ashtech DG14, Справочное руководство

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46 DG14 and DG16 Board & Sensor Reference Manual
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.
Closed-Loop Technique (Advanced Trigger)
The closed-loop technique combines PPS synchronization and shutter timing
(Figure 3.2).
Figure 3.2. PPS Synchronization