From: SMTP%"jimbell@pacifier.com" 27-JAN-1996 03:43:05.83
A peripheral I've long wanted to see, commonly available: ACCURATE time, broadcast to the millisecond/microsecond/nanosecond, available from sources as varied as TV VIR's, FM subcarriers, and other sources, available as an easy input (via a peripheral card) to a computer.
Yup! Do you think it is really possible? If I remember well speed of light is 300.000 Km/s. That means that light takes around 1 ms. to cover 300 Km. If you use a satellite, antenna, whatever to broadcast a timing signal, the accuracy will depend on when do you receive it, and that in turn on your distance from the source. By the time the signal reaches you it may be several milliseconds old (and thousands of microsenconds and millions of nanoseconds). We are so used to think of TV, whatever as an instantaneous broadcast medium that we forget that there are speed limits in the Universe. And you can't exceed them by just paying a ticket. Note that this was a best case scenario: using the speed of light to transfer the information and using the shortest path. In reality most waves won't travel as fast in the air, and will depend on atmospheric circumstances.
I have a 12-year-old Heathkit "Most Accurate Clock" that I assembled myself, and had the foresight to install it with its computer interface option. (receives 5, 10, or 15 MHz signals broadcast from Boulder, Colorado, containing "exact" time.)
Just remember that the best you can get would be microseconds if you're in a 300 meter radius, or milliseconds on a 300 Km. And possibly nanoseconds at 0.3 m. Even then you need to know exactly (i.e. with an accuracy of between centimeters to a few Km depending on the timing you want) your position. And depending on the media you use, possibly the atmospheric conditions in between the emisor and your receptor. Then remains the cypherpunk part on all this: how can you trust the *signal* your receptor receives? How do you know no one is interferring it or sending an inaccurate or false one? So you need a GPS... And a timing source that can be trusted. You'd wantthe signal not to be tamperable or at least to be able to detect when it has been tampered. And that on a broadcast system. A system owned by someone who you may not trust (say a private TV channel, radio or satellite). So you may want to have several sources, and to be able to verify that the signals you receive all come from their respective sources. Yum! a nice problem to think about. One factor is that you wouldn't expect changes in public sources used by sensible systems since those could not pass unnoticed and might raise big protests. But you still have the MITM attack to consider... Oh well, it's too late now. See ya... jr