ashtech GG Surveyor, Reference Manual

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Advanced Operation 33
Advanced Operation
The 1PPS option [L] also controls the output of the Measurement Strobe output. The
Measurement Strobe output is TTL-level into a 75-ohm impedance. Its output is
controlled by the $PASHS,STB command, and is synchronized with GPS or
GLONASS system time, depending upon the constellation selected. The period
depends upon the xxxx value and the setting of the RCI parameter. The ±yyy.yyyy
field allows you to set an offset value from reference time with a resolution of 100
nanoseconds, and also allows you to synchronize the rising or falling edge of the
output with reference time. The accuracy of the measurement strobe output is the
same as the accuracy of the 1PPS pulse (Table 4.4).
Photogrammetry Event Marking
When the photogrammetry [E] option is installed, the GG Surveyor can measure and
record event times with high accuracy. The input signal is TTL level into a 5 K
Ω
impedance. The photogrammetry feature allows the event time to be output by using
the $PASHS,NME,TTT command.
At the rising or falling edge (selectable) of the trigger signal, the time is measured and
output of TTT NMEA message is enabled. The trigger signal can be set to the rising
or falling edge using the $PASHS,PHE (photogrammetry edge) command.
The measured time is accurate to 120 nanoseconds. If the constellation system is set
to GPS, this time is GPS system time; if the constellation system is set to GLONASS,
this time is GLONASS system time, which is equal to UTC + 3 hours; if the
constellation system is set to mixed, this time depends upon the TSC setting (default
is GPS). The time is output as day number, hours, minutes, seconds, and fractional
seconds up to 6 digits.
Table 4.4: PPS and Photogrammetry Accuracy
Condition Accuracy
Setting PPS resolution 100 ns
PPS accuracy GPS+GLONASS stand-alone ~70 ns
PPS accuracy GLONASS stand-alone ~70 ns
PPS accuracy GPS stand-alone ~340 ns
PPS accuracy differential mode ~45 ns
Setting photogrammetry TTT output resolution 100 ns
Photogrammetry accuracy GPS+GLONASS stand-alone ~160 ns
Photogrammetry accuracy GLONASS stand-alone ~160 ns
Photogrammetry accuracy GPS with SA ON stand-alone ~430 ns