What is RSSI, and why should I care?

 

RSSI stands for 'Received Signal Strength Indicator."   If you own a Tandem, Twin or even the older Horus and Taranis  transmitters you can monitor RSSI telemetry. in real time.   What this means is that your receiver is sending back to the transmitter  information about link quality.  This is very useful, and also somewhat confusing.  People often ask me about values.  What seems right, what seems wrong, and why they vary?

I get people asking me questions like this from time to time.   "On one of my flights the RSSI dropped down to 28, is this normal."   What I often ask them to do is give me a telemetry log file.  This gives me the total  flight in question and I can look at the all the parameters of the flight.  Typically I will find that RSSI drops momentarily to a low level but remains in pretty good levels for the majority of the flight.  The problem is that they get a RSSI low or RSSI crucial warning and wonder if they are about to crash?

RSSI Low,/ RSSI Crucial warnings.

RSSI can be better thought of as "Link Quality."   Instead of thinking about it as some hard number that means something specifically important as a snapshot of a flight, think of it more of as an analysis of how your link is doing throughout the flight.   When the warnings go off they are a reminder that you are for whatever reason getting low telemetry readings on your link quality.  It means your signal is degrading and you may want to think about bringing the plane back. It doesn't mean a crash is eminent.  


Causes of Low RSSI warnings

Environment

You may have flown through an era where something is creating a strong signal that is interfering with your signal.   The 2.4ghz band is used for a wide variety of things including industrial, agriculture, infrastructure, government and commercial applications.  If you happen to fly near something that can momentarily swamp your signal it may not be enough to take your plane down, but  it could be enough to affect link quality.    Most often they are intermittent.   I've heard about them working on sprinkler systems at nearby golf courses, light controllers turning on lights for a nearby highway, oil wells and transformers reporting on consumption or production.  

Antenna positioning

The antenna placement and position has a lot to do with the success of your link quality.   There are things in your plane such as batteries, Carbon Fiber tubes or coating, and perhaps turbines on turbine jets which can shade if not outright block the signal from the transmitter.

The correct orientation for the 2.4ghz antenna is one that runs the length of the fuselage and the other is pointed straight up or down.   The 900mhz  T-Antenna should be mounted flush with the floor of the fuselage.   Things like foam don't interfear with the reception.   Coatings like Gunmetal and carbon fiber do. In this situation the antennas should be mounted outside the plane

One thing to keep in mind is that the receiver is often the spot in the middle of the fuselage where you conveniently plug in your servos.  This may not be the best spot for a receiver antenna, but there they usually sit, tethered  close by.

The RB 25 has 18 outputs and works with up to two TD MX receivers or any other modern FrSky FBUS receiver you may want to use.  With this, you can move your receivers further away from the  center of your fuselage, where you may get better radio reception.  If you mount two antennas, mount the second in a very different direction from the first.  I mentioned the 900mhz T Antenna should be on the floor  (horizontal). Mount the second  T-Antenna vertical.  


The second antenna buys you a bit more security and will kick in should the primary antenna have poor link quality.    Keep in mind that each TD MX has built-in redundancy.     You don't really need to go crazy mounting receivers all over your plane if you're flying line of sight.   Someone asked me for my 'too important to ever lose radio contact" setup, and this was it.

Location of the plane

When I look at telemetry logs I can show you where the plane turned simply by watching the 2.4ghz RSSI drop and then come back into normal range in a second or two   This is the nature of 2.4ghz.    One of my planes that I like to fly fairly high tends to throw out an "RSSI Crucial" warning when I make a turn away from me at a specific altitude.  I think it may be the distance away, the plane's signal null and perhaps some shading from the battery that causes  the warning.

Planes the fly directly overhead also tend to be in a location where the signal null could work against you.  It may fire off a warning, but so far nobody has ever told me that they have crashed a plane that was flying directly overhead.

Broken Antenna

Broken antennas are somewhat common.  If the receiver antenna is taped down with no slack and the plane flexes it could be enough to rip the antenna out of the connector.  Replacement antennas are fairly cheap and easy to come by.  You cannot solder them back into place.  I've seen people ruin their receivers trying to do this.  If you break the antenna, don't fly your plane with it broken.  It won't be alright.

The other type of broken antenna  is by people who open the back of the Tandem and Twin transmitters and adjust their gimbals.  FrSky Ben has a good video showing how to do this correctly. 

Just the same, if you break an antenna connection and can't fix it yourself, please send it in for service.

A broken antenna can be be diagnosed with a range check.  You should be performing range checks with each plane, and I would do them once every few months even with planes that are flying well.  With more practice you can spot when a signal looks low in a range check.

Final thoughts on RSSI

I have a Garmin GPS which works pretty well except for on annoying feature.   It keeps track of the speed limit on the road and puts out a loud audible ding when I go slightly over the speed limit.  I think the guys flying by me in the left lane  at 20% higher speeds than mine would be the first one to get a speeding ticket.

RSSI isn't gospel, its just an indication of how your signal is doing. To make matters worse, I've had people send me messages where the RSSI has dropped to 19.  They are of course alarmed and message me about it.  Then I check their telemetry logs and notice that  they had a high VFR count the entire flight and there was no way that the plane was anywhere close to being in peril.   But VFR, or Valid Frame Rates is the topic for another post.