Glide Slope
The ground-based radio transmitter that provides precise vertical guidance to aircraft during the final approach phase of an Instrument Landing System (ILS) approach. The glide slope generates a signal that defines the optimal descent path to the runway, typically at a 3° angle, enabling pilots to maintain the correct vertical profile for a safe landing.
Image disclaimer: For identification and illustration only. It may show a product by another manufacturer and may not depict the exact item described; it is not a statement of any product’s quality, approval or performance. Third-party image and trademark rights belong to their owners. See full notice.
What Is It?
The Glide Slope (also known as the Glide Path) is a ground-based radio navigation system that forms the vertical guidance component of the Instrument Landing System (ILS). It consists of an antenna array and transmitter located to the side of the runway, typically 230 to 380 metres (750–1,250 ft) from the threshold, offset from the runway centreline by roughly 120 to 200 metres (390–660 ft). The glide slope transmits a UHF signal (between 328.6 MHz and 335.4 MHz) that creates two overlapping lobes: one modulated at 90 Hz (above the glide path) and one at 150 Hz (below the glide path). The aircraft's receiver compares the modulation depths to determine whether it is above or below the ideal descent angle (typically 3°), providing the pilot with a deviation indication on the Instrument Landing System indicator (CDI).
Purpose & Function
The glide slope's purpose is to keep an aircraft descending along one specific, obstacle-clearing angle from the point it intercepts the beam all the way to the touchdown zone, rather than leaving vertical profile to the pilot's judgement of sink rate and distance. Because the two overlapping lobes only produce a true on-path indication when the aircraft is exactly on the designed 3° angle, any deviation above or below immediately shows up as an imbalance in modulation depth that the pilot can correct before it develops into a significant vertical error. This precision matters most in the final stages of an approach in reduced visibility, where the pilot has little or no outside visual reference to judge descent rate independently, making the glide slope's guidance the primary means of staying on a safe vertical path to the runway.
Where Is It Used?
The Glide Slope is installed on precision approach runways that support ILS operations. It is a critical component for:
- CAT I Precision Approaches: Providing vertical guidance down to a decision height of 200 feet (60 metres) with RVR of 550 metres or greater.
- CAT II Precision Approaches: Providing vertical guidance down to a decision height of 100 feet (30 metres) with RVR of 350 metres or greater.
- CAT III Precision Approaches: Providing vertical guidance down to a decision height as low as 15 feet (5 metres) with RVR as low as 200 metres (CAT IIIA), 50 metres (CAT IIIB), or even zero (CAT IIIC).
- Military Airfields: Supporting military aircraft ILS operations.
- Civil Airports: Supporting commercial and general aviation ILS approaches.
Installation & Location
The glide slope antenna array is sited beside the runway, offset laterally roughly 120-180 m from the centreline, positioned abeam the desired touchdown point so the projected glide path crosses the threshold at the specified height (typically 50 ft / 15 m). The antenna mast and equipment shelter sit within the runway's obstacle-free zone requirements, and the surrounding ground is graded flat and kept clear of vehicles, snow banks, or structures that could reflect the signal and distort the glide path. The transmitter and monitoring electronics are housed in the shelter at the base of the mast, fed from dual power supplies with automatic changeover, and connected by cable to a remote status and control panel in the ATC tower or navaid maintenance facility.
How Does It Work?
The Glide Slope operates on the principle of amplitude modulation and spatial signal distribution:
- Signal Generation: The glide slope transmitter generates an RF carrier at the assigned UHF frequency. Two modulating signals (90 Hz and 150 Hz) are applied to the carrier.
- Antenna Array: A directional antenna array (typically an end‑fire or broadside array) creates a pattern with two overlapping lobes. The 90 Hz signal is predominant above the glide path, while the 150 Hz signal is predominant below the glide path.
- Glide Path Angle: The glide path angle is typically set to 3° above the horizontal, but can be adjusted (e.g., 2.5° to 4°) based on terrain and obstacle clearance requirements.
- Aircraft Reception: The aircraft's ILS receiver measures the difference in modulation depth between the two signals. The difference is displayed as a vertical deviation indication on the Course Deviation Indicator (CDI), showing the pilot whether the aircraft is above or below the glide path.
- Coverage: The glide slope provides vertical guidance typically from the threshold out to a range of 10 to 18 nautical miles (18 to 33 km), depending on the configuration.
System Architecture & Control Logic
Like the localizer it works alongside, the glide slope facility is not remotely commanded in real time for its RF output — the control logic is a self-monitoring and protection loop. Integral near-field monitors sample the radiated signal directly at the antenna and check course angle, displacement sensitivity and RF power against tolerance, with some installations adding a far-field monitor for independent confirmation. Dual-equipment sites run online and standby transmitters, and the monitor logic executes an automatic changeover to standby on an out-of-tolerance reading; if neither channel can meet tolerance, the facility shuts itself down and radiates no signal rather than risk erroneous vertical guidance. Status, alarms and changeover events are reported to the airport's remote control and monitoring system for ATC and maintenance visibility.
Transmitter → Antenna → Integral Monitor → Changeover Logic → Remote Monitoring
How the Glide Slope Guides the Aircraft Vertically
Antenna Array Radiates Two Lobes
Overlapping RF lobes carrying 90 Hz and 150 Hz modulation
90 Hz Predominant
Upper lobe stronger above the 3° reference angle
150 Hz Predominant
Lower lobe stronger below the 3° reference angle
Aircraft Receiver Compares Modulation Depth
Measures the difference between the 90 Hz and 150 Hz signals
CDI Vertical Deviation Displayed
Equal depths = on path — needle centred on the 3° glide angle
Physical Construction
The glide slope antenna array, typically three to five end-fire or broadside radiating elements, is mounted on a mast alongside the runway at a carefully surveyed offset and height to establish the correct glide path angle, with each element's phasing and power distribution fed from a weatherproof equipment hut at the mast base through low-loss coaxial cable. The equipment hut houses the dual redundant UHF transmitters, modulation electronics generating the 90 Hz/150 Hz course signals, and continuous monitoring equipment, all mounted in a temperature-controlled, weatherproof cabinet similar to other ILS ground equipment. Mast structures are engineered to resist wind loading and are fitted with lightning protection and obstruction lighting per airfield safety requirements, and buried or ducted feeder runs between the hut and antenna array are sealed against moisture ingress. Automatic monitoring continuously checks course alignment, modulation depth, and power output, triggering standby transmitter switchover and remote alarms if a parameter drifts out of tolerance.
Technical Parameters
| Parameter | Typical Value |
|---|---|
| Frequency Range | 328.6 MHz – 335.4 MHz (UHF) |
| Modulation Frequencies | 90 Hz (above path) and 150 Hz (below path) |
| Antenna Type | End-fire or broadside array (typically 3 to 5 elements) |
| Glide Path Angle | Typically 3° (adjustable from 2.5° to 4°) |
| Coverage | ± 2° vertically, up to 18 NM (33 km) range |
| Transmitter Power | 5W – 50W (depending on configuration) |
| Glide Slope Types | Standard, Wide‑aperture (for CAT II/III) |
| Standby System | Dual redundant (hot standby) |
| Monitoring | Continuous self-monitoring with alarm triggering on failure |
| Power Supply | Mains with UPS and generator backup |
| Protection Class | IP65 / IP67 (weatherproof for outdoor installation) |
Electrical Requirements
| Requirement | Typical Value |
|---|---|
| Primary Supply | Dual power feeds with automatic changeover to the transmitter shelter, backed by UPS and standby generator |
| Transmitter Power Draw | Sized to 5W–50W RF output plus transmitter, monitor, and standby-channel overheads |
| Cabling | Dedicated feeder from the airfield distribution point to the shelter, sized to the equipment's rated load |
| Circuit Protection | Dedicated circuit breaker/isolator at the shelter distribution board |
| Surge Protection | SPDs fitted at the power and antenna feed entries into the shelter |
| Insulation Resistance | ≥ 50 MΩ (megger test, cable-to-ground, at commissioning) |
| Earthing/Grounding | Shelter, antenna array, and equipment racks bonded to the airfield earth grid |
| Fault Protection | Continuous self-monitoring with alarm reporting to the remote status/control panel on power or transmitter fault |
Applicable Standards
ICAO Annex 10
Glide slope frequency, modulation, and performance requirements
FAA Order 6750.16
Siting criteria for Instrument Landing Systems, including glide slope antenna placement
EUROCAE ED-82
Glide slope performance and monitoring standards
RTCA DO-192
Glide slope airborne receiver standards
Installation Requirements
Lateral Offset & Touchdown Survey
SiteSet the antenna array 120–180 m laterally from the runway centreline, abeam the desired touchdown point, so the projected glide path crosses the threshold at the specified height.
Mast & Shelter Foundation
CivilInstall the antenna mast and equipment shelter foundation within the runway's obstacle-free zone requirements.
Ground Grading & Reflection Clearance
CivilGrade the surrounding ground flat and keep it clear of vehicles, snow banks, or structures that could reflect the signal and distort the glide path.
Dual Power Supply with Auto Changeover
ElectricalConnect dual power feeds with automatic changeover to the transmitter shelter.
Remote Status/Control Cabling
ElectricalRun the cable link from the shelter to the remote status and control panel in the ATC tower or navaid maintenance facility.
Transmitter & Monitoring Equipment Fit-Out
MechanicalInstall the transmitter and monitoring electronics in the shelter at the base of the mast.
Obstacle-Free Zone Verification
DocumentationConfirm the site meets the obstacle-free zone requirements before commissioning.
Pre-Energisation Sign-off
DocumentationRecord mast position, offset survey, and cable routes as-built before the shelter is closed up.
Testing & Commissioning
Antenna Phasing & Power Distribution Check
Pre-EnergisationVerify each radiating element receives the correct phase and power split from the feed network before the array is energised.
Course Alignment & Modulation Depth Test
CommissioningMeasure the 90 Hz/150 Hz modulation depth and confirm the on-path course aligns with the design glide-path angle.
Glide Path Angle Calibration
CommissioningFine-tune transmitter and antenna settings to set the glide path angle precisely to the design value (typically 3°), verified against a calibrated reference.
Standby Transmitter Switchover Test
CommissioningSimulate a fault on the primary transmitter and confirm automatic, glitch-free switchover to the standby chain with a remote alarm raised.
Monitor & Alarm Function Test
CommissioningVerify the continuous monitoring equipment correctly detects out-of-tolerance course alignment, modulation, or power and raises the appropriate alarm.
Flight Check Verification
Final AcceptanceVerify glide path angle, course structure, and coverage using a calibrated flight-check aircraft before the facility is released for operational use.
Documentation & As-Built Sign-off
Final AcceptanceRecord phasing, modulation, monitor test, and flight-check results in the commissioning dossier for handover.
Inspection & Maintenance
Antenna Array Alignment Check
Annual, per flight check scheduleVerify antenna array alignment through a combination of ground checks and scheduled flight checks.
Site & Terrain Survey
Periodic, or after site changesReassess the site for new terrain features or obstacles that could interfere with the glide path signal.
Modulation Balance Check
Semi-annualVerify modulation balance remains within tolerance to prevent glide path angle errors.
Power Amplifier Performance Check
AnnualTest power amplifier output against baseline to catch degradation before it affects signal coverage.
Common Field Issues
Antenna Array Misalignment
Physical displacement or misalignment of the glide slope antenna array can cause the glide path angle to shift, resulting in a false vertical guidance indication. Regular alignment checks using specialized survey equipment and calibration aircraft are essential.
Terrain & Obstacle Interference
Reflections from nearby terrain, buildings, or vehicles can cause multipath interference, distorting the glide slope signal and creating false indications. Proper site selection and obstruction clearance are critical.
Modulation Balance Drift
Over time, the modulation balance between the 90 Hz and 150 Hz signals can drift, causing the glide path to shift. Regular monitoring and calibration using flight inspection is required to maintain accuracy.
Power Amplifier Failure
The transmitter power amplifiers are subject to wear and thermal stress. Regular maintenance and redundancy (hot standby) are essential to prevent loss of the glide slope service.
Troubleshooting Guide
Flight Inspection Reports Glide Path Angle Shift
Course Roughness or Bends Reported by Aircraft
Glide Path Indication Drifting Off Nominal
Transmitter Trips to Standby or Off-Air
Failure Modes & Root Causes
Antenna Array Displacement
Multipath Interference
Modulation Balance Drift
Power Amplifier Thermal Failure
Safety & Operational Notes
RF Exposure Around the Antenna Array
Personnel SafetyFollow the facility's RF safety procedure and any posted exclusion distance when working near the radiating antenna array, and confirm the transmitter is placed in maintenance/standby mode before working at or near the mast.
Transmitter Shelter Isolation
Electrical SafetyDe-energise and lock out the transmitter before working inside the equipment shelter, and verify zero energy before disturbing internal connections; RF power amplifier stages can carry hazardous voltages that remain present briefly after shutdown.
Mast Work at Height
Personnel SafetyUse appropriate fall protection and follow mast climbing procedures when accessing antenna elements, and confirm the feeder is disconnected from the transmitter before disturbing antenna connections.
Keep the Critical Area Clear
OperationalKeep vehicles, personnel, and equipment out of the glide slope critical area during low-visibility operations or whenever the facility is radiating for an aircraft on approach, since reflections from an obstruction there can distort the signal in flight.
NOTAM for Service Interruption
OperationalIssue a NOTAM before taking the glide slope out of service or below tolerance for maintenance, since aircraft on an ILS approach rely on it for vertical guidance.
Verify Monitor Function Before Return to Service
ComplianceConfirm the near-field (and far-field, where fitted) monitor correctly detects an out-of-tolerance angle, sensitivity, or power reading and executes changeover or shutdown before returning the facility to service after maintenance.
PPE & Grounding
Personnel SafetyUse appropriate PPE and confirm the mast, shelter, and feeder are bonded to the facility earth system and lightning protection before working on internal connections.
Restore to Compliant State
ComplianceDo not leave the facility radiating out-of-tolerance course angle, modulation, or power after maintenance; verify monitored parameters are within tolerance before closing out the work and cancelling the associated NOTAM.
Engineering & Expert Notes
Ground Reflection Plane Changes Shift the Angle
SitingVegetation growth, new drainage work, or even standing water near a null-reference or image-type glide slope antenna can alter the effective ground reflection plane enough to shift the broadcast angle within a tolerance band without any equipment fault. Check the ground condition before chasing an electronics cause.
Flight Inspection and Ground Checks Can Legitimately Differ
VerificationA ground monitor reading within tolerance does not guarantee the in-space signal is equally clean, since near-field ground checks don't fully capture far-field multipath effects an aircraft receiver sees on approach. Treat a flight inspection discrepancy as informative, not as a sign the ground test was wrong.
Vehicle and Aircraft Movement Near the Antenna Matters
OperationalLarge vehicles or aircraft parked or taxiing near the glide slope antenna during a critical area incursion can perturb the radiated signal enough to trip a monitor alarm that has nothing to do with the transmitter itself. Correlate alarm timestamps against ground movement logs before troubleshooting hardware.
Sideband Antenna Balance Degrades Gradually
MaintenanceOn sideband-reference systems, a slow imbalance between clearance and course antenna elements shows up first as a subtle modulation trend rather than a hard alarm. Trending modulation percentage over successive checks catches this before it drifts into an out-of-tolerance condition.
Lightning Events Warrant a Full Parameter Check
TroubleshootingAfter a nearby lightning strike, a facility that comes back up and shows nominal power can still have subtle course or clearance degradation from surge damage to a single module. A full parameter verification after any lightning event catches this rather than relying on the automatic monitor alone.
Correlate Monitor Trends With Weather
ProcessSeasonal ground moisture changes can produce a repeatable annual pattern in monitored course angle at some sites. Logging monitor readings alongside weather conditions over a full year helps distinguish a genuine equipment drift from a recurring, benign seasonal effect.
Frequently Asked Questions
Short answers about the ILS glide slope: how it works, frequencies, standards and maintenance.
Q1What is the glide slope in an ILS?
The glide slope, also called the glide path, is the ground-based radio system that gives the vertical guidance component of the Instrument Landing System (ILS). It uses a transmitter and antenna array beside the runway.
Q2How does the glide slope work?
The transmitter modulates a UHF carrier with 90 Hz and 150 Hz tones. The antenna array creates two overlapping lobes, with 90 Hz predominant above the glide path and 150 Hz below. The aircraft receiver compares the two to show whether it is high or low.
Q3What is the standard glide path angle?
Typically 3° above horizontal, adjustable from about 2.5° to 4° depending on terrain and obstacle clearance requirements.
Q4What frequencies does the glide slope use?
UHF from 328.6 MHz to 335.4 MHz, paired with the localiser frequency.
Q5Which approach categories use the glide slope?
It is installed on ILS precision approach runways and gives vertical guidance for CAT I, II and III approaches. Wide-aperture arrays are used where higher-category performance is needed.
Q6Which standards apply to the glide slope?
The main references are ICAO Annex 10, FAA Order 6750.16 (ILS siting criteria), EUROCAE ED-82 and RTCA DO-192 for airborne receivers.
Q7How often should a glide slope be maintained?
Typically annual antenna array alignment checks tied to the flight check schedule, semi-annual modulation balance checks, and site and terrain surveys periodically or after site changes.
Q8What are the common problems with glide slope systems?
Antenna array misalignment, reflections from terrain, buildings or vehicles causing multipath interference, modulation balance drift between the 90 Hz and 150 Hz signals, and transmitter power amplifier failure.
Related Components
Go Beyond the Component. Understand the Complete AGL System.
You have explored the component. Now understand how it works within the complete Airfield Ground Lighting system.
Learn AGL from fundamentals to design, installation, testing, photometry, commissioning, maintenance and troubleshooting through structured technical training at Precision AGL Academy.
Build practical knowledge. Understand the circuit. Read the standards. Troubleshoot with confidence.
Your next step in AGL starts here.
Need expert guidance on glide slope selection, installation, or troubleshooting?
Precision AGL Academy provides advice on glide slope site planning, signal optimization, and flight inspection preparation.
Follow the Academy
Important Notice, Disclaimer & Limitation of Liability
Please read before relying on any content on this page.
- Purpose and intended users. This page on Glide Slope is an independent educational and informational reference for competent, suitably trained and authorised airfield ground lighting (AGL) and aerodrome electrical personnel. It is not professional engineering, legal or regulatory advice, and using it does not create any professional or client relationship.
- Safety. Work on series circuits, constant current regulators (CCRs), isolating transformers, cables, connectors and lighting equipment can be fatal. Always apply the aerodrome’s permit-to-work and isolation/LOTO procedures, prove dead, use correctly rated instruments and PPE, and follow the manufacturer’s instructions. Nothing here authorises work on live equipment or the bypassing of any safety device or interlock.
- Aviation operations. Nothing here authorises installation, modification, energisation, adjustment, return to service, a change of operating category (CAT I/II/III), a NOTAM or any aerodrome certification or approval decision. These rest with the aerodrome operator, the air navigation service provider and the competent regulator.
- Specifications, ratings and typical values. Descriptions, parameters, ratings, dimensions, photometric values, wattages, service lives, maintenance tasks and intervals, failure modes, certifications (for example CE, FCC, UL or IEC marks) and part or type designations are general, typical and indicative. They may not match any particular product, make, model or version, and they are not a datasheet, design, specification, selection, sizing, procurement or acceptance criterion. The manufacturer’s current documents, the approved design and the project specification prevail.
- Standards and regulations. References to ICAO, FAA, IEC, IEEE, ISO, BIS, DGCA and other standards are paraphrased summaries for orientation only. They may be incomplete or out of date, and standards are revised from time to time. Consult the current official editions. Following this content does not guarantee compliance with any law, standard or regulatory approval, and the regulator’s requirements always prevail.
- No warranty. Content is provided “as is” and “as available”, without warranty of any kind, express or implied, including accuracy, completeness, currency or fitness for a particular purpose. Real installations differ in design, equipment make and model, age and condition.
- Limitation of liability. To the fullest extent permitted by applicable law, Precision AGL Academy, Precision AGL Solutions and their owners, authors and contributors are not liable for any injury, death, loss, damage, downtime, penalty or regulatory consequence, whether direct or indirect, arising from the use of, reliance on, or inability to use this content. Nothing in this notice excludes or limits any liability that cannot lawfully be excluded or limited.
- Independence, brands and no endorsement. This is an independent publication. It is not affiliated with, sponsored by or endorsed by ICAO, the FAA, the IEC, DGCA, AAI, any airport, regulator, supplier or equipment manufacturer. Third-party names, product designations, standard designations and trademarks (including Megger®, Siemens, Eaton, Crouse-Hinds, ADB SAFEGATE, Fluke, ABB and Honeywell) belong to their respective owners and are used only for identification and description. Mention of a brand, product or supplier is not a recommendation, and we do not sell, supply or quote for any component described.
- Images and third-party content. Images and illustrations are shown to help identify the type of component. They may show products of various manufacturers, may not depict the exact item described, and are not a statement about any product’s quality, approval or performance. Copyright and trademarks in third-party images, logos and photographs belong to their respective owners and are not claimed as ours. If you are a rights holder and want an image removed, replaced or credited, email precisionaglsolutions@gmail.com with the page address and we will act promptly.
- Training and services. Mentions of training, courses, consultation, advice, selection, installation, calibration or other support, including the “Learn…Certify” pathway, describe learning stages and are not an offer, quotation, tender or promise of any result. Contacting us or our replying creates no engagement or duty of care; an engagement exists only under a written agreement. We are not a regulator or certification body, and completing any course or reading this content is not a certification, accreditation or authorisation to work on any aerodrome.
- Copyright, privacy and corrections. © 2026 Precision AGL Academy. All rights reserved. Text, structure, compilations and diagrams may be viewed for personal reference and internal training only; any other copying, republication or commercial use needs our written permission. If you email, telephone or message us, we use your details only to respond and handle your matter, in line with applicable Indian law including the Digital Personal Data Protection Act, 2023; please do not send confidential or security-sensitive information. To display this page your browser requests fonts, icons and images from third-party servers, which receive your IP address and browser details under their own policies. Content is reviewed periodically with no obligation to update. To report an error, safety concern or rights issue, write to precisionaglsolutions@gmail.com.