...
The serial messages from the GPS are received on serial port 3, /dev/ttyS3. The pulse-per-second square-wave signal (PPS) from the GPS is also connected to the CTS DCD line of that serial port. A patch has been added to the Linux kernel on the data system so that an interrupt function can be registered to run in response to the CTS DCD interrupts. This interrupt function will be called immediately after the rising edge of the PPS signal has been detected by the serial port hardware.
...