Lassen SQ, Reference Manual

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2 Hardware Integration
2 8 Lassen SQ GPS Receiver
2.5
Pulse-Per-Second (PPS)
The Lassen SQ GPS receiver provides a four microsecond wide,
CMOS compatible TTL level Pulse-Per-Second (PPS). The PPS is a
positive pulse available on pin 4 of the power and I/O connector. The
rising edge of the PPS pulse is synchronized with respect to UTC. The
timing accuracy is ±95 nanoseconds when valid position fixes are
being reported.
The rising edge of the pulse is typically less than 20 nanoseconds. The
distributed impedance of the attached signal line and input circuit can
affect the pulse shape and rise time. The PPS can drive a load up to
5mA without damaging the module. The falling edge of the pulse
should not be used. The PPS is always on (early PPS) and is driven by
the Real Time Clock (RTC) until the receiver acquires GPS time from
the satellite and is getting fixes. The PPS is output immediately after
main power is applied, and continues even if the receiver loses GPS
lock. The drift of the PPS, when the Lassen SQ GPS receiver is not
tracking satellites, is unspecified and should not be used for
synchronization.
Note – Trimble Navigation has measured better than 50 nanoseconds
accuracy on the Lassen SQ GPS receiver PPS signal in static mode.
For more information on the use of the Lassen SQ GPS module in
timing applications, contact your Trimble sales representative.