ILS and MLS working principle 1

ILS and MLS working principle


Working principle of ILS and MLS explained in brief along with sample questions for easy understanding, and to prepare for Part 2 of RTR (A) exam and also for DGCA navigation general exam



It is a very common pilot interpreted Runway Approach aid. It consists of following components.

  1. a) Localizer transmitter
  2. b) Glide Path transmitter
  3. c) Marker beacons
  4. d) Marker locators
  5. e) Airborne equipment

Principle of Operation of Components:

  1. a) Localizer Transmitter: Two overlapping signal lobes on VHF band are transmitted from an antenna located along the centre line of the Runway about 300 m from the up wind end of the runway. To an approaching aircraft, the lobe on the right is modulated by 150 hz and is called blue sector. The lobe to the left is modulated by 90 hz and is called yellow sector. The centre line of the overlapping lobes where ‘Difference in Depth of Modulation (DDM)’ is zero defines the centre line of the runway.

Operations: Aircraft approaching right of the centre line shall receive more of 150 hz modulation, which when supplied to the vertical needle of the airborne ILS meter shall indicate turn left. The approaching aircraft on the left of the centre will get turn-right indication

Glide Path (GP) Transmitter:  It is located approximately 150 m across the centre line, either to left or right, about 300 m upwind from the threshold. It transmits two similar lobes in the vertical plane in UHF band. The upper lobe is modulated by 90hz and the lower by 150 hz. The centre line of the overlapping lobes (DDM zero line) defines the glide path which is between 2º to 4º but generally glide path angle is 3º.

Operation:  Aircraft above the glide path shall receive more of 90 hz modulation which when supplied to the horizontal glide path needle of the airborne ILS meter, shall indicate ‘fly low’ or descend. The aircraft flying below the glide path shall get indication of ‘fly up’ or ascend.

For Glide Path(GP) or Glide Slope (GS)   θ = 3º

Lower limit =.45 x 3 = 1.35º

Upper limit = 1.75 x 3 = 5.25º

Total Coverage = (1.75 – 0.45)x 3 = 3.9º

Marker Beacons:  To give range indication along the centre line of the approach, provision is made for installation of three marker beacons. These beacons transmit a fan shape signal vertically upwards upto a height of 3000 ft. They operate on 75 MHz.

Outer Marker (OM):  Is placed between 3 to 6 nm, preferably at 4 nm from the touch down point. It transmits a similar 75 MHz fan shaped transmission modulated by low pitched 400 hz signal comprising of dashes, two per second, which is heard when the aircraft is above the OM. Also a blue marker light will flash at the same time.

Middle Marker (MM): Placed at approximately 3500 ft from touch down point. Transmits 75Mhz modulated by 1300 hz signal of dots and dashes. Amber light simultaneously flashes over the MM.

Inner Marker (IM):  It is optional.  Placed between 250-1500 ft from touch down point. It transmits 75 MHz with high pitch modulation of 3000 hz, a series of dots (6 per second). White flash light also comes on.

  1. Marker Locaters: Low powered NDB beacons may be placed along with OM and MM. These are called Locator Outer Marker (LOM) and Locator Middle Marker (LMM). Their frequency should not be within 15 KHz and not more than 25 KHz apart. These provide:-
  2. i) assistance in homing in and joining the ILS pattern appropriately
  3. ii) point for holding pattern

iii) a double check along with OM and MM as applicable.

  1. Airborne Equipment: Equipment in the aircraft consists of

1)   Channel Control Box

2)   VHF localizer receiver

3)   UHF glide path receiver (at the nose cone)

4)   75 MHz Marker beacon receiver

5)   3 separate aerials for above receivers

6)   ILS meter or VOR/ILS indicator

ILS Meter-Display and Interpretation:

  1.  Localizer defines the centre line of the approach to the runway. Any deviation from the centre, the vertical needle indicates the direction to turn to intercept the centre line on approach or in other words it behaves as “follow the needle”. On reciprocal i.e. out bound heading it indicates turn towards the opposite direction of the needle placement.

Accuracy:  Complete deflection of the needle occurs at 2.5ºon either side of the centre line. If 5 dots appear on either side, then each dot indicates a deviation of 0.5º in azimuth. Applying 1 in 60 rule:

0.5º means :  380  ft at 7.5nm

760  ft at 15 nm

1520 ft at 30 nm

In case of 4 dot scale, each dot means about 0.6º.

  1. b) Glide Path Indications: The glide path centre line defines the assigned approach angle. When the aircraft is on the correct glide path horizontal needle remains in the centre. Any deviation from the correct flight path is reflected by deviation of the needle in the form of “follow the meter” irrespective of the heading of the aircraft, whether in bound or outbound.

Accuracy: A full needle deflection occurs when the aircraft is 0.7ºor more above or below the correct glide slope. On 5 dot scale, each dot means 0.14º and on 4 dot scale, each dot signifies 0.175º.

This Means: at 15 nm a height of 1064 ft

at  5 nm a height of  355 ft

at  1 nm a height of   71 ft

Half of full scale deflection is to be considered  maximum safe deviation below the glide path. For any position of the needle indicating more than half fly up position, immediate climb must be initiated.

  1. Failure Warning Flags: Two flags, one each for localizer and glide path,

are provided in the meter. When the warning flag appears on the window, the needle returns to the centre indicating:

  1. i) Ground or airborne equipment has failed or is switched off
  2. ii) Signals received are too weak to be used gainfully or aircraft is out of range of lobe patterns of the ILS
  1. Frequency:
  2. Localizer: In the VHF band 108.1 to 112.0 MHz at odd decimals with 50KHz separation i.e. 108.1, 108.15, 108.3, 108.35 etc.
  3. Glide Path: In the UHF band at 20 spot frequencies from 329.3, to 335 MHz at 300 KHz spacing.
  4. The localizer and glide path frequencies are so paired that selecting the localizer frequency on the airborne equipment, automatically selects the corresponding glide path frequency.

NOTE:  108.1-112 MHz is primarily allotted as ILS frequency. ICAO prescribes that this frequency may shared with VOR if not fully subscribed. Odd decimals for ILS and even decimals for VOR.

Type of emission:  A8W for Localizer, Glide path and Marker

Identification: Identification is provided on Localizer frequency. Carrier frequency is AM by horizontally polarized 1020 Hz tone, comprising of 2 to 3 Morse letters at 7 words/min. Ground to air voice communications may be conducted on ILS Category I and II.

  1. Categories of ILS: Operational Performance Category

Category I      Operation down to decision height of 60m (200ft)  with RVR 550 meters

Category II     Operation down to decision height between 200 ft to 30m (100ft) and RVR 300 m

Category III A  Operation down to surface of Runway with RVR 200 m

Category III B  Operation down to surface of Runway and taxiways with RVR 50 m.

Category III C  Operation down to surface of the Runway and taxiway without external visual reference.

RVR means Runway Visual Range or the visibility measured near the surface of the runway by transmissometer.


Back Beam: Not by spillage but by design, some localizer transmitters radiate back beam which is utilized for overshooting or reciprocal approach. GP indication is not available. Localizer indication is less accurate and needle indication opposite.

ROD Calculations:          θ x G/S(kt)

ROD=  ——-      x 100


Given : θ =3.0º, G/S = 120k

3 x 120

ROD=   ——– x 100 = 600 ft/min


False Glide Path:  Twin lobes in vertical plane are radiated several times above the true lobes giving several equi-signal lines.  These lines are much above the normal Glide Path slope, the lowest being at about 6º and do not pose a threat to the precision approach.


  1. A glide slope indicator shows 2 dot fly down indication on a 5 dot indicator at a distance of 3.6 Nm from ILS threshold. Approx how many feet is the aircraft above or below glide slope.
  1. θ = S/R x 60 or 0.28 =(S x 60)/(3.6 x 6080) ft or S = 102 ft above the glide path.

 At a distance of 2.7 Nm a 2 dot fly up indication on 4 dot indicator appears. How many metres is the aircraft above or below the glide path.

 θ = S/R x 60, θ =2 x 0.175, R =2.2 Nm = 4074.4 m Hence S = (4074 x 2 x 0.175)/60 = 23.76 m


  1. At a distance of 2.4 Nm an ILS/DME shows 3 dot fly down indication on a five dot indicator, over a spot height of 150 m on final approach. Find clearance of aircraft while crossing the spot height.


  1. θ =3x 0.14 = 0.42 + 3 deg (GS) = 3.42 R =2.4 Nm = 4444.8 m Hence S=( 4444.8 x 3.42)/60 = 253.35, Ht of hill =150 m Hence clearance = 253.5-150 = 103.5 m


  1. At a distance of 4 Nm localizer shows 3 dot fly right (4 dot ind) and glide slope shows 2 dot fly up indications. Find (a)  How many metres the aircraft is right/left of localiser  (b) How many metres the aircraft is above/below Glide Slope   (c) If GS of aircraft is 120K. Find rate of descent  (d) Runway elevation is 330 ft, what will be the altimeter reading if set at QNH/QFE.


  1. (a) θ =3x 0.625 = 1.875, R = 4 Nm = 7408 m, S= (7408 x 1.875)/60 = 231.5 m

(b)  θ =2.65 , R = 4 Nm = 24320 ft, S= (24320 x 2.65)/60 =1074 ft. For 3 deg GS Ht=1216 ft     (substitute value of θ = 3), Hence 1216-1074 = 142 ft = 43.28 m below Glide Slope.

(c) θ =2.65 , R = 120 K= 2Kts/min =2 x 6080 ft/min S= (2 x 6080 x 2.65)/60 =537 ft/min

(d)  Altitude = 1074 + 300 =1404 ft with QNH/ 1074 with QFE

 At a distance of 3.6 Nm glide slope shows 2 dot fly down indications with GS of 160 Kts. Find (a) How many ft above/below glide slope  (b) rate of descent  (c) Runway elevation is 270 ft find altimeter reading  when QNH/QFE set.


  1. (a) θ =2 x 0.175, R =3.6 Nm = 21888 ft Hence S = (21888 x 2 x 0.175)/60 = 127.68 ft above.

(b)  θ =3.35 , R =3.6 Nm = 21888 ft Hence S = (21888 x 3.35)/60 =1222 ft

(c)  θ =3.35, R = 160 K= 2.67Kts/min =2.67 x 6080 ft/min S= (3.35 x 6080 x 2.67)/60 =906 ft/min

(d) Altimeter with QNH = 270 + 1222 = 1492 ft, with QFE =1222 ft

 THUMB RULE:  ROD in ft/min = Ground Speed (Kts) x 5 for 3° Glide Slope. Remember ROD is a function of Ground Speed, if GS is more ROD will be more.


MICROWAVE Landing System (MLS)

General:  Pattern for the ILS approach flown by large-fixed wing aircraft are not suitable for helicopters and Short Take-off and Landing (STOL) aircraft.  To overcome the difficulties, a more usable system edited Microwave Landing System is being introduced by ICAO to be the primary approach and landing aid .

Principle: In the MLS, approaches may be made anywhere within its horizontal and vertical fan shaped coverage area unlike ILS where fixed localizer and glide path along the extended centre line of the approach are used.

  1. Azimuth Transmitter (like localizer)_: It provides a fan-shaped horizontal approach zone, usually ± 40º of runway centre line.
  1. Elevation Transmitter (like Glidepath)_: It produces a fan shaped vertical approach zone usually ranging from 0.9ºto 20º, but because of aircraft handling, the upper portion of 20º is not practically usable.
  1. Ranging: DME facility (like Marker beacons) is provided as also sometimes a back azimuth facility.
  1.  Operation: With MLS, a time reference scan beam (TRSB) system is used to determine aircraft’s position. Its transmits narrow beams which sweep to and fro through 80º in azimuth and 19º elevation. The airborne equipment measures the time interval between the sweeps to determine position and pilot can select an appropriate approach path. The receiver then creates an ILS like localizer for the chosen approach.

This way a helicopter can make an approach from say 35º to runway and 6º in elevation immediately after a big jet has utilised 0º and 3º azimuth approach.

  1. Frequency: The equipment works an gigahertz (or radar) frequency, therefore eliminates ground effects. It operates on 200 channels and its ground equipment occupies much lesser space.
  1. Advantages: Compared to ILS, MLS has following advantages:
  1. Extremely good guidance capacity
  2.  Insensitivity to geographical site, which makes it possible to be used where ILS can not be installed.

iii) Extensive operational capability

  1. iv) Very wide 3 – dimensional coverage allowing curved flight path and final approaches on different glide slopes.
  2. v) Better means of controlling expediting aircraft movements in the control areas.
  3. Which of the following is an error of MLS (a) Interference from vehicles or aircraft close to the transmitter.  (b)  Siting Error  (c) False glide path at approximately twice the angle of the correct one.  (d) Scalloping.   A. (a)

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  • Arunaksha Nandy Post author

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