Why we use Bluetooth and analogue video transmission in our products

When it comes to transferring data to a drone, there are many possible standards in place. Each standard comes with its own set of advantages and disadvantages. In this article, we would like to discuss some of them briefly.

In model racing, the best known method of control data transfer is the classic RC remote control which is often accompanied by a set of FPV glasses to receive video images from the drone. With the RC remote control, connection over very large distances are possible.

Theoretical Range

The 3km range of the RC remote control, however, can only be achieved in theory since drones and other model aircrafts are required by law to be within the line of sight of the pilot. More information can be found in our article about the drone regulation in Germany.

RC remote controls transmit their signal digitally over the 2.4 GHz band. More often than not, a form of frequency hopping is used. Frequency hopping means that the frequency over which the RC signal is transmitted changes continuously and thus the signal overall becomes less susceptible to disturbances in any one frequency. Any failure in communication due to disturbances in any one frequency lasts for an extremely short time and thus is hardly noticeable. The remote controls themselves are relatively large and unwieldy.

The video image transmission from the drone to the FPV glasses can also work over similar range with the right configurations (directional antennas, diversity receiver). However, any interference with the transmission of an analog video plays a much more significant role as they are directly visible in the picture. Therefore, in order to avoid interference from other signals, the video transfer systems avoid the 2.4GHz frequency band which is already crowded with RC remote control units, WLAN, Bluetooth and other radio transmitters. Video transfer usually occurs on the 5.8 GHz band.

Analog Video Transmission

In general, the analog video image transmission has very low delay (latency) of only about 15 milliseconds (the time between the video recording at the drone and its display at the receiver). This is highly desirable for FPV racing drones, hence, it is very popular despite their susceptibility to interference. The 5.8 GHz band also comes with its own set of disadvantages. With the same transmission power, a much smaller transmission range is achieved because the radio signal attenuates much faster in air. Moreover, obstacles such as trees and walls can cause further problems as the signal at this frequency cannot easily permeate solid objects.

Exclusive Manufacturer Standards

Professional drones (such as the Phantom, Mavic and Inspire models from DJI) use an expensive and exclusive technique that allows for almost latency-free digital video and control signal transmissions. The images can be transmitted with a high bandwidth (even achieving HD quality) over very long distances (theoretically up to 5 km). DJI has branded it the “Lightbridge” technology. The video transmitters are large, heavy and power-hungry, thus, unsuitable for lighter drones. Moreover, with €1.000 price tag, the video transmitter and receiver are no bargain. Other market competitors such as the Connex Prosight from the manufacturer Amimon has similar advantages and disadvantages.

Many beginners drones use WLAN to transmit control and video signals. WLAN works mostly on the 2.4GHz band, but has a low range of only 30 – 100 m. We tested a large variety of WLAN modules for the transmission of the video signal during the early development of the SmartPlane Pro FPV. The results, unfortunately, were very disappointing as the range was miserable and video had significant latency as well as breaks in the stream. Therefore, we decided against using this system.

Bluetooth Smart

Finally, we decided to use the latency-free analog video transmission at 5.8 GHz. The control signals are transmitted via Bluetooth Smart which avoids the need for a large and expensive remote control and can simply be controlled with your smartphone. Bluetooth Smart transmits over the 2.4 GHz band is actually known for rather small transmission ranges. However, with the correct antenna design and appropriate reception module in the drone (where we use the costly Nordic NRF51 series), we were able to attain a ranges of 150 meters without any problems. For small drones like our Moskito and SmartPlane Pro, this proved to be by far the best choice.

We hope, we could give you more insights in this topic. Don’t hesitate to reach out to us if you have more questions or simply post your comment below the article.

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