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Thread: ADSB Out FARs

  1. #51
    Gilbert Pierce's Avatar
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    Default Re: ADSB Out FARs

    Tom, so my transponder reply’s to TCAS interrogations? I assume that the TCAS also receives my ADSB out data?

  2. #52
    6PapaWhiskey's Avatar
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    Default Re: ADSB Out FARs

    Quote Originally Posted by Gilbert Pierce View Post
    Tom, so my transponder reply’s to TCAS interrogations? I assume that the TCAS also receives my ADSB out data?
    Yes, and maybe. TCAS is an airborne Mode S secondary radar. TCAS uses a fixed antenna array rather than a rotating antenna, but it interrogates all transponders and receives/decodes their replies in the same way ATC secondary radar does.

    ADS-B integration into TCAS is likely not complete. It's known as "hybrid surveillance" or "enhanced surveillance". When ADS-B was in its initial development, the folks in FAA Safety (Flight Standards/Aircraft Certification) were opposed to the integration of ADS-B data into TCAS due to the possibility of "spoofing" of ADS-B position data. Spoofing can be deliberate or can happen with equipment failures. During some early flight testing over the Gulf of Mexico in 1994, we encountered an error with a prototype ADS-B avionics installation. The avionics consisted of a modified King KT 70 transponder and a Trimble GPS. The RS232 output of the GPS was sending bad data to the transponder, resulting in false positions (offset by several miles). The GPS display was accurate and the internal integrity checks displayed no errors to the pilot.

    To make a long story a bit shorter, the result was that a TCAS with hybrid surveillance would use interrogation to establish the initial position of the intruder aircraft, use ADS-B primarily at longer distances with occasional interrogations, then rely mostly on interrogations as the intruder got closer.

    You can Google "TCAS hybrid surveillance" and get much more on the topic if you're really interested.

    Tom P.
    Wagabond @ 08A
    Last edited by 6PapaWhiskey; 01-11-2020 at 10:32 AM.

  3. #53
    Gilbert Pierce's Avatar
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    Default Re: ADSB Out FARs

    TCAS hybrid surveillance explanation on Wikipedia was helpful but slightly outdated as it indicted the ground UAT transceivers were only available in Alaska and the East Coast. The FAA site was not clear on the hybrid aspects.
    The Wikipedia site
    Relationship to automatic dependent surveillance – broadcast (ADS–B) was helpful for the curious.

  4. #54
    6PapaWhiskey's Avatar
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    Default Re: ADSB Out FARs

    Quote Originally Posted by Gilbert Pierce View Post
    TCAS hybrid surveillance explanation on Wikipedia was helpful but slightly outdated as it indicted the ground UAT transceivers were only available in Alaska and the East Coast. The FAA site was not clear on the hybrid aspects.
    The Wikipedia site
    Relationship to automatic dependent surveillance – broadcast (ADS–B) was helpful for the curious.
    No surprise on Wikipedia, as it's a crowd-sourced reference. The important thing here is that TCAS has been around since the early 90's after being mandated as a result of the Cerritos CA midair between an AeroMexico DC-9 and a PA-28, and well before ADS-B came along. Since TCAS incorporates a Mode S transponder and a 1090 MHz receiver, the addition of hybrid surveillance using 1090ES ADS-B can be done with a software modification. 1090ES is a global standard, whereas UAT was adopted by the US only. I could be wrong, but doubt that any TCAS manufacturer will incorporate UAT into hybrid surveillance due to the hardware changes (and accompanying certification costs) involved.

    The role of the transponder with TCAS is one of the main reasons that the transponder mandate (and biennial checks) will remain. TCAS (without hybrid surveillance) can only "see" an aircraft that has a transponder with Mode C. TCAS was also the primary driver for the Mode C airspace mandates and the old 30 NM Mode C veil around Class B airspace.

    Tom P.
    Wagabond @ 08A
    Last edited by 6PapaWhiskey; 01-11-2020 at 11:50 AM.

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