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The 1090 ES is the transponder system used in ADS-B. Why does the 1090 ES use the same frequency (1090MHz) for uplink as well as downlink? 1090 ES is an extension of Mode S. The Mode S used 1030MHz for uplink and 1090MHz for downlink. Why did the 1090 ES change that? Having two frequency channels would be better (More space less noise due to message collisions) for communication right?

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There is no uplink in ADS-B. The aircraft is broadcasting information which is received by parties unknown to the ADS-B transmitter. There is no reply and there is no uplink like there is in Mode-S.

The receiving parties listen on 1090MHz for ADS-B broadcast.

A technology related to ADS-B is TIS-B (Traffic Information Service - Broadcast). This is used in the USA to transmit positions of aircraft that are not equipped with 1090ES ADS-B but are detected by FAA radar or UAT ADS-B in an ADS-B like format on 1090MHz. This allows parties listening on 1090MHz for ADS-B traffic to identify non 1090ES equipped aircraft with the same receiver. TIS-B could be seen as form of uplink, but uses the 1090MHz frequency nevertheless as it allows the same ADS-B receiver to be used.

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  • $\begingroup$ what about the TIS-B (traffic info) and FIS-B (weather info) shared by the ATC? Isn't that considered as uplink? If the ATC uses the 1030MHz channel, there would be even less congestion compared to the 1090MHz. That was what I was trying to point out. $\endgroup$ – Clive Sep 18 '14 at 15:50
  • $\begingroup$ @Clive TIS-B is something different than ADS-B. Indeed it causes some congestion on the 1090MHz freq. One of the reason we don't have it Europe. FIS-B is a UAT application, it does not work on 1090ES. $\endgroup$ – DeltaLima Sep 18 '14 at 17:32
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It's a broadcast protocol - devices spontaneously announce themselves for listeners to hear. There are no interrogations to put on a different channel.

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