What are Aircraft Navigation Aids?

Navigation systems help airplanes know where they are at all times. It’s important to understand the difference between systems that assist pilots (navigational aids) in flying from one place to another and the techniques pilots use to navigate.

Aircraft Navigation Aids are infrastructures that give pilots all the information they need about their position and how to get where they’re going. On the other hand, navigation techniques are how pilots use this information to fly the plane.

Let’s look at Aircraft Navigation Aids systems. They can be divided into two main groups:

1. Autonomous systems: These systems only use the equipment onboard the airplane to figure out where it is.
2. Non-autonomous systems: These are external systems that give the airplane information about its position. They help the pilots know where they are and where they’re going.

1. Autonomous Navigation Systems

Autonomous navigation systems rely solely on the aircraft’s own equipment to determine its position and movement. The most advanced navigation technique used with autonomous systems is called dead reckoning. This involves predicting the future position of the aircraft based on its current position, velocity, and course. To do this, a reference or initial position of the aircraft must be known.

There are different methods to determine this reference position. One way is by observing a known point near the aircraft, although this method is quite basic. Another method is by observing celestial bodies, which is also quite rudimentary. Alternatively, autonomous systems can be used, which are capable of determining the velocity and course of the aircraft.

The two principal autonomous systems used for dead reckoning are:

1. Doppler radar
2. Inertial Navigation System (INS)

These systems provide the necessary data for dead reckoning, allowing pilots to predict the aircraft’s future position accurately based on its current state.

1. Doppler Radar

Doppler radar is a specialized radar system that utilizes the Doppler effect to determine the velocity of a moving object at a certain distance. Here’s how it works:

1. The radar emits a microwave signal towards the target, such as a flying aircraft.
2. This signal reflects off the target and returns to the radar.
3. By analyzing the reflected signal, the radar can determine how the frequency of the signal has been altered by the motion of the object.
4. The variation in frequency provides direct and precise measurements of the radial component of the target’s velocity relative to the radar.

In simpler terms, Doppler radar measures how the frequency of the radar signal changes when it bounces off a moving object like an aircraft. This change in frequency helps calculate how fast the object is moving towards or away from the radar. Doppler radar is commonly used in various applications, including weather forecasting, air traffic control, and military surveillance, due to its ability to accurately measure the speed of moving objects.

2. Inertial navigation system (INS).

2. Non Autonomous Systems

Non-autonomous systems rely on information provided by ground-based stations or satellites to determine the position, course, and/or velocity of the aircraft. Here’s how they work:

1. Transmission and Reception: A transmission station (transmitter) emits electromagnetic waves, which are received at a reception point (receptor) onboard the aircraft.
2. Observables: The information provided by these stations is known as observables, which can include distance, course, and other data. These observables help locate the aircraft within what’s called a “situation surface.”
3. Situation Surface: A situation surface is a geometric space that is consistent with the observables. There are different types of situation surfaces:

• Plane perpendicular to the Earth’s surface: If the observable is the course, this surface helps determine the aircraft’s position relative to the Earth’s surface.
• Spherical surface centered on the transmission station: If the observable is the distance, this surface helps pinpoint the aircraft’s position within a sphere.
• Hyperboloid of revolution: If the observable is the distance, this surface is formed when two external transmission centers exchange information, helping determine the aircraft’s position based on the intersecting hyperboloids.

In simpler terms, non-autonomous systems use signals from ground-based stations or satellites to figure out where the aircraft is and how it’s moving. The information provided helps create different types of surfaces that help pinpoint the aircraft’s location.

In general, using information from only one transmitter is insufficient to pinpoint the location of an aircraft; additional sources of information are required. By intersecting the surfaces generated by signals from multiple transmitters, we can determine the aircraft’s position. For example:

• Intersecting two surfaces produces a curve.
• Intersecting three or more surfaces produces a point (ideally).

To locate an aircraft, we typically need either:

1. Two transmitters (generating two surfaces) plus altitude data from the altimeter.
2. Three transmitters (generating three surfaces).

Now, let’s discuss how we obtain these observables, such as distance or course, using terrestrial stations or satellites. These observables are obtained through various techniques based on electromagnetic fields, including:

• Radiotelemetry: Using radio signals to measure distances or transmit data.
• Radiogoniometry: Determining direction or bearing using radio signals.
• Scanning beam: Employing a scanning antenna to detect direction or position.
• Spatial modulation: Modulating signals in space to convey information.
• Doppler effect: Analyzing frequency shifts in signals to determine velocity or direction.

These techniques enable us to gather essential information about the aircraft’s position, course, and velocity, aiding navigation and ensuring safe flight operations.

Aircraft Navigation Aids Techniques

Here’s a simplified explanation of the different navigation techniques mentioned:

1. Radiotelemetry

Radiotelemetry works by leveraging the fact that electromagnetic waves travel at a constant speed—the speed of light, which is about 300,000 kilometers per second—and in straight lines. Based on these principles, if we measure the time it takes for the waves to travel from the transmitter, bounce off the aircraft (acting as a receptor), and return to the transmitter, we can calculate the distance between the transmitter and the aircraft using simple kinematic analysis.

2. Radiogoniometry:

Radiogoniometry relies on the fact that the electric and magnetic fields of an electromagnetic wave are perpendicular to the direction of the wave’s movement. With this technique, we measure the phases of these electric and magnetic fields. By doing so, we can determine the angle formed between the longitudinal axis of the aircraft and the direction of the transmitted wave. This information helps us pinpoint the orientation of the aircraft relative to the transmitted signal.

3. Scanning beam:

Scanning beam navigation relies on the emission of an electromagnetic wave by a transmitter with a dynamic radiation pattern. This pattern consists of a narrow main lobe and smaller secondary lobes. The aircraft, acting as the receptor, is only illuminated or radiated if the main lobe of the radiation pattern is directed towards it.

By understanding the movement pattern of the radiation diagram, we can determine the direction between the transmitter and the aircraft. When the aircraft is illuminated, we can infer the relative orientation and position of the aircraft with respect to the transmitter. This technique allows for directional navigation guidance based on the movement of the radiation pattern.

4. Spatial modulation:

Spatial modulation is a technique developed for air navigation that utilizes two different electromagnetic waves.

Reference Signal: The first wave is known as the reference signal. It generates an omnidirectional magnetic field, ensuring that all points within the region receive the same information. Antennas producing this type of field are called isotropic antennas, and their radiation is termed isotropic or omnidirectional radiation.

Directional Signal: The second wave generates a directional magnetic field, which can be either static or dynamic. This directional field is focused in a specific direction.

By comparing the phases of the reference signal and the directional signal, the direction of the aircraft can be determined. This technique enables navigation by analyzing the differences between the two signals and determining the orientation of the aircraft relative to the directional field.

Doppler effect:

The Doppler effect is a phenomenon based on the change in frequency of a wave caused by the relative movement between the source of the wave (transmitter) and the receiver (aircraft). By analyzing this frequency change, one can determine the distance between the transmitter and the aircraft.

Table 1 and Table 2 provide classifications of various Aircraft Navigation Aids based on different techniques, situation surfaces, and flight phases. These tables help categorize and understand the different navigation systems and aids available for aircraft navigation.

The most important navigation aids using radiotelemetry include:

• DME (Distance Measurement Equipment): Measures the distance between the aircraft and ground-based stations.
• TACAN (Tactical Air Navigation Equipment): Used in military aviation for navigation and air traffic control.
• GNSS (Global Navigation Satellite Systems): Utilizes satellite signals to determine the aircraft’s position.
• Radar (Radio Detection and Ranging): Primarily used for surveillance purposes, detecting and tracking aircraft.