In the aircraft we have:
- ADS-B out/in using 1090ES
- A/C or Mode S transponder for SSR grounding interrogation.
Does the ADS-B out use the same antenna as Mode S or does it use its own antenna?
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Where a Mode S transponder is used, ADS-B will use the same antenna.
To benefit from Satellite ADS-B, at least one antenna needs to be top-mounted and capable of 125 watts (most airliners already support 200 W).
What I find interesting and illuminating for such questions is Mode S'es history—which goes back to the late-60s—and its relation to Mode A/C and ADS-B:
In that history you'll find two important Mode S features that should reveal not just why, but how, ADS-B uses the same frequency:
First was AOPA's argument to the FAA for Mode S to be 1) backward compatible, but then a "fundamental flaw" was revealed in the FAA specifications (see p. 23), which led to an ingenious engineering solution. That workaround also allowed Mode S to be 2) invisible to the Mode A/C SSR radars (ATCRBS).
In other words, when ATCRBS interrogates Mode S, Mode S replies to it. And, Mode S ground sensors can interrogate Mode A/C (backward compatible).
But, when Mode S sends a Mode S message (to Mode S sensor or TCAS or ADS-B), ATCRBS ignores the message without processing it (invisible). Those features essentially paved the way for ADS-B to be on the same frequency as the ingenious Mode S, which had to be on the same frequency as Mode A/C.