GNSS-Denied Navigation with Terrain-Aided Navigation

Tue, 09/03/2024 - 22:20

Reliable navigation is one of the fundamental requirements for autonomous Unmanned Aircraft Systems (UAS). Global Navigation Satellite Systems (GNSS) provide accurate positioning for a wide range of unmanned operations, but their availability cannot always be guaranteed in demanding or contested environments. Electronic interference, jamming, spoofing, terrain masking, or other forms of signal degradation can reduce the reliability of satellite-based positioning. For defense UAS expected to maintain autonomous flight under these conditions, alternative navigation technologies are becoming increasingly important. GNSS-Denied Navigation with Terrain-Aided Navigation (TAN) provides one approach to increasing navigation resilience. By combining inertial navigation with measurements related to the surrounding terrain, TAN can provide an additional source of positioning information when GNSS becomes degraded, unreliable, or unavailable.

Integrated within a multi-sensor Guidance, Navigation, and Control (GNC) architecture, Terrain-Aided Navigation can contribute to more resilient autonomous flight while reducing dependence on continuous access to satellite navigation.

What Is Terrain-Aided Navigation (TAN)?

Terrain-Aided Navigation (TAN) is a navigation approach that uses information associated with the terrain below or around an aircraft to assist its existing navigation solution.

The basic concept involves comparing observations obtained by onboard sensors with previously available terrain or elevation information. The resulting information can then be used as an additional navigation reference.

TAN is particularly valuable when integrated with an Inertial Navigation System (INS). Inertial navigation can operate independently of external signals, but small sensor errors accumulate over time and gradually increase position uncertainty.

Terrain-derived information can provide an additional reference that helps constrain this accumulated uncertainty, improving the overall resilience of the navigation solution.

The Challenge of GNSS-Denied UAS Navigation

GNSS has become one of the primary sources of positioning information for modern unmanned aircraft. Its global availability and accuracy make it highly valuable for autonomous navigation, route management, and other flight functions.

However, GNSS signals can be vulnerable to different forms of degradation, including:

Intentional jamming.
Spoofing.
Electromagnetic interference.
Terrain or structural masking.
Limited satellite visibility.
Temporary signal outages.

In these scenarios, a UAS that depends excessively on GNSS may experience degradation in its navigation solution.

Modern autonomous aircraft therefore increasingly require navigation architectures capable of maintaining sufficient positioning continuity using alternative onboard sources when GNSS is unavailable.

Why Inertial Navigation Is Critical in GNSS-Denied Environments

Inertial Navigation Systems play a central role in resilient UAS navigation because they do not require external radio-frequency positioning signals.

Using measurements from accelerometers and gyroscopes, an INS can continuously estimate parameters related to the aircraft's movement and orientation.

Inertial navigation offers several important characteristics:

Continuous navigation information.
Independence from external radio-navigation signals.
High update rates.
Integration with flight control systems.
Operation across different environments.

However, inertial navigation has an inherent limitation: small errors in sensor measurements accumulate over time. Without an external or complementary reference, this drift can gradually reduce positioning accuracy.

This is where Terrain-Aided Navigation becomes particularly valuable.

How Terrain-Aided Navigation Complements INS

The combination of INS and TAN brings together two complementary navigation technologies.

The inertial system provides continuous estimates of aircraft motion, while terrain-related measurements can provide geographically referenced information that helps constrain accumulated navigation uncertainty.

A typical integrated navigation architecture may include:

An Inertial Measurement Unit (IMU).
An Inertial Navigation System (INS).
Terrain-related sensing.
Digital terrain or elevation information.
Navigation processing and sensor fusion.
GNSS when reliable signals are available.

Rather than treating GNSS and TAN as mutually exclusive technologies, an integrated architecture can use different navigation sources according to their availability and estimated reliability.

This multi-source approach can provide greater navigation continuity across changing operational conditions.

Using Terrain as an Independent Navigation Reference

Terrain provides a naturally available geographical reference that does not depend on satellite signals or external radio-navigation infrastructure.

Variations in elevation and other geographical characteristics can provide information that contributes to estimating the aircraft's position relative to stored terrain data.

For autonomous UAS, this offers an important advantage: terrain-related navigation information can remain available even when the electromagnetic environment prevents reliable GNSS reception.

The usefulness of terrain as a navigation reference depends on factors such as the characteristics of the operating area, the quality of available terrain data, the performance of onboard sensors, and the overall navigation architecture.

For this reason, TAN is most powerful when considered as one component of a broader multi-sensor navigation solution.

Sensor Fusion in Terrain-Aided Navigation

Sensor fusion is fundamental to modern Terrain-Aided Navigation architectures. Instead of relying on a single positioning technology, the navigation system combines information from several sources to maintain a robust estimate of the aircraft state.

Depending on the UAS configuration, these sources may include:

Inertial measurements.
Terrain-related observations.
GNSS positioning when available.
Air data sensors.
Magnetometers.
Vision-based navigation sources.
Other alternative positioning technologies.

The navigation system can evaluate and combine these sources according to their availability and quality.

This approach increases resilience because temporary degradation of an individual source does not necessarily result in the loss of the complete navigation solution.

Transitioning Between GNSS and GNSS-Denied Navigation

A resilient autonomous UAS should be capable of operating across environments where GNSS availability can change during the mission.

The aircraft may begin operating with reliable satellite positioning and later encounter interference or signal degradation. It may subsequently recover GNSS availability as environmental conditions change.

A multi-sensor navigation architecture can support these transitions by combining information from GNSS, INS, TAN, and other onboard navigation sources.

The objective is to maintain navigation continuity rather than treating the loss and recovery of GNSS as completely separate operating modes.

This capability is particularly relevant for autonomous UAS that may operate over extended distances or across environments with varying levels of electromagnetic interference.

Terrain-Aided Navigation and the GNC Architecture

Navigation information is a fundamental input for the complete Guidance, Navigation, and Control architecture of an autonomous aircraft.

Navigation estimates the state of the aircraft, guidance determines the desired flight behavior, and the control system manages the aircraft according to those commands.

Integrating TAN into the navigation architecture can therefore support broader autonomous flight functions, including:

Route management.
Waypoint navigation.
Autonomous mission progression.
Flight path management.
Contingency responses.

The closer the integration between resilient navigation and flight control, the better positioned the UAS is to maintain predictable autonomous behavior when individual navigation sources become degraded.

Advantages of Terrain-Aided Navigation for UAS

Terrain-Aided Navigation can provide several advantages for unmanned aircraft designed to operate in demanding environments.

Reduced dependence on GNSS: terrain information provides a complementary geographical reference.
Support for inertial navigation: terrain-derived information can help constrain the uncertainty accumulated by an INS.
Navigation continuity: multiple navigation sources can support operation through periods of GNSS degradation.
Greater resilience: the navigation architecture is less dependent on the availability of any individual technology.
Support for autonomous operations: a robust navigation estimate helps maintain autonomous flight functions in contested environments.

These characteristics make TAN particularly relevant to defense UAS and other autonomous platforms requiring high levels of navigation resilience.

Considerations for Terrain-Aided Navigation

The effectiveness of TAN depends on the overall system architecture and the operational environment.

Relevant considerations include:

Accuracy and resolution of terrain information.
Performance of onboard navigation sensors.
Characteristics of the geographical environment.
Aircraft altitude and flight profile.
Available onboard processing resources.
Integration with INS and other navigation technologies.

Terrain characteristics can vary significantly between operating areas. Environments containing distinctive variations in elevation may provide different navigation information from areas with relatively uniform terrain.

Consequently, TAN should generally be considered part of a diversified navigation architecture rather than a universal replacement for all other positioning technologies.

TAN and Assured Positioning, Navigation, and Timing

The development of Assured Positioning, Navigation, and Timing (A-PNT) technologies reflects the growing requirement for systems capable of maintaining reliable navigation despite interference or degradation of conventional positioning services.

Terrain-Aided Navigation can contribute to an A-PNT architecture by providing an additional source of geographically referenced information independent of continuous GNSS reception.

Combined with INS and other alternative navigation technologies, TAN can help create a layered navigation architecture in which multiple sources complement one another.

This approach reduces single-source dependency and supports the broader objective of assured autonomous operation in challenging environments.

Terrain-Aided Navigation and Assured Autonomy

Navigation resilience is closely connected to the concept of assured autonomy. An autonomous aircraft cannot operate reliably if its flight control system loses confidence in fundamental information about the state and position of the platform.

By increasing the diversity of available navigation information, TAN can contribute to maintaining a reliable state estimate when GNSS becomes degraded.

This supports autonomous UAS capabilities by helping maintain the navigation information required by guidance, mission management, and flight control systems.

As autonomy increases, resilient navigation will become increasingly important because aircraft will be expected to manage longer and more complex missions with reduced dependence on continuous operator intervention.

The Future of GNSS-Denied Navigation with TAN

The development of electronic warfare technologies and the growing operational importance of unmanned systems are accelerating innovation in GNSS-independent and GNSS-resilient navigation.

Future UAS navigation architectures are expected to increasingly combine multiple technologies, including:

Advanced inertial navigation.
Terrain-Aided Navigation.
Vision-aided navigation.
Alternative positioning sources.
Advanced multi-sensor fusion.
Resilient integrated GNC architectures.

The objective is not simply to provide a backup for GNSS, but to create navigation systems capable of dynamically combining different sources of information throughout the mission.

This evolution will be fundamental for autonomous aircraft expected to maintain reliable operation in increasingly complex and contested environments.

Conclusion

GNSS-Denied Navigation with Terrain-Aided Navigation (TAN) provides an important approach to increasing the resilience of autonomous UAS. By combining terrain-derived information with inertial navigation, TAN can provide an additional geographical reference that helps maintain a robust navigation estimate when GNSS signals are degraded or unavailable.

Its greatest potential comes from integration within a multi-sensor navigation architecture where INS, terrain information, GNSS, and other complementary technologies work together according to their availability and reliability.

As autonomous UAS are required to operate in increasingly demanding electromagnetic environments, technologies such as TAN will play an important role in the development of resilient navigation and assured autonomy.

UAV Navigation-Grupo Oesía develops advanced autopilot and flight control technologies for demanding unmanned applications. Its guidance, navigation, and control solutions are designed to support resilient autonomous flight, multi-sensor integration, and reliable operation in complex environments, including scenarios where GNSS availability may be degraded or denied.