For the ISM300x and ISM420 GPS, we output the 1 PPS as follows:

The first output starts when we the GPS Kalman filter gets a stellite fix. Thereafter, the 1 PPS continues to  be output until the receiver is reset.  If we drop to less than 3  SVs, the Kalman stops using satellites and goes to DR mode.  At that  point, the 1 PPS is no longer steered but free-runs with the 49.107 MHz clock (triple the 16.369 MHz GPS clock). If the Kalman filter again starts using SVs, it will again steer the 1 PPS.

The pulse is 1 μs wide by default. We only have 12 pins on the ISM480 connector so we do not output 1 PPS.