Serial GPS

If this firewall has an available serial port, a Serial GPS may be used to provide a reference clock for the firewall. If the GPS also supports a Pulse Per Second (PPS) signal, that may also be used as a PPS clock reference.


USB GPS units may function, they are not desirable time sources due to USB timing issues. The overhead of USB makes its unreliable as a clock or timing source.

For best results, the best practice is to configure at least three NTP servers under System > General Setup or Services > NTP to avoid loss of sync if the GPS data is not valid over time. Otherwise the NTP daemon may only use values from the unsynchronized local clock when providing time to clients.

Serial GPS Settings

GPS Type

Select the make and model of the GPS unit. If the model is unknown, use the Default choice. If the model is known but not listed, use Custom.

Serial Port

All serial ports detected on the firewall are listed. Select the port with the GPS attached. On-board hardware serial ports start with cuau, USB serial ports are prefixed with cuaU.

Baud Rate

Enter the serial speed for the GPS, typically a low value such as 4800

NMEA Sentences

By default, NTP will listen for all supported NMEA sentences. To limit this to specific types, select them from the list.

Fudge Time 1

Specifies a constant to be added to the GPS PPS signal as an offset.

Fudge Time 2

Specifies a constant to be added to the GPS time as an offset.


Used to configure the stratum of the GPS clock. The default value is 0 so the GPS is preferred over all others. If another clock must be preferred instead, set the stratum value higher than the stratum of the preferred clock.


These options provide additional tweaks to fine-tune the GPS behavior:

Prefer this clock

Marks the reference clock as preferred by NTP.

Do not use this clock

Prevents the clock from being used by NTP for time synchronization, it is only displayed for reference.

PPS signal processing

Enables processing of the Pulse Per Second (PPS) signal in the GPS driver. Only enable this if the GPS is known to output a usable PPS signal.

Falling edge PPS signal processing

When set, the falling edge of the PPS signal is used for timing, rather than the rising edge.

Kernel PPS clock discipline

When set, the OS Kernel will use PPS directly for timing.

Obscure location in timestamp

Obscures the GPS data so the location of the clock cannot be determined.

Log the sub-second fraction of the received time stamp

When checked, this can rapidly fill the log, but can be useful for fine tuning of Fudge Time 2.

Clock ID

A 1-4 character identifier used to change the GPS Clock ID. The default value is GPS.

GPS Initialization

Contains the initialization string sent to the GPS at start up to configure its behavior. When using the Custom GPS type, a proper initialization string for the GPS must be entered manually.

NMEA Checksum Calculator

Calculates a checksum for use when crafting new GPS Initialization values or adjusting existing values.

Configuring a Serial GPS

To configure a GPS for use by NTP:

  • Navigate to Services > NTP

  • Click the Serial GPS tab

  • Configure the settings

  • Click Save