FlightAware Discussions

Poor reception at the airport / inside airplane

At our regional airport, we have observed that 1090Mhz reception is very poor for nearby airplanes. Once they go below 1000ft AGL, signal is all but lost. We run dump1090-mutability on Raspbian without special tuning for gain. We have tested with the standard FlightAware dongles and both whip and outdoor antennas and various cable lengths, with identical results.

Moreover, running dump1090 inside an airplane, it’s next to impossible to see the messages it broadcasts.

Can you please advise what might cause this and what possible workarounds exist? We are mostly interested in ADS-B from around the airport’s traffic pattern and up to 25NM below 5000ft.

Thank you,
Ivan

Try reducing the gain - if the aircraft ADS-B signal is too strong, you won’t see the plane.

I take a portable ADS-B raspi box to my local airport and set the gain to ~30 so I pick up the aircraft in the run-up area at the end of the runway that’s about 1/4 mile away. Experiment with lower gain settings until you get all the planes you want.

Thank you. In my tests right in front of the apron, I lowered gain of dump1090 to 30, 20, 10, 5 down to 1. Only difference noted was that at gain of 1, nothing was received. Also tried auto gain, with similar results.

Sorry if this has been asked: Are gain values meant to be given as negative numbers?

Interestingly enough, a Stratux setup next to my receiver was able to receive nearly perfect data, both on the ground and while in flight. Have you ever seen differences depending on SDR hardware? I haven’t found anything particularly negative about FlightAware’s SDRs.

One more thing: We’ve observed this behavior only at a couple of airports. Identical hardware at other, equally busy/idle airports yields nearby/ground activity without a problem.

gain values should be positive except when you want max gain which is -10 / I think the config file may show MAX instead of -10 .

You have to re-start the software or re-boot the raspi for the new setting to work.

I don’t have experience with other hardware, just the NOOELEC dongle.

Maybe those planes were using 978 ADS-B out instead of 1090. Do you have tuners for both?

Nope, only 1090, but the airplane we were testing with is equipped with a 1090 transponder.

Might be nearby cellphone towers - some antennas need a filter for that. I use DIY 1/4 wave ground plane antennas which have a natural filter due to their design.

May also want to try swapping hardware to see if the problem follows the hardware.

That may be the case, will check what else is around. I will read your article about antenna design, that may yield what we need and actually reduce cost at the same time. Thank you!

If you have a clear line-of-sight you can get great range (+200 miles) with DIY antennas.

very unusual

I am using the blue Pro-Plus stick on my Raspberry with an outdoor antenna.
The other day i edited the dump1090-fa config file and set by fault the gain to “0”

Right after that i was confused that i did not get any planes.
So having your issue seem to have a different root cause.

Once i set the gain to -10 (which is the max for that device) i have close aircraft dropping the signal due to noise.

Do you have by chance the graphs1090 installed where you can see some more details?

To me it sounds more like a technical problem.

You are overloading the front end.
Try a 30dB attenuator before the receiver.

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Nope, I’m just grep-ing the output of dump1090 for specific transponder codes in the pattern/taxiing. In researching the problem realized that transponders transmit with 100W+ of power, so that may have something to do with it.

Thanks @geckoVN. Could you elaborate on what “overloading the front end” means? You also recently referred to the dynamic range of the receiver, which led me to try the NooElec SDRs (arriving today). Have you ever seen differences in reception depending on the manufacturer?

Go to a Deep Purple concert. climb up on the stage and sit in front of a stack of Marshall amps.
Now try your best pickup line on a girl in the back row.

The main difference is if there is an extra LNA in front of the SDR or not.
The FA sticks have an LNA, so does the green AirNav stick.
Other SDRs don’t typically have that extra LNA.

I would suggest getting hold of somebody knowledgeable with a spectrum analyzer, to scan all frequencies. Normal SDR’s have no filtering on the front end, so even a strong signal in the Air Band can overload the front end, making it very difficult to receive anything at 1090MHz. Alternatively you can run a spectrum-scan on and RTL-SDR too, showing you if there are strong signals around, but if it is something out of the frequency range of the SDR, yet still strong enough to overload the front-end, you will just get a huge increase in the noise-floor without any meaningful results.
Lacking a spectrum analyzer, I would run software like SDR#, look for any strong signals as high as the SDR can receive, AND the average noise floor. For all you know, a simple bandpass filter might solve your problems, but you can never know without looking at the whole RF spectrum. If there is a very powerful radar just over the horizon, it might even overload a SAW -filter (>+20dBm).

Thanks @hellhound604 and everyone for your advice. Will conduct onsite testing next weekend with several new SDRs, attenuator and configuration and report findings over here.