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| Re: Canarsie CBTC (100267) | |||
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Re: Canarsie CBTC |
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Posted by AlM on Sat Jun 18 07:02:57 2005, in response to Re: Canarsie CBTC, posted by RonInBayside on Sat Jun 18 00:14:24 2005. "GPS can supply position information (and if you have updated position information, then you can also calculate velocity). Your points about track assignments are correct; GPS could reduce the investment you'd need in wayside equipment."Let's forget about the fact that satellite signals almost certainly aren't strong enough to be detected reliably underground. They're WAY weaker than cellphone signals, and have you every tried using a cellphone in the subway? There is some error in GPS measurements. I believe it is 20 meters, though it is unlikely that two consecutive measurements would be off in radically different ways. So if you take two position measurements a second apart, and they are off by even 5 meters, the resulting velocity estimate would have far too much inaccuracy to be useful. Only over longer term periods is GPS a useful measure of average (not instantaneous) velocity. "A test like this would be well worth an investment from the US Dept. of Transportation." How do you know it hasn't been done? It wouldn't be expensive to take a GPS receiver down into a subway tunnel and check if you can detect a sufficient signal. If the answer is not a chance, it might not have gotten much publicity. |
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