Data link is the term used to describe how the RPAS command and control information is sent and received both to and from the GCS and autopilot. RPAS operations can be divided into two categories: radio frequency line-of-sight (LOS) and beyond line-of-sight (BLOS).
Line-of-sight (LOS) operations refer to operating the RPAS via direct radio waves. Depending on the strength of the transmitter and receiver, and the obstacles in between, these communications can travel several kilometers. Signal strength can also be improved utilizing a directional tracking antenna. Beyond line-of-sight (BLOS) operations refer to operating the RPAS via satellite communications or using a relay vehicle, usually another aircraft. Most RPAS do not have the need or ability to operate BLOS since their missions are conducted within line of sight range. Military BLOS operations are conducted via satellite on an encrypted Ku band in the 12 to 18 GHz range.
The maintenance of the communications is of paramount importance in UAS operations. Without the ability to communicate, the RPAS is reduced merely to a drone system and loses the versatility and wide capability of the RPAS. Loss of communication during operations may result from:
a) Failure of all or parts of the system due to lack of reliability,
b) Loss of line-of-sight (LOS) due to geographic features blocking the signals,
c) Weakening of received power due to the distance from the UAV to the control station becoming too great,
d) Intentional or inadvertent jamming of the signals.
The specifications for communications performance will include two fundamental parameters:
•‘Data rate’ which is the amount of data transferred per second by a communications channel and is measured in bytes per second (Bps).
•‘Bandwidth’ which is the difference between the highest and lowest frequencies of a communications channel, i.e. the width of its allocated band of frequencies and is measured in MHz or GHz as appropriate.