4 ms·
Some transit agencies and railroads are experimenting with GPS, but IIRC the biggest impediment are tunnels,
by Anechoic 11y ago
Some transit agencies and railroads are experimenting with GPS, but IIRC the biggest impediment are tunnels,
- mikeash 11y agoSeems like that would be trivially solved by using dead reckoning in tunnels. You know the exact path of the tunnel, you know when you enter it, and you can count wheel revolutions to see exactly how far you've gone within it. But perhaps I'm missing something that makes this harder.
- iSnow 11y agoIn principle you are right, but precision is an issue: the train control system needs to know the position in the centimeter range. Counting wheel revolutions is notoriously unprecise because as the wheels wear down, they rotate faster. Public transportation is furthermore not an area of "hey I know, I just ask on Stackoverflow for an algorithm that counteracts the wear on the wheels". They do extremely conservative and defensive programming there (I once upon a time read the source code of one of the leading ETCS systems) in extremely restricted and conservative hardware (embedded 8080's in the 2000's) where the production process is audited and nailed down for everything (CF cards costing hundreds of $ for 128MB). Absolutely nothing is trivial there - the usual answer for an improvement idea would be a resounding "no way!". They are mortally afraid of the law of unintended consequences there.
- mikeash 11y agoWhy do trains need centimeter-level precision in tunnels? Especially if we're just talking about figuring out speed limits.
- Someone1234 11y agoI don't see why tunnels would be an issue. If the app has been tracking the object for a while it can calculate projected speed, and as it loses the GPS signal it can use an internal clock to continue to estimate the train's current location. Navigation systems already do this for signal losses (and to save battery).