The Cx link is a crucial piece of technology for RPAS integration in the civil airspace. The C2 link contains the Command and Control, i.e. all the messages used to control the RPA and to retrieve data from its systems (payload excluded). The C3 link also includes voice communications between the GCS and the other airspace users (ATC, aircraft). Finally, some speak about the C4 link to designate the « Command Control Communication and Payload » which includes the payload downlink/uplink.
There is large quantities of data going through these communication links and this requires bandwidth. Such bandwidth is limited and some bands need to be shared with other services. Recently, in 2012, the International Telecommunication Union (ITU) dedicated the C-band (5030-5091 MHz) for civil drones C3-link, but it will be a few years before the ICAO adopts a resolution implementing the use of the C-band.
We provide more information on the available bands for drones communication in this report, written for Engie-Ineo.