Creating a long range/low latency/ low-cost digital radio link for UAVs & aerial photography
As my „not just another drone“ project grew very large over the last year I decided to split it up into different articles.
This article focuses on the communication link between a drone & ground station. As no affordable industry solution existed in 2016 I started to create my own. After spending months developing the software I decided to release it under the name of DroneBridge. DroneBridge is a set of modules plus an Android app that should help to make MultiWii (Betaflight/Cleanflight/iNav) and MAVLink based flight controllers more suitable for aerial photography. It aims to be a real alternative to solutions like DJI Lightbridge or OcuSync.
All DroneBridge modules are embedded in the EZ-WifiBroadcast image. WifiBroadcast runs on the Raspberry Pi and uses the Pi camera module to deliver an HD low latency video feed over more than 2 km. It uses wifi hardware and patched drivers to push the hardware to its maximum. An ideal starting point for DroneBridge.
You can find the most current source code of DroneBridge here:
Why build DroneBridge?
The EZ-WifiBroadcast project has come a long way and is very good at delivering low latency video over long distances. Since release 1.5 there also is the possibility to send RC signals. Without this, the user is forced to switch to 5.8 GHz for video transmission to not interfere with the radio control that usually operates at 2.4 GHz. With video over 5.8 GHz, the range gets halved. The EZ-WBC developers spent a lot of time getting various WiFi adapters to work and build an easy to install image. For that, you have to respect their work.
However, the code and provided documentation of EZ-WifiBroadcast are not developer friendly. In fact, there is no documentation about used protocols and architecture at all. You have to dig through a lot of (unformatted) code to get an idea. The decision process is not transparent. You will have to read 400 pages of forum posts to get a glimpse of why they did certain things. It is not optimized to work with iNav flight controllers (only supports MSPv1). There are no clear interfaces for where to get or to push certain data. A lot is done using a huge shell script and pipes. There is no easy (or documented) way of creating an MSP or MAVLink upload.
The RC implementation only supports eight channels and works with Atheros and some new Realtek cards. There is no way of calibrating the joystick (the default calibration done by Linux sometimes needs adjustment) nor is there a way of seeing what the current channel values are. Some low-cost remotes like the Turnigy i6S have a “strange” button mapping. This needs to be taken care of, otherwise only the analog inputs will work correctly (No button support with EZ-WBC). Changing settings like frequency or FPS of the camera is not possible without removing the SD cards.
And finally, there is no app/user interface out there which is specifically designed to work with the system. Especially when using iNav there is no app or application that delivers the required features for mission planning/execution and video feed.
Instead of trying to fix all these issues I decided that it is better/faster to start an entirely new project and rewrite almost all of the code. By sticking to the base image provided by EZ-WBC I can take advantage of all the driver and OS modifications that were made.
The goal is to build a complete system that works with common UAV hardware. It must be easy to configure and use and capable of enabling low latency video, telemetry, RC and MSP/MAVLink two way communication.
An RC implementation that has more than eight channels, works perfectly with popular hardware and mimics a standard radio control including failsafe. Everything should be easy to configure using an app that acts as the UI for the whole system (drone & ground station)
One open system. A cheap and almost ready to fly solution for UAVs of all kind.
DroneBridge for Android: What can it do?
Display important information using an android app including:
- RSSI of drone & ground station
- low latency live video delivered by the WifiBroadcast project (~112 ms glass to glass latency)
- Monitor autonomos missions (LTM & MAVLink telemetry)
- Positions on a map including: operator, drone, home point
- All kinds of telemetry information
- allows sending any MSP/MAVLink command to the flight controller
- display battery status (voltage, cell voltage, percent using a battery model)
- allows changing wifibroadcast settings (frequency, FEC etc.)
- allows configuring DroneBridge settings
Modular design & custom protocols
With DroneBridge one can send any MSP or MAVLink command directly to the flight controller. For example commands like:
- Calibration of MAG and ACC
- Autonomous missions
DroneBridge features support for the Raspberry Pi and the cheap ESP32.
While the ESP32 only gives about 150m of range it is a cheap & easy way of getting started. Its meant for small & light builds and is supported by DroneBridge for Android out of the box. In the future users can create, edit and upload missions via the app.