Summary

  • The IRS, or Inertial Reference System, is the main navigational system in aircraft, independent of outside signals or input.
  • GPS is crucial for navigation in modern aircraft, with other aids like VOR and NDB used for backup.
  • Aircraft navigational systems are highly independent, with FMS processing multiple positional data for precise navigation.

Have you ever wondered how aircraft navigate through the air? While there are many visual indicators on the ground, such as taxiway and runway signs, how do pilots find their way in the air? A general perception is that pilots use the Global Positioning System (GPS) to navigate. However, most aircraft, particularly large commercial airliners, use a variety of techniques for navigation.

The main navigation system is the Inertial Reference System (IRS), which allows pilots to work with coordinate systems for routes. GPS and other navigational aids, such as Very High-frequency Omni Directional Range (VOR), Distance Measuring Equipment (DME), and Non-Directional Beacons (NDB) are also used. This article focuses on various methods and techniques used for aircraft navigation, as highlighted by Travel Radar.

What is the Inertial Reference System (IRS)

The IRS or the Inertial Reference System is the main navigational system in an aircraft, and it is a self-sufficient system, as it does not require any outside input to know where it is on the earth. The IRS consists of laser gyros and accelerometers, and the purpose of the gyros is to keep the accelerometer's level at the earth's horizon.

In an IRS, the gyros can sense the difference between the aircraft's longitudinal axis and the earth’s horizon and can feed to the accelerometers how much an error is in between them. In the old Inertial Navigation System (INS), the gyros were mechanical, and the accelerometers and gyros were kept on a physical platform that was leveled physically by motors.

Honeywell Navigational Equipment Box
Photo: Honeywell

The IRS makes this less complex as there is no physical leveling. Instead, it simply uses mathematical algorithms to ensure that the accelerometers are always level with the earth's horizon.

Aligning the IRS

For the IRS to be able to initiate navigation, it needs to be aligned first. When pilots start the alignment process, the IRS measures the rotation of the earth on its axis. The earth rotates at a rate of 15 degrees per hour, so, if the IRS can measure this earth rate, it can measure the present latitude of the aircraft. It is just a matter of rearranging the formula for the earth rate, as shown below:

Earth Rate = 15 x Sin latitude/ hour

Once the IRS figures out the latitude, it calculates the aircraft heading relative to True North. However, as airplanes mainly fly using magnetic north as a reference, the difference between true north and magnetic north must be fed to the IRS.

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During the IRS aligning phase, the pilots must input the current aircraft latitude and longitude to the flight management system. In some aircraft, it automatically aligns to the most accurate position coordinates without any input from the pilot. There are two reasons why the IRS needs to know the current location, and one is that the IRS can only figure out the latitude and not the longitude.

IRS control panel
Image: Airbus FCOM

The other reason is that giving the IRS the correct location helps it to make a very accurate alignment. In the early days of inertial navigation, the pilots used to enter gate coordinates in the INS, which is the reason why aircraft gate number boards have the coordinates painted on them.

The IRS is a sensitive piece of equipment and so, during the alignment process, the aircraft must remain stationary. In a perfect world, the IRS should only be able to sense the earth's rotation, but, during high wind conditions, and if the aircraft is shaken too much during the baggage or passenger loading, the IRS can sometimes fail to align.

Post-alignment procedures

After the alignment, the IRS uses a series of integrations to calculate the new aircraft coordinates. The IRS can sense acceleration as the aircraft moves, an this acceleration is converted to velocity. The velocity is then converted to a distance which is subsequently added up to find the change in latitude and longitude.

Lufthansa Airbus A350-900 Aerial View
Photo: Vincenzo Pace | Simple Flying

The IRS is subject to various errors, particularly on long-haul flights, with one of these errors being called transport wander. This happens due to the gyros' ability to maintain their reference to space. As we know, longitudes converge as we move from the equator, which means that the heading to the north or south differs as you move to higher latitudes in a westerly or easterly direction.

If an airplane moves from one place to another, it crosses latitudes and longitudes, and as the gyro maintains its reference to space, there is a difference between the gyro sensing north and the true north. So, as the gyro is 'transported' from one point to another point, transport wandering occurs. There is also apparent wander, which is like transport wander, but it happens due to the rotation of the earth.

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The GPS and conventional navigational aids

In most modern aircraft, the GPS is one of the most important pieces of navigational equipment. Usually, there are two independent GPS receivers, and these take in information from the GPS satellites and find the aircraft's location (latitude and longitude).

The GPS monitor on an A320 FMS.
Image: Anas Maaz | Simple Flying

The navigational aids such as the VOR, DME, and NDB are used only for backup purposes. If, for instance, the GPS and IRS (or just one of them) fail, the aircraft can be navigated using conventional aids.

Independent systems

It is important to realize that each navigation system in an aircraft is completely independent, so the IRS and the GPS calculate their positions separately. The position calculated by the IRS is known as the IR position.

In modern aircraft, there are two or three IRSs. The number one IRS is dedicated to the captain’s instrumentation, the number two IRS is dedicated to the first officer, and the number three IRS remains on standby.

Two Pilots sitting in a cockpit.
Photo: YAKOBCHUK VIACHESLAV | Shutterstock

In an aircraft with three IRSs, each IRS calculates its own IRS position. These positions are then used by the FMS to generate a mixed IRS position. To calculate the errors in the IRSs, it works on a highly democratic system, whereby, if one IRS deviates more than the other two, that IR position is considered incorrect.

How does the Flight management system (FMS) process all the navigational data?

Each IRS also computes a position called a GPIRS position, which combines the IRS and GPS positions. This GPIRS position is then sent to the FMS, and the FMS then chooses the most accurate GPIRS position and uses that position to navigate the aircraft.

The FMS itself also calculates what is known as an FM position. As there are two FMSs (corresponding to the Captain and First Officer), each FMS calculates an FM position based on the three IRS and the onside GPS. Due to this reason, there may be a slight position difference between the two FM-calculated positions.

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When the FMS loses its GPS signals, it navigates using the IRS plus conventional navigational aids. The IRS is continuously updated for errors using nearby navigational sources such as the VOR, DME, and NDB. To reduce errors and to make navigation more accurate, the FMS generates a vector from the mixed IRS position to the GPIRS/ navigational aid position.

This vector, sometimes called a 'bias,' is memorized by the FMS, and if the GPS signal and or the navigation aid coverage are lost in flight, this memorized bias is added to the mixed IRS position. This makes the navigation a little more accurate than purely IRS-based navigation.

On today’s navigational routes, IRS-only navigation is allowed on oceanic corridors where radio navigational aids are not available. However, in most cases, such routes allow IRS navigation for about 6.3 hours, beyond which the IRS position becomes too inaccurate for navigation.

There are still aircraft without GPS operating on many airlines, as IRS navigation is possible in most airways without GPS if the IRS can be updated using conventional navigational aids. Not having a GPS, however, may limit the aircraft in that it might not be able to fly easier arrival and departure routes into and out of airports, as the conventional arrival and departure routes are usually more complex and time-consuming.