So if I’m using PiAware, aren’t I seeing only ADS-B aircraft? If so (I guess if not also), what are the aircraft seen stats that are not ADS-B aircraft seen?
The aircraft not seen are the ones not giving a position so they would appear in your aircraft list but would not show on the chart.
The difference will be aircraft that are responding to Mode S interrogations (so reporting identity, callsign, altitude) from secondary radar, but not broadcasting ADS-B position information.
But that is still ADS-B, is it not? Isn’t it just incomplete ADS-B data?
So this is also broadcast on 1090 MHz? I thought, apparently erroneously, that 1090 MHz was reserved for ADS-B.
I see a fair disparity between the different feeders as well. A nearby Flightfeeder site shows quite a few more aircraft/reports than ADS-B aircraft/reports. Meanwhile my SBS-3 shows nearly the same amount of ADS-B vs other.
Mode S interrogations are on 1030MHz and responses are on 1090MHz; this predates ADS-B.
The Mode S message format was then extended to allow it to carry ADS-B messages (“1090ES”).
(I believe the reasoning was that you can repurpose a lot of the existing Mode S transponder hardware, it mostly just needs a firmware/software upgrade to handle ADS-B, you don’t have to design a whole new receiver/transmitter and maintain an additional antenna etc)
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Very interesting topic, and forced me to read more about ADS-B.
I am not clear on what is being shown on the dump1090 plot - are the planes on the green lines in the table (and planes on the map) the ones which broadcasting the position and the white ones which responding to interrogation but do not broadcast the position? What would be the reason for those planes to not broadcasting the position?
Also, based on the wikipedia, it is appears that FAA was looking to force the planes which are not flying in Class A airspace (i.e. above 18,000 feet) to transmit ADS-B messages on 978 MHz (so called UAT). Does the dump1090 listening on 978MHz as well?
Alex
Yes, that’s exactly it.
What would be the reason for those planes to not broadcasting the position?
I guess they either don’t have ADS-B equipped transponders, or they’re turned off.
Around here (UK) most of the bigger stuff has ADS-B, but smaller stuff often doesn’t. And military often doesn’t transmit a position for some reason 
Not sure what the exact requirements for having ADS-B are.
Also, based on the wikipedia, it is appears that FAA was looking to force the planes which are not flying in Class A airspace (i.e. above 18,000 feet) to transmit ADS-B messages on 978 MHz (so called UAT). Does the dump1090 listening on 978MHz as well?
It does not. The dongle doesn’t have the bandwidth to cover both 1090 and 978 at the same time, and the modulation scheme for UAT is quite different to Mode S/ADS-B.
You’d need different software and a second dongle (and perhaps a second antenna, depending on the bandwidth of your existing antenna) to receive UAT.
I don’t know of any software to do this. I looked into writing something to do this briefly, but there’s no UAT around here to test against, I think it is almost exclusively a US thing.
Some discussion here: forum.flightradar24.com/threads/ … Z-Receiver
I found another discussion ads-b-flight-tracking-f21/978-mhz-uat-in-the-us-t18256.html . Seems that there are a few attempts to build the 978MHz decoder, but no an easily installable package.
My dump1090 plot page usually have 1 to 2 ratio of the planes with position vs planes without position (i.e 5 planes with position and 10 without position). The planes without position often have squawk, flight number, altitude and message counter in thousands. Judging by the flight number, some of the planes are airliners (US Airways for example) yet they do not report position. Is it possible that the position of those planes is transmitted via ADS-B UAT?