UAV Navigation in depth: Inertial Navigation

INTRODUCTION
 
1.   The navigation function forms part of the Guidance, Navigation and Control (GNC) system and consists of calculating a platform's location and velocity (also known as the state vector), as well as its orientation (or attitude). Navigation relies on input from a variety of sensors and subsystems.
 
2.   The output of the navigation function is the input for the control system, which in turn commands deflections of the control surfaces and values for other controls such as the engine.
 

Inertial Navigation System

INERTIAL NAVIGATION
 
3.   Inertial navigation depends only on input from sensors directly contained within the platform and that have no reference to external, artificial input (e.g. GPS) and so is not susceptible to tampering or hacking.
 
4.   Aim.   The aim of inertial navigation, also known as dead reckoning, is therefore to determine the position, velocity and attitude of the platform by using onboard inertial sensors.
 
5.   Basic Components.   The essential components of an Inertial Navigation System (INS) and their setup are as follows:
  • Inertial sensors: to acquire the data.
  • Position: placement of the sensors on the platform in order to determine the trihedral of reference.
  • Computer: to make calculations within the chosen coordinate system.
6.   Main Concepts.   
  • Inertial Measure Unit (IMU).   This device is able to measure and report attitude (roll, pitch and yaw), velocity, changes in altitude and gravitational forces acting on an aircraft. An IMU is typically composed 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 forces of inertia will be added. For this reason, accelerometers are often said to provide a 'noisy' signal.
    • 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 magnetism. 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.
The IMU intelligently compensates for the disadvantages of some sensors by fuzing input from others less affected, thus obtaining an output with reduced noise and less drift. The main problem with IMUs is that they naturally accumulate error during the process of integration of both angular and linear velocity.
  • Air Data System/Air Data Unit (ADS/ADU).   This system (or subsystem) measures ambient atmospheric conditions. It may include some or all of the following sensors:
    • Barometer: measures static pressure. As explained in the Introduction to altimeters, altitude may be derived from air pressure.
    • Static-pitot system:  The pitot tube measures total air pressure, equal to the sum of incidental air and static port pressure. This system is used to measure airspeed.
    • Thermometer: Through different principles it can measure the temperature. The temperature is necessary to estimate the density of the surrounding air. The density is used to calculate the TAS from the IAS.
  • 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 to estimate. The AHRS can typically be found within an Electronic Flight Instrument System (EFIS) as used in many manned aircraft cockpits. When an AHRS also provides air, altitude or external temperature information it is known as an Air Data Attitude & Heading Reference System (ADAHRS).

IMU AHRS Schema

  • Attitude Indicator: shows the orientation of the aircraft in relation to the Earth's horizon. The attitude indicator helps the pilot to fly under low visibility conditions.
  • Inertial Navigation System (INS): estimates the position, velocity and orientation of the aircraft without having to rely on external references. 

IMU AHRS INS

7.   Data processing and the intelligent fuzing of information from a variety of sensors is a core strength of the UAV Navigation AHRS and sets it apart from other, inferior products which may fail to compensate for false readings, out of range readings or the lack of input in a particular environment (e.g. flying in a GPS-denied environment). The UAV Navigation system allows the execution of accurate, robust and reliable flight control under highly dynamic conditions, as well as in degraded environments.