RSSI is the abreviation for Received Signal Strength Indicator - this sounds pretty self explanatory, but let’s look a little bit more into it. With wireless signal the RSSI metric is often used with wireless signal to get an idea about the strength of a received signal.
There is no standardized relationship of any particular physical parameter to the RSSI value, vendors usually define that for themselves. But what is standard is, that higher is better and 0 is the lowest possible (and thus, worst) value.
RSSI is often measured at a point of the circuity before any amplification has been applied, basically as raw as possible. Often the RSSI strength is broken out as an analog DC signal.
In FPV, RSSI is often used in the simplest version of diversity receivers, where the “best” receiver is chosen based on its RSSI value. The RSSI signals are compared and the circuity simply switches between the receiver with the highest RSSI value. This is a obviously a very simplified way to look at it, usually there is a bit more logic that decides when to actually switch to the “best” receiver, and systems like RapidFire add another layer of logic to the processing of the signals from both receivers.
In the Betaflight OSD the frequently used RSSI element for the receiver has a scale from 0 to 99 - 99 being the best.
The problem, or the short coming with RSSI is, that it is just a momentary snapshot of the signal strength, it does not tell you anything about the noise level and you do not know how many data packages have properly arrived and could be processed by the flight controller, this is where LQ - Link Quality comes into play.
Different types of Link Quality
We need to differentiate two different kinds of Link Quality - There is Link Quality that is build into Betaflight and Link Quality that is provided by the receiver itself, like for example the TBS crossfire receivers.
And this is a really big difference: Betaflight calculates Link Quality from the last 16 packets that it received. In case of D16 - which has an average frame length of 25ms - Link Quality will only be calculated for the last 0.4 seconds. So in just under half a second your link quality could drop from best to the worst, this is by no means a metric that you should trust in any way.
To quote one of the Betaflight developers: The “fake” link quality for SBUS is really a hack only meant to be used for Futaba receivers that don’t offer any RSSI. It should not be used for FrSky or other SBUS receivers. It does not provide very accurate data and is not a real link quality.
When Link Quality is built into your receiver, it is a different story. The manufacturer will (hopefully) have added a more reliable way of calculating the Link Quality that at least spans over a meaningful time frame.
If you do not have a receiver that supports proper Link Quality measurements, simply deactivate the warning in Betaflight - this is not enabled by default, but it was enabled with my latest iFlight ready to fly whoops, the Alpha A65 and the Alhpa A85 - this is what made me look into this topic in the first place.
If you are using Crossfire, R9, or any other system that provides native Link Quality calculations, then by any means enable Link Quality, it is definitely a more versatile way of measuring the receivers actual reception and can help you better understand the actual health of your link and when it is time to turn around and come home.