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Appendix II Absolute Time and Laser Firing Time
■ Absolute Time of Point Cloud Data Packets
The absolute time of a Point Cloud Data Packet is the sum of date, time (accurate to the second) and μs time.
· Date and Time can be retrieved either from the current Point Cloud Data Packet (6 bytes of Date & Time), or from the previous GPS Data
Packet (6 bytes of Date and 6 bytes of Time).
· μs time can be retrieved from the current Point Cloud Data Packet (4 bytes of Timestamp)
When using a PTP clock source, the LiDAR does not output GPS Data Packets.
■ End Time of Each Block
The Body of each Point Cloud Data Packet contains 10 data blocks, as detailed in Section 3.1.2 (Point Cloud UDP Data).
Every time the LiDAR sends a command to trigger a round of firing, the measurements are:
· stored in one block in Single Return mode
· stored in two adjacent blocks in Dual Return mode
If the absolute time of a Point Cloud Data Packet is t0, the end time of each block (the time when all the lasers finish firing) can be calculated.
Single
Return
Mode
Block End Time (μs)
Dual
Return
Mode
Block End Time (μs)
Block 10 t0 - 28.58 Block 10 & Block 9 t0 - 28.58
Block N t0 - 28.58 - 55.56 * (10 - N) Block 8 & Block 7 t0 - 28.58 - 55.56 * 1
Block 3 t0 - 28.58 - 55.56 * 7 Block 6 & Block 5 t0 - 28.58 - 55.56 * 2
Block 2 t0 - 28.58 - 55.56 * 8 Block 4 & Block 3 t0 - 28.58 - 55.56 * 3
Block 1 t0 - 28.58 - 55.56 * 9 Block 2 & Block 1 t0 - 28.58 - 55.56 * 4