I read the following on Flightradar24.com's 'How it works' page.

The percentage of aircraft equipped with ADS-B receivers is steadily increasing though, as they will become mandatory for most aircraft around the world by 2020. When mandatory, ADS-B will replace primary radar as the primary surveillance method used by ATC.

Sure, that sounds lovely, but it it seems to me that a large portion of aircraft do not actually use GPS, but instead rely on inertial navigation systems or similar. Of course, the ATC doesn't need millimeter precision, or even an accurate position within a kilometer, but whenever I check flightradar24, there is quite a large bunch of airplanes reporting their position with offsets up to a few kilometers - if I were to judge by the ADS-B data, I would report a major emergency every few minutes.

How will ADS-B actually reliably replace primary radar? Will GPS have become mandatory by 2020? Or is Flightradar24.com just being optimistic?

(Side note: I know that redundancy is very popular in the aviation world, but I don't think you can call something 'replaced' when primary radar is still used for a large portion of the flights)

Edit: some parts of some answers seem to be concentrating on update rate / transmission losses and errors. For clarification, I included a screenshot (which I could literally take the second I looked at fr24.com, indicating how often this happens). enter image description here

At the time this picture was taken, the 36R runway was in use for landing ('yellow' aircraft top right just landed). The selected aircraft is however happily taxiing in a field (pattern matching 36R's adjacent taxiways). This is not caused by update rate or transmission losses (i.e., this can't be old information, the plane was never in that field). Transmission error also seems out of the question, since the entire track is a smooth line (errors sometimes do happen, but are not 'hidden' on fr24 - a large 'jump' in the track can be seen sometimes). So either the plane is reporting a 'wrong' position (how is this handled by ATC?) or fr24 somehow messes up the data (how is that possible?

  • 5
    $\begingroup$ The central premise here is stated incorrectly: what has been observed is NOT that ADS-B is inaccurate. What has been observed is that FlightRadar24 has glaring inaccuracies. There is an assumption being made that ADS-B is the reason for these observed issues, and that is at best an unsupported assumption, and possibly even an unfounded one as well. $\endgroup$
    – Ralph J
    Jun 15, 2015 at 19:33
  • $\begingroup$ See this question for a discussion of FlightRadar24's issues which occur separately from the inherent accuracy of a full-up ADS-B system. Particularly, read the second answer, which is even more on-point here than the first. $\endgroup$
    – Ralph J
    Jun 15, 2015 at 19:44

6 Answers 6



How will ADS-B actually reliably replace primary radar?

It will not.

Will GPS have become mandatory by 2020?

Strictly speaking no, effectively yes.

There are so many problems with that quote from FR24 that I don't know where to start.

Let's take it apart and analyse it sentence by sentence.

The percentage of aircraft equipped with ADS-B receivers is steadily increasing though

Totally irrelevant. An ADS-B receiver allows reception of ADS-B signals from other aircraft. These can be used for example to create a Cockpit Display of Traffic Information (CDTI) or augment TCAS. The number of aircraft with this so called ADS-B IN is increasing, but the equipage percentage is probably below 2% at the moment.

If you are concerned with using ADS-B operationally for ATC then airborne receivers are not of interest. Instead you should be looking at the transmitter side.

[ADS-B receivers ...] will become mandatory for most aircraft around the world by 2020.

ADS-B receivers are not mandatory in any foreseen regulation yet. ADS-B transmitters are / will become mandatory in many parts of the world. The EU and USA rules mandate ADS-B in 2020.

When mandatory, ADS-B will replace primary radar

ADS-B is not intended to replace primary radar. As a new addition to the surveillance technology mix, it will not replace any other technology in the short term.

ADS-B is a form of cooperative surveillance, meaning that the aircraft or vehicle that is supposed to be tracked needs to cooperate by transmitting the ADS-B signal. Primary Surveillance Radar (PSR) on the other hand is a non-cooperative surveillance technology; it works by detecting signals reflecting off the target of interest. PSR can detect aircraft that do not transmit any signals. Since PSR and ADS-B are fundamentally different in their working, they have fundamentally different use cases. ADS-B will not replace primary radar and never was intended to.

will replace primary radar as the primary surveillance method used by ATC

Primary Surveillance Radar is not the primary surveillance method used by ATC in many parts of the world. Since PSR only gives azimuth and range measurements essential information like aircraft identification and altitude is missing. Therefore Secondary Surveillance Radar (SSR) is the primary surveillance method used for ATC since it provides altitude and identity in addition to azimuth and range measurements.

Secondary radar, like ADS-B is a form of cooperative surveillance; it relies on the aircraft replying to an interrogation from the radar. ADS-B can and will replace a number of secondary radars, but not all of them.

Now for the rest of your question.

it seems to me that a large portion of aircraft do not actually use GPS, but instead rely on inertial navigation systems or similar.

Indeed a large number of aircraft do transmit their position on ADS-B that is based on a non-GPS source, usually from the INS. INS can not provide the accuracy and integrity that GPS can provide and therefore you see large position errors on FR24. The fact that the position is not to be trusted is reported in the ADS-B message in the quality indicators, but unfortunately FR24 doesn't use this information.

All the ADS-B mandates require that the system is able to provide high integrity data, which effectively means that GPS is required as position source, although theoretically other means are possible as well. Therefore in 2020 all aircraft subject to the various ADS-B regulations will have to be GPS equipped.

Of course, the ATC doesn't need millimeter precision, or even an accurate position within a kilometer,

ATC does not need millimeter precision (GPS wouldn't provide that anyway) but they do need position accuracy within a kilometre and sometimes within about 10 meters (e.g. runway entry stop bar monitoring). ADS-B with appropriate GPS source will be able to provide that.

  • $\begingroup$ Uh, the whole 'off by miles' part ;) So, how does ATC handle an aircraft reporting itself to be a few miles off course? (especially near airports, since I guess that's where a few miles actually matter) $\endgroup$
    – Sanchises
    Jun 15, 2015 at 17:03
  • 1
    $\begingroup$ Data from those aircraft isn't used. If ATC uses ADS-B position data, then the aircraft has to be certified for ADS-B and the transmitted quality indicators need to be indicating that the position integrity and accuracy are above a certain level (which varies based on the ATC environment). The aircraft from your screenshot does report INS position and at the same time reports that the data is not to be trusted. $\endgroup$
    – DeltaLima
    Jun 15, 2015 at 17:06
  • $\begingroup$ Isn't it as simple to say, that ADS-B is just to enhance and provide meta-data to the already existing systems? $\endgroup$ Mar 25, 2017 at 7:58
  • $\begingroup$ "ADS-B with appropriate GPS source will be able to provide that." - hmm, and then the first enemy nation to bring down GPS will crash the world's economy even more. $\endgroup$ Sep 14, 2021 at 19:06

The linked page also states that as part of ADS-B,

Aircraft gets its location from a GPS navigation source (satellite)

This strongly suggests that any aircraft using INS position only (perhaps with DME/DME updates only at present) would be required to add a GPS receiver as part of the ADS-B upgrade.

While no installed + approved + certified aviation equipment will ever be as cheap as a basic handheld Garmin from your local sporting goods store, the big cost of ADS-B seems likely to be from things other than the GPS piece of it. In E-GPWS systems, the GPS receiver can be dedicated to just that system, without incurring costs of integrating it into the FMC, so it may be possible to do likewise with ADS-B. This would probably reduce costs somewhat, although I don't know how long INS/DME RNAV will be allowed, given the move toward "Next-Gen" everything in ATC.

  • $\begingroup$ Thanks for the answer. However, I disagree with your 'strongly suggests', since evidence strongly suggest the reported position is off by miles. Do you think that 'Next-Gen' aircraft will have compulsory GPS then? $\endgroup$
    – Sanchises
    Jun 15, 2015 at 8:41
  • 1
    $\begingroup$ @sanchises - Can you provide a reference saying that ADS-B will use position information from sources other than GPS? Every piece of technical documentation I've come across states that GPS is used to determine aircraft position and flight vector, meaning that even ADS-B Out kits will have to feature GPS receivers, if the aircraft itself doesn't already. FWIW, if you're seeing reported positions that seem to be off by a few miles, it's not happening due to ADS-B's limitations; I've first flown ADS-B-equipped aircraft nearly 10 years ago and position accuracy really is as good as claimed. $\endgroup$
    – habu
    Jun 15, 2015 at 9:07
  • $\begingroup$ @habu I know that GPS is perfectly accurate enough, as well as ADS-B's position transmission capabilities - hence my assumption that these planes were apparently using INS. Any idea why then the position is off by miles? $\endgroup$
    – Sanchises
    Jun 15, 2015 at 16:49
  • $\begingroup$ @sanchises - Couldn't tell you for sure, but my first guess would be some sort of glitch or limitation in the FR24's code; that's why I was asking for references, thought maybe you'd come across something I hadn't. $\endgroup$
    – habu
    Jun 16, 2015 at 10:46

First of all, what you see on Flightradar24 is not the same as what air traffic controllers see. ADS-B requires a ground station. The FR24 receivers are placed sporadically in peoples homes - far from the sophistication of the proper, more powerful, ATC stations. The coverage on FR24 is often sloppy but the controllers screens are fine.

ADS-B is already mandatory in Australia above FL290. As you can see here, ADS-B brings many benefits - none more so than on page 5 of that booklet. There is a diagram there that compares existing radar coverage to ADS-B coverage. Only a tiny bit of Australia is covered by radar. Radar is very expensive to install and maintain, and doing so in the desert cannot really be justified. But ADS-B provides virtually complete coverage of the country at cruise, perfect for the huge amount of Asia/Europe traffic. But most of this traffic will not appear on FR24 because they don't have many receivers out in the middle of nowhere.

  • $\begingroup$ This post, although interesting, does not answer my question in any way. I'm not asking about coverage, but position accuracy, which is independent of the receiver (either the receiver does or does not receive the airplane position). $\endgroup$
    – Sanchises
    Jun 15, 2015 at 8:37
  • $\begingroup$ True, I was more focusing on showing you that real ADS-B is more accurate than Flightradar24, but I have not actually compared it to radar. I will do some research and come back to you (unless someone else can answer beforehand) $\endgroup$
    – Ben
    Jun 15, 2015 at 9:44

As stated in other answers, the receivers used by FR24 is not the same receivers as ATS use. In a comment you indicate that this is irrelevant:

"I'm not asking about coverage, but position accuracy, which is independent of the receiver (either the receiver does or does not receive the airplane position)."

This is however relevant indeed. Poor reception can lead to a poor update rate (only some of the broadcasts received), leading to old data being displayed. Also, you don't know what signal/data processing occurs between the receiver and you on FR24, and what processing would occur at the ATSU.

The performance specification for ADS-B is laid out in ICAO doc 9871 (which unfortunately comes with a bit of a price tag). Unfortunately, I don't have access to 9871, however, it is reflected in its various national ratifications.

The following are quotes (my emphasis) from Annex 2, Part B in EU 1207/2011 (which most likely is quite harmonized with FAA/other regs, at least within the scope of this question):

§6 The primary data source providing the data items in point 3(h) and (i) shall be at least compatible with GNSS receivers that perform receiver autonomous integrity monitoring (RAIM) and fault detection and exclusion (FDE), along with the output of corresponding measurement status information, as well as integrity containment bound and 95 % accuracy bound indications.


§10. The total latency of the horizontal position data (the data items in point 3(h) and (i)) shall be equal to or less than 1,5 second in 95 % of all transmissions.


§11. The uncompensated latency of the horizontal position data (data item in point 3(h)) shall be equal to or less than 0,6 second in 95 % of the cases and shall be equal to or less than 1,0 second in 99,9 % of all transmissions.

Point 3(h) and (i) are respectively: geodetic position and horizontal position quality indicators

From this, we can deduce that the accuracy of the position input will be bound to GNSS/RAIM, and that the latency of the position data will be in the order of a second, or less. (This however says nothing between time to updates).

Certification requirements of a GNSS-system is controlled by various regs, depending on the installation (ABAS/RAIM, SBAS, GBAS etc) and many of them once again have price tags, but just to get an idea of the precision an ADS-B system could expect we can look to former FAA TSO-129a (GPS/RAIM), in which it states that the equipment 2D position error must be contained to 0.124 nm for oceanic, en-route and terminal ops, at 95% confidence (about 230 meters).

The actual ADS-B equipment requirements are more involved than this (and the possible precision/accuracy of GNSS with a suitable augmentation system (SBAS/GBAS) is much higher), but these figures should at least give a ball park idea of its precision.

It should be mentioned that the article you link to claims that primary radar is the primary surveillance method used by ATC. At least for most civilian ATC units, this is incorrect: Secondary surveillance radar (SSR) is the primary surveillance method, with primary radar sometimes being available as a supplement to SSR-data. As SSR is dependent on a transponder reply from the target aircraft, while a primary radar is able to detect objects without a transponder, ADS-B can not replace the supplementary function of a primary radar per se, but it can replace/extend the function of SSR.

Both FAA and Eurocontrol has stated ADS-B out will be required in certain airspace from 2020. In Australia it has been required above FL290 already since 2013.


This is an older question, but I wanted to add a couple points that I didn't see addressed in any of the answers provided.

As thoroughly explained, Flightradar24 (FR24) provides users with a depiction of near-real time (NRT) civil aviation activity using the aircrafts' ADS-B-out data transmissions. However the website has the capability, albeit it limited, to determine the position of aircraft utilizing a Mode-S transponder without ADS-B out transmissions.

Mode-S transponder transmissions contain a range of data depending on the type and features of the system, but without ADS-B capabilities, they do not provide an aircraft's position. The key piece of data that is included is what is commonly referred to as the "Hex Code". A hex code can be defined and decoded in a bunch of ways, but in it's most simple form it is a unique number that is assigned to a specific aircraft. Yes, there are ways to change it in the avionics shop, but if normally only done if an aircraft is deregistered in one country and registered in another.

Every country which operates under the International Civil Aviation Organization (ICAO) is allotted a block of these codes for use by aircraft registered in their country. So when a Mode-S transmission is received and processed, the system processing it is able to identify the specific aircraft which transmitted the signal. If the the system has at least 4 receivers in an area where all are able to determine that this unique aircraft is transmitting a signal, and the precise positions of those receivers are accurately measured, and those positions are integrated with allowing for the system to use logic algorithms written by people WAY smarter than me, you are able to do something called multilateration (MLAT).

For those of you thinking "that sounds a lot like triangulation", you're exactly spot on. MLAT is a fancier more technically advanced form of triangulation. Historically one would use direction finding equipment which would literally point to the source of a transmission. Get enough arrows pointing, you can figure out an approx. location. With MLAT, the system instead utilizes the extremely small differences in time it takes for the signal to reach each receiver to determine the distance the aircraft is from the receiver. Get enough distances, you can determine a location.

The important point to realize is that MLAT has the potential to be very precise when determining aircraft positions. However, in order to achieve this type of precision the receiver antenna positions and system set-up requires a far higher level of scrutiny and quality control that even the best FR24 receiver host could provide. With certified ATC systems, even antenna cable quality and run lengths are taken into consideration.

So if you consistently see aircraft positions being depicted inaccurately by FR24 in a certain area, check the signal details to determine if MLAT is being used.

The other point is that FR24 overlay maps may have errors or inaccuracies with respect to the geo-referencing. Even a small error can manifest as a significant position indication.

So hope that helps even though it's a little stale!!!


What people are missing here is FR24 is NOT for ATC operation but secondary data source to be used as 'non critical use' by anyone interested. Technology for ADS-B in/out itself is perfectly accurate when operated under certification guideline of aviation authority. In reality FR24 web service or other crowdsourcing data provider never 'guarantee' 100% accuracy for their data. Also FR24 website show intentionally delayed data by couple of minutes.Website user access level also have different accuracy on different time of day between FREE and Paid account. Also if you carefully look on 'settings' of FR24 web interface you will find option for 'Estimations value in minutes' which sometime show aircraft position entirely by estimation of schedule, last received data etc in case of no real data received for any particular scheduled aircraft.

There are other limitation for FR24 data viewing which might give wrong impression. But if ever ADS-B become primary for ATC operation, they will have their own setup for that purpose but ATC operator will never guide aircraft based on screen what you and me are watching on open Internet. TCP data like HTTP/Web traffic will never use on time-critical operation. Switching mode data network is there for time critical network. Even your VoIP traffic normally avoid TCP for voice transmission between two party.

ADS-B is NOT Flightradar24.COM, but Flightrader24.com use ADS-B for sourcing their data. Deciding ADS-B technology by watching Flightrader24.Com is like trying getting experience of traveling on Airbus A380 while traveling on Boeing DC-10:))

Edit adding more info: In nutshell if we compare all other technology available for avation industries for accuring position data at this moment ADS-B is on top of list. If you don't want to beliave FR24 website, try FAA document issue 2010 on this PDF Document. By using FR24 site you got wrong impression about ADS-B. Actually this is very much accurate in operation when use as per regulation guideline and equipment setup both on aircraft and control tower is proffational grade. Just think of it, people now preparing for riding driverless on highways runing hundrudes of kilomiter/hour with much proximity than aircraft would never come close (Even on a parking space). Both position data source is GPS sattelite indeed. Current operating prosidure used guiding allmost all avation traffic is product of Warld War II era. Equipment might become sophisticated but structure is same. You can have more details on ADS-B here in Wikipedia Article.

  • $\begingroup$ This doesn't answer the question. I never said that ATC would use FR24, I was just wondering about the accuracy of ADS-B. The delayed information is no longer the case (that is an outdated FAA thing). You don't pay for accuracy but for access to more data (like wind and temperature aloft as calculated by the aircraft). $\endgroup$
    – Sanchises
    Feb 7, 2018 at 7:40
  • $\begingroup$ Also, 'perfectly accurate' is never true. Only as accurate as the aircraft's navigation, which is quite good in the case of GPS, but rather poor for some older airplanes which haven't bothered to link their GPS to the ads-b transceiver. For example, old KLM Fokker planes which never flew in Australia anyway, and are now retired. $\endgroup$
    – Sanchises
    Feb 7, 2018 at 7:45
  • $\begingroup$ What I wanted to say is FR24 use ADS-B and provide near accurate data as referance for using non-critical public use at this time. As tracking technology ADS-B is most accurate compiring other available technology in operation. Editing answer to add some more description. Thanks $\endgroup$
    – A. Bauani
    Feb 8, 2018 at 22:00
  • $\begingroup$ @Sanchises: "I never said that ATC would use FR24, I was just wondering about the accuracy of ADS-B". True, but your assumptions about the inaccuracy of ADS-B is based solely on your observations of a free web app. I would suggest you get friendly with your local ATC facility and ask if they will show you their displays. You might be impressed... $\endgroup$ Jan 24, 2019 at 23:56
  • $\begingroup$ @MichaelHall As the other answers show, a local ATC facility does not use ADS-B as the primary source of data (and also not primary radar as FR24's website suggest), but secondary radar ('pinging' a transponder). I stand by my observations concerning ADS-B even if they are from a web app, as there is no degree of 'accuracy' involved in reading a digitally broadcast position - either you receive a valid ADS-B position report, or you have some bits wrong and the checksum doesn't add up, and you drop the report.... $\endgroup$
    – Sanchises
    Jan 25, 2019 at 13:27

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .