Although VOR/DME has been the standard airborne navigation aid for over 15 years there are still some features of the system which are far from ideal. In particular, the bearing information from the ground station is noisy and frequently inaccurate. There are a number of sources of error, some avoidable and some not. The quality of maintenance, particularly with respect to alignment and calibration, varies greatly throughout the world – for example there have been a number of reports concerning VOR stations in the Eastern Mediterranean with up to 40° error in the transmitted bearing information.