Inertial Navigation Systems (INS) are a family of devices that exploit inertia properties to determine the position and velocity of moving objects. Inertial Navigation Systems can guide missiles, model rocketry, uncrewed aerial vehicles, underwater vehicles, trains, and boats. For a system to provide accurate navigation data, it must account for several factors, including aircraft/boat movement, acceleration in all axes, and aerodynamic forces. This article will discuss the applications, methods for each inertial navigation system, and their differentiation.
Non-Accelerometer Based Systems
This system does not measure acceleration, but rather it detects velocity changes. It is also known as a “Sighting Navigation” system. This technology uses optical sensors to detect landmarks on Earth’s surface through lasers or cameras, which are then processed by an external database of terrain maps. Once the coordinates for the next destination have been determined, the craft can fly itself autonomously using this information alone. Further, you can effectively use these systems for high-speed flights where aerodynamic forces may cause problems with conventional navigation systems. Applications include uncrewed aerial vehicles (UAV).
Accelerometer Based Systems
This system senses movement and gravity for use in inertial navigation. People see it as the opposite of a Sight Navigation system; it does not require any pre-existing information stored within its database, but rather, this data is continuously collected by an on-board accelerometer that uses gyroscopes to maintain alignment with the current direction of travel. People first developed such autonomous inertial navigation systems during World War II, and their “limitations” we’re compensated for through two sensors – one for unguided movement and another for gravity. Applications include spacecraft, vessels, and submarines.
Gravity Gradient System
The gravity gradient system uses a combination of accelerometer and gravity-sensing technology. An older system, the gradiometer (not to be confused with directional), determines position by measuring the strength of gravitational forces in different directions. Weak variations in Earth’s gravitational field provide useful information for this navigation method. Applications include spacecraft and satellites.
How does an Inertial Navigation System (INS) work?
As we mentioned above an inertial navigation system (INS) uses an inertial measurement unit (IMU) consisting of accelerometers, gyroscopes, and sometimes magnetometers. The gyroscope and magnetometer provide an INS system with the same contributions that they provide to an AHRS. The gyroscope angular rate measurements are integrated for a high-update rate attitude solution, while the magnetometer (if used) provides a heading reference similar to a magnetic compass.
The computational unit is responsible for recording all inertial measurements and performing the necessary calculations, typically through the used of advanced Kalman filtering to determine attitude, velocity and finally position. The following sections will dive into the calculations necessary to determine attitude, velocity and position based on measurements from the IMU and also discuss the various specifications for the sensors and how they impact the overall accuracy of the INS.