A common question for both aviators and non-aviators: Why do some aircraft not appear on flight tracking websites such as Flight Radar 24 and Plane Finder, while others look like the pilots are trying to give passengers a roller coaster experience?

How is it possible that most airline traffic even shows up in the middle of the atlantic ocean, where ‘ordinary’ radar stations cannot even reach in the first place? Meanwhile, why does Cessna X or Robinson Y not show up, and if it does, why does it have a weird looking track?

And to go a step further, how do ADSB transponder tie into this, and what are they exactly? Today we’ll be answering all these questions, but if you want to stay up to date the latest articles, just tap ‘follow’ below so you get notified whenever more content releases!

Radar types

First we need to have a look at what kind of radars are out and about. We can divide the types up in 2 major groups: Secondary Surveillance Radar (SSR) and Primary Surveillance Radar (PSR):

Especially in the early days, the most common type of radar was the PSR, this is your typical ‘rotating disc’ that essentially looks around itself in a circular motion to detect objects.

It’s development was sped up during wartime, to detect enemy aircraft coming into airspace. It sends out very strong electromagnetic waves, and detects when one of the waves get reflected back to itself by any object.

Why do some aircraft not appear on flight tracking websites

The main benefit here is that this type of radar does not rely on the object (plane / helicopter / balloon / and even birds) having a transponder or any other form of communication equipment, or their ‘consent’.

The downside is that it uses a LOT of energy, and it also cannot detect the target’s altitude or other flight variables without relying on other systems. It is still in use today by most ATC units as a backup.

Then there’s SSR, this system relies on the use of transponders. During wartime, PSR did not show the difference between friendly or enemy forces, which is kind of crucial when it comes to monitoring airspace right?

This is where the Identification Friend or Foe system (IFF) came in. IFF used transponders inside aircraft to tell the user whether or not the detected object was an enemy or not.

On top of this, nowadays, SSR provides a lot of other information too, such as altitude, squawk codes, and even the aircraft’s altitude based on mean sea level, which is independent from the pilot’s altimeter setting to avoid confusion when comparing altitudes to other aircraft.

So how does this one work? Well, rather than pinging off as signal and waiting for it to bounce back, SSR sends an ‘interrogation signal’, which is received by an aircraft, the transponder processes this signal and sends a signal back saying ‘well hi there, this is me, I am here, and this is what I am doing’. There’s a two way communication process that is not present for PSR:

Secondary Surveillance Radar

What does radar technology have to do with flight trackers?

Yes yes, we’re getting there. See, the next (and more modern) type of technology that shows us where aircraft are in the world is called ‘Automatic Dependent Surveillance Broadcast’ (let’s stick with ADSB, that name is a mouthful): Let’s briefly breakdown what ADSB means:

Automatic: A signal is sent periodically by the aircraft system, without needing to be interrogated by an external system.

Dependent: The information this signal will carry depends on the availability of the aircraft equipment feeding into it such as GPS, altimeters etc.

Surveillance: The system provides surveillance information to other parties.

Broadcast: The system does not know what will and won’t receive the signal, nor does it have control over it – there is no 2 way contact.

With SSR, the ground station was responsible for that ‘interrogation signal’. This is not the case for ADSB. Let’s go over how ADSB works:

1) The aircraft receives its current position from its onboard GPS system

2) The signal is periodically transmitted by the aircraft

3) The ADSB signal is received by various ground stations, other traffic, or even satellites if no ground stations are available

4) The receivers share this info with the online flight trackers

You can imagine it to be a piece of equipment that constantly shouts to everyone around it where it is and what it’s doing, like a hyperactive instagram influencer.

How does ADSB work?

Of course, for this to work, we do require aircraft to actually be equipped with an ADSB transponder. Rough estimates indicate 70% of all worldwide traffic is now equipped with one, but especially General Aviation traffic (including helicopters) lags behind, and is often not equipped with an ADSB capable transponder.

This percentage is increasing as time passes, and most bigger helicopters available nowadays are all equipped with ADSB technology. To broadcast an aircraft position the transponder needs to have what’s called ‘ADSB-out’, meaning: pushing out a signal with relevant data.

A step further is ‘ADSB-in’, meaning: receiving ADSB signals onboard as well which can be used for EFB purposes.

Flight trackers such as Flightradar24 have over 20.000 ground stations worlwide that can receive ADSB signals, which is why tracking can be rather accurate and includes most traffic. The frequency that these signals are sent on is quite high (1090 MHz), so range is limited to about 150-250 miles.

The higher an aircraft flies, the bigger the range of these transponders become. So what about traffic that do not have ADSB capabilities or are out of range? There are 3 remaining ways to track traffic other than radar and ADSB ground receivers:

  • ADSB Satellites
  • MLAT
  • Open Glider Network

All of these do rely on some sort of transponder nowadays. Turning off a transponder will result in no flight track being shown unless you are in PSR range at an airfield for instance. This is illegal though, with some exceptions in place.

ADSB Satellites

Yep, satellite ADSB is a thing now! Again, an actual ADSB transponder is still required, but satellites that are equipped with ADSB technology can receive aircraft signals now too.

Depending on their location and orbit, specific locations around the globe might still be out of coverage, but most areas that are out of range of ground stations, can now be covered by satellites.

The best examples where this is useful is the the atlantic and pacific ocean where there aren’t any ground based receivers.

It requires a lot of datasources and providers to come together and share data. No flight tracker in the world has ownership of all ADSB capable satellites (luckily). So only with cooperation and the efficient use of resources can this technology be expanded.

MLAT

MLAT stands for Multilateration and can be used in the absence of ADSB receivers (or transponders). MLAT relies on a technique called ‘Time Difference of Arrival’ (TDOA), just a fancy term for saying: how long did it take signal x, y and z to arrive, and what is the time difference?

This is your typical case where you are trying to track some General Aviation traffic that seems to have a trippy tracking line compared to most other traffic, and seems to appear and disappear regularly depending on height and location: you are looking at traffic that is being tracked by MLAT.

To keep things simple:

1) Signals from old transponders (Mode S) are picked up by at least 3 or 4 MLAT stations

2) MLAT stations calculate the time it took for the signal to reach each station

3) Time is converted to distance to calculate the transponder (i.e aircraft) position in relation to each station

4) Aircraft position will be known and shown on screen

How does MLAT work?

Tadaa, now we can still see aircraft without ADSB as well. Your typical traffic being picked up by MLAT are Cessnas, PA28s etc, and older helicopters, but even today also older airline traffic with dated avionics.

The main downside of this technology is the reliance on height and position to these MLAT stations. Some trackers can work with at least 3 stations being in range, but more accurate positions require 4 stations. As soon as you lose 1 of these stations, the position of the traffic will disappear from the map. Most MLAT stations require traffic to be at at least 2000-3000 feet upto 10.000 feet.

If traffic moves away too much from any of these 3 or 4 stations, or lowers their altitude too much (or climbs too much), they will disappear from tracking software.

Open Glider Network

Finally, there is the Open Glider Network (OGN). Technologies such as FLARM, SPOT , FANET adn PilotAware are all services that can be hooked into the Open Glider Network. Depending on the tracker in use, the receivers can then pick up these signals and display them along with the other ADSB and MLAT traffic.

For all of these technologies, a last resort is an ‘estimation’, where the stations can calculate a rough position based on their last track and speed. But the longer time goes on, the less accurate these estimations will be.

Conclusion

There we have it: flight trackers and the technology they rely on! In the future, ADSB will likely become the norm, and regulators are already considering making it mandatory, but we might cover this in a future article. For now, thank you for all the support, and keep sending in your topic requests!


Jop Dingemans

Founder of Pilots Who Ask Why, HEMS Pilot, Flight Instructor, and Aerospace Engineer.

6 Comments

David Wallace · March 20, 2022 at 3:06 PM

Another great article! Looking forward to the next one. Thank you.

George Williams · March 20, 2022 at 8:57 PM

Great work Jop. The other point to note is that you cheaply setup your own receiver which can be set to show live, unfiltered data. Another reason you cannot see some tracks is the owners have asked for them not to be shown eg. business jets that might be filtered out by FR24.

We have recognised in our company that being visible helps just as much as being able to see others so we have carry-on Sky Echo ADS-B transceivers which add ADS-B OUT to our aircraft that don’t have it organically.

Henry · August 17, 2022 at 6:43 AM

Please i have been looking out for an m8 helicopter that always flies over my house, but will never find it on flight radar what could be the problem, am from west Africa

    Jop Dingemans · August 17, 2022 at 10:30 AM

    Hi Henry, it is probably because the transponder inside the helicopter does not have an ADSB capability. ADSB is required if flight radar coverage is poor in your area.

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