Navigating in GNSS-Denied Envrionments

Global Navigation Satellite Systems (GNSS) are ubiquitous today, underpinning critical infrastructure across the defense and aviation sectors.  This dependency has introduced significant vulnerabilities, with a lack of GNSS signal exposing operations to potential risk of failure and worse. It is critical to offset this risk with processes and technologies that allow operations to continue whenever GNSS is denied.

What is a GNSS denied/degraded environment
GNSS is the term used to describe a constellation of satellites that globally broadcast timing, navigation and positioning data to receivers. Denial or degradation of GNSS occurs when the direct line of sight of a satellite’s signal is lost, disrupting or distorting data transmission. This can be caused by obstructions (e.g. tunnels, dense urban environments), jamming (intentional interference to block signals), or spoofing (broadcasting a more powerful, counterfeit, GNSS signal to mislead systems). 

Situations where GNSS signals are denied or degraded are an increasingly common occurrence globally, particularly in extreme weather conditions or in conflict zones. Loss of GNSS signal has serious implications for both commercial and military systems. In the defence sector, loss can lead to disruption in communication systems, reduced intelligence accuracy or increased risk of navigational errors. In aviation, critical activities such as air traffic control are dependent on GNSS for timing, positioning and synchronisation.

Navigating in GNSS-Denied Environments
Operating without reliable GNSS presents significant technical challenges, but these can be addressed using alternative technologies such as LiDAR, vision-based navigation, and inertial navigation systems (INS), typically in combination. 

LiDAR-Based Navigation
LiDAR operates by emitting a laser light toward a target and measuring the time it takes for the reflected light to return to the sensor. This information is used to accurately calculate distances, enabling the construction of high-resolution 3D maps of the environment. To improve positional awareness, many LiDAR systems are integrated with an Inertial Navigation System (INS), which uses accelerometers and gyroscopes to calculate the platform’s position, velocity, and orientation over time. When combined, the LiDAR-INS system can provide geo-referenced data, linking each point on the 3D map to a specific location and orientation in space.

In environments where GNSS signals are unavailable or unreliable, LiDAR-INS can operate using a technique called scan matching, where real-time LiDAR data is compared against previously recorded scans to estimate the platform’s relative motion. The INS supplies information that LiDAR alone cannot provide, enhancing the reliability of the scan data and enabling accurate navigation and mapping while GNSS data is unavailable.

Vision-Based Navigation
Optical data can also determine a platform’s location in GNSS denied environments. Platforms equipped with onboard cameras capture real-time imagery of the terrain below. This imagery is then compared to existing satellite or aerial image datasets to estimate the platform’s position relative to known landmarks.

Similar to LiDAR, accuracy depends heavily on factors such as light conditions and weather. However, the accuracy and robustness of vision-based navigation is also improved by integrating an INS. The INS provides continuous estimates of position, velocity, and orientation, allowing the platform to maintain navigation when visual data is unreliable or unavailable - such as over desert terrain or large bodies of water. 

Inertial Navigation Systems (INS)
INSs are the backbone of resilient navigation and indispensable for navigating when GNSS is unavailable or disrupted.  Where LiDAR and vision-based navigation rely on external references and data can be degraded by poor visibility, low light, or environmental obstructions, an INS operates independently, continuously calculating position, velocity, and orientation using only inertial sensor data.

Short-term GNSS outages can be covered by a tactical-grade INS alone, delivering adequate performance without additional sensors and offering a cost-effective solution. To handle extended outages, and to counteract inertial drift over time, INSs are typically integrated with complementary aiding sources:  odometers on land vehicles, altimeters and barometric pressure sensors on aircraft, and Doppler Velocity Logs (DVLs) for shallow water vessels and seabed survey vehicles.

At the heart of every INS is an Inertial Measurement Unit (IMU), containing accelerometers and gyroscopes that measure linear acceleration and angular velocity. The INS processes this raw motion data using sophisticated algorithms to estimate position, speed, and orientation in real time. Traditionally, high-end INSs have employed Ring Laser Gyroscopes (RLGs) or Fibre Optic Gyroscopes (FOGs) to achieve ultra-precise, low drift navigation. Although highly accurate, these technologies come with significant cost in terms of SWaP-C as they are relatively large, heavy and power-hungry. They also often require environmental conditioning and regular calibration.  All these factors limit their use on smaller, mobile, long-endurance or cost-sensitive platforms.

MEMS IMUs: Meeting the Growing Demand for Compact, Robust Navigation
As modern platforms become more compact and increasingly autonomous, demand is growing for navigation solutions that offer reliable performance without adding significant size, power, or cost burdens. This is particularly evident for platforms such as unmanned aerial vehicles, small ground robots, and autonomous underwater systems, where traditional high-end inertial systems are too large, power-consuming, or expensive to integrate. 

While MEMS inertial sensors have historically been viewed as low precision, this perception is changing. The latest MEMS IMUs are delivering tactical-grade performance, including excellent bias instability, angle random walk, and low noise. As a result, they are now viewed as a realistic alternative to RLGs and FOGs, able to deliver reliable INS functionality within tight SWaP-C constraints. 

These compact and rugged sensors can provide standalone inertial solutions for short GNSS outages, or form part of a multi-sensor system that integrates GNSS, LiDAR, vision, and other aids to maintain robust navigation in any environment. 

Devices such as Silicon Sensing’s DMU41 high performance IMU offer low noise and bias instability characteristics suitable for short-duration GNSS-denied navigation. The DMU41 delivers precise tactical-grade performance where space and power constraints rule out larger systems. Where size or budget constraints exist, compact commercial-grade units like Silicon Sensing’s DMU11 IMU provide stable motion sensing and attitude estimation in less dynamic or shorter-term operations.

The strength of these MEMS-based sensors is how they integrate with other technologies mentioned, including LiDAR and vision-based navigation. In an integrated system these MEMS IMUs significantly enhance system robustness and deliver resilient, drift-free positioning over extended durations.

Key Takeaways: Navigating Without GNSS
The global geopolitical climate highlights the risks of exclusive GNSS dependence and is increasing the demand for an effective, resilient navigation capability for contested or obstructed environments.  Tactical-grade MEMS sensors, with their excellent performance and low SWaP-C, are being adopted as a key component of these navigation solutions. 

To explore how Silicon Sensing’s product family can support your navigation requirements, contact us at [email protected] or check out some of our other articles here.