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(Pocket-lint) - The Holy Grail of positioning, indoor GPS, may have been cracked by Apple and could be on your iPhone soon.

According to app developer Steve Troughton-Smith, Apple left its Indoor Survey app out for all to see. This app, he claims, has the ability to track a user indoors – something GPS cannot do.

GPS uses satellites to triangulate a phone's position, which needs a clear line of sight to the sky. Tracking a phone indoors, to direct a person around a large shopping centre or exhibition hall for example, has not been largely possible. Although Apple uses it iBeacons technology for location based features. This app appears to have tackled the main issue using radio waves.

The app seemingly uses the iPhone's onboard sensors in conjunction with radio frequency signals to position the device. These radio signals could be Wi-Fi or phone network signals that are flying about nearly everywhere now.

The app describes how it works: "By dropping 'points' on a map within the Survey App, you indicate your position within the venue as you walk through. As you do so, the indoor Survey App measures the radio frequency signal data and combines it with an iPhone's sensor data. The end result is indoor positioning without the need to install special hardware."

Back in 2013 Apple bought a company called WiFiSlam which specialised in a similar technology. Apple has also filed numerous patent applications for indoor navigation and positioning.

The Indoor Survey app is still on the App Store, but only via direct link. Be warned, we can't validate the app and don't advocate giving out iCloud details where there's risk, so try this at your own risk. That said the "forgot password" link does go directly to apple.com.

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Writing by Luke Edwards. Originally published on 2 November 2015.