I need to correct myself: TIS-B is transmitted on both 978 and 1090 frequencies. FIS-B (weather and other data) is only transmitted on 978.
This AOPA article (https://www.aopa.org/news-and-media/a...hanges-to-tis-b), in the 7th paragraph seems clear that you receive traffic directly from other aircraft (no matter which freq they're using) -- "Aircraft carrying ADS-B In equipment receive data directly from nearby aircraft with ADS-B Out on the appropriate frequency (1090 MHz or 978 MHz)."
But it, and this article from the FAA (https://www.faa.gov/nextgen/programs/adsb/pilot/), do confound me in that they both imply that TIS-B only contains data on non-ADS-B-Out-equipped aircraft. (See the TIS-B diagram.) So does this article (https://www.aopa.org/news-and-media/a...w-it-works-adsb).
But if true, and if your ADS-B In only receives on one of the two frequencies, then that would leave you blind to aircraft using the other frequency -- so that doesn't seem right. Does every single ADS-B In have dual antennas? If so, why is it you indicate on your setup parms for ADS-B Out which, if any, freq's you can receive on? The same when filing a flight plan. In both cases you can indicate that you only receive on one of the two frequencies.
The problem with these articles is that they gloss over things. For instance, that TIS-B diagram does not show any aircraft-to-aircraft communication. Granted, it's focusing on TIS-B.
Again, if anyone on the forum can chime in here, please do so.
[Edit: Ahhh, Wikipedia to the rescue (https://en.wikipedia.org/wiki/Automat...80%93_broadcast). To quote: "In addition to receiving position reports from ADS-B out participants, TIS-B [USA-only] can provide position reports on non ADS-B out-equipped aircraft if suitable ground equipment and ground radar exist. ADS-R re-transmits ADS-B position reports between UAT and 1090 MHz frequency bands.". ADS-R, which may not be part of TIS-B per se, takes care of the case where an aircraft only receives on one freq.]