Is there a way to check if the iOS device is locked/unlocked?

I have used GPS location updates in my application. I want to detect if the iOS device is in sleep mode so that I can turn off the GPS location updates and optimize the battery use. I have already tried pausesLocationupdates in iOS 6, but it does not work as desired. I want to turn off the GPS location updates as soon as the device goes to sleep mode. I want to detect the lock/unlock event in the device.

Is there any way to achieve this functionality ?

so far I got the darwin notifications as given below

-(void)registerForall
{
    //Screen lock notifications
    CFNotificationCenterAddObserver(CFNotificationCenterGetDarwinNotifyCenter(), //center
                                    NULL, // observer
                                    displayStatusChanged, // callback
                                    CFSTR("com.apple.iokit.hid.displayStatus"), // event name
                                    NULL, // object
                                    CFNotificationSuspensionBehaviorDeliverImmediately);


    CFNotificationCenterAddObserver(CFNotificationCenterGetDarwinNotifyCenter(), //center
                                    NULL, // observer
                                    displayStatusChanged, // callback
                                    CFSTR("com.apple.springboard.lockstate"), // event name
                                    NULL, // object
                                    CFNotificationSuspensionBehaviorDeliverImmediately);

    CFNotificationCenterAddObserver(CFNotificationCenterGetDarwinNotifyCenter(), //center
                                    NULL, // observer
                                    displayStatusChanged, // callback
                                    CFSTR("com.apple.springboard.hasBlankedScreen"), // event name
                                    NULL, // object
                                    CFNotificationSuspensionBehaviorDeliverImmediately);

    CFNotificationCenterAddObserver(CFNotifi