iBeacon: Ranging

iBeacon ranging operates only while your app is in the foreground. You can't use it in the background. The only exception is when an enter/exit event of monitoring occurrs. In this case you have approximately 5 seconds to range beacons and do something with them. After this time the app returns to the background mode and you can't continue ranging.

You can force

If your app also monitors for GPS and it supports Location Updates in background mode, didRangeBeacons is called normally every second when your app is in background, but Location Updates mode must be enabled.

Forcing Ranging in the background this way may cause draining the battery by your app much faster.

From iOS CoreLocation UserExperience

If your iOS app must keep monitoring location even while it’s in the background, use the standard location service and specify the location value of the UIBackgroundModes key to continue running in the background and receiving location updates. (In this situation, you should also make sure the location manager’s pausesLocationUpdatesAutomatically property is set to YES to help conserve power.) Examples of apps that might need this type of location updating are fitness or turn-by-turn navigation apps