Just How Do Aircraft Transponders Work Anyway?

Most of us will have a hazy idea of how radar works to detect aircraft by listening for reflected radio waves. And we’ll probably also know that while radar can detect aircraft, it’s not the most efficient or useful tool in the hands of an air traffic controller. Aircraft carry transponders so that those on the ground can have a clearer picture of the skies, as each one reports its identity, altitude, and position. [Yeo Kheng Meng] was lucky enough to secure a non-functioning aircraft transponder and do a teardown, and his write-up makes for interesting reading as he explains their operation before diving into the hardware.

The 1978 and 1979 date codes on the various integrated circuits and transistors identify it as having been made in 1979, so not having a CPU is not entirely unsurprising given its age. Instead this is a straightforward device that responds to pulse lengths of different timings with sequential bursts of data.

[Yeo Kheng] is mystified by the RF strip and associated components, which look to us like a typical crystal oscillator and frequency multiplier strip from that era, along with some screened boxes that probably contain cavity filters and given that there is also a high voltage power supply present, a tube RF power amplifier. GHz-capable semiconductors were quite exotic in the 1970s, while high-frequency tubes had by then a long history.

It’s evident that the tech behind aircraft transponders has moved on since this unit was built, but one thing’s certain. Hackers in 1978 would have had to go to a lot of work to listen to them and interpret the results, while here in the 21st century it’s something we do routinely.

Es’hail-2: Hams Get Their First Geosynchronous Repeater

In the radio business, getting the high ground is key to covering as much territory from as few installations as possible. Anything that has a high profile, from a big municipal water tank to a roadside billboard to a remote hilltop, will likely be bristling with antennas, and different services compete for the best spots to locate their antennas. Amateur radio clubs will be there too, looking for space to locate their repeaters, which allow hams to use low-power mobile and handheld radios to make contact over a vastly greater range than they could otherwise.

Now some hams have claimed the highest of high ground for their repeater: space. For the first time, an amateur radio repeater has gone to space aboard a geosynchronous satellite, giving hams the ability to link up over a third of the globe. It’s a huge development, and while it takes some effort to use this new space-based radio, it’s a game changer in the amateur radio community.

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Radar In Space: The Gemini Rendezvous Radar

In families with three kids, the middle child always seems to get the short end of the stick. The first child gets all the attention for reaching every milestone first, and the third child will forever be the baby of the family, and the middle child gets lost in-between. Something similar happened with the U.S. manned space program in the 60s. The Mercury program got massive attention when America finally got their efforts safely off the ground, and Apollo naturally seized all the attention by making good on President Kennedy’s promise to land a man on the moon.

In between Mercury and Apollo was NASA’s middle child, Project Gemini. Underappreciated at the time and even still today, Gemini was the necessary link between learning to get into orbit and figuring out how to fly to the Moon. Gemini was the program that taught NASA how to work in space, and where vital questions would be answered before the big dance of Apollo.

Chief among these questions were tackling the problems surrounding rendezvous between spacecraft. There were those who thought that flying two spacecraft whizzing around the Earth at 18,000 miles per hour wouldn’t work, and Gemini sought to prove them wrong. To achieve this, Gemini needed something no other spacecraft before had been equipped with: a space radar.

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Aireon Hitchhikes On Iridium To Track Airplanes

SpaceX just concluded 2017 by launching 10 Iridium NEXT satellites. A footnote on the launch was the “hosted payload” on board each of the satellites: a small box of equipment from Aireon. They will track every aircraft around the world in real-time, something that has been technically possible but nobody claimed they could do it economically until now.

Challenge one: avoid adding cost to aircraft. Instead of using expensive satcom or adding dedicated gear, Aireon listen to ADS-B equipment already installed as part of international air traffic control modernization. But since ADS-B was designed for aircraft-to-aircraft and aircraft-to-ground, Aireon had some challenges to overcome. Like the fact ADS-B antenna is commonly mounted on the belly of an aircraft blocking direct path to satellite.

Challenge two: hear ADS-B everywhere and do it for less. Today we can track aircraft when they are flying over land, but out in the middle of the ocean, there are no receivers in range except possibly other aircraft. Aireon needed a lot of low-orbit satellites to ensure you are in range no matter where you are. Piggybacking on Iridium gives them coverage at a fraction of the cost of building their own satellites.

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Accidental Satellite Hijacks Can Rebroadcast Cell Towers

A lot of us will use satellite communications without thinking much about the satellite itself. It’s tempting to imagine that up there in orbit is a communications hub and distribution node of breathtaking complexity and ingenuity, but it might come as a surprise to some people that most communications satellites are simple transponders. They listen on one frequency band, and shift what they hear to another upon which they rebroadcast it.

This simplicity is not without weakness, for example the phenomenon of satellite hijacking has a history stretching back decades. In the 1980s for example there were stories abroad of illicit trans-atlantic serial links nestling as unobtrusive single carriers among the broad swathe of a broadcast satellite TX carrier.

Just sometimes, this phenomenon happens unintentionally. Our attention was drawn to a piece by [Harald Welte] on the unintended rebroadcast of GSM base station traffic over a satellite transponder, and of particular interest is the presentation from a conference in 2012 that it links to. The engineers show how they identified their interference as GSM by its timing frames, and then how they narrowed down its source to Nigeria. This didn’t give them the uplink in question though, for that they had to make a downconverter from an LNB, the output of which they coupled to an aged Nokia mobile phone with a wire antenna placed into an RF connector. The Nokia was able to decode the cell tower identification data, allowing them to home in on the culprit.

There was no fault on the part of the GSM operator, instead an unterminated port on the uplink equipment was enough to pick up the GSM signal and introduce it into the transponder as a parasitic signal for the whole of Europe and Africa to hear. Meanwhile the tale of how the engineers identified it contains enough detective work and outright hardware hacking that we’re sure the Hackaday readership will find it of interest.

If satellite hacks interest you, how about reading our thread of posts on the recapture of ISEE-3, or maybe you’d like to listen for a lost satellite from the 1960s.

Thanks [Kia] for the tip.

“Alexa, What Plane Is That?”

We’ve all probably done it — gazed up at a passing jetliner and wondered where it was going and what adventures its passengers were embarked upon. While the latter is hard to answer, the former just got a bit easier: just ask Alexa what the plane is.

Granted, [Nick Sypteras]’s Echo Dot isn’t quite omniscient enough to know exactly what plane you’re looking at. His system benefits from the constraints offered by the window of his Boston apartment — from the video below, we’d guess somewhere in Beacon Hill or the West End — that offers a view of the approach to Logan Airport. An RTL-SDR dongle receives the ADS-B transmissions from all aircraft in the vicinity, and a Raspberry Pi does a lookup, picks the closest plane, and scrapes the departure and arrival airports from FlightRadar24. Alexa does the rest, but we have to confess that hearing “Boeing seven hundred eighty-seven” rather than “seven eighty-seven” would drive us nuts.

If you don’t have the limited view of an airport approach that makes [Nick]’s hack workable, maybe a plane-spotting robot camera would work better for you.

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Tracking Nearly Every Aircraft With A Raspberry Pi

FlightAware is the premier site for live, real-time tracking of aircraft around the world, and for the last year or so, Raspberry Pi owners have been contributing to the FlightAware network by detecting aircraft flying overhead and sending that data to the FlightAware servers.

Until now, these volunteers have used Raspis and software defined radio modules to listen in on ADS-B messages transmitted from aircraft. With FlightAware’s new update to PiAware, their Raspberry Pi flight tracking software, Mode S transponders can also be detected and added to the FlightAware network.

Last year, FlightAware announced anyone with a Raspberry Pi, a software defined radio module, and an Internet connection would earn a free FlightAware enterprise account for listening to ADS-B transmitters flying overhead and sending that information to the FlightAware servers. ADS-B is a relatively new requirement for aviators that transmits the plane’s identification, GPS coordinates, altitude, and speed to controllers and anyone else who would like to know who’s flying overhead.

Mode S transponders, on the other hand, are older technology that simply transmits the call sign of an aircraft. There’s no GPS information or altitude information transmitted, but through some clever multilateration in the new PiAware release these transponders and planes can now be tracked.

To get the location of these transponders, at least three other PiAware boxes must receive a signal from a Mode S transponder. These signals, along with a timestamp of when they were received are then sent to the FlightAware servers where the location of a transponder can be determined.

The end result of this update is that FlightAware can now track twice as many aircraft around the world, all with a simple software update. It’s one of the most successful applications of crowdsourced software defined radio modules, and if you’d like to get in on the action, the FlightAware team put together a bulk order of ADS-B antennas.