As technologies like autonomous vehicles shift from science fiction to reality, Inertial Navigation Systems (INS) are becoming critical. INS enables vehicles and devices to determine their position, orientation, and velocity without relying on external signals like GPS. This capability is especially valuable in environments where GPS signals are unreliable or unavailable.
The increasing importance of INS is evident in the autonomous vehicle sector. In 2021, approximately 80 companies were actively testing around 1,400 autonomous cars, trucks, and other vehicles across 36 U.S. states. This growth highlights the need for dependable navigation systems like INS, particularly as we move towards a future with widespread autonomous transportation.
This article will delve into the details of INS, explaining what it is, how it functions, and why it’s a pivotal technology across various industries.
Understanding Inertial Navigation Systems (INS)
An Inertial Navigation System (INS) is a navigation technology that calculates an object’s position, velocity, and orientation by measuring its acceleration and rotation, without needing external references.
Once an INS establishes its initial location, it continuously updates its position based on these measurements to track its movement.
INS Compared to GPS
GPS (Global Positioning System) is a satellite-based navigation system that provides location and time information in all weather conditions, anywhere on or near the Earth. It’s an integral part of [GNSS (Global Navigation Satellite Systems)](https://www.gps.gov/systems/gnss/#:~:text=Global%20navigation%20satellite%20system%20(GNSS,a%20global%20or%20regional%20basis.), a broader term encompassing various satellite navigation systems.
GPS, an example of absolute navigation, determines an object’s position relative to a fixed point using satellite signals, requiring a clear line of sight to function correctly.
INS, in contrast, is a self-contained system, representing a form of relative navigation. It operates independently of external signals, calculating an object’s position relative to its starting point or previous location. This makes INS more reliable in areas where GPS signals are weak or obstructed, such as tunnels or urban environments.
INS systems, however, have their limitations. Small errors in acceleration and rotation measurements can accumulate over time, leading to a drift in the calculated position. To mitigate this, many commercial INS systems are integrated with a GNSS receiver (like GPS) for calibration and enhanced accuracy.
This combination leverages the strengths of both technologies: the independence of INS and the accuracy of GPS. Furthermore, integrating an NTRIP (Networked Transport of RTCM via Internet Protocol) system can provide additional benefits.
NTRIP service providers deliver precise GNSS data, which helps reduce the drift of the INS. This integration ensures that INS maintains its accuracy over extended periods and in challenging environments where GNSS signals may be compromised.
The Inner Workings of INS
The core of an INS is the Inertial Measurement Unit (IMU), which includes gyroscopes, accelerometers, and sometimes magnetometers or GNSS receivers.
The Role of the IMU
The IMU (Inertial Measurement Unit) is the central component of INS. It is a self-contained system that gathers raw sensor data to compute the position, orientation, and velocity of the object to which it is attached.
The primary role of the IMU is to detect changes in motion and orientation, achieved through gyroscopes, accelerometers, and, optionally, magnetometers or GNSS sensors.
Gyroscopes
Gyroscopes within an INS measure the rate of rotation around an object’s axes, determining its orientation or angular velocity. Modern IMUs often use MEMS (Micro-Electro-Mechanical Systems) gyroscopes.
These devices detect changes in orientation by measuring the Coriolis effect on vibrating elements within the gyroscopes.
Accelerometers
Accelerometers measure the acceleration forces acting on an object, indicating changes in speed or direction. By integrating acceleration data over time, the INS calculates velocity and displacements.
This process, known as dead reckoning, allows the INS to track the object’s position relative to a known starting point.
Magnetometers or GNSS Sensors
While not always included, magnetometers can be integrated into an IMU to provide additional data about an object’s orientation relative to the Earth’s magnetic field. Magnetometers are useful for determining heading information, which, combined with gyroscopes and accelerometers, can provide comprehensive orientation data.
Most commercial INS systems also incorporate GNSS sensors to increase accuracy. GNSS, like GPS, provides absolute position information, which can be used to validate or correct the position and velocity estimates obtained from the IMU. This combination is known as sensor fusion.
The GNSS Receiver Advantage
A significant challenge with INS is error accumulation or drift over time, mainly due to small inaccuracies in the measurements of accelerometers and gyroscopes. A GNSS receiver, such as GPS, GLONASS, or Galileo, offers absolute positioning information. When integrated with INS, GNSS data can correct and calibrate the INS outputs, reducing drift and improving overall accuracy.
Although INS can operate independently in environments where GNSS signals are weak or blocked, such as underwater, indoors, or in urban canyons, the availability of GNSS signals provides a reliable external reference to ensure the INS’s accuracy. This integration ensures the navigation system remains functional and accurate in challenging environments, switching seamlessly between GNSS-aided and GNSS-denied models.
The combined INS/GNSS system is essential in applications where precision and reliability are critical, such as autonomous vehicles, aircraft navigation, and maritime navigation. In these applications, even small errors can have significant consequences, making the enhanced accuracy and reliability of the combined systems indispensable.
When integrating GNSS receivers with INS, the decision to use existing systems or build your own, such as an RTK system, can be complex. Ultimately, a professional RTK service provider can help mitigate any issues that may arise when combining INS and GNSS.
Inside INS Calculations
Inertial Navigation Systems perform complex calculations to determine an object’s position, velocity, orientation, and altitude. These calculations are fundamental to the system’s ability to provide accurate navigation information.
Altitude Determination
INS calculates altitude by integrating acceleration data along the vertical axis, measured by the accelerometers. This integration involves calculating the change in velocity over time and then integrating this velocity to determine the change in altitude.
Gyroscopes also contribute by providing the orientation of the accelerometers, ensuring the correct interpretation of vertical acceleration, especially when the vehicle or object is not level.
Position, Velocity, and Orientation Tracking
INS determines position by dead reckoning, calculating how far the object has moved from a known starting point. This is achieved by double-integrating the acceleration data over time, first to find velocity and then to find displacement or change in position.
INS continuously calculates the object’s velocity by integrating the acceleration data. This integration is done in three dimensions to provide a complete velocity vector, indicating how fast the object is moving and in which direction.
Gyroscopes within the INS measure the rate of rotation around the object’s axes. By integrating these rotation rates over time, the INS can calculate the object’s changing orientation in three-dimensional space.
Correcting for Accuracy
Datums and epochs influence the accuracy of these INS calculations. In the context of INS, a datum refers to a reference model of the Earth used for geodetic calculations, including parameters like latitude, longitude, and altitude.
To ensure that position data from the INS is consistent and comparable with other systems, such as GPS, the INS must align its calculations with a specific datum.
Epochs are specific instants in time used as reference points for time-based calculations. In INS, the precision with which time is measured and synchronized plays a crucial role, especially when integrating or aligning data from external sources like GNSS. Accurate timekeeping is essential for the correct integration of INS data over time.
However, small errors in the INS sensor measurements can accumulate over time, leading to drift in the calculated position, velocity, and altitude. To mitigate this drift, periodic corrections using external references, such as GNSS data, are necessary.
Various GNSS correction methods, such as RTK, PPP, and SSR, can significantly enhance the accuracy and reliability of INS systems in various navigational contexts.
Applications of INS Across Industries
INS has numerous applications due to its ability to accurately track position, orientation, and velocity without external references. Here’s a look at some primary uses:
- Automotive: In the automotive sector, particularly in self-driving vehicles, the precision and reliability of INS are essential. Innovations are at the forefront of applying INS in this field, offering precise navigation solutions crucial for the safety and efficiency of autonomous vehicles.
- Aviation: Aircraft use INS for flight control and navigation, especially during long flights over areas without ground-based navigation aids. In drone navigation, the role of RTK in conjunction with INS systems becomes increasingly important, providing stability and accuracy in flight control.
- Marine: Ships and submarines rely on INS for navigation, particularly when traveling in areas where GPS signals are weak or unavailable.
- Space Exploration: Spacecraft use INS to navigate in space, where GPS is not an option.
- Military Applications: INS is widely used in military applications to navigate various vehicles and weapons systems precisely.
Enhancing Inertial Navigation Systems with GNSS
Given INS’s complexities and inherent limitations, such as error accumulation over time, integrating GNSS can significantly enhance navigation systems’ overall accuracy and reliability.
The future of INS appears promising, especially as the technology becomes more affordable. As INS systems become more accessible, we can expect them to be integrated into a wider range of products, from consumer electronics to more advanced autonomous vehicles. This expansion will enhance the capabilities of these products and drive innovation in how we navigate and interact with the world around us.
