8.4 Instrument Landing System (ILS) Ground Components
Key Takeaways
- The Instrument Landing System (ILS) comprises four integrated sub-elements: Localizer (lateral guidance), Glide Slope (vertical guidance), Marker Beacons / DME (range fixes), and Approach Lighting Systems (visual transition).
- The localizer operates on VHF frequencies from 108.10 to 111.95 MHz (odd tenths) and is 4 times more sensitive than a VOR, featuring a full-scale deflection of 2.5° (0.5° per dot) and producing a 700-foot course width at the threshold.
- The glide slope operates on paired UHF frequencies (329.15 to 335.00 MHz) with a standard 3.0° descent angle and a narrow 1.4° total beam width; pilots must capture from below to avoid false harmonic glide slopes (e.g., 6° or 9°).
- Marker beacons broadcast directional 75 MHz cones: Outer Marker (OM, blue light, 400 Hz low dashes at FAF/GS intercept 4–7 NM out), Middle Marker (MM, amber light, 1300 Hz dot-dash at Cat I DA ~200 ft AGL), and Inner Marker (IM, white light, 3000 Hz dots at Cat II DH ~100 ft AGL).
- Under 14 CFR 91.175, pilots may descend below DA/MDA to 100 ft above TDZE using the approach lighting system, but cannot descend below 100 ft without red terminating bars (ALSF-1) or red side row bars (ALSF-2) in sight, or the runway threshold/touchdown zone.
Instrument Landing System (ILS) Ground Components
Quick Answer: The Instrument Landing System (ILS) is a precision approach system consisting of four sub-elements: (1) Localizer (LOC) operating on 108.10–111.95 MHz (odd tenths) providing lateral guidance with a 3° to 6° beam width (4× more sensitive than VOR, full deflection $= 2.5°$, $0.5°$ per dot); (2) Glide Slope (GS) operating on paired UHF channels (329.15–335.00 MHz) providing a 3.0° descent path ($1.4°$ total width); (3) Marker Beacons / DME on 75 MHz for range fixes (Outer Marker = Blue/400 Hz dashes at FAF; Middle Marker = Amber/1300 Hz dot-dashes at Cat I DA; Inner Marker = White/3000 Hz dots at Cat II DH); and (4) Approach Lighting Systems (ALS) enabling visual transition. Under 14 CFR 91.175, descending below 100 ft above TDZE requires the red terminating bars (ALSF-1) or red side row bars (ALSF-2) in sight, or the runway threshold/touchdown zone.
The Instrument Landing System (ILS) is the premier terrestrial precision approach system in worldwide aviation. By combining VHF lateral guidance, UHF vertical guidance, range markers, and approach lighting, the ILS allows aircraft to conduct stabilized instrument descents to decision altitudes as low as 200 feet above ground level (Cat I) or to the surface (Cat IIIc).
ILS Architecture: The Four Core Subsystems
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| ILS SYSTEM ARCHITECTURE |
| |
| 1. LATERAL GUIDANCE: VHF Localizer (LOC) |
| - 108.10 to 111.95 MHz (odd tenths) |
| - Aligned with runway centerline |
| |
| 2. VERTICAL GUIDANCE: UHF Glide Slope (GS) |
| - 329.15 to 335.00 MHz (automatically |
| paired with localizer VHF channel) |
| - Standard 3.0° descent profile |
| |
| 3. RANGE / DISTANCE: Marker Beacons (75 MHz) or DME |
| - Outer Marker (OM) / FAF |
| - Middle Marker (MM) / Cat I DA |
| - Inner Marker (IM) / Cat II DH |
| |
| 4. VISUAL TRANSITION: Approach Lighting System (ALS) |
| - ALSF-1/2, MALSR, SSALR, PAPI/VASI |
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1. The Localizer (LOC): Lateral Guidance
The localizer antenna array is situated approximately 1,000 feet beyond the far departure end of the instrument runway. It transmits two overlapping directional radio lobes modulated at different audio frequencies:
[ LOCALIZER RADIATION PATTERN ]
Runaway Centerline
^
90 Hz Amplitude Lobe | 150 Hz Amplitude Lobe
(Predominates on Left) | (Predominates on Right)
\ | /
\ DDM = 0 /
\ (On Course)/
\ | /
\ | /
===============================[ LOC ANTENNA ]==============================
(Departure End)
Signal Modulation & DDM Principle
- Left of Course (Fly-Right indication): The receiver detects a stronger 90 Hz modulation signal.
- Right of Course (Fly-Left indication): The receiver detects a stronger 150 Hz modulation signal.
- On Course Centerline: The Difference in Depth of Modulation (DDM) is zero ($90\text{ Hz} = 150\text{ Hz}$), causing the vertical needle to center perfectly.
Localizer Beam Width & Sensitivity
- Linear Width at Threshold: The beam is calibrated to produce a linear course width of 700 feet at the runway threshold.
- Angular Width: Tailored based on runway length to between 3° and 6° (nominally 5° total width, or 2.5° on each side of center).
- Sensitivity Comparison: A standard VOR has a full-scale deflection of 10° (2° per dot). A localizer has a full-scale deflection of 2.5° (0.5° per dot)—making the localizer four times more sensitive than a VOR.
Localizer Standard Service Volume (SSV)
LOCALIZER SERVICE VOLUME
35° 35°
\ /
\ /
10 NM \ / 10 NM
+\------/+
10°\ / 10°
\ /
18 NM \/
|
[ RUNWAY ]
- $\pm 35°$ azimuth: Usable out to 10 NM from the antenna.
- $\pm 10°$ azimuth: Usable out to 18 NM from the antenna.
- Vertical coverage: From 1,000 ft above highest obstacle / terrain up to 4,500 ft AGL.
Localizer Back Course & Reverse Sensing
The localizer transmitter radiates energy out the back end of the antenna array down the reciprocal runway heading, creating a Back Course (LOC BC). On a conventional CDI:
- Flying the Front Course Inbound $\rightarrow$ Standard sensing ("fly toward the needle").
- Flying the Back Course Inbound $\rightarrow$ Reverse sensing (needle moves opposite to correction).
- Rule for conventional CDI on Back Course: To correct, fly away from the needle, OR set the front course on an HSI to receive normal sensing.
2. The Glide Slope (GS): Vertical Guidance
The glide slope antenna is located approximately 750 to 1,250 feet down the runway from the approach threshold and offset 250 to 650 feet from the runway centerline. It transmits overlapping 90 Hz and 150 Hz radiation patterns vertically:
[ GLIDE SLOPE BEAM GEOMETRY ]
/ Upper Lobe (90 Hz)
/ Fly DOWN
3.7° /
-----------------------+ (0.7° Above Path)
/ 3.0° Standard Angle /
/----------------------+ On Glide Path (DDM = 0)
/ 2.3° /
/----------------------+ (0.7° Below Path)
/ \ Lower Lobe (150 Hz)
/ \ Fly UP
===[ GS ANTENNA ]====================================================
Technical Parameters of the Glide Slope
- Frequency Band: UHF band (329.15 MHz to 335.00 MHz, 40 paired channels).
- Standard Glide Path Angle: 3.0° above the horizontal plane.
- Beam Width: 1.4° total ($0.7°$ above and $0.7°$ below the 3.0° glide path).
- Full Scale Vertical Deflection: Only 0.7° ($0.14°$ per dot on a 5-dot vertical scale).
- Service Volume: Usable out to 10 NM from the transmitter down to the runway threshold.
Standard Rate of Descent Calculations
To maintain a 3.0° glide slope, the required rate of descent in feet per minute (fpm) is calculated from groundspeed:
| Groundspeed (kts) | Rule of Thumb (fpm) | Exact 3.0° Descent Rate (fpm) |
|---|---|---|
| 90 kts | 450 fpm | 477 fpm |
| 100 kts | 500 fpm | 530 fpm |
| 120 kts | 600 fpm | 636 fpm |
| 140 kts | 700 fpm | 742 fpm |
| 160 kts | 800 fpm | 848 fpm |
False Glide Slopes & Capture Geometry
Ground reflections create harmonic lobes above the true glide path at multiples of the standard angle—most notably at approximately 6° and 9°:
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| FALSE GLIDE SLOPE CHARACTERISTICS |
| |
| - Located at higher angles (e.g., 6.0° and 9.0° above horizontal). |
| - Exhibit REVERSED SENSING (90 Hz lower, 150 Hz upper). |
| - Require an abnormally steep, dangerous descent rate (> 1,200 fpm). |
| |
| DEFENSIVE PROCEDURE: |
| - ALWAYS intercept the glide slope from BELOW at the published |
| Glide Slope Intercept Altitude (indicated by the lightning bolt |
| symbol on the FAA approach chart). |
| - Verify altimeter cross-check at the Outer Marker / FAF. |
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3. Marker Beacons, Compass Locators & DME Fixes
Marker beacons operate on a single universal carrier frequency of 75.00 MHz, projecting an elliptical cone of radio energy vertically upward through the approach path.
| Marker Beacon | Typical Location | Audio Tone & Morse Modulation | Cockpit Light | Associated Fix / Purpose |
|---|---|---|---|---|
| Outer Marker (OM) | 4 to 7 NM from runway threshold | 400 Hz low-pitch tone<br/>Continuous dashes (- - -) at 2/sec | BLUE | Final Approach Fix (FAF); Glide Slope Intercept Altitude check |
| Middle Marker (MM) | ~3,500 ft from threshold (~0.6 NM) | 1300 Hz intermediate tone<br/>Alternating dot-dash (. - . -) | AMBER | Category I Decision Altitude (DA); ~200 ft above TDZE |
| Inner Marker (IM) | Between MM and threshold | 3000 Hz high-pitch tone<br/>Continuous dots (. . . .) at 6/sec | WHITE | Category II Decision Height (DH); ~100 ft above TDZE |
Outer Marker (OM) Middle Marker (MM) Inner Marker (IM)
[ BLUE ] [ AMBER ] [ WHITE ]
400 Hz 1300 Hz 3000 Hz
( - - - ) ( . - . - ) ( . . . . )
| | |
| | |
4 to 7 NM ~3,500 ft ~1,000 ft [ RUNWAY ]
Compass Locators & DME Substitution
- Compass Locators: Low-power Non-Directional Beacons (NDBs) co-located with marker beacons. An NDB at the Outer Marker is a LOM (Locator Outer Marker) and transmits the first two letters of the localizer ID; an NDB at the Middle Marker is a LMM (Locator Middle Marker) and transmits the last two letters.
- DME / Collocated Systems: Where authorized by the approach plate, DME fixes or IFR-approved GPS waypoints may substitute for the Outer Marker.
4. Approach Lighting Systems (ALS) & 14 CFR 91.175
Approach Lighting Systems provide the critical visual transition from cockpit instrument scan to external runway environment recognition during the final seconds of an instrument approach.
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| COMMON APPROACH LIGHT CONFIGURATIONS |
| |
| - ALSF-1 / ALSF-2: High Intensity Approach Light System with |
| Sequenced Flashers (ALSF-2 features RED side row bars). |
| - SSALR / SSALF: Simplified Short Approach Light System with Runway |
| Alignment Indicator Lights / Sequenced Flashers. |
| - MALSR / MALSF: Medium Intensity Approach Light System with Runway |
| Alignment Indicator Lights. |
| - ODALS: Omni-Directional Approach Lighting System. |
| - REIL: Runway End Identifier Lights (synchronized flashing strobes). |
| - PAPI / VASI: Precision Approach Path / Visual Approach Slope Ind. |
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Visual Descent Rules (14 CFR § 91.175)
To descend below the published Decision Altitude (DA) or Minimum Descent Altitude (MDA), three conditions must be met simultaneously:
- The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers.
- The flight visibility is not less than the visibility prescribed in the standard instrument approach procedure being used.
- At least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot:
- The approach light system (allows descent to 100 feet above Touchdown Zone Elevation (TDZE), but NOT below 100 ft unless the red terminating bars (ALSF-1) or red side row bars (ALSF-2) are distinctly visible).
- The threshold, threshold markings, or threshold lights.
- The runway end identifier lights (REIL).
- The visual glideslope indicator (VASI / PAPI).
- The touchdown zone or touchdown zone markings / lights.
- The runway or runway markings / lights.
How does the angular course width and CDI sensitivity of an ILS localizer compare to a standard VOR course?
What audio tone, keying rate, and cockpit annunciator light color are produced when flying over the ILS Outer Marker (OM)?
Under 14 CFR § 91.175, when using the approach light system as the sole visual reference upon reaching Decision Altitude (DA) on an ILS approach, to what altitude may the pilot descend before requiring additional visual references?