iphone - How to constantly check GPS Signal strength -

i want display gps signal strength in form of image like:

can't find documentation or example.
have tried this:

if (self.locationmanager.location.horizontalaccuracy < 0)         {             ...@"no"];         }         else if (self.locationmanager.location.horizontalaccuracy > 163)         {             ...@"poor"];         }         else if (self.locationmanager.location.horizontalaccuracy > 48)         {             ...@"average"];         }         else         {             ...full         } 

but seams code nothing.
how can monitor gps signal strength, possible?

where code located? code should in delegate callback:

- (void)locationmanager:(cllocationmanager *)manager didupdatetolocation:(cllocation *)newlocation        fromlocation:(cllocation *)oldlocation 

or ios 6.0 , above:

- (void)locationmanager:(cllocationmanager *)manager  didupdatelocations:(nsarray *)locations