Haicom HI-405BT - Manual (Page 20)

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6. GPS Technical Data
6.1 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).
As long as enough satellite signals are received to generate valid
position fixes, the 1PPS output remains synchronized to the GPS
second, and the 1PPS Valid Signal remains active. If signal blockage
prevents the receiver from generating valid position fix, the 1PPS
output will drift away from the GPS second and the 1PPS Valid
Signal will become inactive. Upon re-acquiring enough satellites to
generate consecutive valid position fixes, the 1PPS Valid Signal will
become active again, signaling that the 1PPS output is again
synchronized with the GPS second.
For best stable operation of the 1PPS signal, it is to be operated in
static environment having clear view of the sky.