Untethered dead reckoning (UDR): enhancing positioning in urban environments

Navigating urban complexity

Accurate and continuous positioning is essential across a wide range of applications, from automotive navigation and fleet management to asset tracking and mobile mapping. However, urban environments pose significant challenges to GNSS-based positioning systems. Dense cityscapes with tall buildings, tunnels, and overpasses often result in:

• Obstructed satellite visibility
• Multipath effects due to signal reflection
• Signal degradation or loss in narrow urban canyons

When using conventional GNSS receivers, these conditions can lead to degraded positioning performance, including increased time-to-first-fix (TTFF), erratic location updates, and reduced reliability in navigation and tracking applications.

Addressing GNSS limitations

To mitigate these challenges, two key technologies have emerged:

Dual-band GNSS

By leveraging signals on two frequencies, typically L1 and L5, dual-band GNSS improves positioning performance by:

• Reducing ionospheric delay errors
• Minimizing multipath interference
• Accelerating convergence to an accurate position fix

This approach is particularly effective in semi-obstructed environments, such as areas with partial sky visibility.

Assisted GNSS (A-GNSS)

A-GNSS enhances receiver performance in difficult signal conditions by providing satellite orbit and clock data via external sources, such as cellular networks. This enables:

• Faster acquisition of satellite signals
• Improved startup performance when exiting underground parking or starting up in dense urban areas

While both technologies improve GNSS performance, they are not sufficient in scenarios involving prolonged signal loss, such as multi-level parking buildings or tunnels.

Technology: how untethered dead reckoning (UDR) works

Dead Reckoning (DR), also called sensor fusion, estimates position based on motion data from onboard sensors, including accelerometers and gyroscopes, collectively known as an inertial measurement unit (IMU). 

Untethered dead reckoning (UDR) offers a scalable, vehicle-independent solution that combines GNSS and inertial sensor data through advanced sensor fusion. UDR delivers continuous, accurate positioning, even in the absence of satellite signals, without requiring integration with vehicle systems.

UDR combines different data inputs:

• GNSS signals, providing absolute velocity and heading
• Inertial measurement units (IMUs): accelerometers and gyroscopes
• Dynamic model: vehicle dynamics limitations, so called non-holonomic behavior (for instance, a car cannot move laterally or vertically).

A Kalman filter continuously fuses the inputs and estimates the values that are not directly observable. In an initial phase, GNSS signals are compared with inertial measurements to estimate sensor biases and drifts. Alignment is also estimated, if not manually configured. For an easy way to determine alignment angles, check out this guide. The calibration is automatic, and it typically completes within a few minutes under strong GNSS signal conditions. The calibration is faster with increased changes in speed and frequency of turns (e.g., figures of eight).

After completing the calibration, the filter corrects drift, smooths noise, and enables precise position, velocity, and heading estimation through continuous prediction and correction cycles. This enables robust estimation of position, velocity, and heading, even in difficult urban environment and during full GNSS outages. 

UDR integration and optimization

While dual-band GNSS can enhance positioning in urban environments, its benefits diminish in extreme conditions. Combining it with UDR offers limited additional value and increases system cost due to the need for specialized RF components. For applications with continuous urban operation, such as public transportation, ADR remains the preferred alternative to UDR.

UDR systems also support A-GNSS to improve startup performance. Although UDR can output a position immediately after startup, even with no satellite coverage, accuracy in DR-only mode will naturally drift over time and distance until a GNSS fix is achieved. A-GNSS ensures faster satellite acquisition and quicker transition to sensor fusion mode. For the best results, services providing ephemeris and ionospheric correction data, such as AssistNow Live Orbits, are recommended.

Additional benefits of UDR

Beyond bridging GNSS gaps, UDR enhances overall positioning quality:

• Accurate mileage tracking through tunnels and garages
• Reliable heading estimation for improved map matching
• Smooth trajectory visualization for analytics and reporting

These features support advanced navigation, driver assistance, and fleet monitoring applications.

Conclusion

Urban environments will continue to challenge GNSS-based positioning. Untethered dead reckoning provides a resilient, scalable solution that enhances positioning availability and accuracy in complex environments. By intelligently combining GNSS, inertial sensing, and assistance data, UDR enables robust performance across a wide range of mobility applications.

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