RSSI is a measurement of how good the radio signal is between your RC transmitter and receiver. It stands for Received Signal Strength Indication, and is an important safety feature. It helps avoid loss of radio signal, which could lead to unexpected crashes, damage to your quadcopter and other people’s properties.
Understanding what RSSI is
RSSI isn’t actually a linear measurement like voltage or temperature, but a ratio of the current signal to some initial “good” value. It’s measured in dB (the same unit system used for audio levels). dB is a logarithmic measure, and it’s not linear.
This means that a change of +6dB in RSSI, the signal strength has doubled, while a change of +12dB in the RSSI means the signal has increased in power by 4 times.
For instance, if the RSSI reads about 110 at a 1 meter distance, every time you double the distance between the transmitter and receiver, the RSSI level should drop by 6dB. At about 100 meters you should get an RSSI value of about 70dB in ideal conditions.
A good way to see this effect is to use Range Test mode on your radio transmitter. In range test mode the transmitter module operates at 1/30 of full power.
Getting RSSI from RX
Some radio receivers offers RSSI output, which you can connect to the flight controller, or OSD module if supported. With the Frsky Taranis and Horus radios, it’s even possible to pass RSSI through a spare PPM or SBUS channel, without any extra wiring or soldering.
One good use of RSSI is to setup alarms on the radio that warns you when signal strengths drops below certain level.
This is a built-in feature in the Frsky Taranis. It gives me great peace of mind knowing how strong the signal is while flying, and it is a helpful indicator of when to turn back. With this feature I barely need to look at RSSI value in my OSD and I eventually removed it from the OSD.
Getting RSSI from SBUS Protocol (Betaflight Feature)
In Betaflight 3.4 there is a new feature that calculates RSSI automatically when you are using SBUS. I think the way it works is pretty similar to LQ measurement in the TBS Crossfire, where the link quality is determined by the amount of corrupted packets.
When the radio link gets too weak, SBUS packets can become corrupted. SBUS reports these corrupted packets to the FC, however previously these reports were ignored. Now, Betaflight can use these corrupted packets as an indicator of link quality, and displays the result as RSSI in your OSD.
In my opinion this way of measuring RSSI is better than the traditional way which doesn’t take into account ambient noise levels and signal interference in the environment. Because you can have a high RSSI and high number of corrupted data at the same time if noise interference is high.
This new feature is better than “passing RSSI through a spare PPM or SBUS channel”, because it doesn’t take up a channel, and has less delay.
Note that this system of link-quality doesn’t behave exactly like RSSI used to do. As you’re approaching the range limit, the value can drop off a lot faster than you expect. So make sure to do some test flights and get used to the new system.
Analog RSSI vs. PWM RSSI
If you are feeding RSSI signal from your RX to the FC, find out whether it’s PWM or analogue, and whether your FC can read it correctly.
Knowing the difference between digital and analog RSSI
Usually, RSSI is in analogue form (a voltage level), but this is not always the case. RSSI can exist as a PWM signal (pulse width modulated) too. Therefore there could be incompatibility in the system when reading RSSI. (For example, RSSI jumps between values 0, 50 and 100)
You can easily convert PWM signal to an analogue voltage level using a “Digital to Analog converter”, or simply a low pass filter (Resistor-Capacitor filter).
Some OSD modules or flight controllers accept direct connection to the PWM RSSI output because they have a low pass filter built in.
What if OSD doesn’t support RSSI?
If your OSD module doesn’t support native RSSI input, you might still be able to display RSSI with some work around. Some OSD modules have dual battery inputs (BAT1 and BAT2), and are designed to display both the primary and secondary battery voltage levels. Most of the time we only use one and by using the spare battery input we can display RSSI.
Normally the range of the RSSI value is 3.7V to 0V (100% to 0%) when converted from PWM. It might not be a percentage but voltage but gets displayed as a voltage, you can still get a good idea what the RSSI level is. I did this mod with my Hobbyking E-OSD, and it worked like a charm.
- May 2014 – Article created
- Aug 2016 – Added info about “How to use RSSI from receiver”
- July 2018 – Added info about new feature in Betaflight – SBUS RSSI