The role of the telemetry link is to allow communication between the rover and base module. In the case you encapsulate the RTK data into the MAVLink, the telemetry link providing communication between rover and base modules is the telemetry link between your base control station (usually a computer) and your autopilot. In other cases, you may need to use an external telemetry kit to link your rover and base module.
It's possible to use many systems for this role, but in this book, you'll only be introduced to Drotek's homemade 433 MHz telemetry kit and a generic 500 mW telemetry kit.
The 433/915 MHz Telemetry kit can very easily be used with our RTK modules. Already configured in factory, it is plug-&-play with the RTK modules and will offer more than a kilometer of range for your system (using the right antennas). Available in both 433 MHz or 915 MHz, it can output 100 mW, and it's range can be upgraded by using a bi-directional amplifier or installing omni-antennas.
Wirings and connections
The Drotek telemetry kit is built to be plug-&-play. It is not necessary to manually configure the kit, as the values and rates are either configured in factory to match the RTK modules settings or automatically configured by Mission Planner or QGroundControl to be used for the RTK application.
However, if you're not using an autopilot system with you RTK setup, manually configuring your telemetry kit can help achieve what you want to do.
As said above, using an RTK system without an autopilot (without MAVLink then) implies to configure the telemetry kit according to your RTK setup.
It is important to note that all baudrates, for the RTK modules as well as both telemetry boards must be the same.
To configure your telemetry boards, you will need to download the 3DR Radio Config software. You can download it from this link. It doesn't require you to install it. When downloaded, extract the archive to a location on your PC of your choice, and then start the software by opening the 3DRRadio.exe file.
This is almost the same as the Mission Planner telemetry setup menu. You first need to connect to your telemetry board. To do this, simply choose the COM port and baudrate of your telemetry (default is 57600) and click Load Settings in order to get the current setup of your board. You will see the software gathering the informations from the board and display them for you.
If your two telemetry boards are linked (Green LED solid), the software will also display the configuration of the second module on the right Remote panel. It is possible to configure one of the two modules, and then upload the modified settings to the second one by clicking the Copy required items to remote button at the bottom of the window.
The main parameters you can tweak are:
Baud : This will define the baudrate of the telemetry radio. In an RTK setup, it is absolutely necessary to have the same baudrate for all the devices implied in the communication process (GPS chips and telemetry radios). (Note that the values are shown in one byte format, which correspond to the first digits of the desired baudrate. For instance, 57 corresponds to the 57600 baudrate, 115 stands for 115200, etc...)
Net ID : This is the ID of your telemetry board. If the Net ID of your two boards is the same, then the two will be able to connect to each other. If the Net ID is different between one telemetry board and the other, then they won't be able to connect together. This allows to setup different networks when several telemetry boards are being used.
Airspeed : This parameter will select the speed at which the two telemetry boards will communicate with each other. Increasing this value will imply a diminution of the range, whereas diminishing this value will grant a longer range. The value is expressed in Kb/s.
There are other parameters that you can tweak, but these are the main settings to take care of in order to have a stable and customized setup.
You can find informations on all the parameters by following this link. The software used is Mission Planner but the operation is the same.