> How do they deal with many chips crossing the line at the same time? The EPC Gen 2 spec has a few ways of avoiding collisions. One is Random-slotted collisio

> How do they deal with many chips crossing the line at the same time? The EPC Gen 2 spec has a few ways of avoiding collisions. One is Random-slotted collision arbitration, where a tag waits a random amount of time before responding. There's also a somewhat complex algorithm; basically the interrogator says "I heard from tag X, so X be quiet, anyone else?" and repeats until everyone has been inventoried. The interrogator can also ask for subgroups of tags at a time, "Anyone whose number starts with 1? 2? ..." See [1] for details. The tags are designed to work for inventorying when many, many tags are visible at once. I'm not sure which methods are in use during races; taking inventory doesn't have as much time pressure, of course. The tags are fairly omnidirectional, but the detection mats are directional. I read somewhere that the timing system has about 300ms accuracy, but I can't find that now. I assume that for the race winners they use highly accurate finish line technology, rather than RFID. (It would also be very embarrassing if the winner's tag didn't scan: "Sorry, we didn't catch your time.") [1] http://www.gs1.org/sites/default/files/docs/epc/uhfc1g2_2_0_0_standard_20131101.pdf http://www.gs1.org/sites/default/files/docs/epc/uhfc1g2_2_0_...