Table of Contents
Advertisement
The photogrammetry time measures the event time relative to the receiver's time. It
measures only the first event during the period between 2 epochs. See Figure 4.8
Because the 1 PPS signal is being used to measure the photogrammetry events, the
period of the 1 PPS signal needs to be set to a value equal to or less than the period of
the photogrammetry pulse.
The trigger pulse may be TTL-compatible or open collector. Minimum pulse duration
is 100 nanoseconds. The impedance is approximately 2K ohms.
Usage of a coaxial connection cable is recommended.
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.
For instance, if GPS measurements are recorded at the rate of one per second, the
distance the aircraft moves 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.
34
Figure 4.8: Photogrammetry Timing
The GG Surveyor 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 then depen-
dent upon the setting of the RCI parameter.
CAUTION
GG Surveyor GPS+GLONASS Reference Manual
Advertisement
Table of Contents
