Is a distance limitation imposed?


New Member
Hi All,
I hope this is in the right place for a question based on curiosity more than anything else.

I was wondering if Plane Finder assumes data to be invalid once a certain distance is reported, and it consequently doesn't use that data. Now for the why.

I'm quite close to the Pacific Ocean coast of Oz and frequently see aircraft out to beyond 400km, and those aircraft are reported in Plane Finder. Today, during a brief period of Tropospheric Ducting (or bending) to the north east of me (and over the ocean), I recorded planes beyond 600km (639km in one case) but they didn't appear in Plane Finder except for the dead reckoning course of dashed lines. Given that during such ducting (sometimes called tropospheric waveguide*) very strong signals can be received due to a very reduced pathloss, I believe the data was valid and I didn't see ghost aircraft.

*Atmospheric temperature normally changes linearly with changes in altitude. However, under certain meteorological conditions a sudden (step) change in temperature, known as a temperature inversion, can occur and this causes VHF/UHF radio waves to be trapped in the atmospheric duct that is formed, causing those waves to propagate great distances, sometimes thousands of kilometres.

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Interesting question, not sure if they assume about some data and delete or knowingly not store the data?. What do you mean "I didn't see ghost aircraft"?...
Thanks for the reply.

By "ghost aircraft" I meant an aircraft that didn't actually exist, one that was a corruption within the program collecting data - dump1090-mutability in this case. I do have one instance of a ghost plane that appears every few days or so and always from the same direction and distance. I'm guessing that happens overnight because I've never actually seen the event, and that's what was different about the 600+km reports - I saw them in real time, and two different planes being tracked for some distance tends to eliminate a false data possibility. I also heard Brisbane Centre (YBBN) talking with one of the flights on their Oceana VHF frequency, 124.625MHz.

As I said, my enquiry was more curiosity based than anything else.

Happy spotting.
Well, I've discovered that planes out to just under 600km away are reported in PlaneFinder, thanks to a temperature inversion yesterday (Saturday in Oz).

Some background: There are regular Qantas flights between Sydney (also Brisbane and Port Macquarie) and Lord Howe Island, 600km east of Port Macquarie on the Oz mainland, and I can track planes out of Sydney to about 330km. Their normal altitude is 25000ft. Yesterday I saw a flight at 573km and an altitude of 4750ft (yes, 4750ft) on descent in to YLHI. Clearly some atmospheric ducting was taking place, and 4750ft provided a nice entry point into the duct. Given that there was a large high pressure system over the Tasman Sea (part of the Pacific Ocean) between New Zealand and Oz, and a cold front approaching western Victoria, conditions were perfect for ducting.

My original question hasn't been answered but since summer is the perfect time for ducting across the Tasman Sea I'll probably see more planes beyond the 600km mark that I first asked about.