Specifications

Operation 13
Operation
rear panel of the receiver (refer to your individual receiver manual for pinout
information). The event marker feature allows the event time to be stored in memory
and downloaded using the DOWNLOAD program as an M-file, or output by using
the $PASHS,NME,TTT command.
At the rising or falling edge (selectable) of the trigger signal, the time is recorded in
the receiver’s PC card. The trigger signal can be set to the falling edge using the
$PASHS,PHE command.
The measured time is accurate down to 1 microsecond. This is GPS time (UTC + 12
seconds as of 1 July, 1997) and is recorded as the time since the start of the GPS week
(00:00 a.m. Sunday). The output includes day number, hours, minutes, seconds, and
fractional seconds up to seven digits. With each event time, the receiver also records
the site name. One example of the record is:
TEXA 4 21:30:19:4309643
The event time is measured relative to the receiver's GPS time. It measures only the
first event during the period between 2 GPS epochs (1ms). Refer to Figure 2.3. This
allows use of mechanical switches without concern for contact bounces.
The receiver stores only one event time per nav processing cycle (0.5 sec for UB00
and 0.1 sec for UC00 or later). If more than one event time is measured within a data
collection period, the receiver records only the first one.
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 2K ohms.
Time Tagging the Shutter Signal
In this technique, the signal generated by the camera shutter is fed to a GPS unit 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
Figure 2.3: Event Marker Time Measurement