Improve timestamp granularity of different measurements within a scan

[tobiasneumann]

The current message for a RadarScan just stores one time stamp for all returns.
And this time stamp is not clearly defined.

Proposed change

• Add the field stamp_offset to RadarReturn, which will be used relatively to the time stamp of the time stamp in the field header of RadarScan.
• Define the time stamp of the field header of RadarScan to be of the first return.

[SteveMacenski]

Define this better. offset representing what? The time it takes for the total scan to be accumulated? The offset due to time sychronization? etc

[tobiasneumann]

This field should have the same meaning as time_increment of sensor_msgs/LaserScan.
The stamp_offset should be the offset to the time stamp of the first measurement on a collected scan which is defined in the header in the RadarScan message.

When you have a rotating radar scanner, a RadarScan would consist on a collection of RadarReturns that are from different measurements.
With this change the returns can have a precise time stamp.

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I wanted to add an alternative description to this reply.
But I have problems finding other words to describe it (at least not in a substantial way), because I think it is already quite clear.
The ambiguity of the meaning of this sentence might be more introduced by that this message is not supposed to be used by itself, so it needs to be seen as a member of radar_msgs/RadarScan which time stamp is referenced in the comment.

What do you think about the following description?
It basically says the same, but might does it in a clearer way?

Suggested change

	builtin_interfaces/Duration stamp_offset # Offset to the stamp in the RadarScan message.
	builtin_interfaces/Duration stamp_offset # The offset relative to the time stamp set in the header of the outer message
	# containing a collection of these returns.

[SteveMacenski]

This needs to actually describe what its representing. You mention its the offset from the last point in the measurement for the first (e.g. accumulation time).

[tobiasneumann]

I didn't meant to referencing the previous return.
Just the time stamp of the header (of the RadarScan message)

So the time stamp of a RadarReturn would be calculated by

<RadarScan>.header.stamp + stamp_offset

So the RadarScan would store the "base" time stamp.
And every RadarReturn would store the offset to this time stamp.

[SteveMacenski]

Don't explain it to me, explain it in the message precisely :-)