[ygra]

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.