Terrain Referenced Navigation for UAS

Modern Unmanned Aircraft Systems (UAS) are increasingly expected to operate in complex environments where conventional navigation technologies may be degraded, disrupted, or unavailable. While Global Navigation Satellite Systems (GNSS) provide highly accurate positioning under normal conditions, interference, jamming, spoofing, terrain masking, or limited satellite visibility can affect their reliability. For defense and other demanding applications, maintaining autonomous navigation without continuous dependence on GNSS is therefore becoming an important capability. One approach is Terrain Referenced Navigation for UAS, which uses information about the surrounding terrain as an additional navigation reference.

By combining terrain information with inertial navigation and other onboard sensors, a UAS can improve the resilience of its navigation solution and maintain greater positioning continuity when satellite navigation becomes unreliable. Terrain-referenced technologies can therefore contribute to the development of more robust and autonomous unmanned aircraft capable of operating in challenging environments.

What Is Terrain Referenced Navigation for UAS?

Terrain Referenced Navigation for UAS describes navigation techniques that use characteristics of the terrain surrounding an aircraft as a source of positioning information.

Instead of relying exclusively on external radio-navigation signals, the aircraft can compare information obtained by onboard sensors with previously available terrain or elevation data. This information can complement the aircraft's existing navigation solution and help estimate its position relative to the environment.

A terrain-referenced navigation architecture may work alongside technologies such as:

• Inertial Navigation Systems (INS).
• Global Navigation Satellite Systems (GNSS), when available.
• Radar or laser-based altitude sensing.
• Electro-optical sensors.
• Air data systems.
• Digital terrain or elevation data.
• Other onboard navigation sensors.

The objective is not necessarily to replace every conventional navigation source, but to increase navigation resilience by combining complementary sources of information.

Why UAS Need Alternative Navigation Technologies

GNSS has become a fundamental component of modern unmanned aircraft because it provides accurate global positioning with relatively compact and efficient equipment. However, defense UAS increasingly operate in environments where satellite navigation cannot always be assumed to remain continuously available.

Navigation performance can be affected by several factors, including:

• GNSS jamming.
• GNSS spoofing.
• Electromagnetic interference.
• Terrain masking.
• Limited satellite visibility.
• Temporary signal degradation.

When GNSS becomes unreliable, an autonomous aircraft must be able to maintain sufficient knowledge of its position and flight state using other sources of information.

This requirement is driving the development of Assured Positioning, Navigation, and Timing (A-PNT) architectures that reduce dependence on any single navigation technology.

The Role of Terrain in Resilient Navigation

Terrain provides a naturally available reference that cannot be disrupted in the same way as a radio-frequency navigation signal. Mountains, valleys, slopes, elevation changes, and other geographical characteristics can provide useful information for estimating the position of an aircraft.

A terrain-referenced system can use measurements of the environment and compare them with stored terrain information. The resulting data can then contribute to the overall navigation estimate.

This concept is particularly relevant for UAS operating over areas where the terrain contains sufficient distinctive geographical characteristics to provide useful navigation references.

Terrain information can therefore become another layer within a broader resilient navigation architecture rather than functioning as an isolated positioning technology.

Combining Terrain Referenced Navigation with INS

Inertial Navigation Systems are particularly important for autonomous aircraft because they provide navigation information independently of external signals.

An INS uses measurements from inertial sensors to estimate parameters such as:

• Aircraft attitude.
• Velocity.
• Position changes.
• Acceleration.
• Angular motion.

One of the challenges of purely inertial navigation is that small measurement errors can accumulate over time, gradually increasing uncertainty in the estimated position.

Terrain-referenced navigation can provide an additional source of information that helps constrain this accumulated uncertainty. By combining inertial information with observations related to the surrounding terrain, the navigation architecture can maintain a more robust estimate of the aircraft state during periods without reliable GNSS.

The complementary nature of INS and terrain-referenced technologies makes their integration particularly valuable for resilient autonomous navigation.

Sensor Fusion for Terrain Referenced Navigation

Modern autonomous navigation increasingly depends on sensor fusion, where information from multiple sensors is combined to generate a more reliable estimate than would normally be possible from any individual source.

For terrain-referenced navigation, the flight control and navigation architecture may process information from multiple systems, including inertial sensors, terrain-related measurements, air data sensors, GNSS when available, and other onboard navigation sources.

Sensor fusion provides several potential benefits:

• Improved navigation continuity.
• Reduced dependence on individual sensors.
• Greater tolerance to temporary sensor degradation.
• More robust estimation of the aircraft state.
• Improved resilience in contested environments.

Rather than depending on a single navigation technology, the UAS can dynamically use complementary information sources according to their availability and reliability.

Terrain Referenced Navigation in GNSS-Denied Environments

Operating in a GNSS-denied environment represents one of the most demanding scenarios for an autonomous UAS. Without reliable satellite positioning, the aircraft must rely on onboard navigation technologies to maintain sufficient situational awareness for controlled flight and mission execution.

Terrain-referenced navigation can form part of this alternative navigation architecture by providing geographically referenced information independent of GNSS.

Depending on the UAS configuration, a resilient navigation architecture may combine:

• Inertial navigation.
• Terrain-referenced navigation.
• Vision-aided navigation.
• Air data measurements.
• Alternative positioning technologies.
• GNSS information whenever trustworthy signals are available.

This multi-source approach allows the navigation system to reduce reliance on continuous GNSS availability while maintaining autonomous flight capabilities.

Terrain Referenced Navigation and Autonomous Flight Control

A navigation system does more than determine where the aircraft is located. Its information is also essential for the guidance and flight control functions responsible for executing the mission.

Within a Guidance, Navigation, and Control (GNC) architecture, navigation continuously estimates the state of the aircraft. Guidance uses that information to determine the required flight behavior, while the control system manages the aircraft to follow the corresponding commands.

A reliable terrain-referenced navigation solution can therefore support autonomous functions such as:

• Route execution.
• Waypoint navigation.
• Altitude management.
• Autonomous mission progression.
• Contingency management.

The integration between navigation and flight control is particularly important when a UAS must continue operating autonomously despite degradation of external navigation services.

Advantages of Terrain Referenced Navigation for UAS

Integrating terrain information into the navigation architecture can provide several benefits for unmanned aircraft operating in demanding environments.

Among the most relevant advantages are:

• Reduced GNSS dependency: terrain information provides an additional navigation reference when satellite signals are unavailable or unreliable.
• Improved navigation resilience: multiple complementary sources can contribute to maintaining a robust navigation estimate.
• Support for autonomous operations: resilient positioning allows the flight control system to maintain autonomous behaviors during navigation degradation.
• Passive navigation potential: certain terrain-referenced approaches can reduce dependence on external radio-frequency navigation infrastructure.
• Integration with existing navigation systems: terrain information can complement INS, GNSS, and other onboard technologies.

These characteristics make terrain-referenced navigation particularly relevant for defense UAS designed to operate in contested or infrastructure-limited environments.

Challenges of Terrain Referenced Navigation

Terrain-referenced navigation also presents technical challenges that must be considered when integrating the technology into an autonomous aircraft.

Performance can depend on factors such as:

• Availability and quality of terrain data.
• Characteristics of the operating environment.
• Accuracy and performance of onboard sensors.
• Aircraft altitude and flight profile.
• Navigation system processing capabilities.
• Integration with the broader GNC architecture.

Some geographical environments provide more distinctive terrain information than others. As a result, terrain-referenced technologies are most effective when integrated within a multi-sensor navigation architecture capable of using different positioning sources according to operational conditions.

Terrain Referenced Navigation, TAN, TBN, and TERCOM

Several concepts are associated with the use of terrain information for navigation, and terminology can vary depending on the technology, architecture, and application.

Terrain-Aided Navigation (TAN) generally refers to the use of terrain-related measurements to assist an existing navigation solution, particularly inertial navigation.

Terrain-Based Navigation (TBN) can be used more broadly to describe navigation approaches in which geographical or terrain information contributes to aircraft positioning.

Terrain Contour Matching (TERCOM) is a specific terrain-referenced navigation concept based on comparing observed terrain characteristics with stored terrain elevation information.

Although these concepts are related, they should not necessarily be considered interchangeable. Each can represent a different approach to using terrain information within a resilient navigation architecture.

Terrain Referenced Navigation and Assured Autonomy

The evolution toward higher levels of UAS autonomy requires navigation systems that remain reliable even when external infrastructure becomes unavailable.

This concept is closely connected to assured autonomy: the ability of an autonomous platform to maintain predictable and reliable operation despite changing environmental conditions or degradation of individual systems.

Terrain-referenced navigation can contribute to assured autonomy by providing another independent source of positioning information within the overall navigation architecture.

Combined with inertial navigation, sensor fusion, robust flight control, and alternative navigation technologies, terrain information can help create UAS that are less dependent on continuous access to external navigation services.

The Future of Terrain Referenced Navigation for UAS

The increasing importance of operations in GNSS-degraded and GNSS-denied environments is accelerating the development of alternative navigation technologies for unmanned aircraft.

Future navigation architectures are expected to increasingly combine multiple complementary technologies rather than relying on a single positioning source.

Key areas of development include:

• Advanced multi-sensor fusion.
• Improved inertial navigation.
• Terrain-aided navigation.
• Vision-based navigation.
• Alternative positioning technologies.
• More resilient integrated GNC architectures.

As UAS become increasingly autonomous, the ability to determine their position reliably under different operational conditions will remain one of the fundamental requirements for dependable autonomous flight.

Terrain Referenced Navigation for UAS represents an important approach to improving navigation resilience and reducing dependence on continuous GNSS availability. By using information from the surrounding terrain as an additional positioning reference, unmanned aircraft can complement inertial navigation and other onboard technologies when conventional satellite navigation becomes degraded or unavailable.

Its greatest value lies within a broader multi-sensor navigation architecture. The combination of terrain information, inertial navigation, sensor fusion, alternative positioning technologies, and robust flight control can provide the foundation for more resilient autonomous UAS.

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