Inertial Reference System (IRS) | Inertial Navigation

Inertial Reference System (IRS)  image download

Inertial Reference System (IRS) 

The Inertial Reference System (IRS) and Inertial Navigation System (INS) are exceedingly precise navigation systems that do not require any external input, such as ground radio data.

An INS/IRS is a self-contained system that does not rely on radio signals from a ground navigation unit or transmitter to function. The IRS calculates attitude, velocity, and direction based on acceleration measurements taken from a particular starting location. Calculations based on the forces encountered by INS accelerometers are used to keep track of the aircraft's location. An INS converts applied forces into position information using complicated calculations performed by an INS computer. 

While the aircraft is parked on the ground, an interface control head is utilised to enter starting point position data. This is referred to as initialising. In modern INS systems the IRS is initialised using the GPS, therefore the pilot is no longer required to do initialisation. GPS also sends data to the IRS computer for mistake correction.All motion of the aircraft is then detected by the built-in accelerometers and processed by the computer. 

The IRS is a separate entity on modern aircraft that provides data to the FMC (Flight Management Computer). 

IRS Components

The IRS comprises of the following components:

  • Gyroscopes
  • Linear Accelerometers
  • Navigation Computer
  • Clock


Gyroscopes are instruments that detect directional deviation by employing the resistance to turning of a very rapid spinning mass. A gimbal is a structure that has three spinning weights positioned orthogonally. During aircraft movement, this frame permits the three revolving 'gyros' to maintain their spin orientation, showing the'sensed' changes. They are employed in aeroplanes to detect roll, pitch, and yaw angles. 


Accelerometers measure differences in speed (acceleration) along each axis. Three orthogonal acceleration components can be vectorially summed using this three-dimensional accelerometer/gyro setup. The directional acceleration of the vehicle is calculated by adding the gyro-sensed direction change with the summed accelerometer data

Navigation Computer

The navigation computers process all the data from the IRS, GPS, Gyro & Compass and provide the precise navigation. 

System Clock

The role of the system clock is to control how rapidly the navigation computer integrates each directional acceleration in order to calculate the aircraft's velocity vector. The velocity vector is initially integrated with time to create the distance vector.  

Throughout the navigation process, several steps are iterated in order to provide precise location data. The IRS system's initialization is crucial to its correctness. This is the pre-flight gimbal and position alignment procedure that establishes a baseline against which all subsequent in-flight movement can be measured.

The gyros are essentially speed up or run up, the compass heading is aligned, and the origin's latitude and longitude are entered into the computer. This datum can be used to compute any subsequent movement of the aircraft.

Modern IRS/INS System

Modern INS systems have no moving parts and are totally solid-state. In older INS platform systems, three-ring laser gyros or Optical Gyro replace mechanical gyros. Precession and other mechanical gyro flaws are no longer an issue. The inclusion of three solid-state accelerometers, one for each plane of motion, improves accuracy as well. The computer receives the accelerometer and gyro outputs and uses them to calculate the aircraft's position in real time. 

Because there are less moving parts, these are more reliable. Furthermore, GPS data is used to improve precision in commercial  operations. Of course, this isn't necessary for operations, but it can provide useful information.

The satellite GPS is the most modern IRS integration. In and of itself, the GPS is incredibly precise. When used in conjunction with IRS, it creates one of the most precise navigation systems on the market. The IRS is initialised using the GPS, therefore the pilot is no longer required to do initialisation. GPS also sends data to the IRS computer for mistake correction. IRS/GPS is unaffected by occasional service interruptions and GPS system altitude errors. 

ring lase gyro hd image download
Ring Laser Gyroscope


Types of Inertial Reference System

Commonly two types of IRS are in used today:

Gimballed systems   

Gyros were originally mounted on friction gimbals on a spinning platform. Accelerometers were mounted to each gimballed gyro axis and kept in a fixed orientation as a result. The spinning platform sensed any angular motion and maintained the platform's initial orientation. The outer body's movement around the steady platform is measured by pickoffs on the gimbals, while the body's acceleration in the fixed inertial axes is measured by accelerometers.

In certain severe or quick manoeuvres, gimballed systems had a tendency to 'lock up' or 'topple.' They were also reliant on mechanical/moving parts.

The main benefit of gimballed systems is their inherent decreased inaccuracy. Only the vertically oriented of the three orthogonal accelerometers will be sensing gravity because they are maintained in a fixed inertial orientation (and therefore experiencing gravity-related errors). The accelerometers in strap down systems move in three axes, and each one faces potential inaccuracies owing to gravity.

Gimballed systems also have the benefit of being easy to use. In a gimballed device IRS, the primary role of the gyro is to spin at very high Speed and also maintain a high moment of inertia, whereas strap down gyros must monitor the subtended angles of motion accurately.

Strap down systems   

Strap down systems were designed employing advanced computer technologies to have fewer moving parts. Electronics, optics, and solid-state technology advancements have enabled the development of extremely accurate and dependable devices. Integrated circuit technologies are used in modern commercially accessible electronics.

Strap down systems are attached to the aircraft structure, with gyros sensing angular rate changes and accelerometers detecting linear rate changes, both in relation to fixed axes. These three axes are a moving frame of reference, as contrast to the gimballed system's constant inertial frame of reference. This information is used by the system computer to calculate motion in three dimensions in relation to an inertial frame of reference.

The main benefit of the strap down method is its mechanical simplicity. Strap down systems are entirely fixed to the body in motion and are largely solid-state in design; gimballed systems require a complex and expensive design connected to an outer housing via low for its gimbals, pickoffs, and low-friction platform connections; gimballed systems require a complex and expensive design connected to an outer housing via low for its gimbals, pickoffs, and low-friction platform connections.


The IRS runs 24 hours a day, 7 days a week and is totally self-contained within the IRS unit. The IRS element of the system continues to function even if the GPS fails. The size and weight of IRS have been reduced thanks to advancements in electronic technology.

Also Read

Watch Video on YouTube

Post a Comment