The Geolocation API defines a high-level interface to location information associated only with the device hosting the implementation, such as latitude and longitude. The API itself is agnostic of the underlying location information sources. Common sources of location information include Global Positioning System (GPS) and location inferred from network signals such as IP address, RFID, WiFi and Bluetooth MAC addresses, and GSM/CDMA cell IDs, as well as user input. No guarantee is given that the API returns the device's actual location.
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Além disso, a resposta da fmark no StackOverflow tem mais detalhes sobre como implementado no Firefox. A implementação varia entre navegadores e plataformas.
The data sent by Firefox to Google is for all visible access points, public or private. For each access point detected, it sends the following data to https://www.google.com/loc/json:
"mac_address": "01-23-45-67-89-ab", "signal_strength": 8, "age": 0, "SSID": "MyAccessPoint"
{ . . . }