Understand RSSI in RCRSSI is a measurement of how good your radio signal is, which stands for Received Signal Strength Indication. It is an important safety feature as it allows you to judge how the radio link is performing before a control signal failure occurs, that could crash you quadcopter.
When it comes to the hobby RC world, RSSI usually is a telemetry data from the RC Receiver, which means the RSSI value is sent from the receiver back to the transmitter as telemetry data (two way communication). You can also read the RSSI value from receiver and feed it to the OSD, and display it on your FPV screen.
In most cases, the RSSI value is not a voltage level that indicate the strength of the controlling radio link, it’s a PWM signal that used in servos and ESCs. Next I will talk about how to use this signal.
How to use RSSI PWM signalSimplest thing is to get a RC transmitter (or transmitter module) and receiver that are able to do telemetry and output RSSI. Some receiver don’t provide a pin to the RSSI, but requires DIY mod and additional resistors and soldering to obtain the RSSI information.
For example what I am using is the FrSky DJT module and D8R-XP receiver. The pair provides 1.5 km range, telemetry, including RSSI. I simply connect the RSSI pin to my OSD module, and the RSSI value shows on my FPV screen.
The RSSI is not output just as a voltage, it is a pulsed width modulated signal (PWM) with variable high-low ratio depending on signal strength. It can be converted to a voltage easily using a low pass filter (Resistor-Capacitor filter).
Some OSD module or flight controller accept direct connection to the PWM RSSI output, probably because they have a low pass filter built in, and an analogue input reads the voltage to determine the value.
Most OSD, even the most basic ones support voltage level input, so you can see your battery voltage level on screen. With that in mind, we can feed this converted analogue voltage to the OSD. Usually the range of the RSSI value is 3.7V to 0V (100% to 0%). I did this mod with my Hobbyking E-OSD. It has two voltage inputs, I use one to monitor my battery voltage, the other one to monitor RSSI, which works great!
Another good thing about this Frsky transmitter and receiver is, the TX module alarm beeps when the signal is getting weak, as it gives me great peace of mind to know I always have a strong signal while flying. I don’t need to look at the RSSI info unless I have absolutely nothing to do.
For the geeks, What actually is RSSI?Additional Info for those who like to study, but not needed to use RSSI.
RSSI is actually not an absolute value like voltage or temperature but a number that indicates the ratio of the signal to some initial “good” value. It is in dB and is the same measuring system used for audio levels. dB is a logarithmic measure not linear. What this means is that any increase of 6 means the signal has doubled in strength. So a change of +12 in the RSSI means the signal has increased in power by 4 times, by +18 means the signal has increased in power by 8 times, and so on.
For the FrSky system the RSSI should read about 110 at 1m and every time you double the distance between the transmitter and receiver, the RSSI level should drop by 6. Theoretically at about 100 meters you should get an RSSI value of about 70 in ideal conditions.
A good way to see this effect is to use Range Test mode. In range test mode the Transmitter module operates at 1/30 full power. Note where the beeps appear for all three levels. In real flight the distance will be 30 times more.