42 G12 OEM Board & Sensor Reference Manual
Closed-Loop Technique (Advanced Trigger)
The closed-loop technique combines PPS synchronization and shutter timing
(Figure 3.4).
In this technique, the 1PPS output of the G12 triggers a camera shutter. The
camera shutter generates a signal that is fed to the G12 for accurate time tagging,
better than one microsecond.
The delay between the camera receiving the pulse and triggering the
photogrammetry port should be calculated. This may then be applied so as to
advance the 1PPS from the G12 so that the shutter time exactly matches the
GPS time for the epoch. No interpolation between the shutter time and the GPS
position time will be needed.
When the timing pulse option [L] is installed, the G12 can output a timing pulse
synchronized with GPS time to an accuracy of ±1 microsecond. The timing pulse
is a TTL-level square wave signal output on pin 18 of the J301 connector and is
fed into a 75-ohm impedance. The pulse is generated by default once every
second (1PPS, or 1 pulse-per-second) with no offset from GPS time and with the
rising edge of the pulse synchronized to GPS time. Using the $PASHS,PPS
Figure 3.4. PPS Synchronization
G12RevD.book Page 42 Tuesday, April 2, 2002 4:33 PM
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