Civil airways route structures are constructed around en-route and terminal navigation aids such as VOR, DME and ILS. Radio navaids do not always provide accurate information to aircraft and their performance can be seriously degraded by system drift, electronic interference and unrelated changes to the local environment such as man-made obstructions erected in the beam forming area. To ensure efficient and safe operation of air traffic control, ground-based aids must be commissioned and maintained to stringent specifications at all times. Consequently, regular airborne performance checks of all radio/radar navigation aids are essential, and since the most critical time of any flight is the approach and landing phase, the precision aids (PAR and ILS) are checked at regular intervals and with a greater positional accuracy than the general navigation aids.