The Localizer Explained: How the 90/150 Hz Course Works, Frequencies and Identification, Service Volume, Course Width (700 Feet at the Threshold), Critical Areas, and Flying It
When you fly an ILS, the glideslope gets the attention — it’s the part that makes the approach “precision.” But the localizer is the part you can’t do without: it’s the lateral guidance that keeps you on the extended runway centerline from 18 miles out to the threshold, in weather where the runway isn’t visible until the last few hundred feet. It’s also the part that survives on its own — a localizer approach with the glideslope out, a back course, or an LDA all run on the same signal. Understanding how the localizer actually works explains everything about how it behaves: why the needle tightens toward the runway, why it reverse-senses behind the antenna, why ATC holds you short of a special line in low weather, and why “set the front course” is always the right HSI setup.
This post covers what a localizer is, the 90 Hz / 150 Hz signal and how the receiver turns it into a needle, frequencies and identification, the service volume (angular and vertical), course width and the 700-foot rule, how to interpret and set up the indication, the critical areas, the back course and other localizer-type facilities, the LOC-only approach, why localizers persist in a GPS world, and the checkride questions.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
What a Localizer Is
The lateral half of the ILS.
The definition:
A ground-based VHF navigation facility, the lateral component of the Instrument Landing System, that provides course guidance to the runway centerline
The approach course it radiates toward the arriving aircraft is called the front course
It tells you left or right of the extended centerline — nothing about altitude or distance
Paired with a glideslope it’s an ILS; alone it’s a localizer (LOC) approach
Where it sits:
The localizer antenna array is at the far (departure) end of the runway, on the extended centerline, several hundred feet past the stop end
That placement is why the course gets progressively narrower as you approach the threshold — you’re flying toward the antenna, and the angular beam is a smaller number of feet wide the closer you are to its source
The glideslope antenna, by contrast, sits beside the touchdown zone near the approach end
Two antennas, two ends of the runway
What it gives you:
A course aligned with the runway (a few ILSs have a slightly offset localizer, up to 3°, and are still called ILS)
Guidance from the service-volume boundary all the way to the threshold
A needle that behaves like a tightening funnel
The one piece of the ILS every instrument pilot flies in every configuration
How the Localizer Works
Two lobes, one comparison.
The signal:
The antenna array radiates two overlapping lobes on the same carrier frequency
One lobe is amplitude-modulated at 90 Hz, the other at 150 Hz
As seen by an aircraft approaching on the front course, the 90 Hz lobe is on the left of the course and the 150 Hz lobe is on the right
Along the extended centerline the two are received at equal strength
The receiver:
The nav receiver measures the difference in depth of modulation (DDM) between the 90 Hz and 150 Hz tones
Equal → zero DDM → needle centered
More 150 Hz than 90 Hz → you’re right of course → needle deflects LEFT (fly left to the course)
More 90 Hz than 150 Hz → you’re left of course → needle deflects RIGHT (fly right)
Why it’s so precise:
The comparison is a ratio, not an absolute signal strength, so distance and signal fade don’t shift the course
The on-course line is wherever the two lobes are exactly balanced — a line fixed in space by the antenna geometry
The deflection is proportional to how far from that line you are, in angle
It’s a fixed geometric course, not a bearing computed in the airplane
What it isn’t:
There’s no “TO/FROM” and no OBS involvement — the receiver is in localizer mode automatically when you tune a localizer frequency
Setting the OBS on a mechanical CDI changes nothing about the needle on a localizer (set the course anyway as a reference)
It isn’t a radial; it’s a single course line
One course, both directions (which is where the back course comes from)
Frequencies and Identification
The frequency band:
The localizer operates on one of 40 ILS channels between 108.10 and 111.95 MHz
The channels are the odd tenths and the odd tenths plus 50 kHz: 108.10, 108.15, 108.30, 108.35, 108.50, 108.55 … up to 111.95
VORs use the even tenths in the same 108–112 MHz band (108.20, 108.40 …), so a nav receiver knows from the frequency alone whether it’s looking at a localizer or a VOR
Tune a localizer frequency and the CDI switches to localizer behavior
Paired glideslope:
Each localizer frequency is automatically paired with a UHF glideslope frequency (329.15–335.00 MHz); the receiver tunes the glideslope for you when you tune the localizer
You never dial the glideslope frequency
If the glideslope isn’t received, the GS flag shows — you may still have a valid localizer
One tuning action, two signals
Identification:
A three-letter identifier preceded by the letter “I” (•• in Morse), transmitted on the localizer frequency — e.g., I-DIA
Identify it before you use it, every time; an un-identified localizer may be the wrong facility, a test transmission, or out of service
If the identifier is absent or wrong, the facility is unusable — the identification is removed when a localizer is out of service or undergoing maintenance, and a NOTAM announces the outage
The “I” is how you know it’s a localizer (an LDA uses the same convention; an SDF uses three letters without the “I”)
Localizer DME:
Many localizers are paired with a DME (an “ILS/DME” or “LOC/DME”); the DME channel pairs automatically with the localizer frequency
DME fixes on the plate are measured from that DME, usually located at the localizer antenna or the glideslope site — read the plate for which
Replaces markers for the FAF and step-downs on many approaches
Not every localizer has one; many approaches use GPS fixes instead
Service Volume (AIM 1-1-9)
Where the signal is guaranteed.
The angular sectors:
Proper off-course indications are provided within 10° either side of the course out to 18 NM from the antenna
And from 10° to 35° either side of the course out to 10 NM
Beyond 35°, indications are unreliable
Intercept from inside the sector; a needle you see at 40° off course is not information
The vertical band:
Course guidance is provided throughout the descent path to the threshold from 18 NM, between 1,000 feet above the highest terrain along the course line and 4,500 feet above the elevation of the antenna site
Above 4,500 feet above the antenna, or well below 1,000 feet above terrain outside the final, the signal isn’t guaranteed
This is why a high-altitude intercept far from the airport may show a flickering flag
Descend into the volume before you need the needle
Why it matters:
Vectors to final are designed to put you inside the volume at a 30° or shallower intercept
A procedure turn or course reversal does the same
If you’re outside the sector or above the band and the needle looks alive, be suspicious
Unreliable signals may be received outside these areas — the AIM says so explicitly
Signal integrity:
A ground monitor shuts the localizer down (or removes the identifier) if the course shifts beyond tolerance
A flag in the cockpit means no usable signal: don’t fly it
A valid identifier plus no flag is your evidence of a usable localizer
Both, or neither
Course Width and the 700-Foot Rule
The number that explains the needle.
The design standard:
The localizer is adjusted so that the course width — full-scale fly-left to full-scale fly-right — is 700 feet at the runway threshold
That’s 350 feet either side of centerline at the threshold for full-scale deflection
Because runways differ in length (and the antenna is at the far end), the angular width needed to produce 700 feet at the threshold varies
The result is an angular course width that’s typically between 3° and 6° total
The consequence — the funnel:
The width is a fixed angle, so the course measured in feet narrows continuously as you approach the antenna
At 10 NM out, full-scale deflection might be a couple of thousand feet wide; at the threshold it’s 700 feet; over the runway it’s narrower still
The same lateral error in feet produces a bigger needle deflection the closer you get
Small corrections near minimums; the needle will tell you about a 50-foot drift at the threshold
Compared to a VOR:
A VOR CDI is ±10° full-scale (20° total)
A localizer at about 5° total is roughly four times as sensitive, and that ratio holds at every distance
A pilot who flies a localizer with VOR-sized corrections will S-turn down final
Treat each dot as a quarter of what it means on a VOR
Compared to the other localizer-type facilities:
An LDA has localizer-comparable accuracy but isn’t aligned with the runway
An SDF is fixed at 6° or 12° wide — as wide as the widest localizer or twice that — and nearly aligned
The back course is the same angular width as the front course, but you cross the back-course threshold right at the antenna, where it’s only a few feet wide
Same family, different geometry
Reading and Setting Up the Indication
Front course inbound (the normal case):
Needle centered: on the extended centerline
Needle left: you’re right of the course — fly left
Needle right: you’re left of the course — fly right
Normal sensing; fly toward the needle
The HSI rule:
Set the published front course inbound on the course pointer — for every localizer approach, front or back
On the front course inbound, the deviation bar then sits naturally and senses normally
On a back course, the front course on the pointer (now at the tail) keeps the sensing normal; set the back course and you’ve recreated reverse sensing
Front course, always
The CDI reality:
A mechanical CDI with an OBS: the OBS setting has no effect on a localizer needle
Set the inbound course anyway for reference and situational awareness
Normal sensing on the front course inbound; reverse sensing on the front course outbound and the back course inbound (AIM 1-1-9 caution)
Know the four-quadrant rule
Technique:
Intercept at 30° or less from inside the service volume
Establish a heading that holds the needle; make 2°–5° corrections and wait for the needle to respond
Corrections shrink as the course narrows; by the threshold you’re making 1° changes
Don’t chase it — bracket it
Critical Areas: Why You Hold Short of the Other Line
The signal can be bent by what’s on the ground.
ILS course distortion (AIM 1-1-9):
Large aircraft and vehicles near the localizer antenna, or crossing the runway near it, can reflect and distort the signal and bend the course for an aircraft on the approach
The localizer critical area is a defined zone around the antenna; the glideslope has one too, near its antenna
Marked on the airport by the ILS hold-short lines (the ladder-style markings and “ILS” signs), separate from and usually farther from the runway than the standard hold-short line
Those lines exist to keep metal out of the beam
When ATC protects them:
At towered airports during tower hours, when the ceiling is less than 800 feet and/or the visibility is less than 2 miles, ATC keeps vehicles and aircraft out of the localizer critical area whenever an arriving aircraft is inside the outer marker (or the fix used in lieu of the OM) — except aircraft landing, exiting the runway, departing, or executing a missed approach. When the ceiling is below 200 feet and/or RVR is 2,000 or less, the restriction tightens to arrivals inside the middle marker (or ½-mile final)
The glideslope critical area is protected on the same weather trigger whenever an arrival is inside the OM/FAF, unless that aircraft has the runway in sight and is circling or sidestepping to another runway
At or above 800 feet and 2 miles, no critical-area protection is provided — if you intend an autoland or coupled approach in that weather and want the areas protected, tell the tower
At nontowered airports (or with the tower closed), nothing protects the areas — expect possible wobbles if traffic is near the antennas
For the pilot on the approach:
A brief, odd needle excursion with a clear flag in good weather may be a critical-area incursion by ground traffic — hold your heading and let it settle
A sustained deviation with no flag change deserves a go-around decision, not a chase
Autopilots can be fooled by a bent course near the threshold
Monitor, especially below 500 feet
The Back Course and the Localizer Family
Back course:
The same antenna radiates behind itself, off the departure end of the ILS runway; that’s the back course
Inbound on it, a plain CDI reverse-senses; an HSI set to the front course doesn’t
Usable only where a LOC BC approach is published and ATC authorizes it — many localizers don’t radiate a usable back course
Disregard glideslope indications on a back course unless one is specifically charted
LOC-only approach:
The ILS flown without the glideslope — because it’s out of service, the aircraft can’t receive it, or the installation is localizer-only
A nonprecision approach to an MDA with step-down fixes and a MAP (often by DME, timing, or GPS fix)
The plate’s “LOC” minimums line; the Maltese-cross FAF in the profile
Same needle, different vertical discipline
LDA and SDF:
LDA: localizer-comparable accuracy, offset from the runway (straight-in minimums if ≤30°), sometimes with a glideslope; identifier I-xxx
SDF: wider course (6° or 12°), nearly aligned, never a glideslope; identifier xxx
Both flown with localizer technique
Covered in their own posts
Offset localizer ILS:
A few ILS installations have the localizer offset from the runway centerline by up to 3° and are still classified as ILS (noted on the plate)
The runway appears slightly off to one side at DA
Brief it
Rare
Why Localizers Still Matter
Reliability:
A localizer is a fixed ground facility with a monitored signal; it doesn’t depend on satellites, almanacs, or integrity messages
It’s immune to GPS jamming and spoofing — an increasingly real-world concern
It’s the backup when RNAV isn’t available, and the primary at airports without WAAS procedures
Decades of service history
Precision:
700 feet at the threshold is as precise as a WAAS LPV at the same point, and the ILS supports CAT II/III minimums that LPV doesn’t
The lateral guidance is good enough to land on in near-zero visibility with the right aircraft and crew
Standard at essentially every major airport worldwide
Still the global reference for a precision approach
Training value:
Localizer technique — bracketing, shrinking corrections, not chasing — transfers directly to LPV, which uses the same angular scaling
A pilot who can fly a localizer can fly any lateral needle
The back course teaches sensing geometry that applies to every course-based instrument
Foundational
Common Pilot Mistakes
Not identifying the localizer before flying it
Trusting a needle from outside the service volume (beyond 35°, or above 4,500 feet over the antenna far out)
Flying localizer corrections the size of VOR corrections and S-turning down final
Setting the back course on an HSI (reverse sensing you didn’t need), or assuming the OBS changes a localizer needle
Chasing a momentary course wobble caused by ground traffic in the critical area
Crossing the ILS hold-short line (not the runway hold-short line) in low weather
Forgetting the vertical limits of the service volume on a high, distant intercept
Confusing the localizer frequency (odd tenths and +50 kHz) with a VOR’s (even tenths)
Assuming the glideslope must be tuned separately (it’s paired)
Flying a LOC-only approach with ILS (DA) discipline instead of MDA discipline
Checkride Questions — With Answers
“How does a localizer work?”
The antenna at the far end of the runway radiates two overlapping lobes on the same frequency, one modulated at 90 Hz (left of course, as seen on the front course) and one at 150 Hz (right). The receiver compares the two; equal strength is the centerline, and the stronger tone drives the needle toward the course.
“What frequencies does a localizer use?”
40 channels between 108.10 and 111.95 MHz — the odd tenths and the odd tenths plus 50 kHz (108.10, 108.15, 108.30, 108.35 …). The glideslope pairs automatically in the UHF band. VORs use the even tenths.
“How is it identified?”
A three-letter identifier preceded by “I” in Morse on the localizer frequency (e.g., I-DIA). Identify it before use; no ident means unusable.
“What’s the localizer service volume?”
10° either side of the course to 18 NM, and 10° to 35° either side to 10 NM, between 1,000 feet above the highest terrain along the course and 4,500 feet above the antenna elevation. Signals outside those areas are unreliable.
“How wide is the localizer course?”
Adjusted so full-scale deflection is 700 feet wide at the runway threshold, which works out to about 3° to 6° total depending on runway length. It’s about four times as sensitive as a VOR and narrows continuously toward the antenna.
“Where is the localizer antenna?”
At the far (departure) end of the runway on the extended centerline — which is why the course tightens as you approach the threshold.
“What are the ILS critical areas, and when does ATC protect them?”
Zones around the localizer and glideslope antennas where aircraft or vehicles can distort the signal, marked by the ILS hold-short lines. At towered airports during tower hours, ATC protects them when the ceiling is less than 800 feet and/or visibility less than 2 miles and an arriving aircraft is inside the OM/FAF (tighter restrictions below 200 feet / RVR 2,000). At or above 800/2, no protection unless you request it for an autoland or coupled approach.
“How do you set up an HSI for a localizer?”
Set the published front course inbound on the course pointer — for both front-course and back-course approaches. The OBS on a mechanical CDI has no effect on a localizer needle.
Final Thoughts
The localizer is the simplest part of the ILS to describe and the one that rewards understanding most: two tones, one comparison, a course fixed in space by an antenna at the far end of the runway. From that single fact come the funnel-shaped course that’s 700 feet wide at the threshold, the four-times-VOR sensitivity that demands small corrections, the reverse sensing behind the antenna, the service volume you have to be inside of, the critical areas ATC protects in low weather, and the “front course on the HSI” rule that never changes.
Identify it, intercept it from inside the volume, bracket it with shrinking corrections, respect the ILS hold-short line on the ground, and the localizer will take you to the threshold in weather where nothing else on the panel can.
On the Written Test and Checkride
Localizer fundamentals appear on the instrument knowledge test and in instrument orals. The most commonly tested topics:
90 Hz / 150 Hz lobes; which side is which; how the needle is derived
Frequencies: 108.10–111.95 MHz, odd tenths and +50 kHz, 40 channels; glideslope paired
Identification: I + three letters
Service volume: 10°/18 NM, 35°/10 NM; 1,000 ft above terrain to 4,500 ft above the antenna
Course width: 700 ft at threshold, 3°–6° total; ~4× VOR sensitivity
Antenna at the far end of the runway
Back course reverse sensing; HSI set to the front course
Critical areas: when and where ATC protects them
Quick Reference
Localizer (AIM 1-1-9):
Lateral component of the ILS; course guidance to the runway centerline; antenna at the far (departure) end of the runway
90 Hz lobe left / 150 Hz lobe right (as seen on the front course); equal = on course; stronger tone drives the needle toward the course
Front course = the approach course; back course = the same course line behind the antenna
Frequencies & Ident:
108.10–111.95 MHz, 40 channels: odd tenths and odd tenths + 50 kHz (108.10, 108.15, 108.30, 108.35 …); VORs use even tenths
Glideslope (329.15–335.00 MHz) pairs automatically
Identifier: “I” + three letters in Morse (I-DIA) — identify before use; no ident = unusable (ident removed when out of service; NOTAM issued)
Localizer DME pairs automatically where installed; read the plate for the DME location
Service Volume:
10° either side to 18 NM; 10°–35° either side to 10 NM
1,000 ft above the highest terrain along the course to 4,500 ft above the antenna elevation
Outside these: unreliable; intercept from inside at ≤30°
Flag = no usable signal
Course Width:
Adjusted to 700 ft full-scale at the runway threshold (±350 ft)
Angular width 3°–6° total, varying with runway length
Narrows continuously toward the antenna → corrections shrink on final
~4× as sensitive as a VOR (±10°)
Indication & Setup:
Front course inbound: needle left → fly left; needle right → fly right (normal sensing)
HSI: set the front course — always, front or back course
CDI: OBS has no effect; front course outbound and back course inbound reverse-sense
Bracket; 2°–5° corrections far out, ~1° near the threshold
Critical Areas:
Localizer and glideslope critical areas around the antennas; marked by ILS hold-short lines
At towered airports (tower open), ATC protects the localizer critical area when ceiling < 800 ft and/or visibility < 2 SM and an arrival is inside the OM/FAF (tighter below 200 ft / RVR 2,000: inside the MM or ½-mile final); glideslope area likewise
At or above 800/2: no protection unless you request it for an autoland/coupled approach
Nontowered or tower closed: no protection — don’t chase wobbles
The Family:
LOC-only: glideslope out or absent → MDA, step-downs, Maltese-cross FAF
Back course: published and ATC-authorized only; disregard the glideslope; reverse sensing on a CDI
LDA: localizer-comparable, offset (≤30° straight-in), sometimes GS; I-xxx
SDF: 6° or 12° wide, nearly aligned, never GS; xxx
Offset localizer ILS: up to 3°, still an ILS
Why It Persists:
Monitored ground signal; immune to GPS jamming/spoofing; supports CAT II/III; the global precision-approach standard; technique transfers directly to LPV
Key Principle:
A localizer is two tones compared in your receiver, defining a course fixed in space by an antenna at the far end of the runway. Everything follows: the funnel 700 feet wide at the threshold, four-times-VOR sensitivity, reverse sensing behind the antenna, a service volume you must be inside of, and critical areas ATC protects below 800/2 at towered airports. Identify it, intercept it from inside the volume, set the front course on the HSI, bracket with shrinking corrections, and respect the ILS hold line.
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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.