A Road-Mobile GNSS-Disciplined Oscillator for Accurate Synchronization of Vehicular Microwave Measurements

Abstract: Precise synchronization is essential in various technical disciplines, being especially challenging in mobile scenarios. Unfortunately, state-of-the-art global navigation satellite system (GNSS) disciplined oscillators (GNSSDOs) are designed and optimized for stationary operation. We present a novel solution that is optimized for mobile use from the ground up. The centerpiece is a precise oven-controlled crystal oscillator (OCXO) that is optimized for low sensitivity to dynamic accelerations. A state-of-the-art GNSS timing module is used to discipline it. We evaluate the system by comparing it with state-of-the-art test equipment in a real-world test drive through diverse environments. After compensating for the stationary offset, the state-of-the-art devices deviated by up to 2315 ns, while with our devices, the deviation never exceeded 22.6 ns. It is evident that the devices designed for laboratory use perform inadequately in mobile operation and that our novel solution enables a significant leap in accuracy.

• The paper introduces a mobile GNSSDO that achieves an order-of-magnitude improvement in timing error, reducing errors from 2315 ns to 22.6 ns.
• The paper details the integration of a low phase noise OCXO, differential GNSS corrections, and a digital control loop for precise frequency stabilization.
• The paper validates its design through real-world vehicular tests that enhance spatial measurement accuracy for advanced RF applications.

GNSS-Disciplined Oscillator for Vehicular Microwave Measurement Synchronization

Motivation and Problem Statement

Precise temporal synchronization is critical in distributed radio frequency (RF) measurement campaigns, especially when spatial accuracy is paramount. Signal propagation at light speed means that even minor timing errors, on the order of nanoseconds, induce meter-scale spatial discrepancies. Conventional solutions utilize GNSS-disciplined oscillators (GNSSDOs) built around oven-controlled crystal oscillators (OCXOs), but existing commercial implementations such as the Stanford Research Systems FS740 perform poorly in dynamic, mobile environments due to high acceleration sensitivity and lack of differential GNSS capabilities. Previous work in MEMS-based GNSSDOs and advanced phase-coherency mechanisms addresses related challenges but lacks comprehensive real-world vehicular validation and distributed synchronization focus.

System Architecture and Design Choices

The paper introduces a GNSSDO system specifically engineered for mobile vehicular scenarios. Key architectural decisions include:

• OCXO Selection and Characterization: An O-40-ULGS-100M OCXO with low phase noise ($-165\, \mathrm{dBc}$ at $1\, \mathrm{kHz}$ offset) and exceptional acceleration insensitivity ($0.2\, \mathrm{ppb/g}$) underpins the system, providing robust frequency stability during dynamic motion.
• GNSS Receiver and Differential Mode: The u-blox ZED-F9T module, optimized for multi-band timing and supporting RTMC 3.3-based differential operation, is employed. A stationary base station supplies correction messages over IP, minimizing time deviations across distributed nodes.
• Digital Control Loop: The MCU (STM32G474) implements a high-resolution timer and actuates the OCXO via a 16-bit DAC, augmented with dithering and noise suppression for fine-grained frequency discipline. Precision time-tagging is achieved using a TDC7200 chip, ensuring $35\, \mathrm{ps}$ timing resolution for pulse events.
• Clock Distribution: Outputs are LVCMOS logic standard signals, supporting multiple reference frequencies (100, 50, 20, and 10 MHz) via an LMK01801 distribution chip. This configuration mitigates PLL-induced phase noise amplification and reduces zero-cross uncertainty compared to legacy sine wave reference clocks.

Experimental Methodology

Comprehensive real-world validation is performed in a vehicular context. The test setup involves two instances of the novel GNSSDO and three FS740 reference instruments (equipped with rubidium time bases), installed in varying orientations and locations within a vehicle to maximize differential acceleration exposure. GNSS antennas are widely distributed. Synchronized pulse events are exchanged via length-matched coaxial cables and time-stamped by each device, with prior laboratory calibration for static offsets. Devices are allowed extensive thermal and GNSS acquisition stabilization (14 hours garage, 2 hours open sky).

Numerical Results and Analysis

The primary metric is relative node-to-node timing error in vehicular motion across diverse environments, including urban canyons, rural stretches, highways, and deliberate high-acceleration stress tests. Strong numerical results highlight the following:

• State-of-the-art FS740 devices: Deviate by up to $2315\, \mathrm{ns}$, with significant performance instability and frequent warnings of EFC range exceedance.
• Novel GNSSDO instances: Maintain maximum inter-device deviation of only $22.6\, \mathrm{ns}$ through all environments and motion profiles.
• Claim: The new GNSSDO achieves more than an order-of-magnitude reduction in timing error relative to commercial laboratory solutions, with mobile robustness validated empirically.

Practical and Theoretical Implications

The research provides critical advancement for distributed microwave measurement synchronization in mobile contexts, such as vehicular testbeds for wireless communication protocols and OTA characterization. Reduced timing error directly enhances spatial measurement fidelity, enabling finer-grained channel modeling, geolocation, and signal analysis. Differential GNSS and low-noise digital control architectures form a scalable foundation for future distributed test systems.

The implications extend toward ultra-low phase noise and high-coherency distributed node operation, supporting emergent applications like Massive MIMO synchronization in 6G environments. The modular form factor, integration with IMU sensors, and potential for acceleration compensation algorithms pave the way for further reductions in timing instability and expanded deployment scenarios.

Future Directions

Possible future optimizations include advanced digital control strategies exploiting IMU data for real-time acceleration compensation, integration of adaptive GNSS correction schemes responsive to environmental multipath and signal blockage, and miniaturization for wider vehicular or drone deployment. Extension to phase-coherent multi-node synchronization over larger measurement areas may facilitate improved characterization of distributed antenna arrays and cooperative communication systems.

Conclusion

The paper presents a mobile-centric GNSSDO solution for accurate synchronization in vehicular microwave measurement environments. Empirical results demonstrate a significant reduction in relative timing error compared to state-of-the-art laboratory instruments. The design's robustness under dynamic acceleration and adverse GNSS conditions addresses longstanding practical challenges. The theoretical and practical advancements support improved spatial accuracy, expand measurement campaign viability, and lay the groundwork for future distributed synchronization architectures in mobile RF systems (2604.24060).