AirPort provides a firmware option that allows you to turn a regular ExpressLRS transmitter and receiver pair into a bi-directional transparent serial data link, over the air. Which is like turning the ELRS device into Xbees. We can set up a pair of ELRS AirPort devices to transmit the MAVLink command between the ground station and the air. Also, the AirPort has the advantage of a cheaper price and a longer effective range of communication. Cheaper and better, that is why we use it.
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Inside the box of dash lines is what we are trying to achieve. And in this doc, I am going to take RadioMaster Ranger and the RadioMaster RP1 as an example to show you how to set up the ELRS AirPort on TX and RX.
Configure AirPort
Step 1 - Flash the latest version of ELRS onto both devices (v3.3.0 onwards)
If you don’t know how to do that, you can take a look at this page Set up ELRS between controller and receiver
Step 2 - Enable the WIFI on the TX module
By plugging RadioMaster Ranger into power and waiting for 60 seconds, the devices would enter WIFI mode. For Ranger, this can also be achieved by long pressing the middle button and switching the mode to WIFI mode.
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Step 3 - Configure TX AirPort on Web UI
Connect the computer or cellphone to the ExpressLRS TX WIFI, which allows you to enter the Web UI of TX module configuration page. The password of the WIFI is always expresslrs
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In the Web UI, you should see the firmware version of the device to be v3.3.0-RC1 or any firmware version you just flashed to it. Under OPTIONS, the most critical setting for the airport is to check the box of Use as AirPort Serial device. Set up the baud rate of AirPort UART baud.
A good baseline is:
100Hz Full Res at 4800 baud for 868/915MHz
333Hz Full Res at 14400 or 9600 baud for 2.4GHz
For our case, I configured the ranger to run with 333Hz Full Res packet rate with 2.4GHz radio frequency. So I could set up the baud rate to either 9600 or 14400.
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I recommend setting a Binding Phrase for the sake of easier binding. As long as two devices have the same binding phrase, they would connect automatically.
But Notice! The Binding Phrase for the AirPort system has to be different from the Binding Phrase for RC system. This is explicitly mentioned on the ELRS website.
The Web UI also provides the option for the user to customize the actions of the buttons. The picture shown below is the default setup of the buttons.
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Step 4 - Configure RX AirPort on Web UI
I am using the RadioMaster RP1. After powering on the module for 60 seconds, the receiver’s led would fast blink which indicates it is in WIFI mode.
Look for the WIFI called ExpressLRS RX on WIFI detected from your computer. Connect to that WIFI, and the Web UI would pop up.
The RX module should have the same ELRS firmware version, the binding phrase, and the UART baud rate as the TX module with Use as AirPort Serial device enabled.
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Then, go to the MODEL page. Making sure the Serial Protocol is set to CRSF. Otherwise, the AirPort won’t work!
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Step 5 - Configure the Ground Station
Finally, the last step! Connect everything up and build the MAVLink connectivity.
The ultimate goal for the AirPort system is to make the drone able to connect to the Mission Planner remotely.
The Ranger is supposed to connect to the computer and send out the Mission Planner message to the air. And the receiver is supposed to connect to one of the telem ports on the Pixhawk, receive the MAVLink message from Mission Planner, and send out the MAVLink message from Pixhawk.
In order to achieve that, we have to configure the serial port of the telem port that is connected to the receiver such that it receives the same UART baud rate we set to the AirPort system and protocol to be MAVLink 2.
To configure the Pixhawk, first, connect the Pixhawk to a computer through a USB cable. Open up the Mission Planner, set the com port to auto, and hit connect. After the Pixhawk is connected to the Mission Planner, we can open up the Full Parameter Tree list in the Config tab. Search for the serial that the telem port is correlated with. Set the Serial Buad Rate to 9600 or whatever the serial baud rate is set for AirPort. Set the Serial Protocol to 2, which represents MAVLink2. Write Config. Configuration done.
Now, every setup for the AirPort system through ELRS is all done. Enjoy the radio connection brought by ELRS.
References and Useful Resources:
The ELRS AirPort website: https://www.expresslrs.org/software/airport/
How to set up Mission Planner: https://ardupilot.org/plane/docs/first-flight-landing-page.html