A
G
E
H
D
L
-
64
E
S2
and
S2.1
U
s
e
r
’
s
Ma nua
l
Pac
et Format
and
Status Byte
for GPS
T
i
me
S
t
a
mp
i
n
g
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
throu
gh many kinds of sensor information.
1 Status Value 8 bit data as described in Appendix E.
Withi
n 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
S
t
a
mp
i
n
g
Accuracy
R
u
l
e
s
The fol
lowing rules and subsequent accuracy apply for GPS timestamps:
GPS Connection Timestamp Info Accuracy Notes
Ex
pect a drift of about 5
seconds/day
The sensor starts running on The sensor clock does not correct
for leap years. See Appendix E for
more information.
GPS is
n’t connected
its
own clock starting at midnight (GPS Status
0)
J
an 1 2000. This date and time data
is reflected in the H, M, S, D, N,
and Y d
ata values.
GPS is connected
The H, M, S, D, N, and Y data values
are obt
ained 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
+/-50μs 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
a
s
e
r
Firing
Sequence
and
T
i
m
i
n
g
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 senso
point cloud with other data sources.
The upper block and lower block collect distance points
simultaneousl
,
with
each
block
issuing
one
laser
pulse at a time. That is,
each
uppe
block laser fires in sequence and in unison with a laser from the lower block.
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