Dynamic Range: Finding the Balance

What is dynamic range?

ADS-B receivers turn a radio signal into bits (zeros and ones) that computers understand. The transponder radio on an aircraft will send out a radio wave encoded with information.

This wave travels to your antenna and then into the receiver to be digitized. How this happens depends on a circuit called an ADC (analog to digital convertor). Weak signals will have a low value in the digitizer and strong signals will have a high value. The range of values that the digitizer can measure is called the Dynamic Range. Very high dynamic range means your receiver can pick-up a plane very close by and also very far away. (This is generally a good thing!)

We display the signal strength in a value called RSSI (Receiver Signal Strength Indicator) that is viewable on the information panel in Skyview. When you click on an aircraft, its RSSI indicator will show on a scale from 0 to -49.5 dBFS. Values of 0 to -2 dBFS are similar to shouting at the receiver and can overload the receiver. Values close to -38 dBFS are the smallest values that the receiver can measure and decode due to the noise floor. (RSSI values of -49.5 dBFS indicate that we don’t know the signal strength due to the signal coming from other sources such as MLAT.)

You will generally find that planes closer to the receiver will have higher RSSI values and farther away planes will have lower RSSI values. This isn’t always the case. General aviation planes, usually small propeller planes, with low powered transponders will have very low RSSI values even when very close. You will also find large passenger aircraft with very high powered transponders and can have very strong RSSI values even far away. In general, there will be a steady decrease in signal strength as planes fly farther away from your receiver.

Strong signals can actually be bad!

Depending on your setup and how close you are to the aircraft, you might have too much peak signal strength. Peak signal strength refers to the strongest signal received over a period of time. A peak signal close to 0 dBFS can blind the receiver for a short time but too many peak signals can overload the receiver and cause it to be blind for a long time. This is where the PiAware gain setting comes into play if you are using an RTL-SDR dongle such as a FlightAware Pro Stick or Pro Stick Plus. The gain setting can be changed by editing the piaware-config.txt file on your PiAware SD card. We recommend setting the gain such that you have the highest number of messages being decoded in Skyview. You can see the message rate and aircraft count measurements in the top right of the information panel in Skyview.

By default the gain setting is set to maximum gain (represented by "-10") and this will usually give the most messages. If you have a very high gain antenna or are very close to an airport the you will probably want to lower the value. We suggest using a gain of 40 and then try higher or lower values and comparing the message rate in Skyview. For advanced users, there is discussion in our forums about how to get more sophisticated with optimizing the gain setting.

Once you have found the gain setting that maximizes your message rate then you have found your best gain setting. You will also find in Skyview that close planes will usually have an RSSI of around -2 dBFS and far away planes around -38 dBFS. This is close to the full dynamic range of RTL-SDR dongles and FlightAware Pro Sticks.

If you have FlightFeeder, then the gain setting is already configured for you.

For those interested in joining the ADS-B community, FlightAware offers step-by-step instructions to build a PiAware Receiver for under $100.

ADS-B hosts located in areas needing additional coverage might be eligible for a FlightFeeder, a free, prebuilt ADS-B receiver.

FlightAware invites you to join the ADS-B network. You'll become part of a community of aviation hobbyists across the globe and start seeing the skies in a totally different way.