GPS Time Source

A GPS Time Source — also known as a GPS‑disciplined oscillator (GPSDO), GPS receiver with time output, or GNSS time server — is a dedicated device that receives signals from the Global Positioning System (GPS) or other GNSS constellations (GLONASS, Galileo, BeiDou) and generates a highly accurate, stable time reference. Unlike a full NTP server, the GPS Time Source typically provides raw timing signals — such as a pulse‑per‑second (PPS) output and a serial time‑of‑day message (NMEA or proprietary) — which can be used directly by other equipment (e.g., PLCs, RTUs, data loggers) or fed into an NTP server to distribute time over the network.

Key characteristics of the GPS Time Source include:

• High‑Precision Timing: Provides a 1 PPS (pulse‑per‑second) output with an accuracy of typically ±50 ns to ±100 ns relative to UTC, making it suitable for demanding time‑sensitive applications.
• Multi‑Constellation Support: Modern devices support GPS, GLONASS, Galileo, and BeiDou, ensuring reliable time reception even in obstructed environments or during satellite outages.
• Disciplined Oscillator: An internal oven‑controlled crystal oscillator (OCXO) or temperature‑compensated crystal oscillator (TCXO) is disciplined by the GPS signal, providing excellent holdover performance (e.g., < 1 µs per hour) when the satellite signal is lost.
• Multiple Output Interfaces: Typically provides a 1 PPS output (BNC or TTL), an RS‑232/RS‑422 serial port for time‑of‑day messages (NMEA 0183 or proprietary), and optionally an IRIG‑B or NTP output for integration with various systems.
• Time‑of‑Day Data: Outputs UTC date and time via serial data streams, which can be parsed by connected devices to set their internal clocks with high accuracy.
• Antenna Input: Includes a dedicated antenna input (SMA or TNC) with 5 VDC antenna power and built‑in surge protection for outdoor antenna installations.
• Compact and Rugged: Often packaged as a small, DIN‑rail mountable module or a compact enclosure, suitable for installation in control cabinets, equipment shelters, and outdoor enclosures.

In airfield lighting systems, the GPS Time Source is a foundational component for time synchronization. It provides the raw timing signals that are used by the ALCMS server, PLCs, and other devices to maintain accurate time, enabling precise sequence‑of‑events (SOE) recording, alarm correlation, and coordinated system responses. It is often paired with an NTP server or integrated directly into a time synchronization module.

The GPS time source exists to give every device across the AGL network the same, unquestionable definition of "when." Individually, a CCR fault, a network event, and an operator action can each be timestamped by their own local clock well enough for routine logging — but when an incident investigation needs to establish the true order in which several such events occurred across different devices, clocks that have each drifted by even a fraction of a second can make that reconstruction unreliable or simply wrong. By disciplining every connected system to the same GPS-derived reference, accurate to tens of nanoseconds, the time source is what makes a sequence-of-events reconstruction actually trustworthy rather than approximately plausible, which matters directly for fault analysis, alarm correlation, and any compliance record that depends on proving what happened first.

The GPS Time Source is deployed in AGL systems where high‑precision time synchronization is critical. Key applications include:

• ALCMS Server Time Synchronization: Providing the primary time reference for the ALCMS server, ensuring that all alarms, events, and operator actions are timestamped with sub‑millisecond accuracy.
• PLC and Remote I/O Synchronization: Feeding a PPS signal and time‑of‑day data to PLCs and remote I/O units, enabling precise sequence‑of‑events (SOE) logging with 1 ms or better resolution.
• CCR Communication Interfaces: Synchronizing CCR communication modules to timestamp status changes and alarms from Constant Current Regulators with high precision.
• SCADA Integration: Providing a disciplined time source to SCADA systems, ensuring that data from multiple sources is time‑aligned for accurate analysis.
• Approach Lighting Sequencers: Synchronizing sequenced flashing lights and approach lighting systems to ensure precise timing and coordinated operation.
• RWSL (Runway Status Light) Systems: Providing accurate time for RWSL control logic, ensuring that light state changes occur at precisely the right moments during aircraft movements.
• Network Time Servers: Serving as the external reference for NTP servers, which then distribute time to all network‑connected devices in the AGL network.
• Data Acquisition Systems: Providing precise timestamps for data acquisition systems monitoring airfield sensors, weather stations, and power quality monitors.

The receiver unit itself is DIN-rail or rack mounted in the control cabinet, equipment shelter, or control room closest to the devices it will feed, with its PPS and serial time-of-day outputs run on short, direct cabling to the connected PLC, RTU, or NTP server to preserve timing accuracy. Its GPS/GNSS antenna is a separate outdoor component, mounted with a clear, unobstructed view of the sky — typically on a roof, mast, or external wall bracket — and connected back to the receiver by a coaxial antenna cable that includes in-line surge protection at the building entry point, since the antenna cable is the one part of the installation directly exposed to lightning risk. Cable length between antenna and receiver is kept within the manufacturer's rated limit to avoid excessive signal loss, and where multiple receivers in the same facility need a common reference, a splitter or separate antenna runs are used rather than daisy-chaining receivers off one another. Power is typically drawn from the same control cabinet supply as the device it serves, ideally UPS-backed, since a power interruption to the receiver during a GPS outage also removes the holdover oscillator's ability to keep counting accurately.

The GPS Time Source operates as a precision timing generator, leveraging satellite signals to produce a disciplined time reference. Key functional aspects include:

• Satellite Signal Reception: The device receives GPS/GNSS signals via a dedicated antenna. The antenna captures L1 (1575.42 MHz) signals from multiple satellites. The receiver decodes the navigation messages to determine the precise time and position.
• Time and Frequency Extraction: The receiver extracts the highly stable carrier frequency (which is disciplined by atomic clocks on the satellites) and the time‑of‑week information from the navigation message. This provides a reference frequency and a known time epoch.
• Disciplined Oscillator: The internal oscillator (typically an OCXO or TCXO) is disciplined by the satellite‑derived frequency. A phase‑locked loop (PLL) or frequency‑locked loop (FLL) adjusts the oscillator frequency to match the satellite signal, ensuring that the oscillator's output is precisely 10 MHz or a similar reference frequency.
• Pulse‑Per‑Second (PPS) Generation: The disciplined oscillator is used to generate a 1 PPS signal. The PPS pulse is synchronized to the GPS second, with a rising edge that occurs at the start of each UTC second. The jitter of the PPS is typically less than 50 ns, making it suitable for high‑precision synchronization.
• Time‑of‑Day Data Output: The receiver outputs date and time information via a serial port (e.g., RS‑232) using standard NMEA 0183 sentences (e.g., $GPGGA, $GPRMC, $GPZDA). These sentences contain the current UTC time, date, and optionally position information. Connected devices can parse these messages to set their internal clocks.
• Holdover Operation: When the satellite signal is lost (e.g., due to antenna obstruction or atmospheric conditions), the device enters holdover mode. The disciplined oscillator continues to provide a stable time reference, and the PPS output remains accurate for a period determined by the oscillator's stability (e.g., hours or days). The device may also provide a warning or alarm output to indicate that the reference is lost.
• Antenna Management: The device supplies power (typically 5 VDC) to the active GPS antenna via the coaxial cable. It also includes built‑in surge protection and a low‑noise amplifier (LNA) to compensate for cable losses. The antenna status (connected, signal strength, number of satellites) is monitored and can be reported via the serial port or status LEDs.
• Alarm and Status Indication: Front‑panel LEDs or a display indicate the device status, including power, signal lock, holdover mode, and fault conditions. Some models provide a relay output to indicate loss of synchronization for remote alarming.

The GPS Time Source is typically powered from a 24 VDC or 120/230 VAC supply, with optional redundancy for critical installations. The antenna must be installed with a clear view of the sky, and the cable run should be kept as short as possible to minimize signal attenuation and delay.

The receiver electronics are housed in a compact DIN-rail-mount enclosure or a small standalone panel-mount case, built around a multi-constellation GNSS receiver board, an internal oscillator (TCXO or OCXO, depending on the holdover-stability grade), and output-conditioning circuitry for the 1PPS, frequency, and serial time messages. The front or top face carries an SMA or TNC antenna connector supplying 5 VDC to a remote active antenna, BNC or terminal-block outputs for the timing signals, a serial port for NMEA data, and status LEDs showing power, satellite lock, holdover, and fault state at a glance. The GNSS antenna itself is a separate, weatherproof puck- or dome-style unit mounted outdoors with a clear view of the sky — typically on a mast or rooftop bracket — and connected back to the receiver over coaxial cable, with surge protection fitted at the cable entry to the equipment room given the antenna's outdoor exposure. The receiver's own housing is rated for indoor cabinet use (IP20/IP40), relying on the enclosing control cabinet for weather protection, while the internal oscillator is thermally isolated within the case to hold its holdover accuracy if GNSS lock is lost.