Specifications

14 Z-Family Techncial Reference Manual
are interpolated to compute the position of the camera at the time the picture was
taken.
For example, suppose the GPS measurements are recorded at the rate of one per
second while the distance that the aircraft moves in ½ second is about 100 meters.
The induced error 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. 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 as
shown in Figure 2.4.
In this technique, the 1PPS output of the receiver triggers a camera shutter. The
camera shutter generates a signal that is fed to the receiver for accurate time tagging.
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 receiver so that the shutter time exactly matches the GPS system time for the
epoch. No interpolation between the shutter time and the GPS position time will be
needed.
This input is asserted by bringing it to ground with a low-impedance driver, a contact
closure, or an open-collector transistor. The maximum voltage to guarantee assertion
is 0.75 volts, and the current when grounded will be no more than 350 microampere.
The input has an internal pull-up, so it is not necessary to drive it high to make it
inactive. The signal will be de-bounced internally, so only the first falling edge in a
pulse train of up to 100 milliseconds will be detected.
Figure 2.4: Closed Loop Technique