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Packet F
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Status Byte
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GPS Time
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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
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Accuracy
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The following rules and subsequent accuracy apply for GPS timestamps:
GPS Connection Timestamp Info Accuracy Notes
GPS isn’t connected
(GPS Status 0)
The sensor starts running on
its own clock starting at midnight
Jan 1 2000. This date and time data
is reflected in the H, M, S, D, N,
and Y data values.
Expect a drift of about 5
seconds/day
The sensor clock does not correct
for leap years. See Appendix E for
more information.
GPS is connected The H, M, S, D, N, and Y data values
are obtained from the $GPRMC
NMEA record.
GPS time synching runs in
one of two modes:
• 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
+/-50js
of the correct time at
all times.
GPS is disconnected
after being connected
The sensor continues to run on
its own clock.
Expect drift of about 5 seconds/day
L
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Firing
Sequence
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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.
(28 strony)
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