Loosely Coupled & Tightly Coupled INS & GNSS [2024 Guide]

Published by Aaron Nathan // June 1, 2024

In the realm of satellite navigation and positioning systems, the terms “loosely coupled” and “tightly coupled” are often found in discussions of the integration of Inertial Navigation Systems (INS) and Global Navigation Satellite Systems (GNSS). These methodologies play a pivotal role in determining the accuracy and reliability of location-based services, especially in challenging environments like dense urban landscapes where signal interference is likely.

However, the concepts of loose coupling and tight coupling transcend the domain of INS and GNSS integration and find applications across various fields that require system interaction, from agriculture mapping to construction surveying

Generally, loose and tight coupling refers to the degree of interdependence and interaction between various components or modules within a larger system. Understanding these concepts is essential for designing robust, resilient systems that can evolve and thrive in today’s dynamic and interconnected world.

This article will explore the ins and outs of tight and loose coupling, focusing specifically on applications for INS and GNSS.

 

What is loose coupling?

In a loosely coupled system, components have minimal dependencies on each other. They can operate independently and communicate through standardized interfaces or protocols. Changes to one component typically have little impact on others, making the system more flexible, scalable, and maintainable. Loosely coupled systems are often preferred in complex, dynamic environments where adaptability and autonomy are crucial.

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Take, for example, a web-based e-commerce platform. The front-end interface, back-end server, and database can all operate independently. They communicate through well-defined APIs, allowing developers to modify or upgrade each component without disrupting the entire system. The web at large is considered a loosely coupled system since each component can (and must) be manipulated and monitored on its own.

 

What is tight coupling?

On the contrary, tightly coupled systems have strong interdependencies between components. Changes to one module often require corresponding modifications in other parts of the system. While tightly coupled systems may offer efficiency and performance advantages in certain contexts, they can be more challenging to maintain and scale. Moreover, they may lack the flexibility needed to adapt to evolving requirements or technologies.

Take, for example, a tightly coupled software system utilized in the field of drone RTK (Real-Time Kinematic) navigation. In such a system, the various components responsible for flight control, GPS positioning, and sensor fusion are intricately linked and communicate closely to ensure precise and accurate navigation for the drone. 

Any modifications or enhancements to one component, such as the GPS positioning module, would necessitate corresponding adjustments in other modules, like the flight control or sensor fusion algorithms.

Tight coupling offers many advantages in terms of navigation accuracy and reliability, and in certain fields is essential. Yet due to the technical complexities introduced by tight coupling, changes like upgrading a GPS positioning system or integrating new sensor technologies may require extensive testing and validation across the entire system, thereby making it a complicated endeavor to adapt to evolving requirements or technological advancements. 

Luckily, tight coupling for these systems is made accessible due to professional services like Point One that offer pre-calibrated systems and provide ongoing support for users to fully leverage the benefits of tight coupling and sidestep the limitations.  

Point One’s Atlas Inertial Navigation System (INS) combines best-in-class hardware with RTK and Sensor Fusion to deliver ground truth precision right from the box. All at a price that allows you to put them in every car/truck/robot in your fleet.

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Loose Coupling vs. Tight Coupling

The distinction between loose and tight coupling lies in the depth of integration and the nature of information exchange between systems. 

In loose coupling, 

  • Components have minimal dependencies on each other
  • Changes to one component typically have little impact on others
  • Systems are more flexible, scalable, and maintainable
  • Are often preferred in complex, dynamic environments where adaptability and autonomy are crucial

In tight coupling, 

  • Components have strong interdependencies with each other
  • Efficiency and performance advantages are found
  • Can be more challenging to maintain and scale
  • Can lack the flexibility needed to adapt to evolving requirements or technologies

In the rest of this guide, we’ll explore loose and tight coupling as applied to INS and GNSS systems. 

 

How do INS and GNSS work together?

Global Navigation Satellite Systems, or GNSS, works by measuring the range of satellites that broadcast their position in the sky. By precisely calculating where these ranges intersect, users can determine their position on Earth down to a few meters.

This has been the fundamental method for modern satellite-based locations since the first systems went online in the early 1970s. This technique works well in open sky environments because of the direct line of sight to the satellites. However, GNSS signals may be obstructed or degraded in certain conditions like urban canyons, dense foliage, or indoor environments, which lead to inaccuracies or signal loss.

Meanwhile, Inertial Navigation Systems (INS) rely on sensors such as accelerometers and gyroscopes to continuously track the orientation, velocity, and position of a moving object relative to an initial reference point. While INS provides real-time measurements, since it is a relative navigation system, it is prone to inherent errors such as drift over time.

By using the outputs of both an absolute and relative navigation system (GNSS and INS, respectively), GNSS/INS integration yields a far more accurate and robust positioning solution than either system is capable of offering on its own. This integrated approach is widely used in various applications, including navigation for aircraft, ships, autonomous vehicles, and mobile devices, where reliable positioning information is essential for safe and efficient operation.

What is Sensor Fusion?

Sensor Fusion is a sophisticated algorithm that fuses and integrates data from multiple sensors such as GNSS and INS so that they can work together. Sensor fusion can integrate data from satellites, LiDAR, GPS, and cameras to create a comprehensive and accurate representation of the environment or object being monitored. By creating models based on physics, it can predict and correct expected errors in sensor measurement, thereby improving the reliability and accuracy of location information.

Sensor fusion’s ability to blend information from various sensors and incorporate additional contextual data allows users to create a unified framework that enhances the precision of location estimation. Thus, sensor fusion is a critical component in both loose and tight coupling scenarios with integrated navigation systems like GNSS/INSS.

 

What causes inaccuracy in GNSS systems and why is tight or loose coupling necessary?

In cities, traditional GNSS systems struggle due to signal interference caused by buildings. The signals bounce off the structures creating errors in a process called multipath, distorting their accuracy like a bent tape measure.

Consequently, the GNSS receiver must handle these erratic signals and often resorts to averaging them for display. As the receiver moves, the changing reflection angles further degrade the signal, leading to an unreliable and erratic plot.

As a result, multipath error and its effects cause traditional GNSS signals to be inherently inaccurate in dense urban areas. This creates particular challenges for autonomous land vehicles, necessitating tight and loose coupling.

Take, for example, the signals illustrated above. When the satellite signals bounce off buildings, vehicles, or other surfaces before reaching the receiver, they create errors called multipath. As a result, the receiver might get multiple versions of the same signal at different times, leading to errors in position calculation, as the GNSS receiver resorts to averaging them for display. As the receiver moves, the changing reflection angles further degrade the signal, leading to an unreliable and erratic plot.

Advancements in technology have introduced solutions to mitigate these challenges, such as the use of NTRIP (Networked Transport of RTCM via Internet Protocol). NTRIP allows GNSS receivers to access real-time correction data via the internet. By receiving correction data from reference stations located in optimal positions, GNSS receivers can enhance the accuracy of their positioning solutions, even in urban environments.

To integrate NTRIP with INS/GNSS systems, tight or loose coupling is needed for accuracy, reliability, and precision. Tight coupling and loose coupling help address the challenges by combining the strengths of multiple systems. 

 

What is loose coupling in GNSS & INS?

In the context of GNSS/INS integration, loose coupling involves strategies such as averaging GNSS signals and incorporating data from Inertial Measurement Units (IMUs) and sensor fusion algorithms to enhance navigation accuracy. 

As established, GNSS and INS measurements alone carry a degree of uncertainty. Yet when loosely coupled, the degree of uncertainty is greatly diminished, providing highly accurate location information that encompasses parameters such as position, velocity, and attitude from the INS.

Benefits of Loose Coupling

Loose coupling offers a variety of benefits, including simplicity, ease of implementation, and cost-effectiveness. Since loose coupling allows components to operate independently, developers can modify or replace individual components without affecting the entire system. This flexibility makes it easy to adapt to changing requirements or integrate new features. 

Due to its modularity, loose coupling is relatively easy to manage and cost-effective. Each component can be developed, tested, and maintained independently without a tremendous amount of technical expertise.

Loose Coupling Limitations

While loose coupling slightly improves navigation accuracy, it may not be sufficient for all scenarios. Loose coupling relies on integrating accelerometer data to calculate velocity and position, which can introduce drift over time due to the averaging of potentially erroneous GNSS data.

This phenomenon compromises the accuracy of the navigation solution, necessitating careful consideration of its application.

 

What is tight coupling in GNSS/INS?

Another method, known as tight coupling, involves using measurements of aiding signal parameters to mitigate drift in an INS. In comparison to loose coupling, tightly coupled systems are able to update the error states of the INS, even when there is insufficient GNSS data to fix a position.

Tight coupling for GNSS

This can happen when fewer than four GNSS satellites are visible, making it impossible to determine a position solution based solely on GNSS information.

In loosely coupled systems, this situation leads to a complete outage of the data. However, tightly coupled systems can make use of limited GNSS measurements, allowing for partial mitigation of the INS error drift.

To achieve this, tightly coupled systems continuously calibrate the IMU in real-time, particularly when the GNSS signal is unobstructed. This calibration ensures accurate knowledge of the IMU bias and trains the IMU to anticipate the future location of the GNSS signal (anticipatory modeling).

By allowing the IMU to assess the validity and accuracy of the GNSS signal and select the GNSS signal that aligns with its prediction, a tight coupling between the IMU and GNSS can be established.

Benefits of tightly coupled GNSS

Benefits of Tight Coupling for INS 

Essentially, loose and tight coupling differ based on the type of information shared between the individual systems. In loose coupling, a processed GNSS solution is merged with an INS solution; in tight coupling, raw GNSS measurements are combined with INS-predicted measurements.

By actively mitigating drift and leveraging limited GNSS measurements, tightly coupled systems offer improved accuracy and reliability, especially in challenging environments with obstructed or unreliable GNSS signals.

Incorporating RTK into tightly coupled INS/GNSS systems not only enhances positioning accuracy but also improves the system’s overall reliability and robustness, making it an indispensable tool for a wide range of applications, from precision agriculture to autonomous vehicle navigation.

Tight Coupling Limitations

Tightly coupled systems require a sophisticated software design that may involve managing a large number of system data, which can be complex to manage.

Tightly coupled systems are difficult to scale, as adding new components or increasing the size of existing ones often necessitates significant changes to other parts of the system. Moreover, tight coupling may result in more code duplication compared to loosely coupled systems, as components need to communicate more frequently, making maintenance and updates challenging over time. 

Finally, tight coupling leads to high dependency among system components because they are highly connected. Consequently, modifications to one component may trigger changes in the other, limiting flexibility and making it challenging to respond to changing requirements. As the system grows in complexity, high coordination among developers becomes essential.

However, this complexity comes with the benefit of being able to extract valuable data from fewer GNSS satellites. By modeling observation errors on a per-satellite basis from calibrations, these systems significantly increase accuracy and reduce data down time.

Fortunately, the limitations of tight coupling can be effectively mitigated with the assistance of professional services such as Point One. Point One offers solutions that have already addressed the complexities of tight coupling, providing users with pre-calibrated systems that offer optimal performance without the need for extensive setup or calibration. 

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Point One’s team of experts provides ongoing support, allowing users to harness the benefits of tight coupling for INS without encountering any of the associated limitations.

More about Loose Coupling and Tight Coupling

Now that we’ve established an understanding of the many applications of loose coupling and tight coupling, and how they can aid in fusing GNSS/INS data, let’s review the basics.

What is the difference between tight and loose coupling?

Essentially, loose and tight coupling differ based on the type of information shared between the individual systems. In loose coupling, a processed GNSS solution is merged with an INS solution; in tight coupling, raw GNSS measurements are combined with INS-predicted measurements.

What is the difference between no coupling and loose coupling?

No coupling implies the absence of integration or interaction between components within a system. In a system with no coupling, each component operates independently and does not communicate or share data with other components. 

On the other hand, loose coupling implies some level of interaction or integration between components, albeit with minimal dependencies. While components in a loosely coupled system can operate independently, they can also exchange data or communicate through standardized interfaces.

What is another name for loose coupling?

Looser coupling is also known as integration-level coupling. This term emphasizes the minimal dependencies between components in a loosely coupled system, where integration occurs at a higher level through standardized interfaces or protocols rather than direct interactions.

What is tight coupling also known as?

Tight coupling is synonymous with deep integration or high cohesion. These terms highlight the strong interdependencies between components in a tightly coupled system, where components share data and resources directly and have a high degree of reliance on each other for operation.

Access Tightly Coupled INS for Ultimate GNSS Accuracy

The distinction between loose and tight coupling lies in the type of information being shared between the individual systems. While loose coupling involves merging a processed GNSS solution with an INS solution, tight coupling integrates raw GNSS measurements with INS-predicted measurements.

Tightly coupled systems require a sophisticated software design that may involve managing a large number of system data, which can be complex to manage.

This complexity, however, comes with the benefit of being able to extract valuable data from fewer GNSS satellites. By modeling observation errors on a per-satellite basis from calibrations, these systems significantly enhance accuracy and minimize data downtime.

The best way of getting all of the benefits of tight coupling by sidestepping the challenges is to rely on the experts at Point One. Point One Atlas has achieved this technological feat while ensuring affordability, thereby making it accessible to the masses.

You can finally titch the complicated and clunky post-processing workflows and get accurate data right from the device. Atlas’ modern web UI and is equipped with on-device and ethernet based streaming of data.

Embark on your journey toward optimal navigation accuracy.

Aaron is an entrepreneur and technical leader with over two decades of experience in robotics and software/hardware development. He has deep domain experience in sensor fusion, computer vision, navigation, and embedded systems, specifically in the context of robotic applications.
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