Understanding Drone Communication Link Systems at a Glance
An Unmanned Aerial Vehicle (UAV), commonly known as a drone, is an unmanned aircraft operated via radio remote control equipment and built-in program control devices. Technically, drones can be categorized into: unmanned fixed-wing aircraft, vertical take-off and landing aircraft, unmanned airships, unmanned helicopters, multi-rotor drones, and parawing drones, among others. Drones have a wide range of applications, including civilian uses such as aerial photography, pipeline inspection, and crop protection, as well as military applications like reconnaissance, relay operations, and strikes, along with police patrols and surveillance. With the rapid development of civilian drone technology, an increasing number of companies are investing in drone-related projects.
A drone system primarily consists of three major components: the ground station, the flight controller, and the wireless communication link. Below, we will explore the drone wireless communication link system in detail.
The link system is a critical part of the drone system. Its main task is to establish a bidirectional air-ground data transmission channel for long-distance remote control, telemetry, and mission data transmission between the ground control station and the drone. Remote control enables the operation of the drone and its mission equipment from a distance, while telemetry monitors the status of the drone.
Mission data transmission involves sending information such as video and images captured by onboard mission sensors to the control station via the downlink wireless channel. This is key to the drone's mission success, and the quality of this transmission directly impacts the ability to detect and identify targets.
1. Regulations on Drone Frequency Bands in China
Drone communication links require radio spectrum resources. Currently, the primary frequency bands used by drones worldwide are UHF, L, and C bands, with some scattered use of other bands.
2. Components of the Drone Link System
The airborne part of the drone link includes the Airborne Data Terminal (ADT) and antennas. The ADT consists of an RF receiver, transmitter, and a modem connecting the receiver and transmitter to the rest of the system. Some ADTs also include a processor for data compression to meet the bandwidth limitations of the downlink. Antennas are typically omnidirectional, though directional antennas with gain are sometimes required.
The ground part of the link is called the Ground Data Terminal (GDT). This terminal includes one or more antennas, RF receivers and transmitters, and a modem. If sensor data is compressed before transmission, the GDT also requires a processor to reconstruct the data. The GDT can be divided into several components, generally including a local data link connecting the ground antenna to the ground control station, as well as various processors and interfaces within the ground control station.
For long-endurance drones, relay communication is a common method to overcome obstacles such as terrain blocking, Earth curvature, and atmospheric absorption, thereby extending the link's range. When relay communication is used, the relay platform and corresponding forwarding equipment become part of the drone link system. The operational range between the drone and the ground station is determined by the radio line of sight.
3. Frequency Bands for Drone Link Channels
During air-ground data transmission, wireless signals are affected by factors such as terrain, ground objects, and atmospheric conditions, causing reflection, scattering, and diffraction of radio waves, resulting in multipath propagation. Additionally, the channel is subject to various noise interferences, which can degrade data transmission quality.
In measurement and control communications, the impact of wireless transmission channels varies with the operating frequency band. Therefore, it is essential to understand the main frequency bands used for drone measurement and control. Drone measurement and control links can operate over a wide range of carrier frequencies. Lower-frequency bands offer lower equipment costs but limited channel capacity and data transmission rates, while higher-frequency bands come with higher equipment costs but support more channels and higher data transmission rates.
The primary frequency bands used for drone links are microwave frequencies (300 MHz to 3000 GHz), as microwave links provide higher available bandwidths capable of transmitting video footage. The high bandwidth and high-gain antennas used in microwave links offer strong anti-interference performance. Different microwave bands are suitable for different types of links.
Generally, VHF, UHF, L, and S bands are more suitable for low-cost, short-range drone line-of-sight links. X and Ku bands are suitable for medium- and long-range drone line-of-sight links and airborne relay links. Ku and Ka bands are suitable for medium- to long-range satellite relay links.