Flightaware Filter polarity

Hello to everyone!

I’m building a Raspberry Pi ADS-B tracker and I’m going to use the FlightAware filter.
A far as I know, those kind of filters don’t have polarity (input - output) and they can work doth ways with the same performance/characteristics.

Is this true for this filter as well? Does the FlightAware filter have input and output ports?

The FA filter is not polarized.

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Thank you!!!

In other words, I can plug it both ways and I will have exactly the same results! Is this correct?

yes it is correct. I have tested it, performance same.

And something a little bit off topic but its about performance!

Having exactly the same hardware (antenna, filter, connectors, SDR, position etc) but using different Raspberry Pi models (the original one VS the latest RPi 3+) will I get more aircraft positions using the latest and greatest RPi or there is not difference at all?

Messages decoding needs some cpu power and ram. I used a B+ that had a cpu usage around 70% decoding 1200msg. I think some of of first models only had 256mb ram, later ones 512. Even if you did not get any cpu warnings, maybe the decoding was limited by low ram

So, you proposing to get the latest Raspberry Pi 3+?

No, just saying the very first model of the Raspberry Pit might not have enough power if you receive a lot of messages. However, if you do not get any warning messages, it should be ok. All other Pi should do, too.

The original RPi1 would only be able to run the ADSB decoder but not MLAT.
The RPi Zero W, RPi 2B , RPi 3B, RPi 3B+ all can run the decoder and MLAT.

The RPi Zero W and RPi 2B are very close to maxing out the CPU. You will not be able to run extra programs on these boards without degrading the decoder.

The decoder will drop messages instead of decode them if the CPU is maxed. There will be a warning if the CPU is near max.

The RPi foundation has kept the prices of all models the same. There is usually no reason to get the older RPI versions.

Thank you for your reply!

For the MLAT do I have to use and another dongle or any other external device or just the RPi with the SDR dongle and the antenna is fine?

MLAT is a settings on PiAware. It is on by default.

Newer Planes send ADSB message with GPS position.
Older Planes send ModeS message without GPS position.
By using the timing of a ModeS message on multiple receiver there is a way to calculate where the message originated from. The plane tracks are then update with MLAT positions.

About 1/2 the planes in the USA are mode-S. Other countries are usually 5-20%.

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Thank you David! So, with the same hardware I can both “decode” ADS-B and MLAT positions…

Yes, Same hardware for MLAT.

MLAT option has to be turned on (default is on) for MLAT information to be sent to FlightAware. The extra data sent to FlightAware is used to computer MLAT and the MLAT results are returned and displayed on your local Skyview map.

MLAT will usually increase the data requirements from about 400MB/month (ADSB only) to over a 800MB/month (ADSB + MLAT). Some people have limited internet usage and will turn off MLAT to not go over their limit.

MLAT isn’t decoded in the message sent by the plane but computed from the timing information from different sites. Every site in a local area with MLAT turned on will send the extra timing information to FlightAware to be used to compute a MLAT position. If you hasn’t set it’s location on your FlightAware my-ADSB stats page it can not be used for MLAT.


  1. set MLAT on (defaults to on)
  2. set location on your FlightAware my-ADSB page
  3. be in an area with 4 or more receivers with MLAT on
  4. wait for a mode-S plane to pass by
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