I know that one of the biggest advantages of ADS-B is "supposed" to be that it will directly communicate between equipped aircraft without ground support. Has anyone actually seen that in action? That is, seeing another aircraft on an ADS-B display where radar coverage is impossible, say, a remote valley surrounded by mountains with no radar station near.

Obviously both aircraft would need to be ADS-B equipped for this to work.

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    $\begingroup$ Yes, I've seen traffic in areas without radar coverage. $\endgroup$
    – Ron Beyer
    Oct 19, 2017 at 0:00
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    $\begingroup$ I don't understand what you mean by both aircraft have to be ADS-B equipped. One ADS-B OUT equipped aircraft and an ADS-B receiver is enough to verify that ADS-B works $\endgroup$
    – DeltaLima
    Oct 19, 2017 at 7:22
  • $\begingroup$ Use dump1090 with a SDR dongle, this is what all the crowdsourced flight trackers use (Flightaware, Flightradar24, etc) $\endgroup$
    – Steve Kuo
    Oct 19, 2017 at 20:18

3 Answers 3


I can confirm that ADS-B works without radar. I have been involved in several ADS-B trials in areas where there was no or only partial radar coverage. It works. No surprise, it has been designed that way.


Ah, a trick question. You ask about a

remote valley surrounded by mountains with no radar ADS-B radio station near.

Let's make it worse, the valley walls are high enough to also make communicating with the GNSS constellation difficult, further losing one's own location.

For those, you need Wide Area Multilateration (WAM).

A new surveillance system introduced, called multilateration or Wide Area Multilateration (WAM), is now allowing air traffic controllers to track aircraft along the difficult approach to Juneau, Alaska—a mountainous area where radar was not possible (FAA).

ADS-B is not a magic bullet for those very unique locations.

See: What is the purpose of wide area multilateration (WAM)?

Like Secondary Surveillance Radar (SSR) it is classified as a cooperative independent surveillance system; the aircraft has to cooperate (i.e. an active transponder) but position is determined independent from data sent from the aircraft (unlike e.g. ADS-B).


The purpose of ADS-B In is to detect and display the ADS-B Out aircraft and related information. Radar is not a component of ADS-B; so by definition it can operate in the absence of radar. However, since not all aircraft are or will be equipped with ADS-B Out, in the US, a service called Traffic Information Service-Broadcast (TIS-B) is provided. TIS-B depends upon the presence of radar and proximity to an ADS-B (ground) radio station. A transponder only aircraft (no ADS-B Out) is detected by radar. The track information on that aircraft is broadcast to properly equipped ADS-B Out & In aircraft in that proximity. So in a remote valley (or any area with no radar), the ADS-B In aircraft would have to capable of detecting an ADS-B Out aircraft directly on the correct link (1090Mhz or 978Mhz).

  • $\begingroup$ I agree with your technical explaination, but you cannot really say that radar is not a part of it. There are two different frequencies and two different systems (as you said), mode-ES at 1090mhz and UAT at 978Mhz. Instead of unifying on one system and frequency, the FAA decided that they would serve as the "translator" between the systems, that is, when the radar site receives a GPS position transmission on one system, it rebroadcasts it on the other system. $\endgroup$ Nov 22, 2017 at 18:52
  • $\begingroup$ (continued) This is a stunning bad idea, since it not only creates a situation where large and small aircraft cannot see each other directly, but even in radar coverage, it introduces a delay in the reception of the signals between aircraft. ADS-B normally has the interesting property that the signal is stronger and more accurate as aircraft approach each other, because GPS is more accurate if in near locations vs. far. The translation requirement abrogates that. $\endgroup$ Nov 22, 2017 at 18:57
  • $\begingroup$ The translator function is provided by ADS-R (Rebroadcast). It receives the broadcast on one frequency, translates it and broadcasts that information on the other frequency. This functionality resides in a control station for multiple ADS-B ground radios. It does not use any radar input. The delay it introduces in undetectable to a pilot. The accuracy of GPS reported position does not depend on relative proximity of the aircraft to each other. $\endgroup$
    – oldheloguy
    Nov 23, 2017 at 20:10
  • $\begingroup$ By "radar" I was referring to the radar installation. I know what primary and secondary radar is. Sorry, it just isn't true that the "accuracy of GPS reported position does not depend on relative proximity of the aircraft to each other", otherwise WAAS would have no reason to work. GPS has path accuracy issues, which is why WAAS works. It gives you a series of error references so that you get the net error from a station near to you, and can subtract the local accuracy out. Two GPS receivers will give a more similar answer depending on how close they are to each other. $\endgroup$ Nov 25, 2017 at 5:59
  • $\begingroup$ You also neatly side stepped the other issue. I KNOW what a rebroadcaster is. It adds both round trip delay, and several miles of round trip delay (i.e., the path delay times 2) is not NOTHING. Next, it adds another point of failure that didn't exist before, that is, the operation of a ground station, which was not even supposed to be necessary in the original definition of ADS-B. Further, the ground station can be working perfectly, but simply be below reception range, meaning that you and the heavy about to run you over cannot see each other. $\endgroup$ Nov 25, 2017 at 6:08

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