ashtech DG14, Reference Manual

Page: 67 / 390

Basic Operation 47
Basic Operation
In this technique, the 1PPS output of the DG16 triggers a camera shutter. The
camera shutter generates a signal that is fed to the DG16 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 DG16 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 DG16 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
command, the period of the pulse can be changed from 0.05 of a second up to
99.95 seconds, depending upon the receiver update rate, which, in turn, is
dependent upon the installed position update rate and raw data update rate
options.The timing pulse may be offset from GPS time within a range of -999.9999
to +999.9999 milliseconds. GPS time can be synchronized to the rising or falling
edge of the square wave pulse.
Figure 3.3 shows timing pulse characteristics under default conditions. The pulse
occurs when the signal goes high (i.e., goes from zero to five volts). The pulse is
generated within ±1 microsecond of the GPS second and remains high for 1-2
milliseconds. The precision of the epoch between pulses is ±200 nanoseconds in
stand-alone mode with SA off and ±50 nanoseconds when the DG16 is receiving
differential corrections. The DG16 must be computing positions and tracking a
minimum of four satellites in order for the one microsecond accuracy and 50/200
nanosecond precision to be valid.
DEFAULT SETTING
PPS Period 1 second
Offset 0.0000 milliseconds
Synchroniza-
tion GPS time synchronized to the rising edge of the
pulse