Payload Communications Systems
In addition to the primary radio link for the main control system and that for autopilot communications, it is often necessary to fit dedicated radio links for the payload system. Most
Figure 6.22 The SkyCircuits SC2 autopilot (removed from its case (a), and with attached aerials and servo connection daughter board (b). See also Figure 6.3, where the SC2 is fitted with its case and a GPS aerial on top).
commonly, this is a downlink for a camera system, but on-board cameras may also need control inputs to point, zoom, and focus on objects of interest requiring some form of uplink as well. A number of commercial lightweight systems can be used for these roles, varying from low-cost analog items that will only cope with modest image resolution to high-bandwidth encrypted military-grade systems. The precise frequency band to be used and the maximum transmitted power will depend on the regulations applying in the area of operation. Most jurisdictions provide a few public frequency bands where low-powered transmissions can be made without a formal license: in the UK, transmissions can be made at 2.8 and 5.6 GHz for example, although transmission powers are strictly limited. Operators should always confirm the local regulations before transmitting on any frequency band.
Generally, all UAV radio links work on a line-of-sight basis, although this can be to a satellite if required (and affordable). Crucial to the performance of such links is the quality of the antennas used. Assuming that a steerable system cannot be fitted to the airframe, the on-board antenna will be a good-quality omnidirectional unit. On the ground, however, it is often possible to track the aircraft with a high-gain directional antenna so as to maximize reception quality, albeit at the cost of some added complexity. We have tried a range of systems, see for example Figures 6.23 and 6.24.