AirPort

Description¶

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. A TX module can connect via USB to a laptop, and RX modules (or those flashed using RX-as-TX) can connect to a free UART on your device, as per usual. This allows for a wireless serial data link between TX and RX, as if they were connected directly via serial. Note that due to the packet size and sensitive LoRa modulation parameters used by ELRS, the data rate is relatively slow. Also, as the protocol has no knowledge of the data being transported, there is no ACK or retransmission which may result in dropped packets.

Required Hardware¶

• ExpressLRS TX module: A dedicated external TX module is required for the data link. The TX should ideally have an external USB port to make it easy to attach to a computer (assuming a computer is the desired endpoint for the data link). It is also favorable to use a module with a screen and a 5-way button, which makes packet rate and power selection easier. The TX should use an ESP-based microcontroller (for access to the WebUI).
• ExpressLRS RX: Any ExpressLRS ESP-based receiver is compatible with AirPort (technically STM-based receivers can work too, but it will be harder to set up and is therefore not covered in this guide). Receivers with higher telemetry power output (i.e. 100mW telemetry power) will provide better results.

Data rate vs Baud rate¶

AirPort uses a 64-byte buffer to queue bytes that it receives from the FC or GCS, before sending them over the air. If the buffer overflows, the entire 64 bytes are discarded. For this reason, it is VERY important to select a serial baud rate that limits the flow of serial bytes to match the maximum over-the-air (OTA) data rate.

The OTA data rate is determined by the selected packet rate. A faster packet rate will result in a higher data rate at the expense of range. The table below shows the baud rate you should use when configuring the AirPort feature.

A good baseline is:

• 100Hz Full Res at 4800 baud for 868/915MHz
• 333Hz Full Res at 14400 or 9600 baud for 2.4GHz

Configuring AirPort¶

(Optional) Pre-Installation Operation Checks¶

Once you have enabled the AirPort setting on both TX and RX, they should be ready to send bi-directional serial data. If you have an FTDI (USB to serial adapter) handy, you can confirm that the link is working as expected before installing it in a craft.

1. Using a suitable USB cable, connect your TX module to a computer.
2. Open a serial terminal client, such as PuTTY, and connect to the COM port for your TX module using the baud rate you chose in the section above.
3. Connect your FTDI to your computer, and connect the RX to the FTDI
4. Open a second instance of PuTTY, and connect to the COM port for your FTDI using the baud rate you chose in the section above.
5. Type chars into each PuTTY window, and confirm you can see them on the opposite terminal session (i.e. typing into the TX PuTTY should result in the same chars appearing in the RX PuTTY window). If you see comms, everything is working.

Installation¶

Connect the RX to a spare UART on your flight controller, as per normal:

Using the appropriate Configurator tool for your FC firmware, configure your selected UART with your desired telemetry protocol. For example, if using AirPort for MAVLink in ArduPilot, you would set:

• SERIALN_BAUD to the baud you entered in the section above,
• SERIALN_OPTIONS to 0,
• SERIALN_PROTOCOL to 1 or 2 (for MAVLink v1 or MAVLink v2).

Connect the TX to a computer via USB, open the program that you intend to view the telemetry in, then connect to the COM port for your TX module at the baud you entered above. For example, if using AirPort for MAVLink in ArduPilot, you would open Mission Planner / QGC etc. and select the TX COM port, enter the baud, then click Connect.