GNSS oscillator with 24-hour holdover in a postage-stamp size

During a severe sandstorm, a transport aircraft attempts to land at a remote forward operating location. Visibility is nearly zero, forcing the crew to rely on navigation, communications, and landing-assistance systems. If the aircraft were to lose its GNSS signal, local timing sources would drift. Even a few microseconds of drift could cause these systems to lose synchronization.

This can cause communication disruptions, impacts to radar and ranging systems, higher inertial navigation error, and ultimately reduce the accuracy of information displayed to the pilot. Lives and the mission could be at stake. In this scenario, the fallout would not stem from a hardware failure or cyberattack, but from a small timing error.

Take, for instance, MicroPNT GDO-1000, a GNSS-disciplined oscillator (GNSS-DO) module from VIAVI, a supplier of test and measurement, optical, and position, navigation and timing (PNT) technologies. The module, built for situations mentioned above, features MEMS-based Endura Epoch or Elite X oscillators to deliver microsecond-class, 24-hour holdover in a tiny package.

The Viavi µPNT GDO-1000 module nears atomic-clock timing holdover precision, but with a low size, weight, power and cost (SWAP-C) profile—in a footprint the size of a postage stamp. The solution provides greater flexibility to keep networks synchronized, sensors aligned, and mission systems operating with confidence across air, land, sea, space, and cyber domains—even in extreme environments.

What is a GNSS-DO?

GNSS signals can be vulnerable to jamming or spoofing. Signals can be interrupted in urban or geographic canyons and subject to outages. A GNSS-DO combines a local precision oscillator with timing from GNSS to provide accurate and resilient timing required by modern defense systems. Typically, GNSS-DOs are comprised of the following:

  • GNSS receiver
  • High-stability local oscillator (crystal, MEMS TCXO or OCXO, or an atomic clock)
  • Disciplining algorithm (servo loop)
  • 1 pulse per second (PPS) input and a 1 PPS and 10 MHz output

Under normal operating conditions, the GNSS receiver serves as the primary timing reference for the GNSS-DO, providing output signals, such as 1 PPS and 10 MHz synchronized to the GNSS 1 PPS rising edge. During this phase, GNSS-DO continuously learns and characterizes the drift behavior of the local oscillator relative to the GNSS reference.

If GNSS signals become degraded, jammed or denied, GNSS-DO seamlessly transitions to the local oscillator as the timing source. Leveraging this learned behavior, the system actively compensates its outputs to maintain accurate time alignment.

This unique combination of long-term accuracy and short-term stability makes GNSS-DO a critical enabler for military communications, radar systems, electronic defense platforms, sensor fusion networks, and autonomous systems—where precise timing is essential for mission success and operational continuity.

Choosing an oscillator with the right stuff

Drift can be caused by temperature fluctuations, vibration, mechanical stress, power supply variations, and long-term aging effects. Left unchecked, drift accumulates over time, degrading synchronization accuracy and potentially impacting system performance.

The role of GNSS-DO is to continuously correct these errors when a reference signal is available and minimize their impact when it’s not. The better the oscillator’s inherent stability, the longer and more accurately the system can maintain synchronization during a GNSS outage.

This is how the different types of oscillators compare:

  • CSACs

Chip-scale atomic clocks (CSACs) offer great stability and the longest holdover performance, making them the benchmark for resilience in GNSS-denied environments. However, these advantages come with trade-offs in size, weight, cost, and procurement lead times that can limit their practicality for many deployed systems.

  • Crystal TCXOs and OCXOs

Traditional crystal-based TCXOs and OCXOs provide lower-cost alternatives and are widely used across communications, aerospace, and defense applications. While they can deliver strong timing performance, quartz is very susceptible to shock, vibration and mechanical stress, causing them to crack, break, or severely degrade performance. This makes them less suitable for increasingly rugged and mobile platforms.

  • MEMS TCXOs and OCXOs

Micro-electro-mechanical systems (MEMS)-based TCXOs and OCXOs are emerging as a viable solution. They provide robust resistance to vibration, shock, and environmental stress while significantly reducing size and power consumption. In many cases, MEMS OCXOs offer near atomic-level frequency stability over temperature and occupy 75x less volume. This combination enables system designers to achieve precise timing and extended holdover without the SWAP-C penalties.

Figure 1 MEMS OCXOs can outperform the alternatives for SWaP in critical military defense applications. Source: SiTime

About GDO-1000 module

The µPNT GDO-1000 module features dual-frequency L1/L5 GNSS reception with microsecond-class, 24-hour holdover, enabling highly accurate timing even in degraded or contested conditions. “Its holdover performance approaches what customers expect from atomic-class clocks, in a module that fits on a standard M.2 slot and draws approximately half a watt,” said Doug Russell, senior VP and GM for aerospace and defense at VIAVI.

Figure 2 The VIAVI µPNT GDO-1000 module featuring an advanced MEMS oscillator has dual frequency L1/L5 GNSS reception and microsecond-class 24-hour holdover and draws less than half a watt of power. Source: SiTime

Moreover, it integrates directly into modern compute platforms, time appliance cards, and embedded systems without custom mechanical design.

The module performance is enhanced by patented AI and ML algorithms which model and compensate for oscillator behavior across varying environmental conditions. At its core, MEMS oscillators provide improved thermal stability across the full military temperature range compared with traditional quartz OCXOs, while maintaining phase noise and Allan Deviation performance under vibration and shock.

The system also accepts an external 1 PPS input, allowing discipline from M-Code GPS, alternative navigation sources, or other external references without hardware modification. Despite its miniature size, it supports multiple 1 PPS and low-phase-noise 10 MHz coaxial inputs and outputs, providing flexible integration across complex systems.

Modern precision timing solutions such as the advanced OCXO platform and high-performance Super-TCXO architectures are designed to directly address the combined challenges of SWaP constraints and oscillator drift and loss of reference time in GNSS-degraded environments. By improving intrinsic frequency stability, reducing phase noise, and minimizing long-term aging effects, the devices help maintain more accurate local timekeeping when external references are unavailable.

As defense systems evolve from standalone platforms into interconnected networks of sensors, communications systems, and autonomous assets, timing is becoming as fundamental to system design as power and memory management. Precision timing enables everything from PNT to secure communications, sensor fusion, and synchronized operations across distributed missions.

The future belongs to timing solutions that deliver precision, resilience, and performance while minimizing SWaP and system complexity.

Tyler Hohmann is business development director for aerospace and defense at SiTime, where he leads strategy and execution across prime contractors, startups, and sub-system manufacturers to advance precision timing adoption in mission-critical systems. Prior to SiTime, he was VP of sales and marketing and co-founder of Safran Federal Systems.

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