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| Re: Canarsie CBTC (100286) | |||
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Re: Canarsie CBTC |
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Posted by RonInBayside on Sat Jun 18 08:39:18 2005, in response to Re: Canarsie CBTC, posted by AlM on Sat Jun 18 07:02:57 2005. "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?"Let's not forget about it. If you had bothered to read through this thread, you would have seen my posts where I stipulated to the fact that GPS is not useful underground. We acknowledge that - perhaps technology in future will change that (???) But not today. "There is some error in GPS measurements. I believe it is 20 meters," Try 5 meters. The military is no longer demanding to degrade signals for civilian use. And we may see better wiithin 5 years. "though it is unlikely that two consecutive measurements would be off in radically different ways." Agreed - meaning we can quantitate the likely error and account for it. "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." Your caveat is important - but it would be useful to find out where the possibilities and boundaries really are. "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." Refer to the top of this post. |
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