The 1090 Megahertz Riddle: A Guide to Decoding Mode S and ADS-B Signals | Hacker News Reader

The 1090 Megahertz Riddle: A Guide to Decoding Mode S and ADS-B Signals

books.open.tudelft.nl

96 pointstoomuchtodo28 comments

Synopsis:

In the last twenty years, aircraft surveillance has moved from controller-based interrogation to automatic broadcast. The Automatic Dependent Surveillance-Broadcast (ADS-B) is one of the most common methods for aircraft to report their state information like identity, position, and speed. Like other Mode S communications, ADS-B makes use of the 1090 megahertz transponder to transmit data. The protocol for ADS-B is open, and low-cost receivers can easily be used to intercept its signals. Many recent air transportation studies have benefited from this open data source. However, the current literature does not offer a systematic exploration of Mode S and ADS-B data, nor does it explain the decoding process.

This book tackles this missing area in the literature. It offers researchers, engineers, and enthusiasts a clear guide to understanding and making use of open ADS-B and Mode S data. The first part of this book presents the knowledge required to get started with decoding these signals. It includes background information on primary radar, secondary radar, Mode A/C, Mode S, and ADS-B, as well as the hardware and software setups necessary to gather radio signals. After that, the 17 core chapters of the book investigate the details of all types of ADS-B signals and commonly used Mode S signals. Throughout these chapters, examples and sample Python code are used extensively to explain and demonstrate the decoding process. Finally, the last chapter of the book offers a summary and a brief overview of research topics that go beyond the decoding of these signals.

(books is open/freely available for download)

> aircraft surveillance has moved from controller-based interrogation to automatic broadcast

I'd take issue with the phrasing "moved from" and would rather use "supplemented by". Controller-based interrogation is still widely used.

Moreover the advantage of Mode S radar (ground-initiated transactions) is that you're using a self-coherent system: the radar knows what it sent and what it is supposed to receive, and when, in a narrow beam, etc.

It makes lots of spoofing attacks unpractical. Where as relying on GNSS exposes to a whole bunch of 'constellation replay' or fake-adsb-telegrams attacks. Mode S lacks authentication, but there's no reason it couldn't be tacked on one day (through e.g. the already available 'advanced' data-link capabilities or the many available 'gicb' registers...). Even light session-authentication would break most of the remaining spoofing attacks (and most swap problems) on ground-initiated Mode S radar.

I'd further refine "aircraft surveillance" to "air traffic control".

On the defense side ADS-B can provide a baseline air picture of cooperative traffic, but does not otherwise help with detecting non-cooperative aircraft.

This book is a great reference! Junzi has it in website form as well: https://mode-s.org/1090mhz/ Much more affordable than purchasing a copy of DO-260.

I consulted it a lot while working on an open-source ADS-B receiver project that decodes Mode S using RP2040 PIO. https://github.com/CoolNamesAllTaken/adsbee/

The weather model based on ADS-B data was [very recently] discussed here:

https://news.ycombinator.com/item?id=44734515

The article linked to this book, which is very comprehensive.

I've decided to check if it's possible to generate temperature and wind maps in real-time.

Temperature: https://adsb.exposed/?dataset=Planes&zoom=5&lat=45.8124&lng=...

Wind direction (Europe mainly): https://adsb.exposed/?dataset=Planes&zoom=5&lat=46.8734&lng=...

Also very interesting is the data which can be extracted from ADS-B.

There have been several attempts from meterologists to extract wind data from through the actual and target direction values.

Other sites use it to locate GPS-jammed areas because ADS-B can indicate when the reception fails (but still is able to transmit its position due to inertial fallback mechanisms).

Related: Some airliners directly transmit automated weather reports via either AMDAR or AIREP.

AMDAR: https://madis-data.ncep.noaa.gov/MadisAircraftPublic/

AIREP: Go to https://www.1800wxbrief.com/Website/interactiveMap, turn on the "Pilot Reports" layer, and look for "ARP" symbols on the map. You'll mostly see these over oceanic routes, especially in the Atlantic.

The AIREP reports are coded in PIREP format, so you'll need to look up the instructions for manually decoding those online.

That was a huge drama during Covid-19 as weather forecasters heavily rely on these automated weather reports for feeding their models. In Europe there are also special equipped Airbus/Boeings for collecting WMO-grade weather data while flying their standard routes - they were mostly grounded during the pandemic.

They partially replaced the data with the experimental Aeolus satellite which made its data into the productive forecasts within months due to their high quality.

I'll save someone the googling since I just did. You need some hardware to hack: I think this is what you need: https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/

https://www.adsbexchange.com/ways-to-join-the-exchange/build...

https://www.adsbexchange.com/open-source-software/

https://old.reddit.com/r/raspberry_pi/comments/157wd9t/here_...

Better also add a SAW bandpass and a properly designed amplification (LNA) stage to this.

The RTL-SDRs are great - but heavily limited in their dynamics (8 Bit ADC). They get deaf quickly if you are located next to strong transmitters like FM/DAB/DVB-T/etc.

Aliexpress has cheap antennas that include all of this.

Hey, mind looking up a link to one of those antennas? Just don’t want to pick the wrong part, so many abbreviations and acronyms.

I picked up an rtl sdr about a year or two ago and haven’t done much outside of listening to local FM stations.

Would be fun to go a bit deeper. Track satellites or listen to boats/ planes / etc.

https://de.aliexpress.com/item/1005009311806479.html

Plus

https://de.aliexpress.com/item/1005009237905498.html

Maybe you can find an antenna with both integrated.

If you want an easy to use project to get a map and feed to different sites like adsbexchange and flightradar24, I would recommend checking out: https://github.com/sdr-enthusiasts https://github.com/sdr-enthusiasts/docker-adsb-ultrafeeder https://github.com/sdr-enthusiasts/docker-flightradar24 https://adsb.im/home

There is a good community out there on github and discord that have flight tracking as a hobby. In addition to ADS-B there is also voice radio transmissions from aircraft (airband) and text messages (ACARS) etc. If you get hooked there is lots of different things to tinker with.

For optimal ADS-B reception, pair that RTL-SDR with a 1090MHz bandpass filter and place your antenna outdoors with clear sky view, as these signals are line-of-sight and easily attenuated by buildings.

It is perhaps telling the author lives in the Netherlands! My findings were also that it's really super important to have them up high... at which point you need decent cable, termination and general radio knowledge otherwise you are exposing yourself to other issues. Basically, if there's complex topography, give up on this ever giving excellent results unless you have a radio tower or tree.

The most common design for wire antennas at 1090 MHz is the quarter-wavelength monopole (such as a ground-plane or whip). For 1090 MHz, the quarter-wavelength in free space is approximately 68.8mm, but practical build recommendations round this to 65-66mm to account for the velocity factor of the conductor, especially if the antenna element is made from solid wire rather than thin tubing. For a dipole (the total length being half the wavelength), the length is approximately 137mm (ie. 2x68.5mm). Commercial antennas sold for 1090 MHz may appear longer because they are collinear or have internal elements designed for added gain or bandwidth.

> My findings were also that it's really super important to have them up high... at which point you need decent cable, termination and general radio knowledge otherwise you are exposing yourself to other issues.

Well, 1090 Mhz is a crap frequency to run through coax cable for long distances. The attenuation is pretty high at that frequency already. So I wouldn't just mount the antenna on the roof and then coax the RF down a few floors. You will reduce your range.

I would simply mount the SDR itself right up on top of the mast, along with a raspberry pi to decode everything. Then add a PoE HAT and all you need to bring down is Ethernet for which long distances are no issue. The PoE uses 48V so it also bypasses the problem of voltage drop on 5V with high current. Put it all in a waterproof box of course. Ideally metal for heat dissipation because waterproof means no airflow from outside to inside.

Make sure you use real copper Ethernet cable and not that CCA crap (copper clad aluminium) because that has much higher resistance and does have big problems with PoE. All the cheap cables are CCA these days so beware.

I've had good experience with the AirNav Radar FlightStick - it comes with a built-in 1090 MHz filter. It's a smaller package with the same SDR hardware.

If anyone wanted one of those pingGPS portable ads-b receivers (traffic but no gps or weather, $275) but they were sold out, you can get them again. I had been searching for a used one off and on for months and discovered that they are back in stock.

Is Elon still super sensitive about ADS-B? Asking for a friend... who lives next to Austin Bergstrom airport. And has 6 rtl-sdr's on his desk begging for mission...

There's still ADSB Exchange. Though unfortunately they were bought by some company. It was a hobby project before that didn't answer to anyone and they didn't filter anything out. Not sure if that is still the case.

Ps Elon is not against ADS-B but against people sharing the received data and scrutinising his private jet use. Not sure why that would be a deterrent to do that though.

Section 1.2 reads:

>The concept of PSR is fairly simple. It is a rotating radio transponder with an omnidirectional antenna.

Shouldn't it be a directional antenna?

An excellent resource.

Great book that I reference all the time.

(The title is actually "The 1090 Megahertz Riddle".)

It appears that was clipped when I submitted. I will email the mods and request they correct the title. Thank you.

Fixed, thanks!

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