Apple’s iPhones and Google’s Android smartphones regularly transmit their locations back to Apple and Google, respectively, according to data and documents analyzed by The Wall Street Journal.

The widespread collection of location information is the latest frontier in the booming market for personal data. Internet-enabled cellphones allow the collection of user data tied with great precision to specific locations.

Apple said that it collects Wi-Fi and GPS information when the phone is searching for a cellular connection. The data it transmits about location aren’t associated with a unique device identifier, except for data related to its mobile advertising network.

Google seems to be taking a different approach — its location data appear to be transmitted regardless of whether an app is running, and is tied to the phone’s unique identifier,  according to new research by security analyst Samy Kamkar,

This new form of tracking has intensified concerns over privacy and the widening trade in personal data.