Velodyne Acoustics HDL-64E S2.1 user manual Packet Format and Status Byte for GPS Time Stamping

usaGe HDL-64E S2 and S2.1 User’s Manual

Packet Format and Status Byte for GPS Time Stamping

The 6 bytes at the end of the data packet report GPS timing and synchronization data. For every packet, the last 6 bytes are formatted as follows:

Timestamp Bytes in Reverse Order in Microseconds

Bytes	Description	Notes
4	GPS timestamp	32 bit unsigned integer timestamp. This value represents microseconds from the top
		of the hour to the first laser firing in the packet.
1	Status Type	8 bit ASCII status character as described in Appendix E. The status byte rotates
		through many kinds of sensor information.
1	Status Value	8 bit data as described in Appendix E.

Within the GPS status byte, there are 4 GPS status indicators:

•0: no GPS connection.

•A: both PPS and GPS command have signal.

•V: only GPS command signal, no PPS.

•P: only PPS signal, no GPS time command.

Time Stamping Accuracy Rules

The following rules and subsequent accuracy apply for GPS timestamps:

GPS Connection	Timestamp Info	Accuracy	Notes
GPS isn’t connected	The sensor starts running on	Expect a drift of about 5	The sensor clock does not correct
(GPS Status 0)	its own clock starting at midnight	seconds/day	for leap years. See Appendix E for
	Jan 1 2000. This date and time data		more information.
	is reflected in the H, M, S, D, N,
	and Y data values.
GPS is connected	The H, M, S, D, N, and Y data values	GPS time synching runs in
	are obtained from the $GPRMC	one of two modes:
	NMEA record.	• The GPS has an internal clock
		that runs for several weeks that
		is used first. The accuracy is that
		of the GPS device employed.
		• When the GPS achieves lock,
		the sensor clock is then within
		+/-50µs of the correct time at
		all times.
GPS is disconnected	The sensor continues to run on	Expect drift of about 5 seconds/day
after being connected	its own clock.

Laser Firing Sequence and Timing

If the GPS timestamp feature is used, it can be useful to determine the exact firing time for each laser so as to properly time-align the sensor point cloud with other data sources.

The upper block and lower block collect distance points simultaneously, with each block issuing one laser pulse at a time. That is, each upper block laser fires in sequence and in unison with a laser from the lower block.

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