1090 Mhz is currently used for SSR Mode A/C replies and Mode S communication(ELS-EHS replies and ADS-B broadcasts) by civilian aircraft and some of military IFF modes.

With the infamous 2020 ADS-B Mandate about ADS-B usage in certain airspaces(Mandate) , it is expected that this frequency will be very crowded in future.

There are certain methods/precautions to prevent congestion like randomized broadcast timing(between 0.4 s and 0.6 s for Airborne Position for example). F.R.U.I.T. and garble problems are also not new to this frequency.

Are there any discussions in the aviation circles about precautions in anticipation to congested 1090 Mhz band for certain airspaces?


There are some areas where the 1090 MHz frequency is already very busy. The area around Frankfurt am Main in Germany and the Los Angeles Basin airspace is the USA are the most congested in the world.

The direct of impact of ADS-B is an increase in the frequency use of 1090 MHz. At the same time it allows a reduction of number of Mode-S radars, it enables the development of a more effective TCAS with reduced interrogation rates (known as ACAS X), and it reduces the number of interrogations needed by WAM systems. On the long term the impact of ADS-B will likely be positive. But with the increase in number of flights the 1090 MHz congestion will remain a concern.

There are many discussions in the ATC surveillance circles about the topic and the 1090 MHz frequency (and 1030 MHz as well) is are frequently monitored. There are ground based monitors and periodically there are measurement flights to identify areas of high frequency load. Adding new radars to already congested areas is made very difficult by spectrum protection regulations, instead cross-border surveillance data sharing is encouraged. Some countries monitor 1090 MHz transmissions around the clock and the are plans to have a permanent monitoring service of 1030 MHz and 1090 MHz across Europe that will analyse spectrum usages trends and identify rogue transmission.

  • $\begingroup$ Do you mean SSR by "Mode S Radar" or is there a dedicated Mode S Radar? Also I have read somewhere that transponders are designed to reply to hundreds(if not thousands) Mode A/C interrogations. Also , I read from wikipedia that one method of reducing garble is increasing interrogation rate(of mode a/c). Do you have information about estimated percentages of Mode A/C communication compared to Mode S communication? $\endgroup$ Jan 18 '17 at 11:27
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    $\begingroup$ I meant Mode S specific because they are used to downlink information that is included in the ADS-B messages. Reduction of SSR's in general is also possible but then all Mode A/C only transponders need to be phase out. Mode A/C is using a lot of the capacity on 1090MHz and I'd be glad to see it disappear. Increasing de Mode A/C interrogation rate will increase garbling and fruit. The problem with Mode A/C is the high interrogation repetition rate combined with an often wide radar beam. Aircraft manage to reply to 15 successive interrogations sometimes which is polluting the frequency. $\endgroup$
    – DeltaLima
    Jan 18 '17 at 12:12
  • $\begingroup$ @DeltaLima, do you have more information about rogue transmission? $\endgroup$ Mar 25 '19 at 23:38

The FAA was so concerned about ADS-B causing congestion on 1090 that they implemented an entire second data link (978/UAT) and all of the relay complexity that entailed just to mitigate that. Other countries are free to do the same, and it would be easier for them now since there's a working example to copy, but so far none have.


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