Terrain Based Navigation

Sun, 09/03/2023 - 23:20

Autonomous Unmanned Aircraft Systems (UAS) increasingly require navigation technologies capable of maintaining reliable operation in complex and contested environments. Although Global Navigation Satellite Systems (GNSS) provide highly accurate positioning under normal conditions, autonomous aircraft cannot always depend on continuous access to satellite navigation. Interference, jamming, spoofing, terrain masking, and other forms of signal degradation can affect GNSS availability. As a result, resilient UAS architectures are increasingly combining multiple onboard navigation technologies to reduce dependence on any single positioning source. Terrain Based Navigation (TBN) represents one approach to this challenge. By using information associated with the surrounding terrain as part of the navigation solution, an autonomous aircraft can complement inertial navigation and other onboard positioning technologies when external navigation services are limited or unavailable.

Integrated with sensor fusion and Guidance, Navigation, and Control (GNC) systems, TBN can contribute to the development of more resilient autonomous UAS capable of maintaining navigation continuity across demanding operational environments.

What Is Terrain Based Navigation (TBN)?

Terrain Based Navigation (TBN) is a broad navigation concept in which information associated with geographical terrain contributes to determining or refining the position of a moving platform.

For an unmanned aircraft, this can involve processing observations of the environment and relating them to previously available geographical or terrain information.

Depending on the architecture, TBN can operate alongside navigation technologies such as:

Inertial Navigation Systems (INS).
Global Navigation Satellite Systems (GNSS).
Radar or laser-based sensors.
Electro-optical sensors.
Digital elevation and terrain information.
Air data sensors.
Other onboard positioning technologies.

The objective is to provide an additional source of geographically referenced information that can strengthen the overall navigation solution.

Why Terrain Based Navigation Matters for Autonomous UAS

Navigation is one of the fundamental technological foundations of autonomous flight. A UAS must continuously maintain a reliable estimate of its state, including position, velocity, and orientation, for guidance and flight control functions to operate correctly.

In conventional conditions, GNSS can provide an accurate absolute positioning reference. However, UAS designed for demanding applications may encounter environments where satellite navigation becomes unreliable.

Potential causes include:

GNSS jamming.
GNSS spoofing.
Electromagnetic interference.
Signal obstruction.
Terrain masking.
Temporary loss of satellite visibility.

Terrain Based Navigation can contribute an additional source of navigation information that is not continuously dependent on receiving external satellite positioning signals.

This makes TBN relevant to the broader development of resilient and assured navigation architectures for autonomous aircraft.

Terrain as a Navigation Reference

The physical environment contains geographical characteristics that can provide useful information for navigation. Elevation changes, slopes, valleys, ridges, and other terrain features can form part of the geographical information available to an autonomous system.

Depending on the specific TBN approach, onboard sensors can provide observations associated with the surrounding environment. Navigation algorithms can then relate these observations to stored terrain or elevation information.

The resulting information can contribute to the aircraft's estimate of its geographical position.

Unlike satellite navigation, the terrain itself does not depend on the availability of an external radio-frequency signal. This characteristic makes terrain information particularly interesting as one element within a resilient multi-source navigation architecture.

Terrain Based Navigation and Inertial Navigation Systems

Inertial Navigation Systems are fundamental to autonomous UAS because they can continuously estimate aircraft movement without requiring external positioning signals.

An INS processes measurements from inertial sensors to estimate parameters related to:

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

However, small errors in inertial sensor measurements accumulate over time. This phenomenon can progressively increase uncertainty in the estimated aircraft position.

Terrain-based information can provide an additional geographical reference within the navigation architecture. When combined with INS, it can contribute to constraining accumulated navigation uncertainty during periods when GNSS corrections are unavailable or unreliable.

The combination of inertial and terrain-based navigation therefore provides complementary characteristics that are valuable for resilient autonomous flight.

Sensor Technologies Supporting TBN

Terrain Based Navigation is not necessarily associated with a single sensor technology. Different UAS architectures may obtain information about their environment through different onboard sensors.

Depending on platform requirements, these can include:

Radar-based sensors.
Laser-based ranging technologies.
Electro-optical cameras.
Infrared sensors.
Altitude measurement systems.
Inertial sensors.

Each sensing technology presents different characteristics in terms of range, environmental performance, integration requirements, and the type of information it can provide.

A modular navigation architecture allows UAS manufacturers to combine suitable sensor technologies with the aircraft's existing navigation and flight control systems.

Digital Terrain Information and TBN

Digital geographical information can play an important role in terrain-based navigation. Depending on the specific architecture, the navigation system may use representations of terrain elevation or other geographical characteristics as a reference.

This information provides a representation of the operating environment against which onboard observations can be evaluated.

The effectiveness of this approach depends on several factors, including:

Quality of available terrain information.
Resolution of geographical data.
Characteristics of the operating area.
Accuracy of onboard sensors.
Aircraft flight profile.

Terrain information should therefore be considered as one component of the overall navigation architecture rather than an isolated source of positioning.

Multi-Sensor Fusion in Terrain Based Navigation

Multi-sensor fusion is a key enabling technology for resilient navigation. It allows information from different sources to be combined into a unified estimate of the aircraft state.

A TBN-enabled navigation architecture can potentially integrate information from:

INS.
Terrain-related sensors.
GNSS when available.
Air data systems.
Magnetometers.
Vision-based navigation.
Other alternative positioning sources.

The navigation system can use complementary information according to the availability, quality, and reliability of individual sources.

This approach improves resilience by reducing the impact that the degradation or temporary loss of a single navigation source can have on the overall system.

Terrain Based Navigation in GNSS-Denied Environments

GNSS-denied operation is one of the main drivers behind the development of alternative navigation technologies for autonomous UAS.

When reliable satellite positioning is unavailable, an aircraft must depend on onboard systems to maintain a sufficiently accurate estimate of its state.

A resilient navigation architecture can combine several technologies, such as:

Inertial navigation.
Terrain Based Navigation.
Vision-aided navigation.
Air data measurements.
Alternative positioning technologies.

Rather than relying on a single GNSS replacement, this approach creates multiple complementary layers of navigation information.

TBN can form one of these layers, providing geographically referenced information derived from the physical environment.

TBN within Guidance, Navigation, and Control

For autonomous UAS, navigation must be closely integrated with the broader Guidance, Navigation, and Control architecture.

The navigation system estimates where the aircraft is and how it is moving. The guidance system determines the required flight behavior, while the flight control system manages the aircraft according to those commands.

A robust TBN-supported navigation solution can contribute information required for autonomous functions such as:

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

Integrating resilient navigation directly into the GNC architecture allows the flight control system to maintain more predictable autonomous behavior as navigation conditions change.

Terrain Based Navigation and Terrain-Aided Navigation

Terrain Based Navigation (TBN) and Terrain-Aided Navigation (TAN) are closely related concepts, and terminology can vary depending on the application and system architecture.

TAN typically emphasizes the use of terrain-derived information to aid an existing navigation solution, particularly an Inertial Navigation System.

TBN can be understood more broadly as a family of navigation approaches in which terrain or geographical information forms part of the aircraft's positioning architecture.

In practice, both concepts share an important objective: using the physical environment as an additional navigation reference to improve positioning resilience.

The precise implementation and terminology will depend on the sensors, navigation algorithms, geographical data, and system architecture used by a particular platform.

TBN and Terrain Contour Matching (TERCOM)

Terrain Contour Matching (TERCOM) is another concept associated with terrain-referenced navigation. While TBN describes a broader approach to using terrain information for positioning, TERCOM represents a more specific terrain-matching concept.

At a high level, TERCOM uses information related to terrain elevation and compares observed terrain characteristics with stored geographical information to provide a navigation reference.

This distinction is important because Terrain Based Navigation can encompass different sensing and navigation approaches, whereas TERCOM refers to a particular concept within the wider family of terrain-referenced technologies.

Advantages of Terrain Based Navigation for UAS

Integrating TBN into an autonomous UAS architecture can provide several potential benefits:

Reduced dependence on GNSS: terrain provides an additional source of geographically referenced information.
Greater navigation resilience: TBN can complement inertial and other onboard navigation technologies.
Support for GNSS-denied operations: terrain information can remain useful when satellite navigation is unavailable.
Multi-sensor compatibility: TBN can form part of a broader sensor-fusion architecture.
Support for autonomy: resilient navigation provides reliable state information to autonomous guidance and flight control functions.

These advantages make terrain-based technologies particularly relevant for UAS expected to operate with reduced dependence on external navigation infrastructure.

Challenges and Considerations for TBN

As with other alternative navigation technologies, the performance of Terrain Based Navigation depends on both the aircraft architecture and the operating environment.

Important considerations include:

Terrain characteristics.
Availability and quality of geographical data.
Sensor performance.
Environmental conditions.
Aircraft altitude and flight profile.
Onboard processing capabilities.
Integration with existing navigation systems.

Terrain information can be more distinctive in some geographical areas than in others. Different sensor technologies can also present different performance characteristics depending on weather, altitude, visibility, or other environmental factors.

For these reasons, TBN is particularly valuable as part of a diversified navigation architecture where multiple technologies complement one another.

Terrain Based Navigation and Assured Autonomy

Increasing levels of UAS autonomy require navigation architectures capable of maintaining reliable operation when individual sensors or external services become degraded.

This requirement is closely connected to the concept of assured autonomy, where autonomous platforms are designed to maintain predictable and dependable behavior across changing operational conditions.

TBN can contribute to assured autonomy by increasing the diversity of navigation information available to the aircraft.

When combined with inertial navigation, advanced sensor fusion, alternative positioning technologies, and robust flight control, terrain-based information can help reduce dependence on a single external navigation source.

The Future of Terrain Based Navigation

The growing importance of resilient navigation for autonomous UAS is accelerating the development and integration of terrain-based technologies.

Future navigation architectures are likely to combine an increasingly diverse set of complementary technologies, including:

Advanced inertial navigation.
Terrain Based Navigation.
Terrain-Aided Navigation.
Vision-based navigation.
Alternative positioning technologies.
Advanced multi-sensor fusion.

Advances in onboard processing, sensor technologies, geographical data, and navigation algorithms will continue to expand the possibilities for terrain-based positioning.

Rather than functioning as isolated backup systems, these technologies can become integrated components of resilient navigation architectures capable of adapting to the availability and reliability of different information sources.

Terrain Based Navigation (TBN) represents an important approach to increasing navigation resilience for autonomous UAS. By incorporating information associated with the physical terrain into the navigation architecture, TBN can complement inertial navigation and other onboard technologies while reducing dependence on continuous GNSS availability.

Its greatest value comes from integration. Combining terrain-based information with INS, sensor fusion, alternative positioning technologies, and robust Guidance, Navigation, and Control architectures can provide a more resilient foundation for autonomous flight in complex and GNSS-degraded environments.

As unmanned aircraft move toward higher levels of autonomy, resilient multi-source navigation will become increasingly important for maintaining reliable and predictable operation across demanding mission environments.

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, flexible system architectures, and reliable operation in complex environments, including scenarios where GNSS availability may be degraded or denied.