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Comment by jlarocco | Hacker News Reader

Ceilings usually aren't problematic for GPS. They do better out side, but if you get cell service through the ceiling, you'll get GPS signal through it.

What I'm curious about is that GPS is only accurate to within a few meters. If I turn on my GPS and set it on my desk for an hour, the resulting track will look like it's bouncing around the room.

My guess is this app averages a bunch of readings from each wall to correct for the noise, but I'm still surprised that works well enough to give accurate results.

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The cell signal is orders of magnitudes more powerful than the GPS one. In fact, GPS even has a noise floor that's higher than the signal.

As for using GPS, WLAN and others, you can get pretty accurate measurements with sensor fusion, even if the individual sensor values are all noisy or inaccurate.

From past experience I'd also expect GPS to not be available unless you're near a window. But if it's there you can use it to get a more accurate location.

The traditional approach in sensor fusion is to use high accuracy, high drift sensors like the phone's accelerometer to provide accurate short term data and then correct for the drift using less accurate but low drift sensors like GPS. Just integrating the accelerometers will work on short time spans, but you also double integrate the noise of the accelerometer, so that measurement of position blows up quickly unless you correct for it.

> if you get cell service through the ceiling, you'll get GPS signal through it.

Download one of the apps that shows individual GPS satellite signal strengths and tell me if you still believe this. Definitely not true for my phone.

It really depends. I have an older dedicated GPS receiver that absolutely no way will receive a signal indoors. My iPhones all do a pretty good job. (FYI, you can only get signal strength on an Android phone; Apple's CoreLocation API doesn't provide that or some of the other detailed satellite data.)

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