What are C2 Links?

A C2 link (command and control link) refers to the communications link between a UAV (unmanned aerial vehicle) and its ground station that is responsible for managing and controlling the UAV. C2 links are essential in drone operations where the aircraft is being remotely piloted by a human or is programmed to fly autonomously. Drone is a generic term used to refer to any unmanned or remotely piloted aircraft including the RPAS (Remotely Piloted Aircraft System) and UAS (unmanned aircraft system).

Nowadays, Drones are used in precision agriculture, emergency delivery of medical aid, an inspection of oil and gas pipes, power lines, bridges, wind turbines, train lines, solar installations, and preventive maintenance. Trials are being performed to extend their use to public safety, monitoring traffic, and even air taxi services. In the RPAS, C2 links play a pivotal role in the controllability of the aircraft, ATC communication, and conflict detection and avoidance systems (CDA). SESAR (Single European Sky ATM Research Programme) estimates that 20% of traffic will be remotely piloted by 2050.

Communications Technologies used for C2 Links 

C2 links of drones may use various communication technologies. Typically, Line-of-sight (LOS) drones will use an RF (radio frequency) link such as UHF or VHF for the C2 link, and extremely short-range drones may use Wi-Fi or Bluetooth wireless technology. Cellular communication services such as 3G, 4G, and 5G may also be used; but can have limited utility due to “dead zones” in cellular coverage. Beyond visual line of sight (BVLOS) drones, mainly military drones, are likely to use some form of satellite communications (SATCOM) service such as Inmarsat or Iridium for C2 links.

What are the frequencies allocated to C2 Links?

In 2012, ITU Radiocommunication Sector (ITU-R) identified a number of bands as suitable for C2 links of remotely piloted aircraft (RPA), the following bands received the most interest: 

• Terrestrial (C2 link) - 960-1164MHz, 5030-5091MHz
• Satellite (C2 link) – 1545-1555/1645.5-1656.5MHz and 1610-1626.5MHz as well as 5030-5091MHz.

UAV Command and Control Systems

UAV command and control is typically performed by the ground control station (GCS) and may be built using hardware such as a desktop PC or a portable device such as a laptop or tablet, or mobile phone. The GCS features a C2 radio or other communications equipment that allows autonomous commands or remote control and will also allow the drone to send telemetry data back to the GCS. This control station can also send commands to and receive data from payloads installed on the UAV.

C2 link classifications:

Depending on the data flow supported by the RPAS, a C2 link can be classified as either cooperative or non-cooperative and depending on the communication architecture required, a C2 link can be either dependent or non-dependent.

A non-cooperative C2 link supports only the "aviate" function, whereas a cooperative C2 link also supports the "communicate", "navigate" and "integrate" functions. A non-dependent C2 link is a link entirely internal to the RPAS, while the dependent link requires an external communication architecture. Based on this classification, the typical C2 link usage is shown below figure.

Typical C2 link usage

The abbreviations used in the figure above are specified below:

VLOS – Visual Line of Sight

BVLOS – Beyond Visual Line of Sight

UTM – Unmanned Aircraft System Traffic Management (UTM)

VFR – Visual flight rules

IFR – Instrument flight rules