40 G12 OEM Board & Sensor Reference Manual
With the photogrammetry [E] option installed, the G12 can measure and record
events with high accuracy. This is an input signal that is received into a 10K ohm
impedance; the signal must be at TTL levels for proper functioning. In order to
measure an event time, a trigger signal must be sent to pin 27 on connector J301.
This input can be driven with either TTL or a switch that grounds the pin. The
photogrammetry feature allows the event time to be output by using the
$PASHS,NME,TTT command.
After enabling the TTT message, the time is measured at the rising or falling edge
(selectable) of the trigger signal, causing and the TTT NMEA message is output.
The trigger signal can be set to the rising or falling edge using the $PASHS,PHE
command.
The precision of the measured time is 135 nanoseconds (ns) in differential mode
and 280 ns in stand-alone mode with SA on. This is based on GPS time, which is
output as day number, hours, minutes, seconds, and fractional seconds to 6 digits
past the decimal mark.
The photogrammetry time measures the event time relative to the receiver's GPS
time. It measures only the first event during the period between 2 GPS epochs
(Figure 3.3).
DEFAULT SETTINGS
TTT synchronization message output— synchronize with the rising edge
of the trigger signal
Figure 3.3. GPS Epochs
G12RevD.book Page 40 Tuesday, April 2, 2002 4:33 PM
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