Overview
Air-to-ground communication is essential for maintaining orderly flight operations and ensuring flight safety. For example, a busy international airport may handle more than 1,600 daily aircraft movements, averaging one takeoff or landing every 50 seconds. Air traffic controllers coordinate altitude, routing, runways, takeoff and landing times, and parking stands, while pilots carry out checks and procedures. All of these activities require reliable air-ground communication; without it, air traffic could become chaotic.
1. Basic Concepts of Communication
Communication refers to the transfer of information between parties through some medium or action. For instance, on military airfields a green signal flare can indicate "clear to fly" while a red flare can indicate "stop flying".
General model of a communication system
A communication system is the collection of all technical equipment and transmission media needed to implement information transfer. Abstracting common features, a communication system can be modeled as follows:
- Information source: the origin of messages, which converts various messages into raw electrical signals. For example, a microphone converts sound into audio signals and a camera converts images into video signals.
- Transmitter: converts the raw electrical signal into a form suitable for the chosen channel, for example by modulation and coding, and sends the signal.
- Channel: the medium through which the signal is transmitted, such as the atmosphere, underwater, or along the earth's surface.
- Noise source: represents interference from various devices and the channel itself, modeled collectively for analysis.
- Receiver: performs the inverse operations of the transmitter, such as demodulation and decoding, recovering the original electrical signal from the received, possibly corrupted, signal.
- Destination: the endpoint that converts the recovered electrical signal back into the intended message.
2. Channel Selection for Air-to-Ground Communication
Channel choice for air-to-ground communication is constrained by operational requirements and physical propagation characteristics, so there are limited options.
Radio communications commonly use four channel environments: ground surface, underwater, the atmosphere, and free space. Three principal propagation mechanisms are diffraction, refraction/reflection, and line-of-sight. Frequency bands include very low frequency (VLF), low frequency (LF), medium frequency (MF), shortwave (HF), very high frequency (VHF), and microwave. The correspondence is summarized in the table below.
| Ground surface | Underwater | Atmosphere | Free space | |
|---|---|---|---|---|
| VLF | Diffraction | Diffraction | ||
| LF | Diffraction | Diffraction | ||
| MF | Diffraction, Refraction/Reflection | |||
| HF | Refraction/Reflection | Refraction/Reflection | ||
| VHF | Line-of-sight | Line-of-sight | Line-of-sight | |
| Microwave | Line-of-sight | Line-of-sight | Line-of-sight |
From this mapping, air-to-ground communication typically uses HF (shortwave), VHF (ultra-shortwave), and microwave bands, employing refraction/reflection and line-of-sight propagation. Corresponding techniques include radio communication, satellite communication, and data link systems.
This outcome can be explained by the wave-particle duality of electromagnetic waves. Longer wavelengths behave more like waves: they diffract more, can bend around obstacles, attenuate slowly, and propagate farther. Shorter wavelengths behave more like particles: energy is more concentrated, penetrating ability is higher, and propagation tends to be straight-line.
In theory, electromagnetic waves can propagate via diffraction, refraction/reflection, line-of-sight, and scattering, though scattering has limited practical application.
Diffraction mainly occurs near the earth's surface and is often called ground-wave propagation. Because electromagnetic waves below about 100 kHz are easily absorbed by the earth's surface and cannot be effectively transmitted over long distances in other modes, ground-wave propagation corresponds to LF and VLF bands.
Refraction/reflection depends on the ionosphere and is often called sky-wave propagation. The ionosphere extends roughly from 60 km above the surface to the top of the atmosphere; its position and thickness vary with solar activity and are therefore nonuniform and unstable. HF bands are suitable for sky-wave propagation; MF bands may also use sky-wave propagation.
Line-of-sight propagation applies to VHF and microwaves, reflecting the particle-like behavior of the wave. It is used for line-of-sight communication, also called space-wave propagation.
3. Doppler Shift in Air-to-Ground Communication
Besides channel selection, air-to-ground communication must address frequency shifts caused by relative motion between the mobile platform and communication stations. Frequency shift is a common challenge in mobile communications, but it is more severe in air-to-ground scenarios.
Doppler shift in mobile communication
About 180 years ago, Christian Doppler (1803-1853) described the phenomenon that when the transmitter and receiver are in relative motion, the received frequency differs from the transmitted frequency: frequency increases when they move closer and decreases when they move apart. This is the Doppler effect; the resulting frequency change is the Doppler shift.
Studies of high-speed rail communications show that when vehicle speed is below about 250 km/h, Doppler shift can often be neglected; when speed exceeds that threshold, Doppler shift can cause serious connection drops and service loss. Aircraft cruise speeds are significantly higher: large jet airliners cruise around 900 km/h and fighter aircraft can exceed 1,000 km/h, so Doppler effects are more pronounced.
Fortunately, Doppler shift can be estimated from the aircraft's speed and compensated or removed using appropriate signal-processing techniques to restore the transmitted frequency. Many effective algorithms and methods have been developed over the years, and research continues to improve performance to meet evolving air-ground communication requirements.
