Haicom HI-204III, User Manual

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3.2 ONE-PULSE-PER-SECOND (1PPS) OUTPUT
The one-pulse-per-second output is provided for
applications requiring precise timing measurements. The
output pulse is 1usec in duration. Rising edge of the output
pulse is accurate to +/-1usec with respect to the start of
each GPS second. Accuracy of the one-pulse-per-second
output is maintained only when the GPS receiver has valid
position fix.
The 1PPS output is always generated when the GPS
receiver is powered-on. Proper adjustment of the 1PPS
output to align with the GPS second requires calculation of
the receiver clock offset and clock drift-rate as part of the
position-velocity-time (PVT) solution. When enough satellite
signals are received to generate valid position fixes, the
1PPS output is adjusted to align with the GPS second in
several seconds. When the 1PPS output is brought in sync
with the GPS second, the 1PPS Valid Signal on the I/O pin
becomes active (HIGH); when the 1PPS output is not yet in
sync with the GPS second, the 1PPS Valid Signal remains
inactive (LOW).