Modern flight navigation, GPS, ILS, radio

Modern in-flight navigation

Tim Takeoff
05.10.2018
3 pictures
5 minutes

What originated as a seafaring concept is now ubiquitous, even in our cars. Reliable navigation systems are part and parcel of our daily lives. In-flight navigation has also evolved, and today’s systems are highly complex.

The basic principles established by seafarers and aerospace pioneers are still in use in modern aviation. Navigation methods vary in terms of their accuracy, with people tending to choose the most appropriate accuracy level for their needs.

The Well-Known Route: GPS

Of course, today’s jets use the global positioning system (GPS) as their primary source of in-flight navigation information. As with almost all aircraft systems, each aircraft carries multiple GPS systems on board to ensure maximum redundancy. Without being able to determine an aircraft’s position precisely to the nearest metre, the densely packed airspace we know today would be utterly impossible. In the air, just as on the ground, there are precisely delineated “streets”, known as airways, which are connected via fixed waypoints. These are mapped in global on-board databases and enable any destination to be navigated to or from via the GPS.

Inertial Navigation, or Dead Reckoning

Modern in-flight navigation also uses the same dead reckoning method used by the early seafarers. Basically, dead reckoning involves taking a fixed point – back then, it was the stars or the sun – plus a fixed time and an estimated speed and direction (calculated, for example, using a compass and stopwatch). Once you have these three values, you can use dead reckoning to calculate your position from the initial point and to keep recalculating it as you travel. Taking additional, supporting measurements confirms your calculations and facilitates a higher level of precision. In the early days of air travel, a navigator furnished with all kinds of equipment would observe the stars or sun from an astrodome mounted on the aircraft (like that on the Lockheed Constellation, for example) and use them to chart the plane’s course.

In today’s world, this task, which is also known as inertial navigation, is performed by highly precise gyroscopes and accelerometers. This means that, even if (all) other systems, including ground and satellite-based navigation aids, were to fail, the aircraft’s current position could still be accurately calculated.

Ground-Based Radio Navigation

When we talk about ground-based navigation aids, we mean radio beacons. These are radio stations installed at specific fixed points which can carry out precise measurements using a huge variety of signals. Field strength, direction and signal travel time, or a combination of these factors, are used to calculate the result. In aviation, Distance Measuring Equipment (DME) is used to measure distances. If you have two DME stations, you can chart a position.

Another type of radio beacon is known as Very high frequency Omni-directional Range (VOR). It functions in the same way as the traditional lighthouse in maritime navigation: by emitting a signal which radiates out from the station in all directions. Each time the signal passes its northerly point, a new signal is issued. Navigators can use the travel time and direction of the signal they receive to determine their relative direction from and to the station. Combining this with a DME enables them to determine their position with complete precision using just one station. These VOR/DME systems are still used all over the world today.

In addition to directional beacons, non-directional beacons (NDB) are also used. In the simplest cases, these are just radio stations. An instrument inside the aircraft produces readings to indicate the direction of this far-reaching signal, which radiates out from a central point in all directions. Unfortunately, this method is not terribly precise. Nevertheless, if you tune into Radio Moscow and keep flying in that direction, it’s highly likely that you’ll eventually reach Red Square.

Commercial pilots sometimes pass on information about the latest football results or other news during their flights. This shows that someone has been listening to the radio …

The Instrument Landing System (ILS)

Approaching an airport at the end of a flight, in the most adverse weather conditions or at night ­– and even, if necessary, landing the plane automatically – requires precise systems. One of these is known as the Instrument Landing System (ILS). In most cases, this system uses a combination of three signals issued from equipment located on the ground.

A lateral signal, known as a localizer, provides information about “left and right” to enable the aircraft to maintain a course along the central axis of the extended runway. The localizer is usually placed at the other end of the landing strip, where it can continue to signal the aircraft’s direction, even once the plane has touched down. This enables the pilot to maintain an absolutely central course. The vertical element is the “glide slope”. This is a signal tilted at a 90-degree angle which gives the pilot precise altitude information. This is also combined with another DME which provides the distance to the tarmac.

The ILS takes these three pieces of information and gives the aircraft precise instructions to enable it to maintain the correct altitude at the correct distance and to line up with the centre of the runway. In modern commercial aircraft, all these readings are bundled into the flight management system (FMS), where they are compared and evaluated. In most cases, this takes place completely automatically and without intervention from the pilot. Flight management systems ensure that aircraft are always using the most precise signal and that existing position calculations are continuously improved.

Discover the exciting ways in which the adventurous mail planes of the 1920s navigated to their destinations in “Tough Guys and Lame Ducks”.

Image source: Pixabay

by Tim Takeoff

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