How Do Drones Communicate? A Guide to Frequency and Interference

The link system is an essential component of the drone system. Its primary task is to establish a two-way data transmission channel between the air and the ground. This channel is used for remote control, telemetry, and task information transmission from the ground control station to the drone. Remote control allows for long-distance operation of the drone and its mission equipment, while telemetry monitors the drone’s status. Task information transmission sends video and image data collected by onboard mission sensors to the control station via a downlink wireless channel. The quality of this transmission directly affects the ability to identify and discover targets.

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 that connects the receiver and transmitter to the rest of the system. Some ADTs also include processors for data compression to meet the bandwidth limitations of the downlink. The antennas are generally omnidirectional but may sometimes require directional antennas with gain.

The ground part of the link, also known as the Ground Data Terminal (GDT), includes one or several antennas, an RF receiver, a transmitter, and a modem. If sensor data is compressed before transmission, the GDT will also need a processor to reconstruct the data. The GDT can be divided into several parts, generally including a local data line connecting the ground antenna and the ground control station, as well as several processors and interfaces in the ground control station.

Interference Considerations

For long-endurance drones, relays are commonly used to overcome obstacles like terrain, Earth’s curvature, and atmospheric absorption. When using relay communication, the relay platform and corresponding forwarding equipment are also part of the drone link system.

The operating distance between the drone and the ground station is determined by the radio line of sight. The drone link channel frequency is affected by various factors such as terrain, objects, and the atmosphere, causing reflections, scattering, and diffraction of radio waves, leading to multi-path propagation and various noise interferences.

This illustration shows the communication links between ground stations, airports, satellites and unmanned aerial vehicles. Image credit: NASA

Frequency Bands

Drones use a wide range of carrier frequencies for their control links. Lower frequency bands are cheaper but offer limited channel numbers and data rates. Higher frequency bands are more expensive but provide more channels and higher data rates. The primary frequency bands used by drones are microwaves (300MHz~3000GHz), as they offer higher available bandwidths and are capable of transmitting video images. Their high bandwidth and high-gain antennas also offer good anti-interference performance.

Different microwave bands are suitable for different types of links. Generally, VHF, UHF, L, and S bands are suitable for low-cost, short-range, line-of-sight drone links; X and Ku bands are suitable for medium and long-range line-of-sight and airborne relay links; Ku and Ka bands are suitable for medium and long-range satellite relay links.

Interference on Control Links

The control link is an uplink, meaning that the target of interference is the drone itself. For the sake of generalization, let’s consider the following parameters: the control station’s butterfly antenna has a gain of 20dBi, a sidelobe isolation of 15dB, and a transmitter power of 1W. The drone is 20km away from the ground station, and its whip antenna has a gain of 3dBi.

Calculations:

When the interference machine targets the drone, the Effective Radiated Power (ERP) of the useful signal received by the target receiver is: $30 d B m + 20 d B = 50 d B m$ .
The uplink loss is calculated as: $L s = 32.4 + 20 log (20) + 20 log (5000) = 132.4 d B$ .
If the interference is 10km away from the drone, the interference link loss is: $L j = 32.4 + 20 log (10) + 20 log (5000) = 126.4 d B$ .
The ERP of the interference machine is: $EPR j = 50 d B m + 10 d B = 60 d B$ .

Assuming that the receiving antenna on the drone is a whip antenna, which has the same gain in the direction of the ground station and the interference machine, the Signal-to-Interference Ratio (SIR) can be calculated as:

$J / S (d B) = ERP j - ERP s - L j + L s = 16 d B$

Interference on Data Links

The data link is also a downlink, meaning the target of interference becomes the ground station. Assuming that the ground station uses a butterfly antenna, the interference signal usually enters from its sidelobes.

Calculations:

The ERP of the useful signal is $ERP s = 33 d B m$ , and the link loss is $132.4 d B$ .
The ERP of the interference machine is $ERP j = 60 d B m$ .
The ground station’s gain in the direction of the interference machine is 15dB lower than the main lobe gain, which is $20 - 15 = 5 d B i$ .

The Signal-to-Interference Ratio (SIR) is calculated as:

$J / S (d B) = ERP j - L j + G j - (ERP s - L s + G r) = 12 d B$

Real-World Solutions for Drone Communication

While understanding the intricacies of drone communication systems is essential, it’s equally important to have reliable hardware that can handle the demands of modern drone operations. This is where Airmobi’s range of video and data links come into play.

At Airmobi, the focus is on seamless communication, especially for drones and remote applications. Their data and video links are not just ordinary transmission systems; they are designed to bridge vast distances, ensuring that your data and visuals reach their destination without a hitch.