VOR, short for VHF Omni-directional Radio Range, is a type of radio navigation system for aircraft. VORs broadcast a VHF radio composite signal including the station's morse code identifier (and sometimes a voice identifier), and data that allows the airborne receiving equipment to derive the magnetic bearing from the station to the aircraft (direction from the VOR station in relation to the Earth's magnetic North at the time of installation). This line of position is called the "radial" in VOR parlance. The intersection of two radials from different VOR stations on a chart allows for a "fix" or specific position of the aircraft.

Developed from earlier Visual-Aural Range (VAR) systems the VOR was designed to provide 360 courses to and from the station selectable by the pilot. Early vacuum tube transmitters with mechanically-rotated antennas were widely installed in the 1950's, and began to be replaced with fully solid-state units in the early 1960's. They became the major radio navigation system in the 1960's, when they took over from the older radio beacon and four-course (low/medium frequency range) system. Some of the older range stations survived, with the four-course directional features removed, as non-directional low or medium frequency radio beacons (NDBs).

The VOR's major advantage is that the radio signal provides a reliable line (radial) to or from the station which can be selected and easily followed by the pilot. A worldwide network of "air highways", known in the US as Victor Airways (below 18,000 feet) and "jet routes" (at and above 18,000 feet), was set up linking the VORs and airports. An aircraft could follow a specific path from station to station by tuning the successive stations on the VOR receiver, and then either following the desired course on a Radio Magnetic Indicator, or setting it on a conventional VOR indicator (shown below) or a Horizontal Situation Indicator (HSI, a more sophisticated version of the VOR indicator) and keeping a course pointer centered on the display.

VORs also provided considerably greater accuracy and reliability than NDBs due to a combination of factors in their construction -- specifically, less course bending around terrain features and coastlines, and less interference from thunderstorms. Although VOR transmitters were more expensive to install and maintain, today VOR has almost entirely replaced the low/medium frequency ranges and beacons in civilian aviation, and is now in the process of being supplanted by the Global Positioning System (GPS). Because they work in the VHF band, VOR stations rely on "line of sight" -- if the transmitting antenna could not be seen on a perfectly clear day from the receiving antenna, a useful signal would not be received. This limits VOR (and DME) range to the horizon -- or closer if mountains intervene. This means that an extensive network of stations is needed to provide reasonable coverage along main air routes. The VOR network is a significant cost in operating the current airway system, although the modern solid state transmitting equipment requires much less maintenance than the older units.

If a pilot wants to approach the VOR station from due east then the aircraft will have to fly due west to reach the station. The pilot will use the OBS to rotate the compass dial until the number 27 (270 degrees) aligns with the pointer (called the Primary Index) at the top of the dial. When the aircraft intercepts the 90-degree radial (due east of the VOR station) the needle will be centered and the To/From indicator will show "To". Notice that the pilot set the VOR to indicate the reciprocal; the aircraft will follow the 90-degree radial while the VOR indicates that the course "to" the VOR station is 270 degrees. This is called "proceeding inbound on the 090 radial." The pilot needs only to keep the needle centered to follow the course to the VOR station. If the needle drifts off-center the aircraft would be turned towards the needle until it is centered again. After the aircraft passes over the VOR station the To/From indicator will indicate "From" and the aircraft is then proceeding outbound on the 270 degree radial. The CDI needle may oscillate or go to full scale in the "cone of confusion" directly over the station but will recenter once the aircraft has flown a short distance beyond the station.

In the illustration above, notice that the heading ring is set with 254 degrees at the primary index, the needle is centered and the To/From indicator is showing "From" (FR). The VOR is indicating that the aircraft is on the 254 degree radial, west-southwest "from" the VOR station. If the To/From indicator were showing "To" it would mean the aircraft was on the 74-degree radial and the course "to" the VOR station was 254 degrees. Note that there is absolutely no indication of what direction the aircraft is flying. The aircraft could be flying due north and this snapshot of the VOR could be the moment when it crossed the 254 degree radial.

Taking a position fix with a VOR is no easier than with an NDB however. In both cases two stations must be tuned in and their directions found and plotted on a chart. The VOR does offer somewhat better accuracy in this case due to the nature of the signals, but offsets this slightly by the need to rotate the OBS in order to find the direction to the station.

VORs are assigned radio channels between 108.0 MHz (megahertz) and 117.95 MHz (with 50-kHz spacing); this is in the VHF (very high frequency) range.

The VOR system uses the phase relationship between a reference-phase and a rotating-phase signal to encode direction. The carrier signal is omni-directional and contains the amplitude modulated (AM) station Morse code or voice identifier. The reference 30 Hz signal is frequency modulated (FM) on a 9960 Hz sub-carrier. A second, amplitude modulated (AM) 30 Hz signal is derived from the rotation of a directional antenna array 30 times a second. Although older antennas were mechanically rotated, current installations scan electronically to achieve the same result with no moving parts. When the signal is received in the aircraft, the two 30 Hz signals are detected and then compared to determine the phase angle between them. The phase angle is equal to the direction from the station to the airplane, in degrees from local magnetic north, and is called the "radial."

This information is then fed to one of three common types of indicators:

The typical light-airplane VOR indicator is shown in the accompanying illustration. It consists of a knob to rotate an "Omni Bearing Selector" (OBS), and the OBS scale around the outside of the instrument, used to set the desired course. A "course deviation indicator" (CDI) is centered when the aircraft is on the selected course, or gives left/right steering commands to return to the course. An "ambiguity" (TO-FROM) indicator shows whether following the selected course would take the aircraft to, or away from the station.
A Horizontal Situation Indicator (HSI) is considerably more expensive and complex than a standard VOR indicator, but combines heading information with the navigation display in a much more user-friendly format.
A Radio Magnetic Indicator (RMI) was developed previous to the HSI, and features a course arrow superimposed on a rotating card which shows the aircraft's current heading at the top of the dial. The "tail" of the course arrow points at the current radial from the station, and the "head" of the arrow points at the reciprocal (180 degrees different) course to the station.

In many cases the VOR stations have collocated DME (Distance Measuring Equipment) or military TACAN (TACtical Air Navigation -- which includes both the distance feature, DME, and a separate TACAN azimuth feature that provides military pilots data similar to the civilian VOR). A co-located VOR and TACAN beacon is called a VORTAC. A VOR with co-located DME only is called a VOR-DME. A VOR radial with DME distance allows a one-station position fix. Both VOR-DMEs and TACANs share the same DME system.

DME provides the pilot with the aircraft's "slant" distance from the ground station (i.e. the direct distance, not the distance along the ground from a point directly below the aircraft (which can be calculated using the Pythagorean theorem and the aircraft's altitude)); except very close to the station, the difference between direct and slant distance is negligible. By knowing both the distance and radial from the station, the aircraft's position can be plotted on an aeronautical chart from a single station.

Some VORs have a relatively small geographic area protected from interference by other stations on the same frequency -- called "terminal" or T-VORs. Other stations may have protection out to 130 nautical miles (NM) or more. Although it is popularly thought that there is a standard difference in power output between T-VORs and other stations, in fact the stations' power output is set to provide adequate signal strength in the specific site's service volume.

An excellent tutorial on how to navigate using VOR's can be found by clicking here.

Visual flight rules (VFR) are a set of aviation regulations under which a pilot may operate an aircraft in weather conditions sufficient to allow the pilot, by visual reference to the environment outside the cockpit, to control the aircraft's attitude, navigate, and maintain safe separation from obstacles such as terrain, buildings, and other aircraft.

The essential collision safety principle guiding the VFR pilot is "see and avoid." Pilots flying under VFR assume responsibility for their separation from all other aircraft and are generally not assigned routes or altitudes by air traffic control. Near busier airports, and while operating within certain types of airspace classifications, VFR aircraft in Class B & Class C airspace are required to have a transponder. Governing agencies establish specific requirements for VFR flight, consisting of minimum visibility, distance from clouds, and altitude to ensure that aircraft operating under VFR can be seen from a far enough distance to ensure safety.

From a regulatory perspective, airspace is categorized as controlled and uncontrolled. In controlled airspace known as class B, air traffic control (ATC) will separate VFR aircraft from all other aircraft. In most other types of controlled airspace, ATC is only required to maintain separation to aircraft operating under instrument flight rules (IFR), but workload permitting will assist all aircraft. In the United States, a pilot operating VFR outside of class B airspace can request "VFR traffic following" from air traffic control (ATC). This service is provided by ATC if workload permits it, but is an advisory service only. The responsibility for maintaining separation with other aircraft and proper navigation still remains with the pilot.

Meteorological conditions that meet the minimum requirements for VFR flight are termed visual meteorological conditions (VMC). If they are not met, the conditions are considered instrument meteorological conditions, and a flight may only operate under IFR.

IFR operations have specific training, recency of experience, equipment, and inspection requirements for both the pilot and aircraft, and an IFR flight plan, must usually be filed in advance. For efficiency of operations, some ATC operations will routinely provide "pop-up" IFR clearances for aircraft operating VFR, but that are arriving at an airport that does not meet VMC requirements. For example, in the United States, at least California's Oakland (KOAK), Monterey (KMRY) and Santa Ana (John Wayne, KSNA) airports do so routinely when a low coastal overcast forces instrument approaches while essentially the entire state of California is basking in sunshine.

In the United States, VFR pilots also have an option for requesting Special VFR when meteorological conditions at an airport are below normal VMC minimums, but above Special VFR requirements. Special VFR is only intended to enable takeoffs and landings from airports that are near to VMC conditions, and may only be performed during daytime hours if a pilot does not possess an instrument rating.

VFR flight is not allowed in airspace known as class A, regardless of the meteorological conditions. In the United States, class A airspace begins at 18,000 feet msl, and extends to altitudes that can safely be considered near space.

Special visual flight rules (SVFR) are a set of aviation regulations under which a pilot may operate an aircraft.

Flight under SVFR is only allowed in control zones, and always requires clearance from air traffic control (ATC). It usually happens under two circumstances:

In Class A airspace, flight under visual flight rules (VFR) is not permitted and instrument flight rules (IFR) flight is the norm. Pilots may as an alternative to IFR request an SVFR clearance to enter the airspace and fly visually.
In other controlled airspace, when the local weather is less than the minimums required for flight under visual flight rules (VFR) and again IFR would be the norm. Pilots may again as an alternative to IFR request an SVFR clearance to enter the airspace and fly visually.
The aircraft need not necessarily be equipped for flight under IFR, and the aircraft must remain clear of clouds and maintain certain flight visibility minimums (1850 meters according to ICAO, one statute mile in the US). The pilot continues to be responsible for obstacle and terrain clearance.

An example of the use of SVFR is when a flight wishes to leave an airport in a control zone, to fly VFR in uncontrolled airspace, when the visibility is below the minimum for VFR flight in the control zone but not below the lower minimum for VFR flight in uncontrolled airspace.

According to the FAA, SVFR at night requires an IFR-equipped aircraft and an IFR-rated pilot in command.

Instrument flight rules (IFR) are a set of regulations and procedures for flying aircraft whereby navigation and obstacle clearance is maintained with reference to aircraft instruments only, while separation from other aircraft is provided by Air Traffic Control. In layman's terms, a pilot who is rated for IFR can keep a plane in controlled flight solely on the data provided by his instruments, even if that pilot cannot see anything (useful) out the cockpit windows; indeed, one of the benefits of these regulations are the ability to navigate fly through clouds, which is otherwise not allowed.

IFR is an alternative to visual flight rules (VFR), where the pilot is ultimately responsible for navigation, obstacle clearance and traffic separation using the see-and-avoid concept. The vast majority of commercial traffic (any flight for hire) and all scheduled air carriers operate exclusively under IFR. Commercial aircraft providing sight seeing flights, aerial photography, or lift services for parachute jumping usually operate under VFR.

Separation

The distance by which an aircraft avoids obstacles or other aircraft is termed separation. The most important concept of IFR flying is that separation is maintained regardless of meteorological visibility conditions. In controlled airspace, Air Traffic Control (ATC) separates IFR aircraft from obstacles and other IFR and known VFR aircraft by applying a flight clearance based on route, time, distance, speed, and altitude differences between aircraft. ATC monitors IFR flights by relying either on radar or aircraft position reports. Aircraft position reports are traditionally sent as voice radio transmissions, but increasingly also as electronic data exchanges. Aircraft position reports are not necessary if ATC has an aircraft in radar contact. In the United States a flight operating under IFR is required to fall back to position reports if advised radar contact lost.

IFR flights require an ATC clearance for each part of the flight. A clearance always specifies a clearance limit, which is the farthest the aircraft can fly without a new clearance. In addition, a clearance typically provides a heading or route to follow, altitude, and communication parameters, such as frequencies and transponder codes. An aircraft operating VFR must also obtain a clearance to enter class B and class C airspace, and is required to maintain an assigned heading or altitude restriction as long as it does not conflict with the safe operation of the aircraft.

In uncontrolled airspace, IFR aircraft do not require clearances, and they separate themselves from each other by using charted minimum altitudes to avoid terrain and obstacles, standard cruising altitudes to avoid aircraft flying in different directions, and radio reports over mandatory locations.

In the United States and the Southern Domestic Airspace of Canada (SDA), airspace from 18,000 to 60,000 feet (5,586 to 18,288 meters) is designated as class A, requiring an IFR clearance for all aircraft. In other countries class A airspace begins higher or lower. For example, in France class A airspace begins at 19,500 feet (5,850 meters).

In the United States even when on a filed IFR flight plan, if conditions permit the pilot is responsible to maintain a watch for, and avoid other air traffic and obstructions. Separation may also be referred to as 'protection'.

Weather

One main purpose of IFR is the safe operation of aircraft in Instrument Meteorological Conditions (IMC). The weather is considered to be IMC when it does not meet the minimum requirements for Visual Meteorological Conditions. To operate safely in IMC , a pilot controls the aircraft relying on flight instruments, and ATC provides separation.

VMC Flying under IFR Rules

It is important to not to confuse IFR with IMC. The vast majority of IFR flying is conducted in Visual Meteorological Conditions (VMC). Any time a flight is operating in VMC, the crew is responsible for seeing and avoiding other traffic, however, since the flight is conducted under Instrument Flight Rules, ATC still provides separation services.

During flight under IFR, there are no visibility requirements, and as such flying through clouds is permitted. There are still minimum conditions that must be present in order for the aircraft to take off and land; these will vary according to the type of navigation aids available, the location and height of terrain and obstructions in the vicinity of the airport, equipment on the aircraft, and according to qualifications of the crew. For example, landing at mountain airports such as Reno (KRNO) offer significantly different instrument approaches for aircraft landing on the same runway, but from opposite directions. Aircraft approaching from the north must make visual contact with the airport at a higher altitude than a flight approaching from the south, because of rapidly rising terrain south of the airport. This higher altitude allows a flight crew to start a climb earlier in case landing is not feasible.

Although large airliners and, increasingly, smaller aircraft now carry their own terrain- and collision-avoidance systems such as TCAS, these are primarily backup systems providing a last layer of defense if a sequence of errors or omissions causes a dangerous situation.

Navigation

Under IFR, the primary means of navigation are either via radio beacons on the ground, such as VORs and NDBs, or GPS. In areas of radar coverage, ATC may also assign headings to IFR aircraft, also known as radar vectors. Radar vectors are the primary method for ATC to provide separation between aircraft for landing.

Modern Flight Management Systems have evolved sufficiently to allow a crew to plan a flight not only as to route and altitude, but to specific time of arrival at specific locations. This capability is used in several trial projects experimenting with four dimensional approach clearances for commercial aircraft, with time as the fourth dimension. These clearances allow ATC to optimize the arrival of aircraft at major airports, which increases airport capacity, and uses less fuel providing monetary and environmental benefits to airlines and the public at large respectively.

Procedures

There are three stages to an IFR flight: departure, en route, and approach. For each stage there are standard, published procedures to allow IFR aircraft to move in a safe, orderly way, from the moment the wheels leave the runway to the moment they touch down again. These procedures also allow an IFR aircraft to complete a flight predictably in case of communication failure (lost-comm) with ATC, with default altitudes and headings for every stage. An IFR flight typically starts with an IFR clearance, which specifies the departure instructions, and any modifications to the route. Here is an example of an IFR clearance, for a Cessna aircraft traveling from Palo Alto airport (KPAO) to Stockton airport (KSCK).

Detailed explanation:

"Cessna 6253G"
Verifies that only this specific aircraft is cleared.
"is cleared to Stockton Airport."
Clearance Limit: the farthest destination the aircraft is allowed to go under IMC (in most cases it is the destination airport).
"After departure, turn right heading zero-six-zero within one mile of the airport."
The Pilot is expected to execute the turn without further ATC prompting.
"Radar Vectors San Jose"
The departure controller will provide directional guidance to the San Jose VOR.
"Victor-334, SUNOL, Victor-195, Manteca, direct."
After arriving at the San Jose VOR, the pilot will likely resume navigation without ATC prompts along the described airways and intersection to the Manteca VOR and then direct to the destination airport.
"Climb and maintain 3,000 ..."
After takeoff, climb to an altitude of 3000 feet above sea level.
"... expect 5,000 five minutes after departure."
Your final altitude assignment is probably going to be 5000 feet above sea level. However, you must follow actual ATC altitude assignments throughout the flight. This portion of the clearance provides a backup if communications are lost, allowing you to proceed to climb and maintain 5000 feet.
"Departure frequency is 121.3, ..."
Contact with NORCAL Departure on the specified communication frequency, after Palo Alto Tower tells you to switch.
"... squawk 4263."
Program your transponder to 4263 so that ATC can positively identify you on radar.
Departures are described in an IFR clearance issued by ATC prior to takeoff. The departure clearance may contain an assigned heading, one or more waypoints, and an initial altitude to fly. The clearance can also specify a departure procedure (DP), or standard instrument departure (SID) that should be followed unless "NO DP" is specified in the notes section of the filed flight plan.

En route flight is described by IFR charts showing navigation aids, fixes, and standard routes called airways. Aircraft with appropriate navigational equipment such as GPS, are also often cleared for a direct-to routing, where only the destination, or a few navigational waypoints are used to describe the route that the flight will follow. ATC will assign altitudes in its initial clearance or amendments thereto, and navigational charts indicate minimum safe altitudes for airways.

The approach portion of an IFR flight may begin with a Standard Terminal Arrival Route (STAR), describing common routes to fly to arrive at an initial approach fix (IAF) from which an instrument approach commences. Instrument approaches are categorized as precision and non-precision. Despite the names, a precision approach simply indicates that vertical guidance (as well as lateral guidance) is being used. non-precision indicates only lateral guidance.

In either case, an instrument approach will terminate either in visual conditions sufficient for a normal landing of the airplane, or in a missed approach if such conditions are not encountered in time. The point at which the crew of an aircraft has to make a decision to either proceed visually, or "miss" the approach is called either the Decision Altitude (DA) or Decision Height (DH) for precision approaches, and missed approach point (MAP) for non-precision approaches. During precision approaches the altitude of the aircraft is determined by the navigational instruments. For non-precision approaches the crew will descend to specific altitudes during the approach procedure, with the final altitude termed the Minimum Descent Altitude (MDA).

Some commercial aircraft are equipped with automatic landing systems that allow the aircraft to land without transitioning from instruments to visual conditions for a normal landing. Such Autoland operations require specialized equipment, procedures and training, and involve the aircraft, airport, and the crew. Autoland is the only way some major airports such as Paris CDG remain operational every day of the year. Some modern aircraft are equipped with enhanced vision systems based on infrared sensors, that provide a day-like visual environment and allow operations in conditions and at airports that would otherwise not be suitable for a landing. Commercial aircraft also frequently use such equipment for takeoffs when takeoff minimums are not met.[1]

Documents describing the approach procedure are also frequently called approach plates in reference to the plate-like appearance of single-page sheet that it is printed on.

An instrument approach that terminates in a missed approach will continue using missed approach procedure information shown on the approach procedure. Typically it describes a transition to a nearby navigational fix, from which the instrument approach can be attempted again. In practice an approach that terminates in a missed approach rarely flies the missed approach procedure as published. Instead, ATC will assign headings and altitudes that will weave the aircraft into the arriving traffic for a repeated approach attempt. The crew may also request an alternate destination, either a previously stated alternate airport, or other suitable airport considering the prevailing weather conditions.

An ILS consists of two independent sub-systems, one providing lateral guidance (Localizer), the other vertical guidance (Glideslope or Glide Path) to aircraft approaching a runway. Aircraft guidance is provided by the ILS receivers in the aircraft by performing a modulation depth comparison.

The emission patterns of the localizer and glideslope signals. Note that the glideslope beams are partly formed by the reflection of the glideslope aerial in the ground plane.A localizer (LOC, or LLZ in Europe) antenna array is normally located beyond the departure end of the runway and generally consists of several pairs of directional antennas. Two signals are transmitted on a carrier frequency between 108.000 MHz and 111.975 MHz. One is modulated at 90 Hz, the other at 150 Hz and these are transmitted from separate but co-located antennas. Each antenna transmits a narrow beam, one slightly to the left of the runway centerline, the other to the right.

The localizer receiver on the aircraft measures the Difference in the Depth of Modulation (DDM) of the 90 Hz and 150 Hz signals. For the localizer, the depth of modulation for each of the modulating frequencies is 20 percent. The difference between the two signals varies depending on the position of the approaching aircraft from the centerline.

If there is a predominance of either 90 Hz or 150 Hz modulation, the aircraft is off the centerline. In the cockpit, the needle on the Horizontal Situation Indicator, or HSI (The Instrument part of the ILS), or CDI (Course deviation indicator), will show that the aircraft needs to fly left or right to correct the error to fly down the center of the runway. If the DDM is zero the aircraft is on the centerline of the localizer coinciding with the physical runway centerline.

A glideslope or Glidepath (GP) antenna array is sited to one side of the runway touchdown zone. The GP signal is transmitted on a carrier frequency between 329.15 and 335 MHz using a technique similar to that of the localizer. The centerline of the glideslope signal is arranged to define a glideslope of approximately 3° above horizontal (ground level).

Localizer and glideslope carrier frequencies are paired so that only one selection is required to tune both receivers.

These signals are displayed on an instrument in the cockpit. The pilot controls the aircraft so that the indications on the instrument remain centered on the display. This ensures the aircraft is following the ILS centerline and will touch down on the runway at the correct point. Some aircraft possess the ability to route signals into the autopilot, allowing the approach to be flown automatically by the autopilot.

Use of the ILS System

At large airports, air traffic control will direct aircraft to the localizer via assigned headings, making sure aircraft do not get too close to each other (maintain separation), but also avoiding delay as much as possible. Several aircraft can be on the ILS at the same time, several miles apart. An aircraft that has intercepted both the localizer and the glideslope signal is said to be established on the approach. Typically, an aircraft will be established by 6 nautical miles (11 km) from the runway, or just after reaching the Final Approach Fix.

Aircraft deviation from the optimal path is indicated to the flight crew by means of display with "needles" (a carry over from when an analog meter movement would indicate deviation from the course line via voltages sent from the ILS receiver).

The output from the ILS receiver goes both to the display system (Head Down Display and Head-Up Display if installed) and can also go to the Flight Control Computer. An aircraft landing procedure can be either "coupled", where the Flight Control Computer directly flies the aircraft and the flight crew monitor the operation; or "uncoupled" (manual) where the flight crew fly the aircraft uses the HUD and manually control the aircraft to minimize the deviation from flight path to the runway centerline.

A very well written tutorial on how to perform ILS approaches in Flight Simulator can be read by clicking here.

Rate of Descent Formula

A useful formula pilots use to calculate the descent rate on the glideslope.

Rate of Descent = Glideslope Angle × Groundspeed / 60 × 100

Where:

• Rate of Descent is in feet per minute
• Glideslope angle is in degrees from the horizontal (Usually 3 degrees)
• Groundspeed is in knots
• If the glideslope is the standard 3 degrees then the formula can be further simplified to:

Rate of Descent = 5 × Groundspeed

Decision Altitude/Height

Once established on an approach, the Autoland system or pilot will follow the ILS and descend along the glideslope, until the Decision Altitude is reached (for a typical Category I ILS, this altitude is 200 feet above the runway). At this point, the pilot must have the runway or its approach lights in sight to continue the approach.

If neither can be seen, the approach must be aborted and a missed approach procedure will be performed. This is where the aircraft will climb back to a predetermined altitude. From there the pilot will either try the same approach again or divert to another airport.

Aborting the approach (as well as the ATC instruction to do so) is called executing a missed approach.

ILS categories

There are three categories of ILS which support similarly named categories of operation.

• Category I - A precision instrument approach and landing with a decision height not lower than 200 feet (61 m) above touchdown zone elevation and with either a visibility not less than 2,625 feet (800 m) or a runway visual range not less than 1,800 feet (550 m). An aircraft equipped with an Enhanced Flight Vision System may, under certain circumstances, continue an approach to CAT II minimums. [14 CFR Part 91.175 amendment 281]
• Category II - Category II operation: A precision instrument approach and landing with a decision height lower than 200 feet (61 m) above touchdown zone elevation but not lower than 100 feet (30 m), and a runway visual range not less than 1,150 feet (350 m).
• Category III is further subdivided
• Category III A - A precision instrument approach and landing with:
  a) a decision height lower than 100 feet (30 m) above touchdown zone elevation, or no decision height; and
  b) a runway visual range not less than 655 feet (200 m).
• Category III B - A precision instrument approach and landing with:
  a) a decision height lower than 50 feet (15 m) above touchdown zone elevation, or no decision height; and
  b) a runway visual range less than 2,625 feet (800 m) but not less than 165 feet (50 m).
• Category III C - A precision instrument approach and landing with no decision height and no runway visual range limitations. A Category III C system is capable of using an aircraft's autopilot to land the aircraft and can also provide guidance along the runway surface.
  In each case a suitably equipped aircraft and appropriately qualified crew are required. For example, Cat IIIc requires a fail-operational system, Cat I does not. A Head-Up Display which allows the pilot to perform aircraft maneuvers rather than an automatic system is considered as fail-operational. Cat I relies only on altimeter indications for decision height, whereas Cat II and Cat III approaches use radar altimeter to determine decision height.

An ILS is required to shut down upon internal detection of a fault condition as mentioned in the monitoring section. With the increasing categories, ILS equipment is required to shut down faster since higher categories require shorter response times. For example, a Cat I localizer must shutdown within 10 seconds of detecting a fault, but a Cat III localizer must shut down in less than 2 seconds.

Localizer

In addition to the previously mentioned navigational signals, the localizer provides for ILS facility identification by periodically transmitting a 1020 Hz morse code identification signal. For example, the ILS for runway 04R at John F. Kennedy International Airport transmits IJFK to identify itself, while runway 04L is known as IHIQ. This lets users know the facility is operating normally and that they are tuned to the correct ILS. The glideslope transmits no identification signal, so ILS equipment relies on the localizer for identification.

Modern localizer antennas are highly directional. However, usage of older, less directional antennas allows a runway to have a non-precision approach called a localizer back course. This lets aircraft land using the signal transmitted from the back of the localizer array. This signal is reverse sensing so a pilot may have to fly opposite the needle indication (depending on the equipment installed in the aircraft). Highly directional antennas do not provide a sufficient signal to support a backcourse. In the United States, backcourse approaches are commonly associated with Category I systems at smaller airports that do not have an ILS on both ends of the primary runway.

Marker beacons

On most installations marker beacons operating at a carrier frequency of 75 MHz are provided. When the transmission from a marker beacon is received it activates an indicator on the pilot's instrument panel and the tone of the beacon is audible to the pilot. The distance from the runway at which this indication should be received is promulgated in the documentation for that approach, together with the height at which the aircraft should be if correctly established on the ILS. This provides a check on the correct function of the glideslope. In modern ILS installations a DME is installed, co-located with the ILS, to augment or replace marker beacons. A DME continuously displays the aircraft's distance to the runway.

Outer marker

The outer marker should be located 7.2 km (3.9 NM) from the threshold except that, where this distance is not practicable, the outer marker may be located between 6.5 and 11.1 km (3.5 and 6 NM) from the threshold. The modulation is repeated Morse-style dashes of a 400 Hz tone. The cockpit indicator is a blue lamp that flashes in unison with the received audio code. The purpose of this beacon is to provide height, distance and equipment functioning checks to aircraft on intermediate and final approach. In the United States, an NDB is often combined with the outer marker beacon in the ILS approach (called a Locator Outer Marker, or LOM); in Canada, low-powered NDBs have replaced marker beacons entirely.

Middle marker

The middle marker should be located so as to indicate, in low visibility conditions, the missed approach point, and the point that visual contact with the runway is imminent, Ideally at a distance of approximately 3,500 ft (1,100 m) from the threshold. It is modulated with a 1300 Hz tone as alternating dots and dashes. The cockpit indicator is an amber lamp that flashes in unison with the received audio code.

Inner marker

The inner marker, when installed, shall be located so as to indicate in low visibility conditions the imminence of arrival at the runway threshold. This is typically the position of an aircraft on the ILS as it reaches Category II minima. Ideally at a distance of approximately 100 ft (30 m) from the threshold. The modulation is Morse-style dots at 3000 Hz. The cockpit indicator is a white lamp that flashes in unison with the received audio code.

DME

Distance Measuring Equipment (DME) provides pilots with a slant range measurement of distance to the runway in nautical miles. DMEs are augmenting or replacing markers in many installations. The DME provides more accurate and continuous monitoring of correct progress on the ILS glideslope to the pilot, and does not require an installation outside the airport boundary. When used in conjunction with an ILS, the DME is often sited midway between the reciprocal runway thresholds with the internal delay modified so that one unit can provide distance information to either runway threshold. On approaches where a DME is specified in lieu of marker beacons, the aircraft must have at least one operating DME unit to begin the approach, and a "DME Required" restriction will be noted on the Instrument Approach Procedure.

Monitoring

It is essential that any failure of the ILS to provide safe guidance be detected immediately by the pilot. To achieve this, monitors continually assess the vital characteristics of the transmissions. If any significant deviation beyond strict limits is detected, either the ILS is automatically switched off or the navigation and identification components are removed from the carrier. Either of these actions will activate an indication ('failure flag') on the instruments of an aircraft using the ILS.

Approach lighting

Some installations include medium or high intensity approach light systems. Most often, these are at larger airports. The Approach Lighting System (abbreviated ALS) assists the pilot in transitioning from instrument to visual flight, and to align the aircraft visually with the runway centerline. At many non-towered airports, the intensity of the lighting system can be adjusted by the pilot.

Distance Measuring Equipment (DME) is a transponder-based radio navigation technology that measures distance by timing the propagation delay of VHF or UHF radio signals.

It was invented by Edward George "Taffy" Bowen whilst employed as Chief of the Division of Radiophysics of the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Australia. Another Australian world-first, engineered version of the system was deployed by Amalgamated Wireless Australasia Limited in the early 1950's operating in the 200 MHz VHF band. This Australian domestic version was referred by the Federal Department of Civil Aviation as DME(D) (or DME Domestic), and the later international version adopted by ICAO as DME(I).

DME is similar to Secondary Radar, except in reverse. The system was a post-war development of the IFF (Identification Friend or Foe) systems of World War II. To maintain compatibility, DME is functionally identical to the distance measuring component of TACAN.

Operation

Aircraft use DME to determine their distance from a land-based transponder by sending and receiving pulse pairs - two pulses of fixed duration and separation. The ground stations are typically collocated with VORs. A typical DME ground transponder system for en route or terminal navigation will have a 1 kW peak pulse output on the assigned UHF channel.

A low power DME can also be collocated with an ILS localizer where it provides an accurate distance function, similar to that otherwise provided by ILS Marker Beacons.

DME frequencies are paired to VHF omnidirectional range (VOR) frequencies. A DME interrogator is designed to automatically tune to the corresponding frequency when the associated VOR is selected. An airplanes DME interrogator uses frequencies from 1025 to 1150 MHz. DME transponders transmit on a channel in the 962 to 1150 MHz range and receive on a corresponding channel between 962 to 1213 MHz. The band is divided into 126 channels for interrogation and 126 channels for transponder replies. The interrogation and reply frequencies always differ by 63 MHz. The spacing of all channels is 1 MHz with a signal spectrum width of 100 kHz.

Technical references to X and Y channels relate only to the spacing of the individual pulses in the DME pulse pair, 12 microsecond spacing for X channels and 36 microsecond spacing for Y channels.

DME facilities identify themselves with a 1350 Hz morse code three letter identity. If collocated with a VOR or ILS it will have the same identity code as the parent facility. Additionally, the DME will identify itself between those of the parent facility. DME identity is 1350 Hz to differentiate itself from the 1020 Hz tone of the VOR or the ILS localizer.

The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). Utilizing a constellation of at least 24 Medium Earth Orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its location, speed, direction, and time. Other similar systems are the Russian GLONASS (incomplete as of 2007), the upcoming European Galileo positioning system, the proposed COMPASS navigation system of China, and IRNSS of India.

Developed by the United States Department of Defense, GPS is officially named NAVSTAR GPS (Contrary to popular belief, NAVSTAR is not an acronym, but simply a name given by John Walsh, a key decision maker when it came to the budget for the GPS program).[1] The satellite constellation is managed by the United States Air Force 50th Space Wing. The cost of maintaining the system is approximately US$750 million per year,[2] including the replacement of aging satellites, and research and development.

Following the shoot down of Korean Air Lines Flight 007 in 1983, President Ronald Reagan issued a directive making the system available for free for civilian use as a common good.[3] Since then, GPS has become a widely used aid to navigation worldwide, and a useful tool for map-making, land surveying, commerce, and scientific uses. GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.

A typical GPS receiver calculates its position using the signals from four or more GPS satellites. Four satellites are needed since the process needs a very accurate local time, more accurate than any normal clock can provide, so the receiver internally solves for time as well as position. In other words, the receiver uses four measurements to solve for 4 variables - x, y, z, and t. These values are then turned into more user-friendly forms, such as latitude/longitude or location on a map, then displayed to the user.

Each GPS satellite has an atomic clock, and continually transmits messages containing the current time at the start of the message, parameters to calculate the location of the satellite (the ephemeris), and the general system health (the almanac). The signals travel at a known speed - the speed of light through outer space, and slightly slower through the atmosphere. The receiver uses the arrival time to compute the distance to each satellite, from which it determines the position of the receiver using geometry and trigonometry (see trilateration[4])

Although four satellites are required for normal operation, fewer may be needed in some special cases. For example, if one variable is already known (for example, a sea-going ship knows its altitude is 0), a receiver can determine its position using only three satellites. Also, in practice, receivers use additional clues (Doppler shift of satellite signals, last known position, dead reckoning, inertial navigation, and so on) to give degraded answers when fewer than four satellites are visible.

The worlds navigable airspace is divided into three-dimensional segments, each of which is assigned to a specific class. Most nations adhere to the classification specified by the International Civil Aviation Organization (ICAO) and described below. Individual nations also designate Special Use Airspace, which places further rules on air navigation for reasons of national security or safety.

On March 12, 1990, ICAO adopted the current airspace classification scheme. The classes are fundamentally defined in terms of flight rules and interactions between aircraft and Air Traffic Control (ATC). Some key concepts are:

Separation: Maintaining a specific minimum distance between an aircraft and another aircraft or terrain to avoid collisions, normally by requiring aircraft to fly at set levels or level bands, on set routes or in certain directions, or by controlling an aircraft's speed.
Clearance: Permission given by ATC for an aircraft to proceed under certain conditions contained within the clearance.
Traffic Information: Information given by ATC on the position and, if known, intentions of other aircraft likely to pose a hazard to flight.

Click on the PDF icon for the FAA explanation of airspace classes.

The classifications adopted by ICAO are:

• Class A: All operations must be conducted under Instrument Flight Rules (IFR) or Special visual flight rules (SVFR) and are subject to ATC clearance. All flights are separated from each other by ATC.
• Class B: Operations may be conducted under IFR, SVFR, or Visual flight rules (VFR). All aircraft are subject to ATC clearance. All flights are separated from each other by ATC.
• Class C: Operations may be conducted under IFR, SVFR, or VFR. All flights are subject to ATC clearance. Aircraft operating under IFR and SVFR are separated from each other and from flights operating under VFR. Flights operating under VFR are given traffic information in respect of other VFR flights.
• Class D: Operations may be conducted under IFR, SVFR, or VFR. All flights are subject to ATC clearance. Aircraft operating under IFR and SVFR are separated from each other, and are given traffic information in respect of VFR flights. Flights operating under VFR are given traffic information in respect of all other flights.
• Class E: Operations may be conducted under IFR, SVFR, or VFR. Aircraft operating under IFR and SVFR are separated from each other, and are subject to ATC clearance. Flights under VFR are not subject to ATC clearance. As far as is practical, traffic information is given to all flights in respect of VFR flights.
• Class F: Operations may be conducted under IFR or VFR. ATC separation will be provided, so far as practical, to aircraft operating under IFR. Traffic Information may be given as far as is practical in respect of other flights.
• Class G: Operations may be conducted under IFR or VFR. ATC separation is not provided. Traffic Information may be given as far as is practical in respect of other flights.

Classes A-E are referred to as controlled airspace. Classes F and G are uncontrolled airspace.

As of 2004, ICAO is considering a proposal to reduce the number of airspace classifications to three, which roughly correspond to the current classes C, E and G.

Use of airspace classes

Each national aviation authority determines how it uses the ICAO classifications in its airspace design. In some countries, the rules are modified slightly to fit the airspace rules and air traffic services that existed before the ICAO standardization.

United States

The U.S. adopted a slightly modified version of the ICAO system on September 16, 1993, when regions of airspace designated according to older classifications were converted wholesale. The exceptions are some Terminal Radar Service Areas (TRSA), which have special rules and still exist in a few places.

With some exceptions, Class A airspace is applied to all airspace between 18,000 feet (5,500 m) and Flight Level 600 (approximately 60,000 ft). Above FL600, the airspace reverts to Class E (Reference Order 7400.9P, Subpart E). The transition altitude (see Flight level) is also consistently 18,000 feet (5,500 m). All operations in US Class A airspace must be conducted under IFR. SVFR flight in Class A airspace is prohibited.

Class B airspace is used around major airports, in a funnel shape that is designed to contain arriving and departing commercial air traffic operating under IFR, up to 10,000 feet (3,000 m) above MSL (12,000 feet above Denver, Colorado). Class C airspace is used around airports and military air bases with a moderate traffic level. Class D is used for smaller airports that have a control tower. The U.S. uses a modified version of the ICAO class C and D airspace, where only radio contact with ATC rather than an ATC clearance is required for VFR operations.

Other controlled airspace is designated as Class E - this includes a large part of the lower airspace. Class E airspace exists in many forms. It can serve as a surface-based extension to Class D airspace to accommodate IFR approach/departure procedure areas. Class E airspace can be designated to have a floor of 700' AGL (above ground level) or 1,200' AGL. Class E airspace exists above Class G surface areas from 14,500' MSL (mean sea level) to 18,000 MSL. Federal airways from 1,200 AGL to 18,000 MSL within 4 miles (6 km) of the centerline of the airway is designated Class E airspace. Airspace at any altitude over 60,000' (the ceiling of Class A airspace) is designated Class E airspace.

The U.S. does not use ICAO Class F.

Class G airspace (Uncontrolled) is mostly used for a small layer of airspace near the ground, but there are larger areas of Class G airspace in remote regions.

Canada

Canada generally follows the United States in application of airspace with some differences. For example, Canadian class "C" airspace is procedurally equivalent to United States class "B" airspace. Additionally, the term "Class F" is used for Special Use Airspace, this includes Advisory airspace and Restricted airspace.

Germany

In Germany, Classes A and B are generally not used at all. Class C is used for all Airspace above Flight Level (FL) 100 (or FL 130 near the Alps). Airspace is divided into lower airspace below FL 285 and upper airspace above FL 285.

• Class A is not used.
• Class B is not used.
• Class C is used for controlled zones above and around airports and all airspace above FL 100 (or FL 130 near the Alps.)
• Class D is used for controlled zones or above and around airspace class C designated zones where CVFR is not necessary.
• Class E is used for airspace between usually 2,500 ft (760 m). AGL (around airports 1,000 ft (300 m). or 1700 ft. AGL) and FL 100.
• Class F is used for IFR-Flight in uncontrolled airspace.
• Class G is used below 2,500 ft (760 m). AGL (around airports below 1,000 ft (300 m). AGL, then rises via a step at 1,700 ft (520 m). to 2,500 ft (760 m). AGL)

Lithuania

In Lithuania, Classes A and B are generally not used at all. Classes C and D are used in the following areas of controlled airspace of the Republic of Lithuania:

• in control zones (CTR);
• in terminal control areas (TMA);
• in control area (CTA);
• in upper control area (UTA).
• Source: Airfield Guide Lithuania, 29 SEP 2005, ENR 1.1-1

United Kingdom

Class A

• All airways up to FL 195 with the exception of airways lying within the Belfast CTR/TMA and the Scottish TMA.
• The Terminal Control Areas (TMAs) around London and Manchester.
• The London Control Zone around Heathrow and the Channel Islands Control Zone; these areas are thus off-limits to VFR flights (however Special VFR is used as a get-around for this).
• The CTAs of Daventry, Cotswold and Worthing.
• Class C

All UK airspace between FL 195 and FL 660. N.b: The Upper Flight Information Region (UIR) boundary begins at FL 245

Class D

• The CTRs and CTAs around the larger airfields except London Heathrow, such as London Gatwick, Glasgow, Birmingham and Newcastle upon Tyne.
• A few airways in less busy areas allowing mid-level military VFR flights.

Class E

• Parts of the Belfast and Scottish TMAs and a small part of the Durham Tees Valley CTR.

Class F

• "Advisory Routes" (ADRs): regularly used routes similar to airways but where traffic levels are not high enough to warrant establishment of an airway.

Class G

• All remaining airspace, comprising by far the largest part of the airspace below FL 195. The UK is unusual in that IFR flight in Class G airspace is relatively common and ATC units may provide an "as far as is practical" form of separation between some such flights.

In addition the UK has a couple of special classes of airspace that do not fall within the ICAO classes:

• Airdrome Traffic Zones (ATZ) are zones of between 1.5 nm and 2.5 nm from the surface to 2,000 ft (600 m) AAL set up around an airport, where aircraft must obey the instructions of the tower controller (if present), must make radio contact with the Information Officer or Air/Ground radio unit on the airport before entering the zone (in the case of an uncontrolled airfield), or must obey ground signals if non-radio.
• Military Air Traffic Zones (MATZ) are zones from the surface to 3,000 ft (900 m) AAL set up around military air bases in class G airspace. Military aircraft treat these as if they are controlled airspace; civilian traffic is advised but not obliged to do the same.

Australia

Australia has adopted a civil airspace system based on the United States National Airspace System (NAS):

• Class A is used above FL 180 along the populated coastal areas, and above FL 245 elsewhere.
• Class B is not used.
• Class C is used in a 360° funnel shape in the Terminal Control Zones of the major international airports, extending up to the base of the Class A, generally at FL 180 over these airports. It also overlays Class D airspace at smaller airports.
• Class D is used for the Terminal Control Zones of medium sized airports, extending from the surface up to 4,500 feet (1,370 m). Above this, Class C airspace is used, although generally only in a sector, and not 360° around the airport.
• Class E is used along the populated coastal areas, from 8,500 feet (2,590 m) to the base of the overlying Class A or Class C airspace.
• Class F is not used.
• Class G is used wherever other classes are not - almost always from the surface to the base of the overlying Class A, C, D or E airspace.
  In addition, Australia has a non-standard class of airspace for use at the capital city general aviation airports, called a General Aviation Airport Procedures Zone (GAAP Zone). A control tower provides procedural clearances for all aircraft inside the zone. Additionally, any aircraft operating within 5 nm of the zone must obtain a clearance. VFR aircraft arrive and depart using standard arrival and departure routes, while instrument arrival and departure procedures are published for IFR operations. During VMC, IFR aircraft are not provided with full IFR services. During IMC, or marginal VMC, VFR operations are restricted in order to facilitate full IFR service for IFR aircraft.

Airspace classes and VFR

Authorities use the ICAO definitions to derive additional rules for VFR cloud clearance, visibility, and equipment requirements.

For example, consider Class E airspace. An aircraft operating under VFR may not be in communication with ATC, so it is imperative that its pilot be able to see and avoid other aircraft (and vice versa). That includes IFR flights emerging from a cloud, so the VFR flight must keep a designated distance from the edges of clouds above, below, and laterally, and must maintain at least a designated visibility, to give the two aircraft time to observe and avoid each other. The low-level speed limit of 250 knots does not apply above 10,000 feet (3,000 m), so the visibility requirements are higher.

On the other hand, in Class B airspace, separation is provided by ATC to all flights. Now the VFR flight only needs to see where it is going, so visibility requirements are reduced and there is no designated minimum distance from clouds.

Similar considerations determine whether a VFR flight must use a two-way radio and/or a transponder.

Special-use Airspace

Each national authority designates areas of special use airspace (SUA), primarily for reasons of national security. This is not a separate classification from the ATC-based classes; each piece of SUA is contained in one or more zones of letter-classed airspace.

SUAs range in restrictiveness, from areas where flight is always prohibited except to authorized aircraft, to areas that are not charted but are used by military for potentially hazardous operations (in this case, the onus is on the military personnel to avoid conflict). Refer to the external links for more specific details.