{"id":11507,"date":"2026-07-09T14:00:00","date_gmt":"2026-07-09T12:00:00","guid":{"rendered":"https:\/\/fooblog.de\/?p=11507"},"modified":"2026-07-09T14:39:01","modified_gmt":"2026-07-09T12:39:01","slug":"introducing-f3cross-an-electronic-line-judge-for-f3b","status":"publish","type":"post","link":"https:\/\/fooblog.de\/?p=11507","title":{"rendered":"Introducing F3Cross: an electronic line judge for F3B competitions"},"content":{"rendered":"\n

—Deutsch ganz unten —

F3B is one of the most demanding and rewarding disciplines in RC soaring \u2014 thermal duration, spot landings, and a flat-out speed run that pushes glider and pilot to the limit. But the sport has a staffing problem.<\/p>\n\n\n\n

Every aircraft in the distance flight needs two dedicated line judges \u2014 one at Base A, one at Base B \u2014 each watching for the exact instant a part of the aircraft crosses their line. Run a full round and that’s up to a dozen people tied up on judging duty alone. That’s a heavy ask for organizers \u2014 and on a practice day, when nobody’s around to judge, you’re simply flying blind.<\/p>\n\n\n\n

F3Cross<\/strong> fixes that. It’s a small, GPS-based electronics system, currently in development, that replaces the line judges entirely \u2014 instant, tireless, cheap.<\/p>\n\n\n\n

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How it works<\/h2>\n\n\n\n

The system has three parts:<\/p>\n\n\n\n

    \n
  • An airborne unit<\/strong>, small and light enough to fly on top of any glider, fixed with a strip of tape, that continuously tracks its own GPS position.<\/li>\n\n\n\n
  • A ground station<\/strong> that receives crossing events and displays them on a computer<\/li>\n\n\n\n
  • A receiver for every pilot<\/strong> that gives turn feedback (acoustic and visual, maybe vibration) plus optional extras like clocks and laps. (This also eliminates the need for one of the pilots helpers who just watches the lights all flight!)<\/li>\n<\/ul>\n\n\n\n

    The airborne unit knows exactly where Base A and Base B are. The moment the GPS position shows the aircraft has crossed one of those lines, it fires an event and sends it wirelessly to the ground \u2014 almost instantly, with none of the reaction-time lag or subjectivity a human judge has. It works just as well solo on a practice day as it does with a full contest.<\/p>\n\n\n\n

    For reliability, every crossing is sent over two independent radio links<\/strong> at once. If one link drops out, the other one usually still gets through, so a single crossing is less likely to be lost to bad luck on the radio.<\/p>\n\n\n\n

    Proven in the field<\/h2>\n\n\n\n

    This isn’t just a bench project. In testing \u2014 including flights with an electric F3B \u2014 the system has tracked aircraft up to 660 m in altitude<\/strong> and out to nearly 700 m in distance<\/strong>, and kept catching crossings.<\/p>\n\n\n\n

    At the F3B competition site in B\u00fcllingen, Belgium<\/strong>, the setup was flown with two aircraft in the air at once. The pilots and helpers \u2014 Markus, Thilo, and Steffen \u2014 were happy with the accuracy, the low delay, and the handling. They thought it might even be usable in speed as well, but that is no priority for now! A few things still need work, like the antennas, good ergonomics, volume of the beepers, but as a first run it held up very well.<\/p>\n\n\n\n

    What’s next<\/h2>\n\n\n\n

    I’ve been testing this in the field for only a short time yet, but it is already working well. If this gets even one more club running contests with a smaller crew, or gets one more pilot out practicing on a day they’d otherwise have skipped, it’s done its job. Next up: the next integrated PCB edition, with an even smaller footprint and higher reliability and usability. Testing will continue, and hopefully it’s ready to deploy at F3B S\u00f6hl. After that competition I want to make the hardware and software publicly available.
    <\/p>\n\n\n\n

    The picture show the flown prototypes. Please be aware that this were only prototypes with hardware i had lying around anyway.
    The next version will be much smaller and better to mount onto the aircraft!
    (More technical details below, only for the interested!)

    Update 1: To clarify, all hardware will be provided by the competition,. As a pilot you do NOT have to buy anything! Just mount the module before your flight and fly…<\/p>\n\n\n\n

    Follow along here on the blog for updates. Fly more, judge less.<\/p>\n\n\n\n

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