MAVLINKHUD

Telemetry Ratios & Bandwidth Math

Executive Summary

Unlike traditional radios with fixed telemetry, ELRS allows you to tune the Uplink/Downlink Balance. The "Telemetry Ratio" determines how many packet slots are sacrificed to send data back from the drone. For a MAVLink HUD, choosing the wrong ratio (e.g., 1:64) results in unusable lag.

Theory & Concepts

1. The Ratio Math

The ratio defines the frequency of downlink packets.

  • Packet Rate (Air Rate): The fundamental speed (e.g., 250Hz).
  • Ratio: 1:2 means "1 Telemetry packet for every 2 Control packets."
  • Formula: Telemetry_Rate = Packet_Rate / Ratio.

2. Bandwidth Calculation

How many bytes per second do you get?

  • Example: 250Hz, 1:2 Ratio.
    • Telemetry Rate = 125 Hz.
    • Payload per packet = ~6 bytes (standard ELRS) or variable for Airport.
    • Airport Bandwidth: Optimized to fill the packet. Approx 1000 Bytes/Second.
  • Counter-Example: 50Hz, 1:64 Ratio.
    • Telemetry Rate = 0.78 Hz (Less than 1 packet per second!).
    • Bandwidth: ~5 Bytes/Second.
    • Result: Your HUD updates once every 2 seconds. Mission Planner times out.

Architecture (The Engineer's View)

MAVLink is "heavy."

  • Heartbeat: 9 bytes.
  • Attitude: 28 bytes.
  • Global Position: 28 bytes.
  • Sys Status: 31 bytes.
  • Total: A basic stream requires ~500 bytes/sec for 5Hz updates.
  • Conclusion: You MUST use a high packet rate (250Hz+) and a high ratio (1:2) to support MAVLink.

2. Race Mode vs. Standard

  • Standard: Telemetry is always sent.
  • Race: Telemetry is disabled when Armed (to reduce latency jitter).
  • Warning: Never use "Race" mode with a HUD app. Your screen will freeze the moment you take off.

Key Parameters

  • Telemetry Ratio (Lua): Set to 1:2 for MAVLink.
  • Packet Rate (Lua): Set to 250Hz (900MHz) or 333Hz/500Hz (2.4GHz) for best results.

Source Code Reference