31
8. Accurate Time Calculation
To accurately calculate the time of the point cloud data, it is necessary to obtain the
timestamp of the data packet and the UTC of the device package output by the Lidar.
The timestamp and UTC come from the same synchronization source, such as a GPS.
The laser emission interval of the MS03 Lidar is about 3.333 μs, and the measurement
interval of adjacent points is 3.333 μs.
A data packet has a total of 80 measurement data. The packet packing time is about
3.333 μs*80 ~= 0.267 ms,
and the data rate is about
1s/0.143 ms = 6900 data
packets/sec.
8.1 Calculation of Data Packet End Time
The timestamp in the data packet is a relative time in microseconds, which is defined
as the packing time of the laser measurement data of the last channel in the data packet
(packet end time), and its duration is less than 1 second. Therefore, to calculate the
absolute end time of the data packet, it is necessary to obtain the 4-byte microsecond
timestamp in the data packet first, and then obtain the UTC (more than 1 second) from
the device package. The addition of the two will be the exact time when the data packet
ends.
8.2 Accurate Time Calculation of Channel Data
The precise measurement time of each data can be calculated with the exact time of the
end of the data packet and the light-emitting time interval of the 80 data.
Each data packet block of the MS03 Lidar contains 80 measurement data. Therefore,
the end-time interval of each data packet (single echo mode) is 267 μs. Assuming that
the absolute time of the data packet end is T
Packet_end
, the steps for calculating the end
time of the data block
T
Block_end
(N)
are as follows:
T
Block_end
(N) = (T
Packet_end
– 3.333 μs*(80-N)). (N = 1, 2, ..., 80).
