UAV Navigation in Depth: What is an IMU and what is it used for?

An Inertial Measurement Unit (IMU) is a device capable of estimating and reporting specific dynamic states such as angular velocity and accelerations. From these measurements other dynamic states can be inferred, such as attitude angles (roll and pitch), or velocity and position increments of the platform. 

IMUs are the main component of the inertial navigation systems commonly used in aircraft, unmanned aerial vehicles (UAVs) and other unmanned systems, as well as missiles and even satellites.

Where are IMUs used?

Inertial navigation only depends on inputs from different sensors directly contained within the platform, which are independent of an external source (unlike GNSS) and so are not susceptible to external manipulation.

In an inertial navigation system, the raw sensor data collected by an IMU is later processed by a CPU which, by means of different fusion algorithms, is capable of estimating attitude, position and velocity, based on direct measurements and the result of previous iterations. 

The algorithms run by the processor are able to fuse the input data of multiple sensors, detecting at the same time when a sensor is not working properly, or is out of its operating range, e.g. a magnetometer affected by the electromagnetic field from a nearby object. The IMU discards the input from the affected sensors and compensates for that loss with the other sensors available. This makes the system robust against individual (or even multiple) sensor failures. 

Components of an IMU

An IMU typically consists of:

• Accelerometers: measure the gravitational forces in a fixed coordinate system. For example, an accelerometer at rest on the surface of the Earth will measure '-1g', or -9.8 m/s2. When the platform is in motion the inertial forces are also present. For this reason, accelerometers are often said to provide a coupled measurement.
• Gyroscopes: measure angular velocity. A mechanical gyroscope includes a spinning wheel or disc. Thanks to conservation of angular momentum any change in the orientation of the axis of the spinning wheel will be registered by the sensor; the change in orientation of the platform may therefore be calculated. Different technologies and physical principles are used in the construction of gyroscopes. These include the most precise Fiber Optic Gyroscopes (FOG) based on the Sagnac effect and also the less precise Micro Electro-Mechanical (MEMS) units which are based on calculation of the Coriolis force by means of tiny vibratory structures. Gyroscopes are essential for calculation of orientation, but may suffer from drift - even when static. FOGS gyros are generally much more accurate than MEMS units.
• Magnetometers: measure the local magnetic field. A simple type of magnetometer is a compass, which measures the direction of the Earth's magnetic field in 2D. In recent years, magnetometers have been miniaturized (e.g. MEMS sensors). The Earth's magnetic field is a 3-dimensional vector that, like gravity, can be used to determine long-term orientation.

Inertial Navigation System

One of the biggest challenges facing any cutting-edge Attitude & Heading Reference System / Inertial Navigation System (AHRS/INS) is the ability to perform a mission in a degraded environment. In navigation, dead reckoning is the process of calculating a platform’s current position by using a previously determined one, and advancing it based upon known (Air Data System) or estimated speeds over the elapsed time and estimated orientation.
 
UAV Navigation develops high-end autopilots and is completely committed to manufacture products capable of providing precise and reliable estimation. Navigation in degraded environments (e.g. with no GNSS input) is one of our main goals.

The equipment used to produce the following results is the VECTOR, UAV Navigation's cutting-edge range of autopilots. VECTOR autopilots feature a highly advanced, high-end, MEMS-based AHRS/INS. It has been designed for system integration in avionics packages or other attitude sensing applications, and includes:

• Attitude Heading & Reference System (AHRS)
• Inertial Measurement Unit (IMU)
• Inertial Navigation System (INS)
• Air Data System (ADS)
• GNSS receiver

VECTOR family has accumulated thousands of hours of flight time and has proven itself in a variety of highly dynamic environments, giving outstanding results.

Difference between an IMU and an AHRS

Attitude and Heading Reference System (AHRS). Consists of sensors (gyroscopes, accelerometers and magnetometers) that provide attitude information for the platform. The difference between an IMU and an AHRS is the post processing system. The IMU reports data to an additional device that computes attitude and heading. These computers usually use Kalman filters for estimation. The AHRS can typically be found within an Electronic Flight Instrument System (EFIS) as used in many manned aircraft cockpits.