FlightAware Discussions

Ducting and long distance reception

I’m on a hillside in Southern California looking out over the Pacific Ocean and we’ve got some heavy duty refraction going on this evening. This was earlier tonight, it’s a French Blue A350 inbound to SFO from French Polynesia, caught it at 405 nautical miles. A few minutes later got a Quantas inbound to SFO at 413 nm.

The irregular contour is my line of sight at 36K feet altitude so they are nearly a 100 nm past that.

Unfortunately neither showed up on the Flightaware tracklog as I was hoping to get a screenshot. Does FA toss out tracks as erroneous beyond some distance?

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Yes, there is a max range on positions we’ll accept for flight tracking purposes.

That makes sense as the farthest I’ve seen from my site in tracklogs is around 350 nm. Too bad as this time of year we get strong tropo ducting over the ocean (both RF and optical) with some really long range hits.

In case anyone finds it interesting, here’s the tropo prop forecast for yesterday which shows why these long distance hits were showing up. That dark orange band is categorized as “very strong” and aligns pretty much exactly with what I was seeing.

The forecast website is here: http://www.dxinfocentre.com/tropo_wam.html

When these get strong it also causes optical phenomena that makes the offshore islands pop up higher that they actually are.

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I am based in Bay Area. This explains why I had a good number of positions this morning over the Pacific ocean – more and further out than I usually have (no 413 miles though)

You can increase the display distance in dump1090-fa (FA has there own internal limits as OBJ has stated above).
edit /etc/default/dump1090-fa

DECODER_OPTIONS="–max-range 360"

I remember there was a guy in rural NSW or Vic (Australia) that was getting out really long distances(500+Km). I think he had a nice tower, height and very little electrical or RF noise at his location.

Summary

I don’t think that’s a display limit.

That’s used for CPR relative to receiver location.
ADS-B positions are encoded in two messages.

But if you know approximately where the position is you can sometimes decode it, that’s what this range is used for.

Above a range of 360 nm that doesn’t work anyway so receiver relative location decoding is disabled by default.
Any setting above that has no effect if i remember the code i looked at correctly.

Edit: Yeah what i wrote above was wrong, it indeed limits displayed positions to that range.

It still is the case that using a lower range might give you better position decoding because less messages from the aircraft are needed.
But if your actual range is bigger for any reason then that can produce wrong positions and will limit your view.

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I set mine up using jprochazka’s scripts (ADSB Receiver project) and he includes a step in the setup that lets you change the maximum range value (probably changes the config item jonhawkes mentions). I increased it to something like 500 nautical miles just to see what happened.

So far the long distance hits look legitimate as I’ve double-checked the position I recorded against where FA thought the aircraft was. I can see why FA would limit max range as it’s a pretty simple filter to throw out spurious reports. As a radio DX’er though, it’s a shame these aren’t counted.

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I also had new highs for positions and number of aircraft so things were definitely hopping of the California coast.

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