Air to ground communication

What Is Air to Ground Communication?

Air to ground communication, abbreviated A2G, is the wireless exchange of voice, data, and telemetry between an airborne platform and a station on the surface of the earth. The airborne end may be a transport aircraft, a general aviation aircraft, a small unmanned aircraft, a high altitude platform, or a balloon, and the ground end may be an air traffic control facility, a dedicated ground station, or a cellular base station. The subject covers the propagation physics of a link whose geometry changes continuously, the radio systems built to work within that geometry, and the regulatory framework that assigns them spectrum.

The discipline grew out of aeronautical radio in the 1920s and has since split into two loosely coupled communities: civil aviation communications, which is standards-driven, safety-certified, and conservative about change, and unmanned aircraft communications, which reuses commercial cellular and satellite infrastructure and moves quickly. Both contend with the same physical situation: a mobile terminal moving at high speed with an unobstructed or partially obstructed path to a distant fixed station.

Propagation and Channel Characteristics

An air to ground link usually has a dominant line of sight component, so its small-scale fading is Rician with a K factor that rises with altitude and elevation angle. The most common model is a two-ray geometry combining the direct path with a specular ground reflection whose phase difference produces periodic fading as the aircraft moves. Path loss exponents fall close to the free space value over water and open terrain and rise where buildings or vegetation intrude at low elevation angles. High platform velocity produces Doppler shifts of hundreds of hertz at ultra high frequencies and correspondingly short channel coherence times, which constrains symbol design and pilot spacing. Airframe shadowing during banked turns can interrupt a link entirely. A survey of channel modeling for UAV communications reviews the deterministic, stochastic, and geometry-based approaches used to represent these effects, and multi-frequency air-to-ground channel measurements for UAV systems provide empirical path loss and fading statistics across several bands.

Civil air traffic control voice uses double-sideband amplitude modulation in the very high frequency band from 118 to 137 MHz, with 25 kHz channel spacing in most of the world and 8.33 kHz spacing in congested European airspace. Amplitude modulation persists in this role because a stronger transmission overrides a weaker one audibly rather than capturing the receiver silently, which lets controllers detect simultaneous transmissions. Oceanic and polar regions beyond very high frequency line of sight fall back on high frequency single sideband and on satellite communications. Digital data links carry an increasing share of routine traffic: ACARS for operational company messages, VHF Data Link Mode 2 as the subnetwork beneath it, and Controller Pilot Data Link Communications for clearances and route amendments. The FAA Data Communications program describes how text clearances loaded directly into a flight management system reduce readback errors and controller workload compared with voice.

Unmanned aircraft need a command and control link with tightly bounded latency and a separate payload link with much higher throughput, and both are increasingly served by terrestrial cellular networks. Serving an aerial user from a network engineered for ground users creates a specific interference problem: base station antennas are downtilted, so an aircraft above the main lobe sees many cells through sidelobes at similar strength, which raises handover rates and uplink interference. Third Generation Partnership Project work beginning with the study on enhanced support for aerial vehicles addressed these effects and evaluated aerial service at altitudes up to 300 m above ground, as reviewed in a survey of UAV cellular communications and standardization. Satellite links and dedicated air to ground broadband networks serve passenger connectivity on commercial aircraft.

Applications

Air to ground communication has applications in a range of domains, including:

  • Air traffic control voice and clearance delivery
  • Airline operational control and maintenance reporting
  • In-flight passenger broadband and entertainment
  • Unmanned aircraft command, control, and video downlink
  • Flight test telemetry and range safety
  • Search and rescue, firefighting, and disaster response coordination