The most common system today used in aviation for instrument approaches is the Instrument Landing System ILS. Even it was already invented back in 1929, it is still today the most common landing system you'll find on all major airports. The ILS uses a robust and reliable technique which has always been adapted to the state of the art. The ILS is very popular with the pilots, an outstanding majority of them requests the ILS for landing at low visibility conditions. In the future more and more the satellite navigation will be used for landing. But until today it is not approved in Switzerland to replace an ILS. 

With the help of the ILS an aircraft can determine its course, altitude and distance relative to the runway and to the touch down zone. The ILS information enables a landing in low visibility conditions following instrument flight rules IFR. There are 3 categories for an ILS: CAT1, CAT2 and CAT3. A CAT3 ILS combined with the adequate airport infrastructure makes a landing nearly without visibility possible. ILS CAT1 can be used down to a height of 200 feet above ground, ILS CAT2 down to 100 feet and ILS CAT3a down to 50 feet (ILS CAT3b even down to below 50 feet). At this height (Decision Height) the pilot has to see the runway touch down zone and continue the landing visually, otherwise he has to abort the landing and perform a go around / missed approach.

An ILS consists of these components:

  • Localizer (LOC / LLZ)
  • Glide Path / Glide Slope (GP / GS)
  • Markers (Outer Marker OM and Middle Marker MM) or Distance Measuring Equipment (DME)

The Localizer is located after the end of the runway on the extended runway axis. The LOC antenna array consists of 12 to 24 antennas, which can be a "normal" dipole, a logarithmic-periodic dipole LPD or a frame antenna.

The radiated signal is an amplitude modulated signal in the frequency bandwidth of 108 to 112 MHz. Depending on the aircraft's position relative to the extended runway axis it receives the Fly Left (modulation depth 150 Hz > 90 Hz) or Fly Right (modulation depth 90 Hz > 150 Hz) information. If the aircraft is exactly on the extended runway axis, the 90 Hz and 150 Hz have the same modulation depth. The "needle" of the cockpit instrument is in the middle and the pilot / auto pilot follows this course.


The Glide Path is located perpendicular to the touch down zone on the left or right side of the runway. Typically the GP consists of 2-3 antenna arrays mounted on a frangible mast. At the airport of Zurich 2 of the 4 GPs have a different antenna system, the End-Fire antenna. It consists of slotted "cables". The big advantage is that no mast is needed. So the End-Fire antenna can be used where the typical GP mast cannot be installed.

The radiated signal is an amplitude modulated signal in the frequency bandwidth of 329 to 335 MHz. Depending on the aircraft's position relative to the defined approach angle (glide angle, typically 3.0°) it receives the Fly Up (modulation depth 150 Hz > 90 Hz) or Fly Down (modulation depth 90 Hz > 150 Hz) information. If the aircraft is exactly on the approach angle, the 90 Hz and 150 Hz have the same modulation depth. The "needle" of the cockpit instrument is in the middle and the pilot / auto pilot follows this course.


The Markers are located on the extended runway axis on defined distances to the runway threshold. The Outer Marker has a distance of approx. 7'200 m and the Middle Marker of approx. 1'050 m to the threshold. Each Marker radiates a signal vertically into the sky. When the aircraft crosses these cones, the information OM or MM is shown in the cockpit and the pilot knows his distance to the threshold. Very rarely there are also Inner Markers (IM) installed, which can be found short before the begin of the runway.

The radiated signal is an amplitude modulated signal with the frequency of 75 MHz. The OM signal is modulated with 400 Hz and has the Morse code "---". The MM signal is modulated with 1'300 Hz and has the Morse code "-.-.-.". The IM signal is modulated with 3'000 Hz and has the Morse code "...".

In Switzerland instead of Markers there is always a Distance Measuring Equipment used. The big advantage is that the pilot gets permanently the distance information instead of only when crossing the Marker cones. The distance information ranges from approx. 25 NM (equals typical LOC range) down to 0 NM.

The DME is located either perpendicular to the touch down zone on the left or right side of the runway (co-located with the GP) or at the end of the runway (co-located with the LOC). Typically in Switzerland the omnidirectional DME antenna is used. It gives the distance information to the aircraft not only during the approach, but also during a go around and missed approach.

The DME receives and transmits pulse pairs with defined pulse durations and on defined receive and transmit frequencies. The frequency bandwidth ranges from 962 to 1'213 MHz.

The DME Interrogator in the aircraft sends an interrogation pulse pair to the DME Trans-ponder on the ground, which is processing the signal and sending a reply pulse pair back to the aircraft. By measuring the time between sending the interrogation signal and receiving the reply signal, the aircraft computer can calculate its slant distance to the DME Trans-ponder.

Air Traffic Safety Electronics Personnel ATSEP in this domain are working as maintenance technicians or engineers. The defined roles following ESARR5 (Eurocontrol Safety Regulatory Requirement 5) are: Maintenance Technician, Equipment Specialist and Equipment Engineer.

skyguide has two maintenance teams for all civil and military Navaids in Switzerland: One performs the maintenance in East- and the other in West-Switzerland.
There is the periodic maintenance, which is composed of ground measurements (measurements at the Navaids racks and in the field) and flight inspection. During flight inspection a special aircraft performs twice a year aerial checks and measurements, while the maintenance team controls the Navaids equipment and makes the fine tuning.
And there's also the corrective maintenance in case of a failure or outage of a Navaids. If a system fails, the on-standby maintenance service gets called by the 24h system administrator and starts immediately the fault finding and repair.
Additionally, the maintenance personnel is participating in the project teams led by the engineering.

To support the two maintenance teams, skyguide has an engineering team which is located in Geneva and Zurich. It helps the maintenance teams in fault finding, hardware and software changes and training; and it also manages the projects including all project engineering when old Navaids are replaced.

There is a very good knowledge about Navaids in these 3 teams. So usually for installation, commissioning, flight inspection, fault finding, periodic and corrective maintenance there is no external support needed.

03.04.2013, Andreas Pfisterer, ATSEP & SATTA member, skyguide

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The Ground Based Augmentation System (GBAS) is an augmentation system to a satellite navigation system. GBAS provides an enhanced level of service supporting all phases of approach and landing within the systems area of coverage. In particular, the main driver for the installation of GBAS will normally be to provide a precision approach service (CAT I, II, or III). The GBAS project in LSZH starts off with a CAT I appraoch to RWY 14. CAT II/III operataions are possible follow-up projects.

The GBAS is divided in into three distinct sub-systems:

  • The satellite constellation, which provides both the aircraft GNSS receiver and the GBAS ground station with ranging information. Current developments of GBAS use GPS and/or GLONASS, and will potentially use other constellations (such as Galileo) in the future;
  • The GBAS ground station, which monitors the satellite signals, calculates and broadcasts a number of parameters and corrections to improve the accuracy and integrity of the signals. The GBAS ground station also broadcasts the Final Approach Segments (FAS) data which defines the final approach path in space (both laterally and vertically) to enable Precision Approach operations. GBAS broadcasts are normally via VHF Data Link (VDL);
  • The aircraft receiver, which receives both the satellite signals and the GBAS datalink signals, supplying navigation output/guidance to both the pilots’ displays and to the autopilot;

GBAS is part of the medium to long term strategy as a technology to support landing and take off operations in the European region. In the medium term GBAS operations need to be envisaged in a “mixed equipage” operational scenario (i.e. some aircraft using ILS, some aircraft using GBAS). In the long term, GBAS may potentially replace ILS.


The benefits of GBAS operations compared to ILS operations are listed below:

  • Siting Criteria: Contrary to ILS, which may only be installed adjacent to the runway, GBAS offers more flexibility in the vicinity of the airfield, however certain siting and signal protection criteria must still be met. The GBAS protection area is named Local Object Consideration Area (LOCA). This may still present a challenge at highly congested airfields;
  • Multipath: The need to protect the ILS signal from multi-path effects places restrictions on building developments and aircraft movements. In ILS CAT II/III operations a large LSA is required to protect these operations. The GBAS ground station must be sited to avoid multipath effects, but this is likely to be less onerous than the requirements for ILS and the GBAS ground station is likely to be situated away from the runway;
  • LSA: The biggest restriction on runway capacity during ILS CAT II/III operations is normally the LSA. This restriction does not apply to GBAS (as the GBAS Ground Station can be located farther away from the runway); therefore, potentially allowing higher movement rates than ILS during LVP (although it is considered unlikely that rates equivalent to the full CAT I movement rate will be possible in CAT II/III operations due to other considerations such as the need to protect the OFZ and the position at which landing clearance is given). The concept of Optimised Operations uses this benefit to maximise capacity in LVP at high density airfields;
  • False capture: ILS localizer false captures are situations where the aircraft prematurely initiates a turn onto the localizer centreline. This phenomenon of false capture cannot happen with GBAS;
  • Single GBAS ground station: A single GBAS ground station can serve multiple runways, potentially reducing installation and maintenance costs compared to ILS;
  • Flight Inspection: GBAS should need significantly less periodic flight inspections than ILS as most of the checks can be realised on the ground; and fewer checks are required;
  • Enabling “All runway ends”: With GBAS all runway ends can be enabled simultaneously, allowing a higher flexibility in runway operation at a given airfield with single or multiple runways. It is also possible to select and de-select specific approaches according to operational needs.

GBAS will be implemented as an “ILS look alike” straight in approach. This greatly simplifies the transition phase from other approach aids due to:

  • Standardisation of precision approach procedures;
  • Limited requirements for pilot training;
  • Lower cost aircraft architecture implementation;
  • The certification process is reduced;
  • Changes to ATC procedures and training requirements are minimised.


Implementation of GBAS CAT I has commenced and development of GBAS CAT II/III is underway.

The differences between ILS and GBAS operations are minimised by the use of “ILS look alike” approaches, but there are still some significant operational differences:

  • The system is dependent on the satellite constellation and GBAS ground station rather than the ground based navaid (ILS or MLS);
  • One GBAS ground station can serve multiple runways. This has the benefit of providing maximum operational flexibility, however the failure of the GBAS ground station could affect multiple approaches;
  • For the foreseeable future multiple systems (ILS, MLS and GBAS) may be providing precision approach and landing operations for one runway, requiring procedures to support mixed equipage operations;
  • Positioning of the GBAS ground station should ensure that GBAS is not sensitive to multipath around the runways, but suitable protection will be required around the GBAS antennae;
  • Different chart terminology and phraseology; the GBAS approach is referred to as GLS (GBAS Landing System).

From the ATC point of view, a GLS is considered to be operationally identical to an ILS approach to the same runway. ATC operational procedures are the same, e.g. with ATC vectoring the aircraft to intercept the final approach track in the same manner as for the ILS. The only difference is that the aircraft is “cleared GLS approach”.

The term “localiser” is replaced with the term “approach course”.

Runway changes may be easier and more efficient due to the ability to broadcast the approaches to all runways. The facility to disable certain selected approaches will be provided where required (e.g. to disable the approach to a closed runway).

There are likely to be changes to LVP to accommodate the mixed equipage operations (ILS/MLS and GBAS) particularly if special procedures such as Optimised Operations are used to maximise capacity.

The position of CAT II/III runway-holding positions may be reviewed. As GBAS does not have a critical or sensitive area around the runway, on a GBAS only runway, the CAT II/III holding position may be located closer to the runway (e.g. at the same position as the CAT I holding position). The size of the OFZ also has to be taken into consideration. This is only likely once the ILS has been removed. In the interim period, the ILS LSA is likely to be the factor controlling the location of the CAT II/III holding position.

From the pilots perspective, the ILS Look-alike concept uses similar operational procedures for all landing functions so as to minimise the impact on the crew. The cockpit interface is the same, except that the pilot selects the GLS approach rather than the ILS or MLS approach and the aircraft performance is the same.

Mixed equipage operations with more than one approach aid

The introduction of new technology approach and landing aids (MLS and GBAS) will, in many cases, be on runways which are already equipped with ILS. Due to the length of time required for fleet equipage or renewal, operations with mixed equipage are likely to be required for a considerable period of time.

Other cases where new technology can provide benefits are at runways where no precision approach currently exists. GBAS can reduce the risk of Controlled Flight Into Terrain (CFIT) and may improve the regularity of service with reduced aircraft operating minima.

Where GBAS is introduced, one GBAS ground station may enable new or improved approach and landing operations on more than one runway. E.g. with mixed equipage GBAS and ILS operations on the primary runway, but with a new GBAS approach on a subsidiary runway where previously no approach aid was available, or a lower category of approach was available (e.g. NPA on the subsidiary runway upgraded to GBAS CAT I).

The upgrading of any runways to a higher approach category will require the full range of facilities (e.g. AGL) and runway holding positions to be re-assessed based on the new category of operations.

On runways with mixed equipage ILS/MLS/GBAS operations, the requirements for all the approach aids will need to be considered carefully, in particular the protection requirements for the landing aids, as ILS, MLS and GBAS may all have different sized critical and sensitive areas (GBAS may have none in the vicinity of the runway). The most practical solution in most cases will be for the runway holding positions to be established to protect the most demanding protection requirements (the largest areas). Separate CAT I and CAT II/III holding positions may be required.

When conducting mixed equipage operations the pilot needs to be informed of the status of the approach aids. This information may be provided individually to each aircraft or via ATIS. The pilot should then request the preferred approach aid and the controller clears the aircraft for this type of approach.

Controllers should be provided with information on the aircraft equipage either automatically (e.g. flight plan information displayed on the flight strip or radar data block) or manually via RTF. In cases where an automated system is provided, the actual type of approach being flown should be confirmed by the pilot. The controller then clears the aircraft for the appropriate type of approach.

At airfields which are not capacity limited, the most straightforward mode of operations would be to protect the most restrictive areas regardless of the type of approach being conducted (e.g. to protect the ILS LSA even when aircraft are using GBAS). This has the advantage that a single and simplified set of procedures can be applied to all aircraft. The disadvantage is that this may be unnecessarily restrictive and have an impact on runway capacity.

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