ekf2_timestamps.msg: add new message with ekf2 timestamps

msg/CMakeLists.txt

@@ -50,6 +50,7 @@ set(msg_file_names
 	distance_sensor.msg
 	ekf2_innovations.msg
 	ekf2_replay.msg
+	ekf2_timestamps.msg
 	esc_report.msg
 	esc_status.msg
 	estimator_status.msg

msg/ekf2_timestamps.msg

@@ -0,0 +1,24 @@
+# this message contains the (relative) timestamps of the sensor inputs used by
+# ekf2. It can be used for reproducible replay.
+
+# timestamp             # ekf2 reference time. This matches the timestamp of the
+                        # sensor_combined topic.
+
+
+int16 RELATIVE_TIMESTAMP_INVALID = 32767 # (0x7fff) If one of the relative timestamps
+                                         # is set to this value, it means the associated sensor values did not update
+
+# timestamps are relative to the main timestamp and are in 0.1 ms (timestamp +
+# *_timestamp_rel = absolute timestamp). For int16, this allows a maximum
+# difference of +-3.2s to the sensor_combined topic.
+
+
+int16 gps_timestamp_rel
+int16 optical_flow_timestamp_rel
+int16 distance_sensor_timestamp_rel
+int16 airspeed_timestamp_rel
+int16 vision_position_timestamp_rel
+int16 vision_attitude_timestamp_rel
+
+# Note: this is a high-rate logged topic, so it needs to be as small as possible
+