The Glideslope
The glideslope subsystem radiates a signal from the ground providing elevation (vertical) guidance for a landing aircraft.

The signal is transmitted to a visual display on the aircrafts VOR-LOC receiver, to assist the aircraft in easily finding the runway centerline. The ground-to-aircraft signal depicts the aircraft's approach position relative to the runway centerline.

When the aircraft strays above or below the designated glide angle (normally 3 degrees) while on final approach, the pilot's cockpit display will indicate that the pilot, or the autopilot, must take the proper correction to allow landing safely.

The glideslope antenna tower is located approximately 335 meters (1100 feet) down the runway from the threshold and approximately 122 meters (400 feet) off to the side of the runway centerline. A compact, environmentally controlled shelter is located behind the antenna and houses the glideslope electronics, allowing technicians to work comfortably and protecting the system from the elements

 

Electrical/Hardware Details

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The Glideslope component of the ILS operates on one assigned frequency in the range of 328.6 to 335.4 MHz and is paired with the frequency selected for the localizer.

Three glideslope antenna systems are available from Thales ATM to suit a variety of installation requirements. The Null Reference system is the standard for most installations. As siting becomes a factor, and where tighter tolerances are needed to meet Category II and III commissioning requirements, the Capture Effect or Modified Sideband Reference system may be needed. Thales ATM will survey your site to determine which system is required.

The glideslope antenna is available in four configurations designed for various terrains.

Null Rreference: standard for most installations.

Sideband Reference: used when level terrain is available in front of the antenna and terrain drops off shortly beyond the end of the runway.

Capture Effect: used when terrain beyond the end of the runway rises slightly.

End-fire: non-imaging system: used when siting is not possible with other systems; employs a special slotted-cable transmitting antenna not more than 1.5 meters tall and requires no modification to airborne avionics.

The glideslope antenna consists of two bays of three colinear dipoles (three bays for the capture effect system) spaced 3/4 wavelength apart (at 332 MHz) backed up by a 90 degree corner reflector. The arrangement concentrates the radiated signal in the direction of the approaching aircraft and reduces the effects of reflection from nearby objects.

Each dipole antenna incorporates integral monitoring for greater reliability, corner reflected co-linear dipoles for maximum signal concentrations and proven reliability in extreme conditions.