is often a timestamp or QoS problem before it is a code problem.

Practical Guidance

• Start with approximate sync and tune tighter only when data quality demands it.
• Verify each individual topic first before blaming the synchronizer.
• Keep queue sizes bounded and justified.
• Use synchronization as a deliberate data contract, not as a bandage for sloppy timestamp handling.

Corroborating References

• message_filters documentation
• ROS 2 QoS concepts

When to Read the Original Source

Go to the original skill when you want exact Python and C++ synchronizer code, cache examples, and the practical tuning advice for slop, queue size, and sensor-rate mismatch.