Instrument Landing System

The ILS, or Instrument Landing System, is a ground based radio navigation system giving pilots horizontal and vertical guidance. It consists of a localizer antenna providing horizontal guidance and a glideslope antenna providing vertical guidance.

To fly the ILS an aircraft has to be fitted with an ILS receiver in order to display and convert the picked-up signals on the cockpit instruments.

The two antennas emit a signal on one tuneable frequency.

Positioned on the far end of the runway, the localizer antenna is generally made of several pairs of directional antennas that emit two lobes frequency modulated to 90Hz (left) and 150Hz (right). Where the two lobes intersect marks the alignment of the extended runway centreline to which your instrument indicates your relative position.

To avoid tuning in to a wrong ILS frequency (i.e. other nearby airports), localizers transmit an ‘ILS Facility Identification Code’ (a unique morse code). During instrument training your flight instructor will teach you how to self tune the ILS frequency by listening to the morse code and comparing it to your approach plate.

Some airports will allow the use of localizer back course, meaning you can approach the runway from the opposite side, but without glideslope indication.

NOTE: If your airplane is not fitted with instrument capable of switching to the localizer back course approach, the indications will be opposite.

The glideslope antenna is similar to the localizer, except it sends out signals to the vertical axis of the runway. The glideslope equipment is usually positioned on the side of the runway, near the aiming point markers.

Standard glideslope angle is 3º, which is an angle presenting an adequate vertical descent rate in coherence with the approach speed. The glideslope beam is 1.4º thick (0.7º on either side).

Objects below 5,000 ft. AGL reflect its signals, creating a false glideslope.