Instrument Landing Systems (ILS) provide aircraft guidance information during the most critical phase of the flight profile. Almost regardless of weather conditions and visibility, the pilot of a suitably equipped aircraft can be reassured that the steering commands provided by his instruments during landing will permit him to avoid local obstructions and to position his aircraft correctly for a safe landing. Data on the required profile in elevation (glidepath) and the required track (localiser) are derived from transmissions from the ground. To ensure correct and continued alignment of these transmissions, initial and periodic calibration of the ILS facilities is required. This checking function, following guidelines laid down by ICAO, is usually performed using accurately calibrated receivers on board a flight inspection aircraft along with an independent aircraft position measuring system that should itself be sufficiently more accurate than the ILS. In this way, any discrepancies between the received ILS data and the independent measuring system can be assumed to be errors in the ILS.