MAVLINKHUD

Link Symmetry & TX-as-RX

Executive Summary

Standard ELRS systems are Asymmetric. The Transmitter (Ground) has 1000mW of power. The Receiver (Air) often has only 100mW (telemetry). This means you can control the drone at 20km, but you lose telemetry (HUD data) at 5km. To solve this for professional MAVLink use, we use the "TX-as-RX" strategy: flashing a high-power 1000mW Transmitter Module with Receiver firmware.

Theory & Concepts

Wireless range is determined by the Link Budget.

  • Received Power = Transmit Power + Gains - Losses.
  • Uplink (Ground -> Air): 1W (+30dBm) TX -> -110dBm Sensitivity = 140dB Budget.
  • Downlink (Air -> Ground): 100mW (+20dBm) TX -> -110dBm Sensitivity = 130dB Budget.
  • The Gap: The Uplink is 10dB stronger (10x power). You will lose MAVLink long before you lose Control.

For a reliable MAVLink HUD, you need a Symmetrical Link (1W Uplink, 1W Downlink).

  • The Hardware: The RadioMaster Bandit (or similar 1W modules) contains the exact same RF chips (ESP32 + SX1280 + PA/LNA) as a high-end receiver. The only difference is the firmware.

Architecture (The Engineer's View)

1. Flashing TX as RX

ExpressLRS Configurator allows you to flash "RX" firmware onto a "TX" target.

  • Logic: The firmware re-maps the pins (DIO) to act as a receiver.
  • Wiring: You must wire the "TX Module" to the Flight Controller UART.
    • Note: TX modules usually don't have solder pads for this; you may need to hack the case or use the module bay pins (if supported by the "RX" firmware map).

2. Power & Thermal Management

A 1W transmitter gets hot.

  • Cooling: Standard receivers rely on airflow. A 1W "TX-as-RX" unit might need a fan or heatsink, especially if buried inside a fuselage.
  • Power Supply: A 1W module draws ~0.5A at 5V. Ensure your Flight Controller's BEC can handle this load, or power it directly from the battery (if the module supports VBAT).

Key Parameters

  • Telemetry Power (Lua): Set to "Match TX" or a fixed high value (e.g., 250mW, 500mW) to maintain the link.
  • Fan Threshold: Configure if using a module with an active fan.

Source Code Reference