UAV Navigation in depth: External Datalink Selection
1. General. A datalink is a means of transmitting information from one point to another. When the transmission is accomplished using a wireless means, the datalink may also be referred to as a 'radio link' or 'radio modem'.
2. Datalinks in the UAV Navigation System. Used to connect the autopilot to the Ground Control Station (GCS). The datalink allows the transmission and receipt of information between the aircraft and the GCS as follows:
- Uplink: commands from GCS to autopilot.
- Downlink: telemetry information from autopilot to the GCS.
3. A standard setup for wireless data transmission consists of two radio modems: the onboard and the onground segments. These will both work on the same frequency and will usually have the capacity both to transmit and receive data.
4. Communications Logic. Datalink manufacturers will usually use their own proprietary communications logic for their products, each manufacturer's products having a different startup, handshaking procedure, buffer size, number of retries for bad packets etc. However, the transfer of data via this proprietary procedure is transparent to the autopilot and to the rest of the system.
Generic radio modem (left), TELEM05 (right)
5. Types of Datalink. There are three basic types of datalink:
- Simplex: communication in one direction only (e.g. from GCS to autopilot).
- Half Duplex: communication in both directions, but not simultaneously.
- Full Duplex: simultaneous communication in both directions.
6. The VECTOR autopilot can make use of a half or full duplex datalink when using RS-232, RS-422 or Ethernet protocols.
7. The type of communication used by the datalink depends on the transmitter and the protocol used. The UAV Navigation system can be used with a datalink which transmits telemetry using half duplex communication. In this case, although the VECTOR uses the datalink as if it were full duplex, the datalink must have an internal logic with a buffer which is capable of managing the data and sending it as half duplex.
8. Frequencies. Datalinks may use different frequencies, depending on their intended purpose. These frequencies are strictly regulated and controlled in each country. See 'Local Radio Frequency Regulation' for further information.
SELECTION OF A DATALINK
9. The first step in selecting a datalink is to identify the permitted frequencies in the country where the equipment is to be used.
10. Next, the range of operation should be considered.
11. Datalink Characteristics. When using a third party datalink, UAV Navigation recommends the following characteristics for optimum performance:
|115200 bps-8N1, no flow control||10/100Mbps, auto-negotiated|
|MINIMUM DATA THROUGHPUT|
|4000 Bytes/s||1000 Bytes/s|
|AVERAGE PACKET SIZE*|
|Ideally 50 Bytes (max 111, min 21)|
|PERIODICAL PACKET RATES|
|20Hz, 5Hz, 1Hz and 0.5Hz||50Hz, 5Hz and 1Hz|
*statistical values from a standard mission
12. Connection Type. RS-232 (COM_DBG, COM_0 y COM_1 in VECTOR) and Ethernet are preferred.
13. Connection Speed. The UAV Navigation system requirement for the datalink is 115200 bps for a serial connection. The normal data throughput of the system is around 4000 bytes/s downlink and 1000 bytes/s uplink average, so the datalink selected should ideally ensure delivery at these speeds.
14. Retries. Datalinks may split longer telemetry messages and may also need to make some retries in order to communicate an entire message correctly. The ideal average packet size for the UAV Navigation system is 50 bytes. By way of orientation, the TELEM05 datalink offered by UAV Navigation splits data transmitted into a maximum packet size of 178 bytes, and will make a maximum of 2 retries when transmitting. These parameters are normally (but not always) configurable in a datalink.
15. Datalink Manufacturers. There are multiple manufacturers on the market; UAV Navigation recommends contacting a manufacturer with the minimum specification shown above in order for the manufacturer to propose the best solution for a particular platform.
16. Other Peripherals. In addition to the considerations above, the payload and the complete communications setup should be taken into account because these systems may have an impact on datalink performance. See also Camera Setup for further information.
17. Finally, UAV Navigation has a long history of working with the best datalink manufacturers around the world. Our engineers will be delighted to make recommendations and to assist in selection of a suitable datalink.