Waypoints mean VORs in aircraft navigation
An important aspect of safe flight operations is knowing where you are at all times. The basic task is to get from start to finish. In addition, it should be done safely, as efficiently as possible, without disturbing residents and gently for the aircraft. In order to achieve all these different goals, a lot of technical solutions have emerged since the beginning of commercial aviation. This article gives an overview of the story and is intended to help you understand the context.
Flying by sight
The simplest and most original navigation is on sight. There are various methods that can be combined with one another as required.
The pilot uses a map to orient himself to clearly visible landmarks such as motorways, towns or high obstacles (such as radio towers). This way of navigating is still widespread among recreational pilots. That is why it is also required in our pilot tests. But passengers were also transported with this aid in the past (sometimes even today!).
The pilot uses the existing radio navaids (e.g. VORs or NDBs), if necessary by means of cross bearings.
- Give me a stopwatch and a map, then I'll fly through the Alps in a plane with no windows. Gregoriy Kamarov, navigator of "Red October".
With dead reckoning you follow a course for a certain time at a certain speed in order to get to a navigation point. Depending on the instrumentation and accuracy, a measurement error due to wind or variation, for example, can be minimized.
Flying by instruments
Of course, navigation by sight is only good as long as visibility is good enough. As soon as the pilot can no longer recognize sufficient visual features, he is flying blind and can no longer adequately determine his position and his position in space. This resulted in instrument flight, according to which a large part of flight movements are handled in commercial aviation today.
The first and simplest methods of guiding aircraft are based on radio stations that enable the pilot to find bearings. The simplest form is the non directional beacon (NDB). This station radiates its bearing evenly in all directions, and enables the pilot to measure the direction under which he sees the station. In the early days of instrument flight, airways were nothing more than connecting lines between these NDBs, and required the pilot to constantly monitor the navigation equipment, set new frequencies and fly the bearings accurately. Not least because of this, navigators and flight engineers were necessary to fly the aircraft. The accuracy of the NDBs is generally not particularly high and is further restricted by numerous effects, for example at night or when reflecting off mountains. Since the NDB only shows the bearing in the direction of the station, it is difficult to approach in crosswinds, the wind must be constantly corrected by the pilot.
The VOR, the high-frequency rotary radio beacon, provides a remedy. While the NDB radiates its bearing evenly in all directions, i.e. non-directionally, a beacon rotates in the VOR. In principle, you can imagine it a little like a lighthouse: there a white light rotates 360 degrees once a minute, and exactly when the white beam passes through to the north, a red light flashes. From the observation of the two flashes of light, the ship can now determine its own position on a line away from the station. The VOR works in a similar way, except that it is not a matter of flashes of light and the beam does not even rotate every minute, but 60 times a second. The VOR is a much more precise and robust device for navigation, but it is also expensive to operate and maintain, which is why both VOR and NDB are still used in parallel today.
The NDB and the VOR together offer quite good navigation support. However, they alone are not sufficient to correctly determine their own positions, so the pilot can determine a line on which he is, but he can only determine his specific positions by means of a cross bearing, i.e. the simultaneous observation of at least two stations.
In order to remedy this restriction, a third technology is used in addition to the NDB and VOR: the distance measuring equipment (DME). To put it simply, a DME receiver can use the radio signal from a ground-based DME system to evaluate the inclined distance to the station and display it to the pilot. To do this, an aircraft-side device sends out an interrogation pulse, which is answered by the DME station. The distance to the station is calculated from the time that elapses between request and response. These stations are often used in conjunction with a VOR, which is then referred to as a VOR / DME. With this combination, it is possible to determine your position quite precisely using a flight map and a single device. However, this determination has to be made by hand again and again (after all, the aircraft is always moving), and the manual re-rotation of frequencies is not omitted.
The last important ground-based navigation system supports the pilot during landing. The flight segment of the landing is still the most complex and also the most accident-prone to this day. Especially here the pilot is prevented from successfully executing the maneuver in a critical flight phase (low and slow) due to poor visibility. As early as the 1930s, experiments were therefore carried out with ground-based landing systems, from which the instrument landing system (ILS) was ultimately developed. The ILS in its current design includes the localizer, which takes over the lateral guidance, and the glide slope, which marks the descent. On-board receivers evaluate the radio signals and show the pilot in which direction he has to steer in order to get onto the runway.
Nowadays it is common practice to operate the landing course and glide path transmitters together with a DME. The installation is then called ILS / DME and offers comfortable and safe navigation for landing. With modern aircraft, it is possible to use the ILS for landings automatically carried out by the autopilot with visibility around 75 meters and decision heights below 50 feet. This increases the reliability with which aircraft can land at their destination, transport passengers safely and earn money with it.
In addition to these classic earth-based navigation systems, there are a few that should be mentioned for the sake of completeness. Since the range of VOR, DME and NDB are very limited, long-range navigation methods were developed. One representative was OMEGA, this system has not been in operation since 1997. The principle is similar, but LORAN-C is still in operation. This system is based on a chain of transmitters that emit an identifier at precisely defined intervals. The observer on board an aircraft can calculate his position from the time interval between the individual signals. Using LORAN-C, an aircraft can determine its position to within a few kilometers, thanks to transmission chains distributed around the world, almost anywhere in the world. The system is still used by shipping today, primarily as a backup in the event of failure of satellite navigation.
In addition to the ground-based methods, there are also navigation methods that rely solely on devices installed on board the aircraft. The principle here is again quite simple and can be explained with an example. Suppose I know my current location on a map and have a compass. If I start walking in any direction, at the same time I measure my stride length and the direction in which I walk, I can help determine my position by adding it up and entering it on the map. This form of navigation is called dead reckoning.
This technique was refined by precise gyroscopic instruments. The property of gyroscopes is that they maintain their axis of rotation when they are not forced to move differently. If you hang a top freely in all three directions and turn the suspension, the top remains stable. The angle that the gyro axis has to the housing can be measured and from this angle the position of the aircraft in space can be determined. The simplest application is the course gyro, which measures and displays the rotation of the aircraft quite precisely in the horizontal plane. A gyro and three accelerometers are often combined to form a real navigation device. The accelerometer and the gyro work according to the principle of inertia and conservation of angular momentum, which is why we speak of an inertial navigation system (INS).
Inertial navigation has long been a widely used long-range navigation method, so you can read the position of the aircraft directly on the INS without great effort and without constant manual recalculation of the position by the navigator. One disadvantage, however, is that small errors in the evaluation of the gyroscope and accelerometer add up over time. After a while, a position error occurs that can be several miles on long flights and is therefore too imprecise for precise navigation.
The third pillar of navigation methods are space-based methods. The top dog and namesake is the NAVSTAR global positioning system, popular under the name GPS. However, it is far from the only satellite navigation system! Other systems run under the name GLONASS (Russia), Galileo (European Union) or COMPASS (China). In general, the term global navigation satellite system (GNSS) is often used, so the term is a collective term for all of the above-mentioned systems.
They all work on the same principle: satellites in orbit continuously emit a time signal. The receiver can now calculate its distance to the satellite by comparing its own clock and the time signal. To determine your position, you actually need three satellites. However, since high-precision atomic clocks are rather expensive and impractical to install in commercially available GPS receivers, so-called pseudo transit times are used - the receiver calculates the distance to the satellite using its own (faulty) clock. Four satellite signals must be evaluated so that the GPS calculation can provide accurate results despite this restriction. The accuracy of satellite navigation is then around 15 meters.
These accuracies are quite good for navigation in cross-country flights, but this accuracy is not sufficient for instrument approaches. For this reason, support systems have been developed for GPS over the past 10 years, so-called differential GPS systems (DGPS). These systems send correction signals either from space or from the ground, which can significantly reduce errors in GPS positioning. These systems are under the technical term space based augmentation system (SBAS) or ground based augmentation system (GBAS) or under the "trade names" Wide Area Augmentation System (WAAS, in the USA), European Geostationary Navigational Overlay System (EGNOS, European Union ) or Multi-functional Satellite Sensor System (MSAS, Japan). They can be used to correct errors in the GPS signal, for example from the ionosphere, in real time. With DGPS it is possible to fly landings up to the accuracy otherwise only known from ILS of CAT 1, so the accuracy of the position determination is in the decimeter range.
And how does that look practically now?
All of the options listed above are technically available today. And how do we pilots now use it for navigation?
First of all, the navigation depends mainly on how the aircraft is equipped. While almost all aircraft from E-Class and heavier have an ADF and a VOR receiver, expensive inertial navigation systems are only installed on commercial and business aircraft. GPS receivers, on the other hand, have become such a cheap and easy to retrofit alternative that many gliders now use GPS to navigate. Other aircraft, on the other hand, have no navigation equipment at all, in very rare cases there are even aircraft without a radio!
In fact, in practice, an aircraft is equipped with different equipment depending on the purpose and budget of the aircraft owner and operator. This is no different with a Piper Archer than with an Airbus A380. Therefore, in practice there is a mix of different forms of navigation.
The simplest form is of course to navigate terrestrially using a compass and map. In well-equipped general aviation aircraft, this navigation is then often supported by a VOR receiver and ADF, and completed with a GPS. The equipment is then very comfortable for visual flight.
Nowadays, IFR pilots are designed for area navigation in most cases. The term area navigation (RNAV) means that the waypoints controlled during the flight do not have to be linked to ground-based navigation systems. So waypoints are no longer the intersection of two radials, or a DME distance on a certain radial. Rather, the waypoints are given names and entered into the RNAV system with their coordinates before the flight.
Such an RNAV system can determine the position of the aircraft using very different methods. The classic is GPS because it provides the position with high accuracy and high availability. Another possibility is to pinpoint your own position over several DME distances. If there are not enough stations available, a standard VOR / DME will do the same. In areas without coverage by radio navigation equipment, navigation by inertia, i.e. by an IRS or INS, is still possible.
What all these systems have in common is that they calculate the position of the aircraft in terms of longitude and latitude. The autopilot then uses this information to calculate the course to the next programmed waypoint.
When landing, the pilots also use all of the aforementioned position solutions, depending on the system available. The classic approach, particularly popular in the United States, is the visual approach. But also NDB, VOR, ILS or an RNAV approach are common all over the world.
When it comes to lateral navigation, all technologies deal with the one question: where am I in comparison to my surroundings? The existing systems were partly developed one after the other or in parallel and are now more or less in use side by side. Depending on the equipment of the airport and the aircraft (and of course the skills of the pilot), different combinations of the individual modules are used to achieve safe and reliable navigation overall.
Comparison of radio-based navigation systems
This table only compares the properties of the most common radio navigation systems. In addition, there are of course other ways to navigate.
|parameter||NDB||IN FRONT||DME||LORAN C||GNSS|
|information||Bearing to the station||Baseline||Distance to the station||2D position||3D position / speed|
|frequency||200-1750 kHz||108-117.95 MHz||962-1213 MHz||100 kHz||1227.6 / 1575.42 MHz|
|accuracy||1-5°||2°||0.1 nm||30-400 m||100-300 m (civil)|
|Main use||Arrival / departure||Arrival / departure, backup for area navigation||Cross-country flight||Arrival / departure, cross-country flight|
|reliability||Well||very good||Well||very good|
|Device effort||low||medium to high||high|
|Range||200 nm||2000 nm||20,000 km|
|Ground stations||approx. 5000||approx. 2000||about 1000||33 chains||24*|
|Users||approx. 200,000||approx. 80,000||approx. 2000||approx. 50,000|
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