Why do we need to perform time synchronization?

[kerwin112ME]

I'm new to VIO, and I am confusing that why do we need time synchronization?
Aren't the timestamps given by ROS system of all topics already time-synchronized?
Thanks!

[m0oz]

ROS messages have timestamps, yes.

But the accuracy of this timestamps can only be so good as the time reference that they are created with.

For example, consider what a global shutter camera does when capturing images:

• Open shutter
• Integrate charges on the individual pixel diodes of the image sensor
• Close shutter (Ideally we want to have the timestamp exactly in the middle between opening and closing the shutter)
• Step through the image lines and read out the charges
• Send an interrupt to the CPU to signal it has a new frame
• Send the actual data over USB, MIPI-CSI or whatever interface
• Data gets written to memory of the computer
• ROS node that send image messages might now read this data and create a timestamp based on when it received the data. But that timestamp is not reflecting when the camera shutter was actually open

Without physically adding a wire that signals "The shutter is now open/close", there is no way that the computer can now exactly how much time passed since the image was taken to when it receives the interrupt or when it is written to memory.