What in the purple hell was that???

[Russell Holton]

No, Russell, ADS-B out transmits every second, regardless of radar coverage.

Either way, in a congested airspace, packets will get stepped on.

[Scott Dyer DXR/CT]

Yep, and radar used to operate in what is now the FM broadcast band. Shorter wavelengths equal more fun with reflections, absorbtion, and interference patterns, regardless of the information which modulates the carriers.

Today's primary radar is S band, 2.42.7 GHz. Nothing like 978 or 1090 MHz.

[Scott Dyer DXR/CT]

As I understand it, ADS-B is usually triggered by a radar sweep. In a congested airspace with multiple radars, two transponders could be triggered at the same time. The parity bits may not catch the corruption every time.

What we're seeing is a "still". I'd like to see what the "movie" it comes from reveals.

No, Russell, ADS-B out transmits every second, regardless of radar coverage.

[Russell Holton]

Shorter wavelengths equal more fun with reflections, absorbtion, and interference patterns, regardless of the information which modulates the carriers.

As I understand it, ADS-B is usually triggered by a radar sweep. In a congested airspace with multiple radars, two transponders could be triggered at the same time. The parity bits may not catch the corruption every time.

What we're seeing is a "still". I'd like to see what the "movie" it comes from reveals.

[Ray Tackett]

Ray -- ADS-B operates at 978 and 1090 MHz. The echoes that are discussed are from primary radar, not DME/Mode C transponders, as I understand it.

Yep, and radar used to operate in what is now the FM broadcast band. Shorter wavelengths equal more fun with reflections, absorbtion, and interference patterns, regardless of the information which modulates the carriers.

Back in the mode C days, when I was flying traffic watch on weekday evenings, I almost got in Class B trouble due to a flaky transponder. It had me at illegal altitudes, and very erratic speed indications (evidently variations in return latency). Eventually PNE Tower got hold of me on the radio, and asked me to make a pass over the airport at a particular altitude and airspeed. Their radar display was showing crazy stuff coming from my transponder, but looking out the window, it was obvious I was doing what I had been told.

That, and the fact that I interacted professionally with towers at PHL, PNE and sometimes TTN several times every day gave me a far better reputation than any piece of hardware from Happy Herbie's Used Airplane Lot. (That's what I called the excessively low-budget flight school/rental/charter biz I flew for.)

[Scott Dyer DXR/CT]

The ADS-B transmits a sequence of positions, not speed.

Bill, I'm reading/understanding that ADS-B data packets include speed as well as position and altitude. https://mode-s.org/decode/adsb/airborne-velocity.html

"ADS-B Out works by broadcasting information about an aircraft's GPS location, altitude, ground speed and other data to ground stations and other aircraft, once per second." https://www.faa.gov/nextgen/equipads...ties/ins_outs/

From: https://www.icao.int/APAC/Meetings/2...B%20Basics.pdf

[Scott Dyer DXR/CT]

Yeahbut ... It's still a LOT of transmitters in one area, all operating at radar frequencies. The exact type of transmission, 1940s radar, Mode C transponder squawk, or ADS-B doesn't matter. The basic physics of wave interference still apply. I think the absence of a sonic boom is pretty strong evidence.

Ray -- ADS-B operates at 978 and 1090 MHz. The echoes that are discussed are from primary radar, not DME/Mode C transponders, as I understand it.

[B.Butler]

But who is calculating it? The aircraft or the ADS-B receiver?

Taking a quick look at the packet format, there is a "Airborne velocities" field.

The ADS-B transmits a sequence of positions, not speed.

[Russell Holton]

R = D/T ?

But who is calculating it? The aircraft or the ADS-B receiver?

Taking a quick look at the packet format, there is a "Airborne velocities" field.

[B.Butler]

R = D/T ?

[B.Butler]

Someone remind me how ADS-B works? Isn't speed self-reported and comes from the on-board GPS receiver? Of course, I wouldn't rule out a scrambled data packet. All those ADS-B packets flying around, I wouldn't be surprised if two stepped on each other.

The real question is what was the "hits" just before and after that screen shot?

No. Speed is calculated from target displacement. D = RT

[Russell Holton]

Someone remind me how ADS-B works? Isn't speed self-reported and comes from the on-board GPS receiver? Of course, I wouldn't rule out a scrambled data packet. All those ADS-B packets flying around, I wouldn't be surprised if two stepped on each other.

The real question is what was the "hits" just before and after that screen shot?

[Ray Tackett]

Ray, it's ADS-B only, so no live radar with echoes and all. Stephanie, I got a speed mod...it was a bad hit and scrambled my hex code.

Yeahbut ... It's still a LOT of transmitters in one area, all operating at radar frequencies. The exact type of transmission, 1940s radar, Mode C transponder squawk, or ADS-B doesn't matter. The basic physics of wave interference still apply. I think the absence of a sonic boom is pretty strong evidence.

[Scott Dyer DXR/CT]

Ray, it's ADS-B only, so no live radar with echoes and all. Stephanie, I got a speed mod...it was a bad hit and scrambled my hex code.

[Ray Tackett]

When TW800 exploded in the same area, there was a missile theory with a realistic-looking radar track. The unlabeled, seemingly primary return, was acually an interference pattern from some combination of multiple radars and echoes. I believe it was Bill English (NTSB) who posted a detailed explanation here once it had been solved.

That is an area with a lot of radar coverage and a lot of aircraft, so I could believe another interference pattern. Did that track originate anywhere? Did it end anywhere? Did it even enter ane leave the image area, or did it just appear? Any reports of a sonic boom? Any real object going that fast at that altitude would produce one.