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General information
The THEY ( Instrument Landing System ) is the means of the most precise radionavigation used for the landing in IFR.
It includes/understands 2 elements:
- a localizer which provides the variation of the plane compared to the axis of the track;
- a glide path which provides the variation of the plane compared to the nominal slope of approach (generally 3 degrees).
This two information is provided either in the form of needles on an indicator VOR or better on a plate of road HSI , or in the form of index (bars, triangles,…) on 2 scales, one horizontal, the other vertical, located on both sides artificial horizon (traditional or EFIS).
The beams localizer and glide path being very narrow and sensitive to the disturbances, their interception must always be validated using an other source of navigation. For the localizer that can be carried out using a VOR, ADF or RNAV. For the validation of the glide path , one used one or two markers (marks out with very specific vertical emission), more and more often replaced by a DME (Distance Measuring Equipment) whose advantage is to provide information of distance uninterrupted. The DME Co-is generally established with the glide path , giving thus directly the distance to the threshold of track, which is very practical; but it happens exceptionally that he is established with the localizer .
A luminous indicator and an aural signal are activated in the passing of each markers . The information of distance DME as for it is provided on bill-poster DME.
From a point of view practices, the user posts only one frequency, that of the localizer , included/understood in range VHF 108,0 - 111,95 MHz. The frequencies glide path and DME when there exists are in bands frequency different (UHF) but paired with that of the localizer thus that remains transparent for the user.
The certified range is from 15 to 20 Nm for the localizer (30 to 50 Nm in practice); slightly less for the glide path . The DME of one THEY, less powerful than a DME on the way can nevertheless be received to 50 even 100 Nm.
Advantages of THEY:
- very high degree of accuracy
- under certain conditions (release of the critical surfaces, separations increased between planes, electric help, specific beaconing,…), allows to carry out automatic landings and thus to be posed with very low visibilities.
- sensitive to the disturbances of the electrical looms (by vehicle or plane on the ground or plane in flight)
- occasional existence of false axes per reflection of the beam on a relief
- narrow beams requiring a help for the capture.
Localizer
The localizer is consisted a whole of antennas located after the end of the track which emit a carrying VHF between 108 and 112 MHz, first odd decimal (: 111.75 MHz for track 26 with Orly for example).
It is modulated by 2 low frequencies one with 90 Hz and the other with 150 Hz. On the right of the axis of the track, the modulation factor of the 150 Hz is higher than that of the 90 Hz and conversely on the left of the axis. The difference in rate makes it possible to deduce from it a variation which is posted on the receiver of edge.
He is added a modulation to: 1020 Hz which transmits the code Morse of identification of the station generally corresponding to 2 or 3 letters of the alphabet (OLW for track 26 in Orly for example) transmitted at least 6 times per minute.
The diagram of radiation is open of approximately 35° in the horizontal plane and 7° in the vertical plan. The linear zone of guidance covers as for it only one opening maximum of +-3° around the position of axis.
This system is sensitive to the multi-ways (reflections, diffractions,…). To reduce the phenomenon the majority of THEY are bi-fréquence (two very close frequencies VHF). A frequency VHF for guidance in the axis of track (called Directing) and a frequency for the cover in the horizontal plane (called Clearance). The receiver of edge carries out the capture of the signal most extremely.
Glide Path
The glide path is consisted a whole of antennas generally located between 120 m and 150 m on with dimensions one of the track, close to the threshold, which emit a carrying UHF between 328,65 and 335,40 MHz appairée at the frequency of the localizer.
It is modulated by 2 low frequencies one with 90 Hz and the other with 150 Hz. Below the plan of descent, the modulation factor of the 150 Hz is higher than that of the 90 Hz and conversely above the plan. The difference in rate makes it possible to deduce from it a variation which is posted on the receiver of edge.
They ensure an adjustable plan of descent and generally about 3° (between 2,5 and 3,5).
The diagram of radiation is open of approximately 16° in the horizontal plane and 7° in the vertical plan.
The beam of the glide is not exploitable any more starting from 15m (50ft). For the automatic atterissage the plane is guided by information of the radiosonde and the vario.
Markers
The markers are radio beacons with vertical emission placed on the final trajectory of the Avion S which emit on 75 MHz. They are replaced little by little by DME paired at the frequency of the localizer.
Historically, to the pioneers of the instrument flying such as Gaston Génin, a radio operator operator transmitted since the ground in Morse 2 times letter Z when he intended them to fly over the ground in the fog. They knew whereas they could set in motion their procedure of approach.
The outer marker located at approximately 8 km of the threshold is modulated to 400 Hz, it lights a blue indicator in the cockpit and emits a tonality Morse of 2 features a second (- -).
The middle marker located at approximately 1 km of the threshold is modulated to 1300 Hz, it lights an orange indicator in the cockpit and emits a tonality Morse of 2 times 1 feature and 1 point (- * - *), but 1 feature and 1 point a second (- *).
The inner marker located at approximately 100 m of the threshold is modulated to 3000 Hz, it lights a white indicator in the cockpit and emits a tonality Morse of 6 points a second (*** ***). There is no inner marker established on the French aerodromes.
This kind of beacons is not any more installed in France, and is replaced by the use of DME ATT.
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