How the Glideslope Works: 90/150 Hz Lobes, Antenna Location and TCH, the 1.4° Beam, Coverage Limits, False Glideslopes, Glideslope Failure, and Flying It Precisely
When the weather drops below visual minimums, the ILS is the approach that gets airplanes to the runway, and the glideslope is the part that makes it a precision approach. Most instrument pilots can fly the needle; far fewer can explain what’s driving it — which matters, because the glideslope’s failure modes (false glideslopes above the true path, reflections from vehicles near the antenna, a flagged needle at the worst moment) are all explained by how it’s built. A pilot who knows that the 90 Hz lobe is on top, that the antenna sits beside the touchdown zone, that the beam is only 1.4° wide, and that the path crosses the outer marker at about 1,400 feet will recognize a bad glideslope before it does any harm.
This post covers what the glideslope is and the geometry of a 3° path, where the transmitter sits and why, how the two-lobe signal becomes a needle, coverage and its limits, sensitivity, the threshold crossing height and what it means for your wheels, false glideslopes and the intercept-from-below rule, the altitude cross-check, the critical area, glideslope failure and the localizer-only fallback, autopilot coupling, and the checkride questions.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
What the Glideslope Is
The vertical half of the ILS.
The definition:
A UHF ground-based transmitter that radiates a vertical descent path in the direction of the localizer front course, giving the aircraft above/below guidance to the runway touchdown zone
Paired with the localizer (lateral) to form a precision approach flown to a decision altitude
Provides descent information from glideslope intercept down to the lowest authorized DH for the procedure
The glideslope is what separates an ILS (DA) from a localizer approach (MDA)
The geometry of a 3° path:
The glide path projection angle is normally adjusted to 3° above horizontal
At 3° the path descends roughly 300 feet per nautical mile (318, precisely), which works out to about 500 feet per minute at 90 knots groundspeed, 640 at 120, and 740 at 140
On a 3° path the glideslope intersects the outer marker at about 1,400 feet above the runway elevation and the middle marker at about 200 feet
Some installations use steeper angles (up to about 3.5° and occasionally more) for terrain or noise; the plate shows the angle
Where it fits on the plate:
The profile view shows the glideslope as the sloping line from the glideslope intercept altitude (the lightning-bolt FAF) to the runway, with the angle and TCH printed
Minimums: the ILS line gives a DA; the LOC line beneath it gives an MDA for when the glideslope is unavailable
The glideslope intercept altitude and the published crossing altitude at the FAF/OM are your cross-check numbers
Brief all three: angle, intercept altitude, DA
Where the Signal Comes From
The transmitter location (AIM 1-1-9):
The glideslope antenna is located between 750 and 1,250 feet from the approach end of the runway, down the runway
It’s offset 250 to 650 feet laterally from the runway centerline — on whichever side the site allows, not a fixed side
It radiates toward the localizer front course, so an aircraft on the back course gets no valid glideslope
The antenna is a tall mast with stacked antennas beside the touchdown zone — the structure you see off to one side as you cross the threshold
Why beside the touchdown zone:
The path has to originate at the point where you’ll touch down, so the antenna sits abeam that point, not at the threshold and not at the far end
The lateral offset keeps the mast clear of the runway and the obstacle-free zone
The ground in front of the antenna is part of the system: the signal reflects off the terrain to form the vertical pattern, which is why the glideslope site is graded and why snow or vehicles in front of it can bend the path
The glideslope is as much a ground reflection as a radiated beam
Why it matters to you:
The path is referenced to the glideslope antenna site; your threshold crossing height depends on it (below)
The critical area is the ground in front of the antenna (below)
The angle is the same across the localizer course, so the glideslope is only meaningful when you’re on the localizer
Lateral first, then vertical
How the Signal Is Generated and Read
Two lobes stacked vertically.
The transmission:
The glideslope operates on one of 40 UHF channels between 329.15 and 335.00 MHz
Each channel is paired with a localizer frequency; tuning the localizer tunes the glideslope automatically — you never dial it
The antenna radiates two overlapping lobes, one modulated at 90 Hz and one at 150 Hz, stacked vertically
The 90 Hz lobe is above the glide path; the 150 Hz lobe is below it; they’re equal along the path itself
The receiver:
Your glideslope receiver compares the depth of modulation of the two tones, just as the localizer receiver does with its two lobes
More 90 Hz than 150 Hz → you’re above the path → the needle deflects DOWN (fly down to the path)
More 150 Hz than 90 Hz → you’re below → the needle deflects UP (fly up)
Equal → on the path → needle centered
What drives the needle:
The glideslope needle (the horizontal needle on a CDI, or the vertical deviation scale/pointer on an HSI or PFD)
The flight director’s pitch command bars
The autopilot’s pitch channel in approach mode
All from the same 90/150 comparison
What it doesn’t know:
The glideslope doesn’t measure your altitude, airspeed, or distance — it only knows the angle between you and the antenna
A valid “on glideslope” indication at the wrong place (a false glideslope, or far outside coverage) is still geometrically “on a path,” just not the one you want
Your altimeter is the independent check
Geometry and signal ratio, nothing more
Coverage and Limits (AIM 1-1-9)
Usable distance:
The glideslope is normally usable to 10 NM from the antenna
Some installations are certified for an extended service volume beyond 10 NM, with the distance noted
Outside the usable distance, indications are not reliable — a centered needle 15 miles out is not information
Intercept inside the coverage, at the published intercept altitude
Lateral coverage:
The glideslope is usable within 8° either side of the localizer course
Outside that sector the vertical indication may be present but is not valid
Get established on the localizer first; a glideslope indication while well off the localizer is meaningless
Localizer, then glideslope — always
Vertical limits:
Descent information is provided down to the lowest authorized decision height in the approved procedure
Below the lowest authorized DH, the glidepath may not be suitable for navigation; any reference to it must be supplemented by visual reference to the runway environment
Glidepaths with no published DH are usable to the runway threshold
At DA you transition to visual; you don’t ride the needle to the flare
Beam width:
The glide path beam is 1.4° wide vertically — 0.7° above the path to full-scale “fly down,” 0.7° below to full-scale “fly up”
Far narrower than the localizer’s 3°–6° course, which is why the glideslope needle is the more sensitive of the two
At the outer marker (roughly 5 NM) full-scale deflection is about 350 feet above or below the path; at 1 NM it’s about 70 feet above or below; near the threshold a single dot is a handful of feet
The funnel is steep
Sensitivity and Needle Behavior
As you approach the runway:
The beam is a fixed angle, so a given needle deflection represents fewer feet the closer you are to the antenna
The same pitch error that moved the needle a dot at the OM moves it full-scale near minimums
Corrections must shrink continuously — a 100-fpm change at the OM, a 50-fpm change inside the marker, a pitch nudge near DA
Chasing the glideslope close in produces the same porpoising that chasing the localizer produces S-turns
Technique:
Establish the descent rate that holds the needle (groundspeed × 5 is a usable approximation of the fpm for a 3° path: 90 knots → 450 fpm), then trim
Small pitch changes to correct the path, power to hold airspeed (or the reverse, per your SOP — stay consistent)
A half-dot deviation near the OM is a trend to watch; a half-dot near DA is a correction to make now
Set the descent rate early so the capture is a trim change, not a chase
Why the sensitivity is deliberate:
Tight guidance near the runway is what makes a 200-foot DA safe — the obstacle-clearance surfaces are built around a tight path
It also makes a stabilized approach almost automatic: if you’re on the needle at DA, you’re on a normal descent to the touchdown zone
It forces discipline precisely where discipline matters
The needle is strict because the ground is close
Threshold Crossing Height (TCH)
The number on the plate that tells you where your wheels will be.
What it is (AIM 1-1-9):
The published TCH is the height above the runway threshold at which the aircraft’s glideslope ANTENNA would cross if the aircraft stays on the trajectory formed by the glidepath segment from about four miles out to the middle marker
It is a planning reference, not a measurement of the actual on-course indication at the threshold
Typically 50 to 60 feet, set by the largest aircraft category normally using the runway, the antenna siting, and terrain
Printed in the profile view next to the glide path angle (“GS 3.00° / TCH 55”)
Why the antenna-to-wheel distance matters:
The TCH is where your antenna crosses; your main gear is below the antenna by the aircraft’s antenna-to-gear height in the landing attitude
On a light airplane that’s a few feet; on a large aircraft it can be 20 feet or more
A comfortable wheel crossing height is about 20 to 30 feet, depending on aircraft type
Know your airplane’s antenna-to-gear height and compare it with the TCH
The high-TCH consideration:
A higher-than-optimum TCH at the same angle means the path crosses the threshold higher, and an aircraft that holds the path to the flare touches down farther down the runway
On a short runway, a high TCH eats into landing distance
Brief the TCH on short runways and in large aircraft
It’s on the plate for a reason
False Glideslopes and the Intercept-From-Below Rule
The most important glideslope limitation.
What a false glideslope is:
The antenna’s radiation pattern produces additional lobes at angles considerably steeper than the published path — spurious “paths” at roughly multiples of the true angle (around 6° and 9° for a 3° glideslope)
On the first false glideslope (about 6°), the sensing is reversed; on the next (about 9°), it’s normal — both are far too steep to fly safely
The glideslope flag can be clear on a false glideslope — the receiver sees a valid-looking balance of tones
False courses exist at angles well above the true path, never below it
How you’d end up on one:
Intercepting the glideslope from ABOVE — descending onto it from a high altitude close to the airport, where the first lobe you meet is a false one
Being high and fast on vectors, then diving for the needle when it centers
Arriving inside the FAF above the intercept altitude
The common thread is being above the path when you capture
The rule:
Intercept the glideslope from BELOW, at the published glideslope intercept altitude, level, with the needle coming down from the top
If the needle shows “on glideslope” while you’re well above the intercept altitude, or the descent rate needed to hold it is far more than the ~300 ft/NM a 3° path requires, suspect a false glideslope
The AIM: be alert approaching glidepath interception; false courses and reverse sensing occur at angles considerably greater than the published path
Never fly below the glide path to “find” it either — obstacle clearance depends on staying on or above the path (AIM caution)
The altitude cross-check:
At the outer marker (or the FAF/glideslope-intercept fix), compare your altimeter to the published glideslope-crossing altitude on the plate — on a 3° path, about 1,400 feet above the runway at a typical OM
A mismatch of hundreds of feet with a centered needle means a false glideslope or a mis-set altimeter
Thirty seconds, every ILS
The single best defense
The Glideslope Critical Area
What it is:
The ground in front of the glideslope antenna forms the signal; vehicles or aircraft there — taxiing on a parallel taxiway, holding near the antenna, or crossing the runway near it — can distort the path for an aircraft on the approach
A defined critical area around the antenna is marked on the airport by ILS hold-short lines
Separate from the localizer critical area at the far end of the runway
The reason for “hold short of the ILS critical area”
When ATC protects it:
At towered airports during tower hours, when the ceiling is less than 800 feet and/or visibility is less than 2 miles, ATC keeps vehicles and aircraft out of the glideslope critical area whenever an arriving aircraft is inside the outer marker (or the fix used in lieu of it) — unless the arriving aircraft has reported the runway in sight and is circling or sidestepping to another runway
At or above 800/2, no protection is provided unless a crew requests it for an autoland or coupled approach
At nontowered airports, nothing protects it
Expect momentary glideslope wobbles in good weather when something crosses near the antenna; don’t chase them
For the pilot:
A sudden, brief glideslope excursion with a clear flag in good weather may be ground traffic near the antenna — hold the pitch attitude and let it settle
A sustained deviation, or a flag, is a go-around decision
Autopilots can follow a bent path toward the ground; monitor the raw data below 500 feet
On the ground, respect the ILS hold line even when the tower isn’t requiring it
Glideslope Failure: The Localizer-Only Fallback
When the glideslope flags:
A glideslope flag (or a removed indication) means no usable vertical guidance — the transmitter is off, out of tolerance, or you’re outside coverage
The approach doesn’t end; it changes: you’re now on a localizer approach to the LOC MDA
You must be at or above the LOC step-down altitudes and the MDA from that point
Go missed if you can’t make that transition safely
Brief it in advance:
Every ILS brief includes the LOC minimums and step-downs, the LOC MAP (usually by DME, timing, or a fix — not the DA point), and the decision: “glideslope fails before the FAF → fly the LOC approach; fails inside the FAF below a LOC step-down → missed”
Set the LOC MDA as a reminder on the altitude preselect bug or in your brief
Know the LOC MAP’s definition — it’s different from the ILS’s
A glideslope failure inside the FAF is a busy moment; the plan has to exist already
NOTAMs:
“ILS RWY 27 GS U/S” (glideslope unserviceable) means the ILS is a LOC approach today
The localizer may still be fine; the minimums are the LOC line
Check NOTAMs for both components
A glideslope-out ILS is common and routine, if you’ve briefed it
Autopilot and Flight Director Integration
Coupled approaches:
In approach mode, the autopilot arms glideslope capture and, when the beam is captured, uses pitch to hold the 90/150 balance; the flight director displays the same commands as pitch cues
Capture from below: most autopilots will not capture a glideslope from above, and shouldn’t
The altitude selector should be set to the glideslope intercept altitude until capture, then to the missed approach altitude (per your SOP) — not left at an altitude the autopilot will level at on the glidepath
Monitor the raw glideslope needle, not just the flight director
CAT II/III systems:
Dual or triple autopilots, redundant receivers, and monitoring logic that compares channels and alerts on disagreement
Radio altimeter DH instead of a barometric DA
Fail-passive and fail-operational architectures depending on the category
Beyond GA scope, but the same 90/150 signal underneath
What modern systems cross-check:
Receiver health and flag status
Signal validity (the balance of tones within limits)
Aircraft position relative to the localizer (no glideslope capture off-course)
Attitude and vertical speed reasonableness
Why Understanding the Glideslope Matters
Error recognition:
A centered needle with the wrong altitude at the OM: false glideslope or altimeter error
A needle that wanders briefly in good weather: critical-area traffic
A needle that demands 900 fpm at 90 knots: you’re on a steep false path
A flag inside the FAF: the LOC plan, now
Automation management:
Knowing the autopilot captures from below, holds a tone balance, and can follow a bent beam tells you what to monitor and when to disconnect
The raw needle and the altimeter are the independent checks
The autopilot doesn’t know about false glideslopes either
Raw data below 500 feet
Approach discipline:
The 1.4° beam makes the glideslope strict; a stabilized approach is the only way to stay on it
Configured before intercept, descent rate set at capture, small corrections
Decide at DA
The system rewards exactly the habits the ACS tests
Common Pilot Mistakes
Intercepting the glideslope from above and capturing a false glideslope
Skipping the altitude cross-check at the outer marker / glideslope-intercept fix
Chasing the needle close in with large pitch changes (porpoising)
Flying below the glide path to “get established” — obstacle clearance depends on staying on or above it
Trusting a glideslope indication while off the localizer or outside 10 NM
Trusting glideslope indications on a back course
Not briefing the LOC minimums and LOC MAP before every ILS
Freezing when the glideslope flags inside the FAF instead of executing the briefed LOC plan
Ignoring the TCH on a short runway in a large aircraft
Leaving the altitude preselect where the autopilot will level off on the glidepath
Checkride Questions — With Answers
“How does the glideslope work?”
A UHF transmitter beside the touchdown zone radiates two overlapping lobes toward the localizer front course, one modulated at 90 Hz above the glide path and one at 150 Hz below. The receiver compares them: equal is on path; more 90 Hz means you’re above (fly down); more 150 Hz means you’re below (fly up).
“Where is the glideslope antenna?”
750 to 1,250 feet down the runway from the approach threshold, offset 250 to 650 feet from the centerline, beside the touchdown zone (AIM 1-1-9).
“What frequencies, and how do you tune it?”
40 UHF channels from 329.15 to 335.00 MHz, each paired with a localizer frequency; tuning the localizer tunes the glideslope automatically.
“What’s the standard glide path angle, and where does it cross the markers?”
Normally 3°; on a 3° path the glideslope intersects the outer marker at about 1,400 feet above runway elevation and the middle marker at about 200 feet.
“How wide is the glideslope, and how far is it usable?”
The beam is 1.4° wide vertically; usable within 8° either side of the localizer course, normally to 10 NM (extended at some sites), and down to the lowest authorized DH — below that, supplement with visual reference.
“What’s a false glideslope, and how do you avoid it?”
Spurious paths at angles considerably steeper than the published one (around 6° and 9° for a 3° glideslope), the first with reverse sensing. Avoid them by intercepting from below at the published intercept altitude and cross-checking the altimeter against the charted crossing altitude at the OM/FAF.
“What is the TCH?”
Threshold crossing height: the height above the threshold at which the aircraft’s glideslope antenna crosses if it stays on the glidepath trajectory — a planning reference, typically 50–60 feet. Your wheels cross lower by the antenna-to-gear height; a comfortable wheel crossing height is about 20–30 feet.
“The glideslope flags inside the FAF. What do you do?”
Revert to the localizer approach: you must be at or above the LOC step-down altitudes and continue to the LOC MDA and LOC MAP — if you can’t make that transition safely, go missed. Brief the LOC minimums before every ILS.
Final Thoughts
The glideslope is a simple needle driven by an elegant system: two tones stacked vertically, radiated from a mast beside the touchdown zone, compared in your receiver to tell you whether you’re above or below a 3° line that crosses the outer marker at 1,400 feet and the threshold at the TCH. It doesn’t know your altitude, your airspeed, or your intentions — only geometry and signal balance. That’s why it can be fooled by a false path above the true one, bent by a truck in front of the antenna, and silent below the DH.
Intercept it from below at the published altitude, cross-check the altimeter at the marker, keep corrections small as the beam narrows, stay on or above the path, know your TCH and your wheels, brief the localizer-only plan before every ILS, and transition to visual at DA. Understand the invisible architecture, and the needle becomes the most trustworthy thing on the panel.
On the Written Test and Checkride
Glideslope fundamentals appear on the instrument knowledge test and in instrument orals. The most commonly tested topics:
90 Hz above / 150 Hz below; receiver comparison; needle direction
UHF 329.15–335.00 MHz, paired with the localizer
Antenna location: 750–1,250 ft down the runway, 250–650 ft off centerline
3° path; ~1,400 ft at the OM, ~200 ft at the MM; ~300 ft/NM
1.4° beam width; ±8° lateral coverage; usable to 10 NM; down to the lowest DH
False glideslopes above the path; intercept from below; altitude cross-check
TCH and antenna-to-wheel geometry
Glideslope critical area and ATC protection
Glideslope failure → LOC minimums
Quick Reference
Glideslope (AIM 1-1-9):
Vertical guidance for the ILS; radiated toward the localizer front course (no valid glideslope on the back course)
90 Hz lobe above the path, 150 Hz below; equal = on path; more 90 → above → needle down (fly down); more 150 → below → needle up (fly up)
Doesn’t measure altitude or distance — geometry and signal balance only
Frequencies:
329.15–335.00 MHz UHF, 40 channels, paired automatically with the localizer frequency
Antenna:
750–1,250 ft down the runway from the approach end; 250–650 ft off centerline (either side), beside the touchdown zone
The ground in front of it forms the signal → glideslope critical area
Geometry:
Path angle normally 3° (some steeper — read the plate): ~300 ft/NM; fpm ≈ groundspeed × 5
Crosses the OM at ~1,400 ft and the MM at ~200 ft above runway elevation
Beam 1.4° wide vertically (±0.7° full-scale) → far more sensitive than the localizer; ~350 ft above/below at the OM, ~70 ft at 1 NM; narrows toward the antenna
Coverage:
Usable to 10 NM (extended at some sites); 8° either side of the localizer course
Usable down to the lowest authorized DH; below that, supplement with visual reference; no published DH → usable to the threshold
TCH:
Height of the aircraft’s glideslope antenna over the threshold on the glidepath trajectory — a planning reference (~50–60 ft typical)
Wheels cross lower by the antenna-to-gear height; comfortable wheel crossing ~20–30 ft
High TCH → longer landing; brief on short runways
False Glideslopes:
Spurious paths at angles considerably steeper than the published one (~6° reverse-sensing, ~9° normal for a 3° GS); flag may be clear
Intercept from below at the published intercept altitude
Cross-check the altimeter at the OM/FAF against the charted crossing altitude
Never fly below the glide path (AIM caution)
Critical Area:
At towered airports (tower open), ATC protects it when ceiling < 800 ft and/or visibility < 2 SM and an arrival is inside the OM/FAF (circling/sidestep exception); at/above 800/2 only on request (autoland/coupled); nontowered: no protection
Brief wobbles in good weather = ground traffic; don’t chase; sustained deviation or flag = go-around decision
Glideslope Failure:
Flag → LOC approach: at/above LOC step-downs, to the LOC MDA and LOC MAP; can’t transition safely → missed
Brief LOC minimums, step-downs, and the LOC MAP before every ILS; “GS U/S” NOTAM = LOC approach today
Autopilot:
Approach mode arms GS capture from below; pitch holds the 90/150 balance; FD shows the same
Altitude preselect: intercept altitude until capture, then the missed altitude per SOP
Monitor the raw needle and altimeter below 500 ft
Key Principle:
The glideslope is two tones compared in your receiver, radiated from a mast beside the touchdown zone, defining a 1.4°-wide path at 3° that crosses the OM at 1,400 feet and the threshold at the TCH. It can be fooled by false paths above it, bent by traffic near the antenna, and is silent below the DH — so intercept from below, cross-check the altimeter at the marker, stay on or above the path, keep corrections small as it narrows, brief the localizer-only plan, and go visual at DA.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.