Operation 41
Operation
Because the 1 PPS signal is used to measure the photogrammetry events, the
period of the 1 PPS signal must be set to a value equal to or less than the period
of the event pulse.
The trigger pulse may be TTL-compatible or open collector. Minimum pulse
duration is 100 nanoseconds when the signal is not terminated at the receiver
input. The impedance is approximately 5 KW.
Use a coaxial cable with BNC connectors to connect the camera trigger output to
the photogrammetry input connector of the G12.
Time Tagging the Shutter Signal
In this technique, the signal generated by the camera shutter is fed to a GPS
receiver for accurate time-tagging which can then be post-processed with the
GPS observations. Since the time of the picture is not synchronized with the time
that the GPS measurement is taken, the two position computations before and
after the shutter time are interpolated to compute the position of the camera at the
time the picture was taken.
If GPS measurements are recorded at the rate of one per second, the average
distance an aircraft travels in ½ second is about 100 meters. Therefore, the
distance between the position of the camera at the time the picture was taken and
the GPS position fixes can be as much as 50 meters. The motion of the aircraft
during this time may be in the meter range.
To minimize the errors discussed above, the closed loop technique is
recommended.
CAUTION
The G12 measures only one event time per data collection period. If more than
one event time is measured within a data collection period, the receiver measures
only the first one. The event time record rate is dependent upon the setting of the
RCI parameter.
G12RevD.book Page 41 Tuesday, April 2, 2002 4:33 PM
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