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| Re: TA is obsessed with CBTC, and ''New'' tech for no reason. (581272) | |||
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Re: TA is obsessed with CBTC, and ''New'' tech for no reason. |
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Posted by Stephen Bauman on Wed Mar 5 18:00:52 2008, in response to Re: TA is obsessed with CBTC, and ''New'' tech for no reason., posted by LuchAAA on Wed Mar 5 15:34:57 2008. I asked to see the calibration sticker. That's a polite way of my questioning the accuracy of the speedometer reading.I have a couple of reasons to be skeptical, not the least being that I have made my own measurements. First, the reading, the T/O sees, has been "processed" to eliminate jitter. So, even if the sensor were providing an accurate measurement there is the question of what the TA is doing with these readings. There's also the accuracy of the sensor itself. Doppler speedometers compare the frequency of a transmitted frequency with that of its echo. The Doppler receiver detectors have a lot of problems, if there are multiple echoes. Doppler speedometers measure Doppler's velocity not the vehicle's velocity. The Doppler's velocity is the vehicle's velocity multiplied by the cosine of the between the Doppler path and the train's velocity. If the train were heading into a wall, and the Doppler were aimed directly at the wall, then the two would coincide. They aim the Doppler onto the ground, to make them useful for more applications. Thus, there is a fixed angle between train's horizontal motion and the point on the ground that defines the Doppler path. They then divide by the cosine of this angle to display the vehicle's velocity. What happens if the angle changes? The bracket can become bent. Also the ground can "become bent", i.e. undergoes a vertical curve like the bottom in a tunnel. How much of an angle is required to throw the reading off by 10%? Suppose the desired angle were 45°. Then a change of 6° is enough to throw the reading off by 10%. It's my understanding from reading this forum that these mountings are maintained men noted for their brawn and not by men in white coats, equipped with shims and levels. |
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