How RSSI and LQ Actually Work on ExpressLRS: A Beginner’s Guide

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Understanding RSSI and LQ in ExpressLRS

If you’re into DIY RC projects or flying drones with ExpressLRS (ELRS), you’ve probably come across terms like RSSI and LQ. These two values are crucial for understanding your radio link quality and ensuring a safe, responsive flight. But what do they actually mean, and how does ExpressLRS handle them? Let’s break it down.

What is RSSI?

RSSI stands for Received Signal Strength Indicator. It’s a measure of how strong the signal is that your receiver is picking up from the transmitter. In simple terms, it’s like the volume of the radio signal.

In ExpressLRS, RSSI is typically represented as an 8-bit value (0-255), where a higher number means a stronger signal. A strong RSSI value generally means your transmitter and receiver are close together or have a clear line of sight.

How ExpressLRS Uses RSSI

ExpressLRS continuously measures RSSI to monitor the signal strength between your transmitter and receiver. This helps you understand when you’re getting too far away or if obstacles are interfering with the signal. Many radio transmitters and goggles can display RSSI in real-time, letting you know how strong your link is during flight.

What is LQ (Link Quality)?

Link Quality (LQ) is a bit more complex. Instead of just measuring how loud the signal is, LQ tells you how clean or error-free the signal is. Think of it as the clarity of a phone call rather than just the volume.

LQ is calculated based on the number of errors detected and corrected in the data packets sent between your transmitter and receiver. ExpressLRS uses forward error correction (FEC), so it can fix some errors before they cause real problems.

How ExpressLRS Calculates LQ

ExpressLRS examines the incoming data packets and tracks:

  • Packet loss (how many packets are missed)
  • Corrected errors (how many bits were fixed)
  • Uncorrectable errors (packets that can’t be fixed)

Using this information, ELRS derives a percentage value for LQ, usually between 0% and 100%. A high LQ percentage means your link is clean and reliable, while a low percentage indicates interference or weak signal quality even if RSSI is high.

Why Both Matter

RSSI and LQ together give you a full picture of your radio link health:

  • High RSSI + High LQ: Excellent signal strength and quality — perfect for flying.
  • High RSSI + Low LQ: Strong signal but lots of interference or noise — watch out for glitches.
  • Low RSSI + High LQ: Weak signal but clean data — usable but approaching range limits.
  • Low RSSI + Low LQ: Weak and noisy signal — risk of failsafe or loss of control.

Practical Tips for UK RC Makers and Pilots

  • Monitor Both Values: Don’t rely on RSSI alone. Use LQ to gauge actual link reliability.
  • Use Good Antennas: Proper antennas improve both RSSI and LQ by boosting signal strength and reducing noise.
  • Adjust Power Settings: ELRS allows you to tweak transmit power. Increasing power can improve RSSI and sometimes LQ, but be mindful of local regulations.
  • Test in Different Environments: Buildings, trees, and weather affect link quality differently. Check both RSSI and LQ in real flying conditions.

Summary

ExpressLRS’s RSSI tells you how strong your radio signal is, while LQ shows how clean and error-free it is. Both are critical for safe, reliable RC flying. By understanding and monitoring these metrics, UK makers and pilots can optimise their setups for the best possible performance.

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