The use of ‘traditional’ radio frequency (RF) communications, comonly in recent years at the Very High Frequency (VHF) band, is widespread for RPAS flight control and telemetry.  Most RF transmissions follow a direct point to point path (cf ’straight’) but as the frequencies get lower (eg High Frequncy or HF), they are subject to refraction and exhibit some ‘bending’ to allow limited over-the-horizon (OTH) communications.  VHF tends to be used where there is direct Radio Line of Sight (RLOS) between the Remote Pilot Station (RPS) and the RPA.

In some environments, such as in built up areas, VHF can relect off buildings and similar surfaces to deliver ‘multi-path’ communications (hence not direct RLOS) but these tend to be unreliable since an RPA will be moving and not always able to receive any multi-path signal continuously.

The use of radio relays can extend RF communications beyong the Line of Sight (LOS).  Such relays may be terrestial (normally in high points) or airborne themselves.  The technical requirement is for the RF signal to be able to have direct LOS to and from the RPS, relay stations and the RPA.

Generally, lower VHF frequencies propogate more successfull at longer distances.  It is understood that effective RPA flight control can be achieved, under direct RLOS conditions, using VHF at distances up to 350km.

When RPAS flight control is regarded as a safety critical function (depending on the safety case, scenario and operational approvals), there is usually a requirement for flight control communications to use ‘protected frequencies’, which are reserved for aviation and are not available for general use.  This reduces the risk of communications errors, such as interference and other loss of integrity.

Typically flight control communications using VHF are considered an integral part of an RPAS and as such are delivered as part of a system deployment and are owned and implemented by the RPAS Operator.  In some countries, there have been proposals for a Regulator to arrange for provision of RF communications serrvices under contract to Operators.

For much more local use, such as for Visual Line of Sight operations (VLOS), publicly available frequencies, such as those used for model aircraft (35MHz, 2,4 and 5 GHz) are commonly used.

For operations beyond the feasible range of VHF and similar RF communications, the use of Satellite communications provides options.

Further Reading

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