ILS, GP & GLS

062 Radio NavigationChapter 4

Schematic

LOC provides azimuth; G/S typically 3 degrees; markers OM, MM, IM.

ILS characteristics

Key fact

Emission A8W, horizontally polarised, VHF 108–112 MHz (odd tenths) — e.g. 108.1–111.95 MHz. 3-letter Morse ident. Principle: Difference in Depth of Modulation (DDM). ILS produces a radiation pattern that is amplitude modulated.

Component	Band	Position
Localiser	VHF	300 m beyond runway end
Glideslope	UHF 329–335 MHz	Abeam TDZ, 300 m from threshold, offset 120 m from centreline
Markers	75 MHz	Outer, middle, inner

Key fact

LARD: Left and Above = 90 Hz; Right and Down = 150 Hz. Equal DDM = centreline. DDM doubles as angular displacement doubles.

Localiser and glideslope coverage

Element	Coverage
Localiser	25 nm within 10°; 17 nm within 35°; approach azimuth 40° either side to 20 nm
Glideslope	10 nm, 8° either side of centreline; captured normally between 3–10 nm
Elevation	150 ft to 20,000 ft

Key fact

Glideslope vertical coverage: bottom at 0.45 × G/S angle, top at 1.75 × G/S angle. Example 3° G/S: vertical coverage 1.35° to 5.25° over 10 nm, 8° either side.

CDI scale deflection

Element	Per dot	Full scale
Localiser	0.5°	2.5°
Glideslope	0.14°	0.7°

Key fact

Once established, maximum ½ scale deflection. Within half-scale = established. Localiser needle indicates deviation in correct sense regardless of OBS/course selector — measures DDM, not phase difference. Good practice to set runway QDM.

ILS markers

Marker	Modulation	Distance	Colour	Morse
Outer (OM)	400 Hz	4 nm	Blue	Dashes
Middle (MM)	1,300 Hz	0.6 nm	Amber	Dots and dashes
Inner (IM)	3,000 Hz	0.04–0.24 nm	White	Dots

Key fact

Middle marker marks DH for missed CAT I approach. Markers transmit at 75 MHz (metric wavelengths).

ILS categories

CAT	DH	Guidance	RVR
I	200 ft	200 ft	550 m
II	100 ft	50 ft	300 m
IIIA	100 ft or none	To runway	200 m
IIIB	50 ft or none	To runway	75 m (fail-operational rollout)
IIIC	None	To runway	No limit

Key fact

CAT II and III require autopilots. Backbeam approaches are non-precision — no glideslope, less accuracy, no markers, needle sense reversed. Backbeam can provide centreline guidance after take-off or go-around.

Failure and monitoring

Key fact

Localiser fail = NAV flag. Glideslope fail = G/S flag. Warning flag when either 90 or 150 Hz signal is lost or percentage modulation of either reduces to zero. ILS monitor: second transmitter brought online; if none — transmissions stopped, ident removed, maintenance alerted.

False glideslope and bending

Key fact

Lower 150 Hz lobe sometimes reflected from ground — false glideslope above the correct path. Approach always made from below glideslope to avoid intercepting the false one. Bending of localiser and glideslope from other aircraft/vehicles — Critical Area for CAT I; Sensitive Area for CAT II/III.

Formula — Rate of descent on glideslope

ROD = (Glideslope × GS × 100) ÷ 60
ROD ≈ 5 × GS   (for 3° glide slope)

Formula — Glideslope height

G/S Height = G/S Angle × Range × 100
1-in-60: (Glide slope × distance × 6080) ÷ 60

Key fact

FM immunity: FM transmission near 108 MHz may interfere with ILS LOC and G/S — modern aircraft fitted with FM filters. Immune filter makes localiser (not glideslope) less susceptible.

MLS (Microwave Landing System)

Key fact

Developed to overcome ILS shortcomings (expensive, beam bending, poor runway occupancy during LVPs) but largely superseded by GPS. SHF 5,031–5,091 MHz, 200 channels (500–699). Azimuth and elevation use same frequency; DME on paired frequency. Principle: Time Reference Scanning Beam (TRSB) — difference between TO and FROM scan determines position.

MLS feature	Detail
Glidepaths	Pilot selects any path 0.1°–15° (no fixed glidepath)
Approach paths	May vary 60° or more from runway direction
Accuracy (95%)	Laterally ± 50 ft; vertically ± 12 ft at 200 ft above datum
DME-P	Co-located; failure requires straight-in approach
Advantages	Simpler install, not terrain-sensitive, immune to scalloping/vehicles, no false glidepaths

Exam trap

Back course: align CDI/HSI needle to ILS inbound course — OBI reverses sensing; HSI correct with front course set. Sensitive area for vehicles/aircraft active during CAT I/II operations. CDI glideslope: horizontal bar above centreline — decrease descent rate / pitch up to recapture. MLS with no DME = ILS capabilities only.