General

    The term  ILS is the acronym of  Instrumen Landing System. ILS is a terminal navaid  which provides informations to the pilot of an aircraft , in cooperation with the respectives aircraft equipments , for a precision approach and landing in the airport in which is available this navaid. 

    The ILS stations are usually installed in the airports which have full traffic and contracy weather. Today, ILS stations are installed in Greek international Airports of Athens, Thessaloniki and Rhodes. ILS is used to give to the pilot, precision informations of the aircraft direction so as he drives the aircraft, if it's possible, to touch the ground in the specific point. 

    The system reliability up to the equipments reliability, the installation quality and  the enviromental conditions (mountains, buildings, climatologic conditions.For this reason there is a theoritical study all the above conditions,  which must not change after the installation. 

In propotion with the reliability, the ILS stations classify in the followed thre categories : 
 

ILS stations include the followed equipments : 

1. Localizer

Localizer is a transmitter which gives information about  azimuth  in regard with the Center Line of the airport landing runway.Together with the glideslope transmitter (Glide path) , a precision approach can be performed. 

The localizer antennas are located at the far end of the runway, as seen by the aircraft on approach to landing. The antenna installation is a linear array of multi-element antennas, with thick, staggered elements. Localizer  transmit between 108 and 118 MHz. 

2. Glide path

Glide path is a transmitter which gives informations of the correct angle slope in regard with the horizontal level of the straight of aircraft slide, during the landing. The angle is 3⁰. 

3. ILS Marker Beacon and Compass Locator Stations

Marker Beacons are two or three transmitters which give information about the pecision approach, as control points for the aircraft correct direction of the landing runway extension. In the above stations is possible are installed and Compass Locators. Marker beacons are VHF transmitters operating at 75 MHz.The Outer Marker (OM) is used to indicate that an aircraft should intercept the glide path when over the transmitter. The Middle Marker is used to indicate that the aircraft is at the Decision Height (DH) for most approaches. 
 

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V.O.R(VHF Omni Directional Range) is a ground-based navaid which gives to the aircraft pilot the capability to know the aircraft azimuth site È from the magnetic North and needle VOR, independently from the route and in regard with aircraft receiver. 

VOR is a transmitter operating in the frequency range of 108-118MHz. 
VOR stations classify propotionately with its installation site: 
 

1. Terminal	This is installed into the airport with transmition power 50W. Terminal VOR transmits in the frequency range of 102-112MHz
2. En Route	This is usually installed on mountains with transmition power    100W. Terminal VOR transmits in the frequency range of  112-118 MHz.

VOR stations are usually co-located with a DME(Distance Measuring Equipment) station.
 

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Distance Measuring Equipment (DME) stations transmit in response to requests from aircraft-based transmitters. The delay between the request and response allows the aircraft equipment to calculate the distance to the DME station. 

The DME stations are often co-located with the Glide Path for a runway. The DME antenna is a mast antenna mounted on top of the VOR Antenna. 

Sometimes, DME is co-located with a VOR, or even in rare cases with an NDB. 

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NDB Stations

NDB transmitters are used for several purposes. General aviation uses them as a non-precision approach to some airports, and they can also be used enroute. Also, low power versions of NDB transmitters can be co-located with an instrument landing outer marker, in which case they are called Compass Locators. 

An NDB is a transmitter operating at a carrier frequency between 190 and 415 kHz. 
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