Improvement methods of short range and low bandwidth communication for small range UAVs

Abstract

Parameters of the onboard electronics of a small sized (0.5–3m) and light weight (0.5–5kg) UAV is highly limited. Usually there is only a low powered, narrow bandwidth 2.4 GHz radio modem (e.g. XBee or XStream) for the communication. The useful range of these modules is only 1–2 km. Over that distance, the noise of the channel is very high. There is no way to use a high gain, physically large omnidirectional antenna on the UAV. High gain and large directional antenna (e.g. yagi or patch) can only be used on the ground station only. These radio modems can operate only in half-duplex mode. The specificity of the telemetry is that usually the UAV is transmitting and the ground control station is receiving. Telemetry data contains GPS, navigation and internal state messages of the UAV and sensor data. For the human readability, these messages are usually in ASCII format. There must be a little time window between the sent messages to capture possible commands from the ground. The problem is the already flooded telemetry channel continuously deteriorating while the UAV flies away, so the transmitted messages became unrecognizable. The communication can be improved with an adaptive dynamic message compression process. The onboard computer can detect channel failures and it can automatically tune the compression so telemetry range can be increased. For this, it is necessary to use a message framing protocol and to set up a hierarchy of the messages and its contents. This compression process needs to be implemented in the UAV tracker module at the ground control station also to improve the stability of the communication. The first task of the module is to forward the UAV messages to the ground control station. The second task is to continuously track the plane with the high gain directed antenna with the knowledge of its own and the position of the UAV from the messages. If the plane is close to the ground control station the module switches to an omnidirectional antenna. With the preprocessing of the telemetry the independent operation of the module and long range communication in bad conditions can be ensured.