Guide to Aviation Approaches: Types and Technologies Explained

Approaches are a critical part of aviation, guiding pilots through safe and controlled descents, often under challenging conditions. This article explores the types of approaches used in aviation, how technology has evolved to enhance precision and safety, and the global standards set by ICAO (International Civil Aviation Organization) to ensure consistency.

Evolution of Approach Technology

Over the decades, technological advances have dramatically changed how pilots approach and land at airports, especially in low-visibility conditions. Here’s a timeline of key navigation technologies and their impact on aviation:

• Non-Directional Beacons (NDBs): Early in aviation history, NDBs guided aircraft using simple radio transmitters. However, these systems required considerable skill from pilots and were vulnerable to environmental interference.
• VOR (VHF Omnidirectional Range): By the mid-20th century, VOR became the backbone of radio navigation, providing radial information for airways and approach paths. Although VOR provided lateral guidance, it required additional instruments to assist with descent, limiting its precision.
• Instrument Landing System (ILS): Introduced in the 1930s, ILS revolutionized approach technology by offering both lateral (localizer) and vertical (glide slope) guidance. Over the decades, it became the primary approach technology for commercial aviation, allowing for extremely precise landings. ILS categories (CAT I, II, and III) support different levels of visibility, with CAT III enabling landings in nearly zero-visibility conditions. ILS remains a vital tool, especially at major airports.
• Global Navigation Satellite Systems (GNSS) and Augmentation: GNSS technologies, including GPS and augmented systems like WAAS (Wide Area Augmentation System) and GBAS (Ground-Based Augmentation System), enabled Area Navigation (RNAV) and approaches with vertical guidance (APV). These satellite-based systems provide a flexible alternative to ILS, often without requiring ground-based transmitters at each airport.
• Required Navigation Performance (RNP): The introduction of RNP in recent years allows aircraft to navigate with extreme precision, using onboard performance monitoring. RNP has opened up new, safer approaches, particularly for complex or terrain-heavy airfields. It allows aircraft to follow curved approach paths, adapting to challenging environments and runway layouts.

ICAO Standards for Aviation Approaches

The ICAO sets global standards for aviation approaches to ensure consistency, safety, and interoperability across international airspace. These standards define approach categories and establish requirements based on visibility conditions, equipment, and pilot certification.

Categories of Aviation Approaches

Approaches are categorized into three main types based on the guidance they provide:

1. Precision Approaches
   • Instrument Landing System (ILS)
   • Ground-Based Augmentation System (GBAS)
   • Microwave Landing System (MLS)
2. Non-Precision Approaches
   • VOR (VHF Omnidirectional Range) Approach
   • NDB (Non-Directional Beacon) Approach
   • Localizer (LOC) Approach
   • RNAV (Area Navigation) Approach
3. Approaches with Vertical Guidance (APV)
   • LPV (Localizer Performance with Vertical Guidance)
   • LNAV/VNAV (Lateral Navigation/Vertical Navigation)
   • RNP-AR approaches

Historically, ICAO classified approaches into two main categories: Precision Approaches (PA) and Non-Precision Approaches (NPA). With the introduction of Performance-Based Navigation (PBN) in 2014, ICAO updated its approach classifications to better reflect the capabilities of modern navigation systems, which now include approaches both with and without vertical guidance. The new classification consists of Type A and Type B approaches.

Classification of ICAO Approach Types

ICAO has introduced a revised classification system for instrument approach operations to simplify and more accurately reflect the variety of approach and landing operations. This system harmonizes classifications with performance-based navigation (PBN) and optimizes runway requirements for both PBN and conventional approach operations.

Instrument approach operations are now classified by their specified minimum operating minima, below which an approach can only proceed if required visual reference is established.

There are two methods for executing instrument approach operations:
Two-dimensional (2D) Instrument Approach Operations and Three-dimensional (3D) Instrument Approach Operations.

A two-dimensional (2D) instrument approach operation, using lateral navigation guidance only; and A three-dimensional (3D) instrument approach operation, using both lateral and vertical navigation guidance.

Lateral and vertical navigation guidance refers to the guidance provided either by: a ground-based radio navigation aid; or Computer-generated navigation data from ground-based, space-based, self-contained navigation aids or a combination of these.

Instrument approach operations are classified based on the designed lowest operating minima, below which an approach shall only be continued with the required visual reference, as follows:

• Type A Approaches: A minimum descent height or decision height at or above 75 meters (250 feet). Type A approach operations may be either 2D or 3D operations.
• Type B Approaches: A decision height below 75 meters (250 feet). All Type B approach operations are 3D operations. Type B instrument approach operations are further categorized as:
  • Category I (CAT I): A decision height not lower than 60 meters (200 feet) and with either a visibility not less than 800 meters or a runway visual range not less than 550 meters.
  • Category II (CAT II): A decision height lower than 60 meters (200 feet) but not lower than 30 meters (100 feet) and a runway visual range not less than 300 meters.
  • Category III (CAT III): A decision height lower than 30 meters (100 feet) or no decision height and a runway visual range less than 300 meters or no runway visual range limitations.

Types of Approaches in Aviation

Landing safely requires selecting the right approach type based on equipment, weather, and pilot capabilities. Let’s explore each in more detail:

1. Precision Approach (PA)

Precision approaches provide both lateral (left-right) and vertical (up-down) guidance to the pilot. This type of approach is typically used in conditions of low visibility, where precision guidance is essential. Key precision approach systems include:

• Instrument Landing System (ILS)
• Ground-Based Augmentation System (GBAS)
• Microwave Landing System (MLS)

Instrument Landing System (ILS)

The most common precision approach, ILS uses radio signals to guide aircraft to the runway, even in poor visibility. With different categories (CAT I, II, and III), ILS can accommodate landings in varying visibility conditions.

Components of ILS

1. Localizer (LOC): The Localizer (LOC) provides lateral (side-to-side) guidance, helping pilots align the aircraft with the runway centerline during approach and landing. The localizer antenna is positioned at the far end of the runway, emitting VHF signals on a specific frequency between 108.1 MHz and 111.95 MHz. These signals create two overlapping lobes that form a narrow beam down the center of the runway. The aircraft’s onboard ILS receiver interprets these signals to determine if it is left or right of the runway centerline, guiding the pilot or autopilot to stay on course.
2. Glide Slope (GS): Provides vertical (up-down) guidance, guiding the aircraft along an ideal descent angle. The glide slope antenna is located to the side of the runway, near the touchdown zone. It emits signals that create an angled radio beam, typically at a 3° descent angle, guiding the aircraft down to the runway at a safe, steady rate. The Glide Slope (GS) transmits UHF signals (329.15 MHz to 335.0 MHz). The onboard receiver interprets this beam, indicating whether the aircraft is above or below the correct glide path.
3. Marker Beacons (less common today but historically important): Marker beacons transmit signals on a standard frequency of 75 MHz. Each beacon (outer, middle, and inner) emits a distinct signal pattern to provide distance information to the runway, giving pilots audible tones and visual cues that confirm their position along the approach path.
   • Outer Marker (OM): Located about 4-7 miles from the runway, typically where the aircraft should intercept the glide slope.
   • Middle Marker (MM): Positioned around 3,500 feet from the runway threshold, indicating that the aircraft is close to the decision height (where the pilot must decide to land or go around).
   • Inner Marker (IM): Very close to the runway, indicating that the aircraft is at or very near the landing threshold (used only in CAT II/III approaches).
4. Approach Lighting System (ALS) (not technically part of ILS but often associated): ALS is a series of high-intensity lights positioned along the approach path, helping pilots visually align with the runway during the final moments of the approach, especially in low visibility.

The outer marker has often been combined with a non-directional beacon (NDB) to form a Locator Outer Marker (LOM). This combination was a common practice in the past, particularly in the United States. The LOM served as a navigational aid for instrument approaches, providing pilots with both a radial signal from the NDB and a marker beacon signal to indicate their position relative to the runway. However, with the advent of more precise navigation technologies like GPS, the use of LOMs has declined. Many countries, including Canada, have phased out marker beacons altogether, relying on GPS signals or other modern navigation systems.

How ILS Works

• As the aircraft approaches the airport, it first intercepts the localizer beam, typically around 5-10 miles from the runway. The ILS receiver on the aircraft displays deviations from the centerline, allowing the pilot to adjust and align precisely with the runway.
• Once aligned with the localizer, the aircraft begins to intercept the glide slope, which usually happens around the outer marker. This descent angle guides the aircraft down the approach path at a safe rate, following a 3° glide path toward the touchdown zone.
• Throughout the descent, the ILS system continuously provides lateral and vertical guidance. As the aircraft nears the runway, it reaches the decision height (DH) or decision altitude (DA), which is typically around 200 feet for CAT I approaches. If the runway environment is visible, the pilot continues the landing; if not, a missed approach is initiated.

Ground-Based Augmentation System (GBAS)

The Ground-Based Augmentation System (GBAS) is an advanced navigation aid that enhances GPS accuracy, providing high-precision guidance for aircraft approaches and landings. Unlike the Instrument Landing System (ILS), which relies on ground-based radio beams, GBAS uses satellite signals that are augmented by ground stations to improve the precision of the data received by the aircraft. This system is particularly effective for providing Category I through III precision approaches, and it allows for flexible approach paths that can be tailored to specific airport needs and environmental factors.

Components of GBAS

1. GPS Satellites: GPS satellites transmit signals that the aircraft and ground-based receivers use to determine accurate position coordinates. However, these signals alone have inherent inaccuracies due to atmospheric and signal errors, which GBAS helps to correct.
2. Ground Reference Stations: A network of highly accurate ground reference stations at or near the airport continuously monitors GPS satellite signals. These stations compare the satellite data to their known, fixed locations, identifying any deviations due to GPS signal errors. They then calculate correction data to improve GPS accuracy for the approach path.
3. GBAS Ground-Based Transmitter (VHF Data Broadcast, or VDB): The ground-based transmitter receives the correction information from the reference stations and broadcasts it via VHF radio signals to aircraft within the airport’s vicinity. This signal is broadcast continuously and covers a radius of about 23 nautical miles around the airport. The aircraft’s GBAS receiver captures this data and applies the corrections to its GPS-derived position, resulting in highly accurate positioning.
4. GBAS-Equipped Aircraft Receiver: The onboard GBAS receiver in the aircraft interprets both the raw GPS signals and the VHF corrections. It provides the pilot or autopilot with accurate lateral and vertical guidance along the predefined approach path, similar to ILS but with improved flexibility and accuracy.

How GBAS Works

1. Receiving GPS Signals: The aircraft’s GPS receiver and the ground-based reference stations each receive signals from multiple GPS satellites. These signals provide basic position data for navigation but contain slight inaccuracies due to atmospheric delays, satellite clock errors, and signal reflections.
2. Correction Calculation by Ground Stations: The GBAS ground stations, which are precisely located and know their exact positions, measure the GPS signals and calculate the errors between their actual positions and the positions calculated from GPS data. These errors are translated into correction factors for each satellite in view.
3. Broadcasting Corrected Data: The ground stations send the correction data to the VHF transmitter, which then broadcasts the augmented signal over the airport area. This signal includes precise corrections and approach guidance information, providing a seamless GPS-based approach path to the runway.
4. Receiving Augmented Data Onboard: The GBAS receiver on the aircraft receives the VHF data broadcast and applies the corrections to the aircraft’s GPS data, resulting in highly accurate position information. The system guides the aircraft along a smooth, pre-programmed descent path, allowing for approaches with precision comparable to, or better than, CAT III ILS.
5. Executing the Approach: With the corrections applied, the aircraft can follow a highly accurate approach path to the runway, with lateral and vertical guidance provided by the augmented GPS data. Unlike ILS, which limits aircraft to straight-in approaches, GBAS enables curved or segmented approach paths that can navigate around terrain, noise-sensitive areas, or restricted airspace, increasing operational flexibility and safety.

Unlike ILS, which requires separate localizer and glide slope equipment for each runway end, one GBAS installation can support multiple approach paths to multiple runways, reducing infrastructure requirements.

Microwave Landing System (MLS)

The Microwave Landing System (MLS) is a precision approach system that guides aircraft during landing, especially in challenging terrain or environments where flexibility in approach path angles is needed. Unlike the Instrument Landing System (ILS), which uses VHF signals for straight-line approaches, MLS operates in the microwave frequency band, enabling a wider range of approach angles and descent paths, including curved and segmented approaches. This system is particularly useful in airports where standard ILS approaches may be limited due to obstacles or airspace constraints.

Components of MLS

1. Azimuth Antenna: Provides lateral (left-right) guidance to align the aircraft with the runway. This azimuth antenna is typically positioned near the approach end of the runway and emits a highly focused microwave beam that scans horizontally. As the beam sweeps back and forth, the aircraft’s MLS receiver captures it and calculates its position relative to the centerline, guiding it laterally toward the runway.
2. Elevation Antenna: Provides vertical (up-down) guidance for an optimal descent path. This elevation antenna is located near the runway threshold, the elevation antenna emits a vertically scanning microwave beam. The aircraft’s receiver detects this beam and calculates its vertical position, ensuring a stable descent angle to the runway. Unlike the fixed glide slope in ILS, MLS can support a range of descent angles, typically 3° but adjustable if needed for terrain or operational requirements.
3. DME (Distance Measuring Equipment): Measures the distance from the aircraft to the runway threshold. DME operates as part of the MLS system to provide accurate distance information. By receiving distance data, the aircraft’s MLS receiver combines this with azimuth and elevation signals to calculate its precise position in three dimensions, supporting a smooth and accurate descent.

How MLS Works

1. Receiving Azimuth Signal: As the aircraft approaches the runway, the MLS receiver first captures the azimuth signal. The azimuth antenna transmits a microwave beam that sweeps horizontally across the approach sector (usually up to ±40° on each side of the centerline). The receiver on the aircraft detects when the signal reaches it, calculating its lateral position relative to the runway centerline.
2. Receiving Elevation Signal: Simultaneously, the elevation antenna emits a vertically scanning beam covering approach angles between 0.9° and 15°. The MLS receiver detects this vertical scan and calculates the aircraft’s position along the vertical descent path, guiding it down at a stable angle toward the runway. MLS’s flexible beam allows for varying descent angles, making it adaptable to different approach scenarios.
3. Distance Information from DME: Throughout the approach, the DME component provides real-time distance information, enabling the pilot or autopilot system to precisely gauge the aircraft’s position relative to the runway threshold. This allows for smooth, continuous adjustments in descent rate and lateral positioning.
4. Approach Path Adjustments: Unlike ILS, which restricts aircraft to a single, straight-in approach, MLS can support curved or segmented approach paths, adapting to airport or airspace needs. For example, if the runway is surrounded by obstacles or if noise abatement procedures require curved paths, MLS allows the aircraft to follow a curved descent path with accuracy.
5. Landing or Missed Approach Decision: The MLS receiver continues to provide precise guidance down to the runway threshold. If visibility or runway conditions are below acceptable limits, the pilot can initiate a missed approach, using MLS guidance to safely climb and reattempt the approach or divert.

Although MLS is less common than ILS today due to the widespread adoption of GPS-based systems, it remains valuable at certain airports where flexible approach paths, complex terrain, or stringent noise abatement procedures make traditional ILS approaches impractical.

2. Non-Precision Approach (NPA)

Non-Precision Approaches are types of instrument approaches that provide only lateral guidance to the runway, without the vertical (glide path) guidance found in precision approaches like ILS. As a result, NPAs require pilots to manage their descent visually or with calculated descent rates, often making these approaches more challenging, especially in low-visibility or adverse weather conditions.

NPAs are commonly used when precision approach equipment is unavailable, or when the airport is smaller and lacks full Instrument Landing System (ILS) infrastructure. Non-Precision Approach systems include:

• VOR (VHF Omnidirectional Range) Approach
• NDB (Non-Directional Beacon) Approach
• Localizer (LOC) Approach
• RNAV (Area Navigation) Approach

VOR (VHF Omnidirectional Range) Approach

Purpose: Provides lateral guidance using VOR ground stations.

How It Works: The VOR approach uses a VOR station (usually located at or near the airport) to transmit radio signals in a 360° radial pattern. The aircraft’s VOR receiver captures this signal and calculates its position relative to the station, guiding the pilot toward the runway. The pilot adjusts the aircraft’s course based on this radial information. However, VOR approaches offer no vertical guidance, so the pilot must visually manage the descent using altitude steps (step-down altitudes) and a published Minimum Descent Altitude (MDA) where they must have runway visibility to continue.

NDB (Non-Directional Beacon) Approach

An older technology, NDB approaches require pilots to use directional antennas to track the beacon. Due to limitations like environmental interference, NDBs are being phased out.

Purpose: Provides lateral guidance using non-directional radio beacons.

How It Works: An NDB is a ground-based transmitter that sends non-directional signals in all directions. The aircraft’s Automatic Direction Finder (ADF) detects these signals and continuously points to the beacon, allowing the pilot to “home in” on the NDB. The pilot uses this bearing to navigate toward the runway. NDB approaches require careful pilot monitoring, as they can be influenced by atmospheric and magnetic interference. Like VOR, NDB provides no vertical guidance, so pilots follow altitude steps to reach the MDA, where visual runway contact is required.

Localizer (LOC) Approach

Purpose: Provides lateral guidance using the localizer portion of an ILS system.

How It Works: A LOC approach uses only the localizer (lateral component) of an ILS. The localizer antenna, positioned at the far end of the runway, emits a lateral guidance signal, which the aircraft’s receiver detects. This signal guides the pilot to align the aircraft with the runway centerline. Without glide slope guidance, the descent is managed manually by following step-down altitudes. Once at the MDA, if the pilot has sufficient runway visibility, they can continue to land.

RNAV (Area Navigation) Approach

Purpose: Provides lateral guidance through satellite-based or on-board navigation.

How It Works: RNAV approaches use satellite navigation (such as GPS) or on-board systems to guide the aircraft to the runway. Unlike VOR or NDB, RNAV does not depend on ground-based stations near the airport. Instead, the aircraft’s GPS or other navigation systems guide it along a defined flight path with waypoints leading to the runway. RNAV NPAs are typically easier for pilots, as the descent and lateral navigation are pre-programmed into the aircraft’s Flight Management System (FMS), though they still require altitude step-downs. RNAV approaches can be used almost anywhere, providing flexibility for airports lacking traditional navigation aids.

When discussing Non-Precision Approaches (NPAs) that use satellite-based or on-board navigation systems, the on-board system typically refers to the Flight Management System (FMS), Inertial Navigation System (INS), or GPS receiver integrated into the aircraft’s avionics.

3. Approaches with Vertical Guidance (APV)

Approaches with Vertical Guidance (APV) are non-precision approaches that provide both lateral (horizontal) and vertical guidance to pilots. However, they do not meet the stringent requirements of a full-precision approach like an ILS. APV approaches rely heavily on satellite navigation, typically through GPS and augmentation systems, to offer a stable descent path to the runway without requiring extensive ground-based equipment.

How APV Works

APV approaches guide the aircraft on both lateral and vertical paths, similar to precision approaches, but the vertical guidance provided is less precise than that of an ILS glide slope. This guidance allows pilots to maintain a steady descent path, reducing the workload in the final stages of approach and enhancing landing safety, especially in low-visibility conditions.

Key Components of APV Approaches

1. Satellite-Based Positioning (GPS)
   • APV approaches use GPS as the primary source of position information. GPS provides accurate latitude, longitude, and altitude data, allowing aircraft to follow a designated approach path in three dimensions.
   • With GPS data, the aircraft can navigate directly to predefined waypoints that make up the approach, regardless of ground-based navigation aids.
2. Augmentation Systems (e.g., WAAS, EGNOS)
   • Wide Area Augmentation System (WAAS) in the U.S. and European Geostationary Navigation Overlay Service (EGNOS) in Europe are examples of augmentation systems that enhance GPS accuracy and integrity.
   • WAAS and similar systems broadcast correction data that improve the reliability of GPS signals, enabling vertical guidance for approaches. This correction data allows APV approaches to achieve the level of accuracy needed for safe descent without the need for a ground-based ILS system.
3. Flight Management System (FMS) and Flight Display
   • The Flight Management System (FMS) integrates GPS data and calculates the descent profile based on the waypoints of the APV approach. The FMS then displays this calculated path on the pilot’s navigation display, showing the aircraft’s position relative to the intended path.
   • Pilots can follow both the lateral path to stay on course and the vertical path to descend smoothly toward the runway, which is particularly helpful in low-visibility conditions or unfamiliar airports.

Types of APV Approaches

1. LPV (Localizer Performance with Vertical Guidance)
   • LPV is a highly accurate APV approach that uses WAAS to provide lateral and vertical guidance similar to an ILS. LPV can support approach minimums as low as 200 feet above the ground, similar to a Category I ILS approach.
   • It is popular in the U.S., especially at regional airports lacking ILS installations, offering safe approaches without costly infrastructure.
2. LNAV/VNAV (Lateral Navigation/Vertical Navigation)
   • LNAV/VNAV uses GPS and barometric altitude data to provide a stable descent path. While it provides both lateral and vertical guidance, LNAV/VNAV typically has higher minimums than LPV.
   • This approach is often used in areas where WAAS is not available or as a backup if WAAS augmentation temporarily fails.
3. RNP (Required Navigation Performance) AR (Authorization Required) Approaches
   • RNP-AR approaches offer high precision with both lateral and vertical guidance and are typically used in complex or terrain-challenged airports.
   • These approaches require special certification for both aircraft and pilots and are ideal for airports where other approach types are limited due to obstacles or airspace constraints.

Factors Influencing Approach Selection

Several factors influence approach type selection:

• Airport Equipment: Availability of ILS, GBAS, or RNAV systems can dictate the approach type.
• Aircraft Capabilities: Advanced navigation systems, such as RNP, require specific onboard equipment and certification.
• Pilot Certification: Only pilots certified in certain approaches (e.g., CAT III) can perform them.
• Weather Conditions: Low visibility or adverse weather conditions may necessitate precision approaches.

Comparison of Common Approaches

Discover the key differences between ILS, RNAV, VOR, RNP, APV, and NDB approach types in airline operations. This in-depth comparison covers guidance types, infrastructure needs, certification requirements, and common challenges, helping pilots and aviation enthusiasts understand when and why each approach is used.

Future Trends in Approach Technology

The future of approach technology is heading towards even greater precision, safety, and flexibility in aircraft landing procedures. Emerging trends include the integration of satellite-based augmentation systems (SBAS), such as GBAS and SBAS, which offer enhanced accuracy for vertical and lateral guidance without extensive ground infrastructure, making them highly adaptable for remote and underserved airports. There’s also a growing focus on Required Navigation Performance (RNP) with Authorization Required (AR) approaches, which enable aircraft to navigate complex paths with high precision, even in challenging terrain or weather. With increasing reliance on these advanced satellite navigation systems, the industry is looking to reduce dependency on legacy systems like ILS and VOR, which require costly maintenance and can be susceptible to environmental interference.

The International Civil Aviation Organization (ICAO) is actively supporting this evolution through its Global Air Navigation Plan (GANP), which emphasizes Performance-Based Navigation (PBN). ICAO’s goal is to standardize and streamline the implementation of satellite-based and performance-driven navigation procedures across member states, improving interoperability and safety globally. Additionally, ICAO is encouraging the adoption of resilient navigation technologies that integrate multiple GNSS constellations to ensure continuity and reliability in increasingly crowded skies. These initiatives are setting the stage for a future where approach technology is more robust, precise, and globally harmonized.

Performance-Based Navigation (PBN)

PBN is a modern navigation framework developed by ICAO that allows for the design and implementation of flight paths based on the performance capabilities of the aircraft and its navigation systems. PBN can be applied to both en-route and terminal operations, including approaches. PBN includes two main navigation specifications: Area Navigation (RNAV) and Required Navigation Performance (RNP).

PBN includes two main navigation specifications: Area Navigation (RNAV) and Required Navigation Performance (RNP). RNAV allows aircraft to fly on any desired flight path within the coverage of navigation aids or within the limits of self-contained systems. RNP, on the other hand, includes onboard performance monitoring and alerting capabilities, providing a higher level of precision and integrity.

PBN aims to standardize navigation practices globally, ensuring consistency and interoperability across different regions and airspaces. The ICAO PBN Manual (Doc 9613) provides detailed guidance on the implementation and application of PBN.