ILS localizer vs glideslope explained: compare lateral and vertical guidance, cockpit indications, paired frequencies and common capture failures.
In aviation, an ILS localizer provides lateral guidance, showing whether the aircraft is left or right of the runway centreline. The glideslope provides vertical guidance, showing whether it is above or below the published descent path. Together they form the precision-guidance core of a full Instrument Landing System approach.
How do the localizer and glideslope differ?
The localizer guides the aircraft horizontally towards the runway centreline, while the glideslope guides it vertically along the approach angle.
| Feature | Localizer | Glideslope |
|---|---|---|
| Guidance | Lateral: left or right | Vertical: above or below |
| Reference | Extended runway centreline | Published descent path, commonly near 3 degrees |
| Typical transmitter position | Beyond the opposite end of the runway | Beside the runway near the touchdown zone |
| Radio band | VHF, 108.10–111.95 MHz on assigned ILS channels | UHF, 329.15–335.00 MHz |
| Receiver tuning | The pilot or avionics selects the localizer frequency | The paired glideslope frequency is selected automatically |
| Use without the other signal | May support a published localizer-only approach | Not normally used as a stand-alone approach aid |
You do not tune a separate glideslope frequency. Each localizer channel has a corresponding UHF glideslope channel, as explained in our guide to ILS frequency bands and automatic channel pairing.
The localizer and glideslope are components rather than two different types of ILS. Our comparison of a complete ILS and localizer-only guidance covers that related distinction.
What do the localizer and glideslope indications mean?
The localizer deviation indicator shows where the runway course lies horizontally, while the glideslope pointer or diamond shows where the descent path lies vertically.
- Localizer indication left: on a normal front-course approach, the centreline is to the aircraft's left.
- Localizer indication right: the centreline is to the aircraft's right.
- Glideslope indication above centre: the aircraft is below the glideslope.
- Glideslope indication below centre: the aircraft is above the glideslope.
Flight-director bars are command indications, not necessarily raw localizer and glideslope needles. Display formats also vary between conventional instruments, an HSI and glass-cockpit primary flight displays. For the lateral part of the procedure, see our practical explanation of how to intercept and track a localizer without chasing the needle.
Can a localizer work without a glideslope?
Yes. An aircraft can receive and track a valid localizer even when the glideslope is unavailable, out of range or not part of the procedure.
A published localizer-only approach normally uses step-down fixes and higher minimums than the associated ILS. Pilots must not substitute an estimated descent path and continue to ILS minimums when valid glideslope guidance is absent. The published chart, clearance and aircraft procedures determine what may be flown.
The localizer can also become active before the glideslope because its useful coverage extends farther along the approach. Back-course localizer procedures normally provide no usable glideslope and require the specifically published back-course procedure.
Why does the localizer capture but not the glideslope?
The usual causes are joining above the descent path, arming lateral mode without approach mode, receiving the localizer before entering glideslope coverage, or using mismatched approach data.
- Confirm the approach and navigation source. Check the runway, ILS frequency, identifier, inbound course and selected navigation receiver. A wrong course setting may not prevent radio reception, but it can confuse the display or aircraft-specific autopilot logic.
- Intercept from below. Level at the charted glideslope intercept altitude and allow the pointer to move down towards centre. Joining from above risks a missed capture and may expose the receiver to false glideslope lobes above the normal path.
- Arm the correct mode. A localizer or LOC mode controls lateral guidance only. Approach or APP mode normally arms both localizer and glideslope capture, although the exact annunciations and capture sequence depend on the aircraft.
- Check for a valid signal. A warning flag, missing diamond or persistent empty scale means the avionics may not have usable vertical guidance. The glideslope transmitter could be unavailable, or the aircraft may still be outside its coverage.
- Check simulator data. In a flight simulator, outdated navigation data, incompatible airport scenery or simplified avionics can place the runway and ILS beam out of agreement. FSX users can work through our specific localizer-captures-but-glideslope-does-not checklist.
Is glideslope guidance a fixed descent rate?
No. A glideslope is an angular path through space, not a fixed vertical-speed command.
The descent rate needed to remain on it changes with groundspeed: a faster aircraft needs a greater rate of descent. For a glideslope near 3 degrees, groundspeed multiplied by roughly five gives a useful feet-per-minute estimate, but the published angle and actual guidance remain authoritative.