ILS vs SLS explained: compare radio and satellite guidance, SBAS/LPV minima, cockpit indications, failure modes and simulator support.
In real-world aviation, ILS uses runway-based radio transmitters—a localiser for lateral guidance and a glideslope for vertical guidance. SLS uses GNSS corrected by a satellite-based augmentation system to generate comparable guidance, often on ILS-like scales. ILS depends on airport equipment; SLS depends on satellite coverage, approved avionics and a coded procedure.
ILS vs SLS at a glance
The fundamental difference is ground-transmitted radio guidance versus aircraft-computed satellite guidance.
| Feature | ILS | SLS |
|---|---|---|
| Full name | Instrument Landing System | Satellite Landing System |
| Guidance source | Localiser and glideslope radio transmitters near the runway | GNSS position, SBAS corrections and a coded final approach path |
| Airport equipment | Dedicated transmitters, monitoring equipment and protected critical areas | No local approach-guidance transmitter; SBAS still relies on a regional ground-monitoring and satellite network |
| Pilot setup | Select or tune the ILS, confirm the course and identify the radio aid as required | Load the correct SBAS procedure from the navigation database; there is no ILS frequency to tune |
| Typical procedure | A published ILS approach | Usually an RNP approach with LPV or jurisdiction-specific SBAS minima |
| Lowest capability | Can support Category I, II or III operations when the runway, aircraft, crew and operator are approved | Usually provides LPV or Category-I-class capability; the published minima and approvals control what may be flown |
| Main vulnerabilities | Signal reflections, terrain, transmitter faults and vehicles or aircraft entering the ILS critical area | GNSS interference, SBAS outages, insufficient integrity, antenna or receiver faults and incorrect navigation data |
How does a Satellite Landing System work?
SLS compares an SBAS-corrected GNSS position with a precisely coded final approach path and generates lateral and vertical deviation guidance aboard the aircraft.
SBAS services such as WAAS or EGNOS use a ground-monitoring network to calculate corrections and integrity information, which is then rebroadcast by satellite. The avionics combine that information with the final approach segment stored in an approved navigation database.
The resulting indications may resemble an ILS localiser and glideslope, but no radio beams are coming from the runway. An ILS needle responds to received ground signals; an SLS indication represents the aircraft's computed displacement from the coded path.
SLS is often an avionics-function name rather than the title printed on an approach chart. The chart may instead describe an RNP approach with LPV minima, and another aircraft may label the same type of SBAS capability as LPV rather than SLS.
Is SLS the same as RNAV, LPV or GLS?
SLS is a way of flying certain SBAS-enabled RNAV approaches; it is not another name for every RNAV approach, and it is not GLS.
- RNAV is the broader method of navigating between defined points without tracking conventional ground beacons directly. Our comparison of ILS and RNAV approaches covers that wider distinction.
- LPV is a line of minima supported by SBAS lateral and vertical guidance. An SLS-equipped aircraft may use this data while presenting familiar ILS-like indications.
- GLS uses satellite positioning corrected by an airport-local GBAS station and transmitted over a VHF data link. SLS instead uses wide-area SBAS corrections rebroadcast by satellite.
An approach having ILS-like accuracy does not automatically make it an ILS or give it the same regulatory classification. Always use the procedure name, minima and equipment requirements shown on the applicable chart.
Can SLS replace ILS?
SLS can fill the Category-I-like role of an ILS at many runways, but it does not replace every ILS capability.
- Use the cleared ILS when it offers the required minima, company procedures call for it, or satellite guidance is unavailable. Approved ILS installations can also support Category II and III operations that an ordinary SLS/LPV installation cannot.
- Use the published SLS-capable procedure when the aircraft, crew and operator are authorised, SBAS integrity is available, and its minima suit the weather. It is especially useful at airports without a conventional ILS.
- Compare actual minima, not the appearance of the cockpit needles. Terrain, obstacles, lighting and procedure design can leave an LPV approach with higher minima than a nearby ILS.
Neither SLS nor LPV automatically authorises autoland. Autopilot coupling, automatic landing and rollout depend on the aircraft certification, avionics installation, runway facilities and operational approval.
Why is SLS unavailable or showing no vertical guidance?
SLS normally remains unavailable when the aircraft cannot validate the required SBAS procedure, position integrity or avionics capability.
- The selected approach has only LNAV minima rather than an SBAS-supported vertical path.
- The aircraft supports ordinary GNSS navigation but not certified SBAS or SLS guidance.
- The approach was not loaded correctly, the wrong navigation source is selected, or the final approach sequence is incomplete.
- The navigation database is missing the procedure or contains data that do not match the chart being used.
- SBAS integrity is unavailable, or GNSS reception is being affected by interference or equipment failure.
A mistake we see constantly is treating familiar-looking deviation scales as proof that ILS is active. Check the navigation-source and flight-mode annunciations: the shape of the needles alone does not identify the guidance system.
How do ILS and SLS differ in flight simulators?
Flight simulators generally model conventional ILS more widely than SLS, while satellite approach capability depends on the aircraft avionics, navigation data and depth of the add-on's systems modelling.
An LPV procedure appearing in the simulator's database does not guarantee that every aircraft can display or couple to it. Garmin-style avionics may identify the guidance as LPV, while a suitably modelled Airbus may use SLS terminology. Our MSFS 2024 ILS tuning and capture walkthrough shows how the radio-based side behaves in practice.
Some add-ons use satellite-derived data to create ILS-like guidance at runways without transmitters. An older satellite-assisted landing add-on illustrates that basic simulator concept, but such software should not be assumed to reproduce certified SLS logic, integrity monitoring or real-world operational approval.