between aircraft and on the maneuvering area between aircraft and obstructions and to

expedite and maintain an orderly flow of air traffic [3]. To properly manage the traffic in

the system, the jurisdiction of control is divided into three parts: en route, terminal, and

oceanic. Terminal air traffic control may be divided into terminal radar approach control

(TRACON) and air traffic control tower (ATCT) operations at an airport. Each part has a

specific function.

Air route traffic control centers

Air route traffic control centers (ARTCC) have the responsibility of controlling the

movement of en route aircraft along the airways and jet routes and in other parts of the

airspace. Each of the 20 air traffic control centers within the continental United States has

control of a definite geographical area which may be bigger than 100,000 mi2. At the

boundary point, which marks the limits of the control area of the center, control of aircraft

may be transferred to an adjacent center or an approach control facility, or radar service

may be terminated and aircraft using VFR are free to contact the next center. Air traffic

control centers are normally not located at airports. Air traffic control centers can also

provide approach control service to non towered airports and to non terminal radar

approach control airports. The ARTCC is concerned primarily with the control of aircraft

operating under instrument flight rules (IFR).

Under IFR pilots are required to file a flight plan indicating the route and altitude they

desire to fly. The ARTCC will then check to determine whether the flight plan, as filed, can

be approved so that a safe separation between aircraft can be ensured. Changes in flight

plans en route are permitted if approved by each ARTCC along the route of the flight.

Each ARTCC geographic area is divided into sectors. The configuration of each sector is

based on equalizing the workload of the controllers. Control of aircraft is passed from one

sector to another. The geographic area is sectored in both the horizontal plane and the verti-

cal plane. Thus there can be a high altitude sector above one or more low altitude sectors.

Each sector is staffed by one or more controllers, depending on the volume ,and complexity

of traffic. The average number of aircraft that each sector can handle depends on the

number of people assigned to the sector, the complexity of traffic, and the degree of

automation provided.

Each sector is normally provided with one or more air route surveillance radar (ARSR)

units which cover the entire sector and allow for monitoring of the separation between

aircraft. In addition, each sector has data on the identification of the aircraft, destination,

flight plan route, estimated speed, and flight altitude, which are posted on pieces of paper

called flight progress strips or may be superimposed on the radarscope adjacent to the blips

which specify the position and identity of the aircraft. The strips are continuously updated

as the need arises.

At present, communication between the pilot and the controller is by voice. Therefore each

ARTCC is assigned a number of very high and ultra high radio communication frequencies.

The controller in turn assigns a specific frequency to the pilot.

Terminal approach control facility

The terminal approach control facility monitors the air traffic in the airspace surrounding

airports with moderate to high density traffic. It has jurisdiction in the control and

separation of air traffic from the boundary area of the air traffic control tower at an airport

to a distance of up to 50 mi from the airport and to an altitude ranging up to 17,000 ft. This

is commonly referred to as the terminal area. Where there are several airports in an urban

area, one facility may control traffic to all the airports. In essence, the facility receives

aircraft from the ARTCC and guides them to one of several airports. In providing this

guidance, the facility performs the important function of metering and sequencing aircraft

to provide uniform and orderly flow to airports.

The radar approach control facility is referred to as TRACON, an abbreviation for terminal

radar approach control. There are various degrees of automation in an approach control

facility depending on the volume of traffic normally handled. Various abbreviations are

used to designate the type of hardware in an approach control facility. As an example,

ARTS III is an acronym for automated radar terminal system. The designation III denotes

the highest level of automation, while I is the lowest level of automation. Thus one can

have ARTS 1, II, III automation capability in a TRACON facility. ARTS IIIA and ARTS

IIIE are updated enhancements of the ARTS III system capability to accommodate

automation data.

The organizational structure of an approach control facility is very similar to that of the

ARTCC. Like the ARTCC, the geographic area of the facility is divided into sectors to

equalize the workload of the controllers. The approach control facility transfers control of

an arriving aircraft to the airport control tower when the aircraft is lined up with the runway

about 5 mi from the airport. Likewise control of departing aircraft is transferred to the

approach control facility by the airport control tower.

If the flow of aircraft is greater than the facility can handle, traffic management and the air

traffic system command center (ATSCC), formerly called the central flow control facility

(CFCF), manipulate aircraft on the ground and en route to adjust, or meter, the arrival flows

to their destination airports. This may result in delays to departing aircraft or delays en

route. In the past such aircraft were delayed by either reducing their speed en route or

detaining them at specified radio fixes within the area of the destination facility. The latter

method is referred to as stacking. In a stack, aircraft navigate around a fix in a racetrack

pattern, a holding pattern, and are separated vertically by 1000 ft intervals. There may be as

many as 10 aircraft in a stack, and each is directed in turn to a landing by the approach

control facility. As a matter of procedure, stacking is no longer performed except when the

arrival capacity at an airport is reduced due to unexpected events.

In 1990 there were more than 200 various types of terminal area approach control facilities

operated by the FAA in the United States.

Airport traffic control tower

The airport traffic control tower is the facility which supervises, directs, and monitors the

arrival and departure traffic at the airport and in the immediate airspace within about 5 mi

from the airport. The tower is responsible for issuing clearances to all departing aircraft;

providing pilots with information on wind, temperature, barometric pressure, and operating

conditions at the airport; and controlling all aircraft on the ground except in the

maneuvering area immediately adjacent to the aircraft parking positions called the ramp

area. In the United States in 1990, 400 air traffic control towers were operated by the FAA,

and 25 air traffic control towers operated under contract to the FAA.

Flight service stations

The flight service stations (FSS) are located along the airways and at airports. Flight service

stations are not air traffic control facilities but provide essential information to pilots. Their

principal function is to accept and close flight plans and to brief pilots, before flight and in

flight, on weather, navigational aids, airports and navaids that are out of commission, and

changes in procedures and new facilities. A secondary function is to relay traffic control

messages between aircraft and the appropriate control facility on the ground. The FAA

operated 183 domestic and international flight service stations in 1990. Flight service

stations are in the process of being converted to automated flight service stations (AFSS),

and when the transition is completed, there will be 60 AFSS in the United States.

Air Traffic Separation Rules

Air traffic rules governing the minimum separation of aircraft in the vertical, horizontal or

longitudinal, and lateral directions are established in each country by the appropriate

government authority. The current rules described in this text are those prescribed by the

FAA for use in the United States. The separation rules are prescribed for IFR operations,

and these rules apply whether or not IMC conditions prevail. Minimum separations are a

function of aircraft type, aircraft speed, availability of radar facilities, navigational aids, and

other factors such as the severity of wake vortices [3].

Vertical separation in the airspace.

The minimum vertical separation of aircraft outside the terminal area from the ground up to

and including 29,000 ft above mean sea level (AMSL) is 1000 ft. Higher than 29,000 ft

AMSL the minimum separation is 2000 ft. Within a terminal area a vertical separation of

500 ft is maintained between aircraft, except that a 1000 ft vertical separation is maintained

below a heavy aircraft.

Use of VFR altitudes

VFR altitudes below 18,000 ft AMSL are designated as the odd 1000 ft altitudes plus 500 ft

beginning at 3500 ft AMSL for course headings from 0° to 179° magnetic azimuth and the

even 1000 ft altitudes plus 500 ft beginning at 4500 ft AMSL for course headings from

180° to 359° magnetic azimuth. The authorized VFR altitudes between 18,000 ft AMSL

and 29,000 ft (flight level 290, or FL 290) are designated as the odd 1000 ft altitudes plus

500 ft beginning at FL 195 for course headings from 0° to 179° magnetic azimuth and the

even 1000 ft altitudes plus 500 ft beginning at FL 185 for course headings from 180° to

359° magnetic azimuth. The authorized VFR altitudes above FL 290 are designated as the

even 1000 ft altitudes. at 4,000 ft intervals beginning at FL 300 for course headings from 0°

to 179° magnetic azimuth and the even 1000 ft altitudes at 4000 ft intervals beginning at FL

320 from 180° to 359° magnetic azimuth.

These altitudes are in effect under VFR at altitudes of 3000 ft above the surface in both

controlled and uncontrolled airspace.

Assigned IFR altitudes

The assigned IFR altitudes below 18,000 ft AMSL are designated as the odd 1000-ft

altitudes for course headings from 0° to 179° magnetic azimuth and the even 1000-ft

altitudes for course headings from 180° to 359° magnetic azimuth. The assigned IFR

altitudes from 18,000 ft AMSL up to but not including 29,000 ft (flight level 290, or FL

290) are designated as the odd 1000-ft altitudes for course headings from 0° to 179°

magnetic azimuth and the even 1000-ft altitudes plus 500 ft for course headings from 180°

to 359° magnetic azimuth. The assigned IFR altitudes at and above FL 290 are designated

as the odd 1000-ft altitudes at 4000-ft intervals beginning at FL 290 for course headings

from 0° to 179° magnetic azimuth and the odd 1000-ft altitudes at 4000-ft intervals

beginning at FL 310 from 180° to 359° magnetic azimuth.

Longitudinal separation in the airspace

The minimum longitudinal separation depends on a number of factors; among the most

important are aircraft size, aircraft speed, and availability of radar for the control of air

traffic. For the purposes of maintaining aircraft separations, aircraft are classified by the

FAA as heavy, large, or small based upon their maximum certified takeoff weight. Heavy

aircraft have a maximum certificated takeoff weight of 300,000 lb or more. Large aircraft

have a maximum certificated takeoff weight in excess of 12,500 lb but less than 300,000 lb.

Small aircraft have a maximum certificated takeoff weight of 12,500 lb or less. Aircraft size

is related to wake turbulence. Heavy aircraft create trailing wake vortices which are a

hazard to lighter aircraft following them.

The minimum longitudinal separations en route are expressed in terms of time or distance

as follows:

1. For en route aircraft following a preceding en route aircraft, if the leading aircraft

maintains a speed at least 44 kn faster than the trailing aircraft, 5 mi between aircraft

using distance-measuring equipment (DME) or area navigation (RNAV) and 3 min

between all other aircraft

2. For en route aircraft following a preceding en route aircraft, if the leading aircraft

maintains a speed at least 22 kn faster than the trailing aircraft, 10 mi between air' craft

using DME or RNAV and 5 min for all other aircraft

3. For en route aircraft following a preceding en route aircraft, if both aircraft are at the

same speed, 20 mi between aircraft using DME or RNAV and 10 min for all other

aircraft

4. When an aircraft is climbing or descending through the altitude of another aircraft, 10

mi for aircraft using DME or RNAV if the descending aircraft is leading or the

climbing aircraft is following and 5 min for all other aircraft

5. Between aircraft in which one aircraft is using DME or RNAV and the other is not, 30

mi

The minimum longitudinal separation over the oceans is normally 10 min for

supersonic flights and 15 min for subsonic flights, but in some locations it can be slightly

more or less than these values [3].

When the aircraft mix is such that wake turbulence is not a factor and radar coverage is

available, the minimum longitudinal separation for two aircraft traveling in the same

direction and at the same altitude is 5 nmi, except that when the aircraft are in the terminal

environment within 40 nmi of the radar antenna, the separation can be reduced th 3 nmi.

For this reason the minimum spacing in the terminal area is 3 nmi because the airport is

almost always within 40 nrni of a radar antenna. Under certain specified conditions, a

separation between aircraft on final approach within 10 nmi of the landing runway may be

reduced to 2.5 nmi [3].

If wake turbulence is a factor, the minimum separation in the terminal area between a

small or large aircraft and a preceding heavy aircraft is 5 nmi. The spacing between two

heavy aircraft following each other is 4 nmi. The spacing between a heavy aircraft and a

preceding large aircraft is 3 nmi.

For landing aircraft, when wake turbulence is a factor, the longitudinal separation is

increased between a small aircraft and . preceding large aircraft to 4 rni and between a

small aircraft and a preceding heavy aircraft to 6 mi.

The instrument flight separation rules for consecutive arrivals on the same runway

which are used when wake vortices are a factor are shown in Table 4-1. This table also

displays the observed average spacings maintained by pilots in VFR at Chicago O'Hare

International Airport. Note that the VFR separations in Table 4-1 are not maintained by air

traffic control but are the average observed separations maintained by pilots and, as such,

are not valid measures upon which runway or airspace capacity studies can be based. They

are simply presented for comparative purposes.

TABLE 4-1 Horizontal Separates in Landing for Arrival-Arrival Spacing ell Aircraft on

Same Runway Approaches in VFR and IFR Condition.% nmi

Leading aircraft type

VFR* IFR (wake vortex)

Trailing aircraft type Trailing aircraft type

Heavy Large Small Heavy Large Small

Heavy 2.7 3.6 4.5 4.0 5.0 6.0

Large 1.9 1.9 2.7 3.0 3.0 4.0

Small 1.9 1.9 1.9 3.0 3.0 3.0

*These values are shown to appropriately represent these operations and are not regulatory

in nature.

souace: Federal Aviation Administration [181

TABLE 4-2 Separation for Same-Runway Consecutive Departures in VFR and IFR

Conditions, s

Leading aircraft type

VFR* IFR

Trailing aircraft type Trailing aircraft type

Heavy Large Small Heavy Large Small

Heavy 90 120 120 120 120 120

Large 60 60 50 60 60 60

Small 50 45 35 60 60 60* These values are shown to appropriately represent these operations and are not regulatory

in nature.

Source: Federal Aviation Administration [181.

The visual and instrument flight separation rules for consecutive departures from the same

runway are expressed in terms of time and are shown in Table 4-2.

Lateral separation in the airspace

The minimum en route lateral separation below 18,000 ft AMSL is 8 nmi, and at arid above

18,000 ft AMSL the minimum en route lateral separation is 20 nmi. Over the oceans the

separation varies from 60 to 120 nmi depending on the location [3].

Genera considerations

The longitudinal separation standards significantly influence the capacity of the airspace

and the airport runways since separations reflect the size of headways between aircraft. The

significant influence of radar in reducing headways can be illustrated as follows. With radar

the minimum en route separation is 5 nmi for aircraft equipped with DME or RNAV. If

radar is not available, the separation must be increased to 20 mi for aircraft equipped with

distance-measuring or area navigation equipment and to about 30 mi if none of this equip-

ment is installed in the aircraft. Over the oceans the longitudinal separation varies from 60

to 120 nmi. These large separations reduce capacity of the airspace and increase delays, and

for this reason efforts are being made to reduce aircraft spacing. Figure 4-5 illustrates the

impact of a variety of minimum-spacing rules for arriving aircraft on runway capacity.

Navigational Aids

Aids to aerial navigation can be broadly classified into two groups: those that are located on

the ground, or external aids, and those installed in the cockpit, or internal aids. Some aids

are primarily for flying over the oceans, other aids are only applicable to flight over land

masses, and still other aids can be used over either land or water. Some aids are used only

during the en route portion of the flight, while other aids are necessary in terminal areas or

near airports.

External overland e route aids

Very high-frequency omnirange radio. The advances in radio and electronics during and

after World War II led to the installation of the very high-frequency omnirange radio

(VOR) stations„ These stations are located on the ground, and they send out radio signals in

all directions. Each signal can be considered as a course or a route, referred to as a radial,

that can be followed by an aircraft. In terms of 1O intervals, there are .360 courses or routes

that are radiated from a VOR station, from 0° pointing toward magnetic north increasing to

359° in a clockwise direction. The VOR transmitter station is a small square building

topped with what appears to be a white derby hat. It broadcasts on a frequency just above

that of FM radio stations. The very high frequencies it uses are virtually free of static. The

system of VOR stations establishes the network of airways and jet routes and is essential to

area navigation. The range of a VOR station varies but is usually less than 200 nmi.

Aircraft equipped with a VOR receiver in the cockpit have a dial for tuning in the

desired VOR frequency. A pilot can select the VOR radial or route to follow to the VOR

station. In the cockpit a position deviation indicator (PDI) specifies the heading of the

aircraft relative to the direction of the desired radial and whether the aircraft is to the right

or left of the radial. Figure 4-6 shows schematically the type of information the PDI

provides. At A the aircraft is on the selected radial, and the needle is pointed vertically and

passes through the cross, which is a symbol for the aircraft. In other words, the aircraft is

heading in the same direction as the desired radial. At B the aircraft is flying parallel to but

to the right of the desired radial. At C the aircraft is to the right of the radial and is heading

across the radial.

Distance-measuring equipment. Distance-measuring equipment (DME) has been

installed at nearly all VOR stations in the United States. Those so equipped are called

VORTAC facilities. The DME shows to the pilot the slant distance between the aircraft and

a particular VOR station. Since it is the air distance in nautical miles that is measured, the

receiving equipment in an aircraft flying' at 35,000 ft directly over the DME station will

read 5.8 nmi.

An en route air navigation aid which best suited the tactical needs of the military

was developed by the Navy in the early 1950s. This aid is known as TACAN, which stands

for tactical air navigation. This aid combines azimuth and distance measuring into one unit

instead of two and is operated in the ultra-high-frequency band. As a compromise between

civilian and military requirements, the FAA replaced the DME portion of its VOR facilities

with the distance-measuring components of TACAN. These stations are known as VOR-

DMET. If a station has full-TACAN equipment, both azimuth- and distance-measuring

equipment, and VOR, it is designated as VORTAC.

Air route surveillance radar. Long-range radar for tracking en route aircraft has been

established throughout the continental United States and in other parts of the world. While

in the United States there is complete radar coverage in the 48 contiguous states, this is not

the case elsewhere in the world. These radars have a range of about 250 nmi. Strictly

speaking, radar is not an aid to navigation. Its principal function is to provide air traffic

controllers with a visual display of the position of each aircraft so they can monitor the

spacings and intervene when necessary. However, radar can be and is used by air traffic

controllers to guide aircraft whenever necessary. For this reason it has been included as an

aid to navigation.

External overland terminal aids

The principal aids in the terminal area are used for landing aircraft,' and these are

described below.

Instrument Banding system. The most widely used method is the instrument landing

system (ILS). It consists of two radio transmitters located at the airport. One radio beam is

called the localizer, and the other is the glide slope. The localizer indicates to pilots whether

they are left or right of the correct alignment for approach to the runway. The glide slope

indicates the correct angle of descent to the runway. Glide slopes measure on the order of

2° to 3° minimum to 7.5° maximum.

To further help pilots on their ILS approach, two low-power fan markers, called ILS

markers, are usually installed so that pilots will know just how far along the approach to the

runway they have progressed. The first is called the outer marker (LOM) and is located

about 3.5 to 5 mi from the end of the runway. The other is called the middle marker (MM),

and it is located about 3000 ft from the end of the runway. For category II operations, when

visibility is quite poor, an additional marker called the inner marker (IM) is located 1000 ft

from the end of the runway. This inner marker is placed so as to alert pilots that they must

have visual reference with the ground at that point and if they do not, abandon the

approach. When the plane passes over a marker, a light goes on in the cockpit and a high-

pitched tone sounds. The configuration of the ILS is shown in Fig. 4-7. At many locations

the fan markers have been replaced with DME using the airport VORTAC or TACAN at

the airport.

The localizer consists of an antenna, which is located on the extension_ of the

runway centerline approximately 1000 ft from the far end of the runway, and a localizer

transmitter building located about 300 ft to one side of the runway at the same distance

from the end of the runway as the antenna. The glide slope facility is placed 750 to 1250 ft

down the runway from the threshold and is located to one side of the runway centerline at a

distance which can vary from 400 to 650 ft. The functioning of the localizer and the glide

slope facility is affected by the close proximity of moving objects such as vehicular and air-

craft traffic. During inclement weather, the use of the ILS critical areas keeps aircraft and

vehicles from entering areas that would impede an aircraft inside of the outer marker from

receiving a clear signal. Stationary objects nearby can also cause a deterioration of the

signals. Abrupt changes of slope in proximity to the antennas are not permitted, or else the

signal will not be transmitted properly. Another limitation of the ILS is that the glide slope

beam is not reliable below a height of about 200 ft above the runway.

Microwave landing system. The. ILS has a number of problems which have made

the development of more sophisticated landing systems necessary. The ILS is based on

signals reflecting from the surface of the ground. Thus the area adjacent to the antennas

must be relatively smooth and must be kept clear of any obstructions such as buildings and

taxiing aircraft; otherwise the beams are distorted. There have been improvements in the

transmission of the localizer beam brought about by the installation of a waveguide

antenna, which confines the beam spray and reduces the probability of reflections from

buildings and other obstructions. But this has not solved all the problems associated with

the ILS. The ILS provides only one path in space, which all aircraft must follow if they are

using the system. Some aircraft, particularly the STOL (short takeoff and landing) type, can

use a steeper approach angle, about 7°, than conventional aircraft, which use 2.5° to 3°

approaches. Other aircraft may wish to make a two-segment approach to reduce noise

beneath the flight_ path. The ILS is unable to provide for these types of operations. Finally,

only a limited number of frequency channels are available for the ILS, and as the number of

installations has increased, it is becoming difficult to provide the necessary discrete

channels required.

To overcome these limitations, the microwave landing system (MLS) was

developed. Instead of providing only one glide slope as the ILS does, the MLS provides for

a number of slopes. In the horizontal plane, the MLS provides for any desired routes as

long as they are within an area that is from 20° to 60° on each side of the runway cen-

terline, whereas the ILS provides only one route to the runway. Distance-measuring

capability can be incorporated into the MLS, providing the pilot with continuous

information on the aircraft distance from the end of the runway and removing the need for

establishing markers as in the ILS. The MLS is far less susceptible to interference from

surrounding objects than the ILS. With the MLS a pilot can choose any desired route to the

runway at any glide slope within the vertical coverage of the system. A microwave landing

system is shown schematically in Fig. 4-8.

From the standpoint of airport planning, one of the most significant advantages of

the MLS is the potential reduction of noise since aircraft can be kept at higher altitudes

before they make the descent to the airport or follow curved routes which do not affect as

much land as the ILS routes do. The difference between an ILS and an MLS approach into

Kennedy (JFK) and LaGuardia (LGA) airports in the New York area is shown in Fig. 4-9.

Another advantage is the elimination of the requirement that all aircraft, large or small,

follow a common approach route to the runway.

Precision-approach radar. At a number of military airports, another landing aid

known as ground-controlled approach (GCA) has been installed. The GCA operates either

with the airport surveillance radar alone or with both the airport surveillance radar and

precision-approach radar (PAR) The latter equipment was developed by the military during

World War II in order to provide a mobile unit that is not dependent on airborne navigation

equipment. The precision-approach radarscope gives controllers a picture of the descending

aircraft in both plan and elevation; i.e., one-half the radarscope is in plan, and one-half is in

elevation. Thus controllers can determine whether an aircraft is on the glide path and

whether it is on the correct alignment. Instructions from controllers to pilots are given by

voice communication, and thus no airborne navigation'\equipment is necessary.

Commercial airline pilots use the ILS almost exclusively, because using PAR places too

much dependence on the controller and does not provide any direct information to the pilot.

At airports where there are both ILS and PAR facilities, commercial airline pilots use ILS

but often request that they be monitored by PAR.

Airport surveillance radar. To provide the MACON and control tower operators

with an overall picture of what is going on within the airspace surrounding the terminal,

airport surveillance radar (ASR) has been installed at many of the major U.S. airports. The

ASR rotates through 360°, and the information is received by an ARTS (automated radar

terminal system) type of computer system in the TRACON and relayed to a bright radar

tower equipment (BRITE) radarscope in the control tower. The primary range of ASR is

from 30 to 60 mi_ It shows the aircraft in their relative horizontal positions on the

radarscope as blips. With non mosaic radars, the blips of moving aircraft leave a luminous

trail and indicate the direction in which the aircraft are moving. ARTS can also determine

and show an indication of the aircraft speed. ASR does not indicate the altitude of aircraft

since it simply responds to the reflection df the signal from the skin of the aircraft. This

type of return radar is called primary radar or skin paint.

Approach lighting systems. The most critical point of approach to landing comes

when the aircraft breaks through the overcast and the pilot must change from instrument to

visual conditions. Sometimes, only a few seconds are available for the pilot to make the

transition and complete the landing. To aid in making this transition, lights are installed on

the approach to the runways and on the runways themselves. These are generally termed

approach lighting systems (ALS). A number of types and configurations are used, and

others are under experimental testing today. More details concerning these systems are

contained in Chap. 13 on signing, marking, and lighting.

Airport surface detection. At large high-density airports, controllers have difficulty

in regulating taxiing aircraft because they cannot see the aircraft in poor visibility

conditions. A specially designed radar, called airport surface detection equipment (ASDE),

often referred to as ground radar, has been developed to aid the controller. The system

gives the air traffic controller in the control tower a pictorial display of the runways,

taxiways, and terminal area, with radar indicating the positions of aircraft and other

vehicles moving on the surface of the airport.

Visual-approach slope indicators. These systems (VASI and PAPI) provide, through

a system of lights, the proper approach slope to the runway much the same as the glide

slope of an ILS system. VASI systems are intended for day or night use during good (VFR)

weather conditions, and they cannot be used under very poor visibility conditions. A

refined version of the visual approach slope indicator the precision approach path indicator

(PAPI) system is presently being installed at airports in the United States. The PAPI system

gives a more definitive indication of approach slope to the pilot and uses only a single set

of electronic devices at one point down the runway. A more detailed explanation of the

VASI and PAPI systems is contained in Chap. 13 on signing. marking, and lighting.

Runway end identifier lights. Runway end identifier lights (REIL) are installed to

give the pilot positive visual identification of the approach end of the runway when there

are no approach lights.

The relative location of terminal-area navigational aids is shown in Fig. 4-10. The

siting requirements for visual aids and the ILS antennas can be found in the References [1].

External verwater en route aids

The principal overwater aid to navigation is LORAN, which consists of stations

located on the ground. LORAN stands for long-range aerial navigation. The system was

developed during World War II. LORAN stations are located in all parts of the world. The

particular system in use today is designated LORAN-C. The principle of the LORAN

system is as follows. Each element consists of a master station and a slave station located

some distance from the master. The master station sends radio signals into space, and at the

same time one of the signals goes to the slave station, where it is delayed a specified

amount of time and then sent into space. At any point in space, there is a difference in time

between the original signal from the master and its intersection with the delayed signal

from the slave. Thus a contour of equal time differences can be drawn in space. The same

thing can be done from another master and slave station, resulting in another contour. The

intersection of the two contours establishes a position in space. In the aircraft the LORAN

receiver tunes in on two master and slave stations, establishing an intersection of two time-

difference contours in space. The range of LORAN is affected by the time of day, being

greater at night than during daylight. LORAN requires the use of a navigator in the cockpit.

Internal overwater en route aids

There are two principal aids used in overwater operation: the doppler navigation

system and the inertial navigation system (INS). A third system which derives from use on

ships at sea is celestial navigation. It was quite popular prior to the development of the

doppler and inertial navigation systems. The advantages of these later systems are related to

the economics of aircraft operation: They do not require the use of a navigator.

Doppler navigation system. This is a long-range-radar type of aid that provides the

pilot with the ground speed, angle of the aircraft axis relative to the desired course (drift

angle), distance of the aircraft right or left of the desired course, and distance to the

destination or way point. Suppose an aircraft is to fly from point A to point B via a great

circle route of length L. The length L is usually divided into several shorter lengths or

segments. The ends of these segments are established in space by way points. A way point

is an imaginary point in space. Inputs into the system are the latitudes and longitudes of

points A and B and of all way points along the route. The number of way points depends on

the length of the trip.

The doppler system is based on the following: The aircraft sends to the ground four

beams of continuous wave energy (8800 Mc), two forward and two toward the rear. The

change in frequency of the energy return from the ground is measured.. This change in

frequency is known as the doppler shift and is proportional to the aircraft speed in the

directions the beam is pointing.. By checking the speed in the four directions in which the

beams are pointing, the system derives the ground speed and drift angle. The smoother the

surface, the less chance there is for the radiated energy to reflect to the aircraft's antenna.

This is a limitation of the system and is encountered over smooth bodies of water.

Inertial navigation system. The inertial navigation system is by far the most popular

overwater long-range aid. It provides all the information that the doppler system provides

as well as the wind speed and direction, latitude and longitude of the aircraft at any instant,

and time to reach the next way point. As for the doppler system, the inputs are the latitudes

and longitudes of the origin, destination, and way points. The inertial guidance system is a

development of the space program. It is quite accurate and reliable. Both the inertial and

doppler navigation systems provide azimuth information referenced to true north, not

magnetic north.

Internal overland en route aids

Both the doppler and the inertial navigation systems can be used over land masses.

There are also area navigation systems (RNAV) which can be used only over land. They

use as inputs the distance and azimuth information provided by VORTAC stations. The

desired course is referenced to way points, as for the other systems. The way points are

established by distances and azimuth from the nearest adjacent VORTAC station.

Information provided in the cockpit is similar to that for the doppler and inertial navigation

systems.

Internal overland terminal aids

Area navigation systems used for en route navigation can also be used in the terminal area.

In addition to guidance in the horizontal plane, these systems provide guidance in the

vertical plane. This latter capability is particularly useful for guidance to runways.

Global positioning system

The global positioning system (GPS) is a space-based satellite radio positioning and

navigation system. The system is designed to provide highly accurate position and velocity

information on a continuous global basis to an unlimited number of properly equipped

users. The system by weather and provides a common worldwide grid reference system.

The GPS concept is predicated upon accurate and continuous knowledge of the spatial

position of each satellite in the system with respect to time and distance from the

transmitting satellite to the user. It is expected that the full GPS will consist of 24 satellites

in near-circular orbit about the earth. The GPS receiver automatically selects the

appropriate signals from the satellite which is in view of the receiver and translates these

signals to a three dimensional position, velocity and time. It is expected that the GPS will

have a horizontal accuracy on the order of 100 m. The global positioning system offers

considerable assistance in navigation by providing precise position information to aircraft,

and therefore it is likely to become an external navigational aid in the near future.

A variation of the system, called the relative global positioning system, has the potential to

assist aircraft in conducting precision instrument approaches to airports. Relative GPS is

based on the concept that the knowledge of the location of the GPS receiver in an aircraft

relative to the GPS receiver at an airport is more accurate than the knowledge of the

absolute position of each receiver. An aircraft on approach would receive GPS satellite

position signals, and the beacon radar at the airport would relay its own position at the

airport. By comparing the beacon radar position with the position of the aircraft, the aircraft

can compute a vector to the beacon radar site. Knowledge of the range and bearing from the

beacon radar site to the point of touchdown allows the aircraft system to compute a vector

to touchdown. It is expected that the relative GPS will reach an accuracy on the order of 2

m, which would make it useful in precision instrument approaches to runways [20]. The

FAA is currently conducting tests on another variation of the system, the differential global

positioning system, for use in precision instrument approaches to runways.

The global positioning system has the capability of providing external overland and

overwater precise navigational assistance to aircraft as well as assistance to aircraft in

conducting terminal-area precision approaches to runways.

Aids for the Control of Air Traffic

The principal aids for the control of air traffic are voice communication and radar. The

controller monitors the spacing between aircraft on the radarscope and instructs pilots by

voice communication. There are two types of radar: primary and secondary. The primary

radar returns appear on the radarscope as small blips. These are reflections from the aircraft

body. The primary radar as it appears on a radarscope is shown in Fig. 4-11. Primary radar

requires the installation of rotating antennas on the ground, and the range of the primary

radar is a function of its frequency. Beacon radar, sometimes referred to as secondary radar,

consists of a radar receiver and transmitter on the ground that transmit a coded signal to ban

aircraft if that aircraft has a transponder. A transponder is an airborne receiver and trans-

mitter which receives the signal from the ground and responds by returning a coded reply to

the interrogator on the ground. The coded reply normally contains information about the

aircraft's identity and altitude. The interrogator (receiver and transmitter) is the beacon

radar antenna. It is usually installed as an integral part of the primary radar antenna. Beacon

radar returns are presented on the radarscope as two slashes if they are decoded and a single

slash if they are not decoded. Prior to the development of ARTS, which, provides

alphanumeric information about the flight in a data block, controllers would decode only

those aircraft that they were controlling. The slashes always appear at a right angle to the

radial from the location of the antenna to the aircraft, as shown in Fig. 4-11. The center of

the slash closest to the antenna is the location of the aircraft. Prior to the installation of

ARTS, the primary and beacon radar presentations did not provide identity of the aircraft or

its altitude. This was obtained by voice communication and was recorded on flight progress

strips.

To overcome the deficiencies of the original beacon radar presentation, and to reduce the

amount of communication, a video presentation which includes identity and altitude was

developed. This is shown in Fig. 4-11 and is referred to as the alphanumeric display. The

first line shows the identity of the aircraft; the second line shows its altitude and ground

speed; and the third line gives the beacon code transponder number and the aircraft track

number. To be able to have this information presented on the radarscope, the aircraft must

carry a mode C transponder that has the capability of reporting altitude along with aircraft

identity. All commercial airline aircraft carry a mode C transponder, which satisfies the

requirement for reporting altitude. One problem with the original beacon radar system was

its inability to selectively interrogate aircraft. A modified beacon radar system has been

incorporated into ATC to alleviate this difficulty in congested areas through the use of a

discrete-address beacon system (DABS) using a mode S transponder [15].

If all aircraft, including general aviation, were equipped with transponders, there would be

no need for primary radar except possibly in a backup role.

Automation in Terminal and En Route

Air Traffic Control

There are a number of reasons for updating and automating the air traffic control system.

The more important are as follows [17]:

1. Having an operational system that is capable of being technically expanded in

incremental steps to meet the needs of aviation as time requires

2. Accommodating increasing demands in a manner that allows users to operate in the

airspace with minimal regulatory constraints and in a fuel-efficient way

3. Reducing the risks of midair and surface traffic collisions, landing and weather-related

accidents, and collisions with the ground

4. Increasing the productivity of air traffic control personnel in terms of the amount of air

traffic handled

5. Decreasing the technical staff required to maintain and operate the system

6. Maintaining the overall operating costs of the system at reasonable levels