It's certainly better than it used to be, since there are more satellites, and a lot of work goes into doing well in these environments (I used to do the GPS in

It's certainly better than it used to be, since there are more satellites, and a lot of work goes into doing well in these environments (I used to do the GPS integration for Google Maps). There are two big tricky parts about urban canyons. First, yeah, you still have quite a few satellites, but they're all in the same directions. Satellites directly above you don't really tell you anything. Satellites in front of and behind you can only tell you how far you are along that axis. You don't have any satellites to the sides, because you're in a canyon. So in an idealized version of a really bad urban canyon, the GPS can tell how far you are on the street, but not which street you're on, which is what the user actually wants to know. (I did a lot of work on knowing this from context, but it's still quite a mystery on a cold start.) The second is that, unlike natural canyons, signals in urban canyons tend to bounce. So instead of seeing the satellite missing, you see it giving a signal that's consistent with if you were in the next street over. Which is the same problem we had in the first place. There's a lot of outlier detection done in the chips, but in a really noisy environment, sometimes that'll inevitably end up throwing out the good data and choosing the outlier.