Flightradar24 (and other trackers) Thread Part IV

I’m probably about to tell many of you what you already know, and there’s plenty on Wikipedia that’s going to be a lot more informative than me!

https://en.m.wikipedia.org/wiki/Secondary_surveillance_radar

Also my training took place a long while ago, and things have progressed plus my memory ain’t as good as it was!

Ordinary “Primary” radar works by sending out a radio pulse and then listening for the echo. The radar knows where it was pointing when it sent the pulse and received the echo, so it can display the direction of the object, and how long the pulse took to hit the object, so it can tell how far away the object is. One of the problems with primary radar is that it picks up trees, hills, masts, you name it, as well as aircraft; so-called “ground clutter” or “permanent echoes”.

A bit of a diversion. When I was first involved, you used some of the permanent echoes - a tv mast for example, to ensure the radar was set up properly. And there were metal structures adjacent to the touchdown points on the runways, which were visible on the radar, which you could align the final approach track with, for talkdowns.

Secondary Surveillance Radar (SSR) had its origins ( like many things!) in WWII. To sort out which radar returns were which, IFF or Identification Friend or Foe was introduced. A pulse is sent out, and a receiver on the aircraft hears it, and responds with an answering pulse. Hence “Transponder”. It’s a bit like calling out, “who goes there; friend or foe” and the friend responds with a code

To keep this secret from Gerry, the transponder box on the aircraft was called the “Parrot” and the instruction to turn it on was “Squawk your parrot”. And the first part of that still survives in phraseology such as “blah blah Liffey5A departure squawk 3423...”

At first, there were only 64 possible transponder codes. The numbers can only be between 0 and 7 - you’ll never hear a squawk with an 8 or 9 in it. Thats all to do with the pulses representing 0, 1, 2, or 4, or any combination of those, and at one end of the scale it could be a 0, or if all the pulses are sent, it’s 1+2+4 = 7

This evolved into the present system, where the squawk code consists of 4 digits, each one being between 0 and 7, so there are 4096 possible codes. Some are reserved for emergencies etc, and there is a regime for allocating codes so that the same code isn’t allocated to two aircraft where the possibility of confusion exists

Transponders can also send altitude information; this is called Mode C, and is basically an enhancement of the original system.

There’s a huge amount of information on Eurocontrol’s Skybrary pages, rather than me blather on all evening!

I’m not sure if this is useful, please let me know (PM welcome!)

smurfjed wrote: »

Robert6920, thank you for the explanation, to answer your question, you stated that the original transponders had the capability to have 64 possible numbers, this was later increased to 4096 variations. The rest of the technology behind the messages has also evolved over time, transponders had to comply with requirements called DO-260, later DO-260A and since 04Jan20 in the USA DO-260B, this standard was supposed to be implemented in EASA land next month, but due to COVID its delayed until the end of the year.

If you look at https://tar1090.adsbexchange.com/ and pick an aircraft, there is now a wealth of information available to the controller from the aircraft, due to the fact that this information isn’t encrypted, its also available to any person who has an ADS receiver and these people are the source of the information provided to most tracking sites.

DO-260B ADS-B-OUT also has a sister called ADS-B-IN, I believe right now that this is only available on the 787 and A350, but this allows the crew to see all the same information as you/I get to see on a tracking site, they can literally pick any aircraft and see all of the data about that aircraft, its pretty cool.

My original comment that the aircraft may not have the correct transponder is based around the DO-260B mandate, if they had an older “box” they should have had it scheduled for replacement by June 2020.

Thanks Smurfjed. I misunderstood/mixed up in my own mind ADS-B and SSR transponders. Apart from the wealth of information contained in an ADS-B message (Informative article here https://www.skybrary.aero/index.php/Automatic_Dependent_Surveillance_Broadcast_(ADS-B)) The aircraft has a unique 24 bit address. So do ground vehicles operating on the manoeuvring area - at the moment, the only thing visible at DUB is the airport’s Ops3 vehicle, which itself has a unique 24 bit address.

As usual, I learn far more than I contribute to these threads!