Chapter 7 - Navigation Systems

Introduction

This chapter provides the basic radio principles applicable to navigation equipment, as well as an operational knowledge of how to use these systems in instrument flight. This information provides the framework for all instrument procedures, including departure procedures (DPs), holding patterns, and approaches, because each of these maneuvers consist mainly of accurate attitude instrument flying and accurate tracking using navigation systems.

A radio wave is an electromagnetic wave (EM wave) with frequency characteristics that make it useful in radio. The wave will travel long distances through space (in or out of the atmosphere) without losing too much strength. The antenna is used to convert it from an electric current into a radio wave so it can travel through space to the receiving antenna which converts it back into an electric current.

All matter has a varying degree of conductivity or resistance to radio waves. The Earth itself acts as the greatest resistor to radio waves. Radiated energy that travels near the ground induces a voltage in the ground that subtracts energy from the wave, decreasing the strength of the wave as the distance

Sky Wave

The sky wave, at frequencies of 1 to 30 MHz, is good for long distances because these frequencies are refracted or “bent” by the ionosphere, causing the signal to be sent back to Earth from high in the sky and received great distances away. [figure 7-1] Used by high frequency (HF) radios in aircraft, messages can be sent across oceans using only 50 to 100 watts of power. Frequencies that produce a sky wave are not used for navigation because the pathway of the signal from transmitter to receiver is highly variable. The wave is “bounced” off of the ionosphere, which is always changing due to the varying amount of the sun’s radiation reaching it (night/day and seasonal variations, sunspot activity, etc.). The sky wave is not reliable for navigation purposes.

For aeronautical communication purposes, the sky wave (HF) is about 80 to 90 percent reliable. HF is being gradually replaced by satellite communication which is more reliable.

Space Wave

Radio waves of 15 MHz and above (all the way up to many GHz), when able to pass through the ionosphere, are considered space waves. Most navigation systems operate with their signals propagating as space waves. Frequencies above 100 MHz have nearly no ground or sky wave components. They are space waves, but (except for global positioning system (GPS)) the navigation signal is used before it reaches the ionosphere so the effect of the ionosphere, which can cause some propagation errors, is minimal. GPS errors caused by passage through the ionosphere are significant and are corrected for by the GPS receiver system.

Space waves have another characteristic of concern to users. Space waves reflect off hard objects and may be blocked if the object is between the transmitter and the receiver. Site and terrain error, as well as propeller/rotor modulation error in very-high omnidirectional range (VOR) systems is caused by this bounce. Instrument landing system (ILS) course distortion is also the result of this phenomenon, which led to the need for establishment of ILS critical areas.

Generally, space waves are “line of sight” receivable, but those of lower frequencies will “bend” over the horizon somewhat. Since the VOR signal at 108 to 118 MHz is a lower frequency than distance measuring equipment (DME) at 962 to 1213 MHz, when an aircraft is flown “over the horizon” from a VOR/DME station, the DME will normally be the first to stop functioning.

Disturbances to Radio Wave Reception

Static distorts the radio wave and interferes with normal reception of communications and navigation signals. Low-frequency airborne equipment such as automatic direction finder (ADF) and long range navigation (LORAN) are particularly subject to static disturbance. Using very-high frequency (VHF) and ultra-high frequency (UHF) frequencies avoids many of the discharge noise effects. Static noise heard on navigation or communication radio frequencies may be a warning of interference with navigation instrument displays. Some of the problems caused by precipitation static (P-static) are:

• Complete loss of VHF communications.
• Erroneous magnetic compass readings.
• Aircraft flies with one wing low while using the autopilot.
• High-pitched squeal on audio.
• Motorboat sound on audio.
• Loss of all avionics.
• Very-low frequency (VLF) navigation system inoperative.
• Erratic instrument readouts.
• Weak transmissions and poor radio reception.
• St. Elmo’s Fire.

Propeller/rotor modulation error: Critical areas: Most ILS Certain propeller RPM settings or installations are subject to signal helicopter rotor speeds can cause the interference by surface vehicles, VOR course deviation indicator aircraft, or both. As a result, areas are (CDI) to fluctuate as much as ±6°. established near each localizer and Pilots should check for this glide-slope antenna so air traffic phenomenon prior to reporting a control (ATC) can steer aircraft away VOR station or aircraft equipment from these areas. for unsatisfactory operation.

The RMI differs from the movable-card ADF in that it automatically rotates the azimuth card (remotely controlled by a gyrocompass) to represent aircraft heading. The RMI has two needles, which can be used to indicate navigation information from either the ADF or the VOR receivers. When a needle is being driven by the ADF, the head of the needle indicates the MB TO the station tuned on the ADF receiver. The tail of the needle is the bearing FROM the station. When a needle of the RMI is driven by a VOR receiver, the needle indicates where the aircraft is radially with respect to the VOR station. The needle points to the bearing TO the station, as read on the azimuth card. The tail of the needle points to the radial of the VOR the aircraft is currently on or crossing. figure 7-5 indicates a heading of 005°, the MB to the station is 015°, and the MB from the station is 195°.

Function of ADF

The ADF can be used to plot your position, track inbound and outbound, and intercept a bearing. These procedures are used to execute holding patterns and nonprecision instrument approaches.

Orientation

The ADF needle points TO the station, regardless of aircraft heading or position. The RB indicated is thus the angular relationship between the aircraft heading and the station, measured clockwise from the nose of the aircraft. Think of the nose/tail and left/right needle indications, visualizing the ADF dial in terms of the longitudinal axis of the aircraft. When the needle points to 0°, the nose of the aircraft points directly to the station; with the pointer on 210°, the station is 30° to the left of the tail; with the pointer on 090°, the station is off the right wingtip. The RB does not by itself indicate aircraft position. The RB must be related to aircraft heading in order to determine direction to or from the station.

Station Passage

When you are near the station, slight deviations from the desired track result in large deflections of the needle. Therefore, it is important to establish the correct drift correction angle as soon as possible. Make small heading corrections (not over 5°) as soon as the needle shows a deviation from course, until it begins to rotate steadily toward a wingtip position or shows erratic left/right oscillations. You are abeam a station when the needle points to the 90° or 270° position. Hold your last corrected heading constant, and time station passage when the needle shows either wingtip position or settles at or near the 180° position. The time interval from the first indications of station proximity to positive station passage varies with altitude—a few seconds at low levels to 3 minutes at high altitude.

Homing

The ADF may be used to “home” in on a station. Homing is flying the aircraft on any heading required to keep the needle pointing directly to the 0° RB position. To home into a station, tune the station, identify the Morse code signal, then turn the aircraft to bring the ADF azimuth needle to the 0° RB position. Turns should be made using the heading indicator. When the turn is complete, check the ADF needle and make small corrections as necessary.

Homing: Flying the aircraft on any heading required to keep the needle pointing directly to the 0° RB position.

figure 7-8. ADF interception and tracking outbound.

figure 7-7. ADF tracking inbound.

1. Positively identifying any malfunctions of the RMI slaving system or ignoring the warning flag.
2. Dependence on homing rather than proper tracking. This commonly results from sole reliance on the ADF indications, rather than correlating them with heading indications.
3. Poor orientation, due to failure to follow proper steps in orientation and tracking.
4. Careless interception angles, very likely to happen if you rush the initial orientation procedure.
5. Overshooting and undershooting predetermined MBs, often due to forgetting the course interception angles used.
6. Failure to maintain selected headings. Any heading change is accompanied by an ADF needle change. The instruments must be read in combination before any interpretation is made.
7. Failure to understand the limitations of the ADF and the factors that affect its use.
8. Overcontrolling track corrections close to the station (chasing the ADF needle), due to failure to understand or recognize station approach.
9. Failure to keep heading indicator set so it agrees with magnetic compass.

Very-High Frequency Omnidirectional Range (VOR)

Description

VOR is the primary navigational aid (NAVAID) used by civil aviation in the National Airspace System (NAS). The VOR ground station is oriented to magnetic north and transmits azimuth information to the aircraft, providing 360 courses TO or FROM the VOR station. When DME is installed with the VOR, it is referred to as a VOR/DME and provides both azimuth and distance information. When military tactical air navigation (TACAN) equipment is installed with the VOR, it is known as a VORTAC and provides both azimuth and distance information.

The courses oriented FROM the station are called radials. The VOR information received by an aircraft is not influenced by aircraft attitude or heading. [figure 7-10] For example, aircraft A (heading 180°) is inbound on the 360° radial; after

Radials: The courses oriented FROM the station.

crossing the station, the aircraft is outbound on the 180° radial at A-1. Aircraft B is shown crossing the 225° radial. Similarly, at any point around the station, an aircraft can be located somewhere on a VOR radial.

The VOR receiver measures and presents information to indicate bearing TO or FROM the station. In addition to the navigation signals transmitted by the VOR, a Morse code signal is transmitted concurrently to identify the facility, as well as voice transmissions for communication and relay of weather and other information.

VORs are classified according to their operational uses. The standard VOR facility has a power output of approximately 200 watts, with a maximum usable range depending upon the aircraft altitude, class of facility, location and siting of the facility, terrain conditions within the usable area of the facility, and other factors. Above and beyond certain altitude and distance limits, signal interference from other VOR facilities and a weak signal make it unreliable. Coverage is typically at least 40 miles at normal minimum instrument flight rules (IFR) altitudes. VORs with accuracy problems in parts of their service volume are listed in Notices to Airmen (NOTAMs) and in the Airport/Facility Directory (A/FD) under the name of the NAVAID.

The ground equipment consists of a VOR ground station, which is a small, low building topped with a flat white disc, upon which are located the VOR antennas and a fiberglass cone-shaped tower. [figure 7-11] The station includes an automatic monitoring system. The monitor automatically turns off defective equipment and turns on the standby transmitter. Generally, the accuracy of the signal from the ground station is within 1°.

VOR facilities are aurally identified by Morse code, or voice, or both. The VOR can be used for ground-to-air communication without interference with the navigation signal. VOR facilities operate within the 108.0 to 117.95 MHz frequency band and assignment between 108.0 and 112.0 MHz is in even-tenth decimals to preclude any conflict with ILS localizer frequency assignment, which uses the odd tenths in this range.

TO/FROM indicator. The TO/FROM indicator shows whether the selected course will take the aircraft TO or FROM the station. It does not indicate whether the aircraft is heading to or from the station.

Flags, or other signal strength indicators. The device that indicates a usable or an unreliable signal may be an “OFF” flag. It retracts from view when signal strength is sufficient for reliable instrument indications. Alternately, insufficient signal strength may be indicated by a blank or OFF in the TO/FROM window.

The indicator instrument may also be a horizontal situation indicator (HSI) which combines the heading indicator and CDI. [figure 7-13] The combination of navigation information from VOR/Localizer (LOC) or from LORAN or GPS, with aircraft heading information provides a visual picture of the aircraft’s location and direction. This decreases pilot workload especially with tasks such as course intercepts, flying a back-course approach, or holding pattern entry. (See Chapter 3, for operational characteristics.)

Horizontal situation indicator (HSI): A flight navigation instrument that combines the heading indicator with a CDI, in order to provide the pilot with better situational awareness of location with respect to the courseline.

Function of VOR

Orientation

The VOR does not account for the aircraft heading, it only relays the aircraft direction from the station and will have the same indications regardless of which way the nose is pointing. Tune the VOR receiver to the appropriate frequency of the selected VOR ground station, turn up the audio volume, and identify the station’s signal audibly. Then rotate the OBS to center the CDI needle, and read the course under or over the index.

In figure 7-12, 360° TO is the course indicated, while in figure 7-14, 180° TO is the course. The latter indicates that the aircraft (which may be heading in any direction) is, at this moment, located at any point on the 360° radial (line from the station) except directly over the station or very close to it, as in points I to S in figure 7-14. The CDI will deviate from side to side as the aircraft passes over or nearly over the station because of the volume of space above the station where the zone of confusion exists. This zone of confusion is caused by lack of adequate signal directly above the station due to the radiation pattern of the station’s antenna, and because the resultant of the opposing reference and variable signals is small and constantly changing.

The CDI in figure 7-14 indicates 180°, meaning that the aircraft is on the 180° or the 360° radial of the station. The TO/FROM indicator resolves the ambiguity. If the TO indicator is showing, then it is 180° TO the station. The FROM indication indicates the radial of the station the aircraft is presently on. Movement of the CDI from center, if it occurs at a relatively constant rate, indicates the aircraft is moving or drifting off the 180°/360° line. If the movement is rapid or fluctuating, this is an indication of impending station passage (the aircraft is near the station). To determine the aircraft’s position relative to the station, rotate the OBS until FROM appears in the window, then center the CDI needle. The index indicates the VOR radial where the aircraft is located. The inbound (to the station) course is the reciprocal of the radial.

If you set the VOR to the reciprocal of your course, the CDI will reflect reverse sensing. To correct for needle deflection, you will need to fly away from the needle. To avoid this reverse sensing situation, set the VOR to agree with your intended course.

Zone of confusion: Volume of space Reverse sensing: When the VOR above the station where a lack of needle indicates the reverse of normal adequate navigation signal directly operation. This occurs when the above the VOR station causes the aircraft is headed toward the station needle to deviate. with a FROM indication or when the

A single NAVAID will only allow you to determine your position relative to a radial. You need to cross-reference the indications from a second NAVAID in order to narrow your position down to an exact location on this radial.

To track to the station, rotate the OBS until TO appears, then center the CDI. Fly the course indicated by the index. If the CDI moves off center to the left, follow the needle by correcting course to the left, beginning with a 20° correction.

Course interception is illustrated in figure 7-15. 5. Failure to parallel the desired radial on a track interception problem. Without this step, orientation to the desired radial can be confusing. Since you think in left/right terms, aligning your aircraft position to the radial/course is essential.

1. Overshooting and undershooting radials on interception problems.
2. Overcontrolling corrections during tracking, especially close to the station.
3. Misinterpretation of station passage. On VOR receivers equipped without an ON/OFF flag, a voice transmission on the combined communication and navigation radio (NAV/COM) in use for VOR may cause the same TO/ FROM fluctuations on the ambiguity meter as shown during station passage. Read the whole receiver—TO/ FROM, CDI, and OBS—before you make a decision. Do not utilize a VOR reading observed while transmitting.
4. Chasing the CDI, resulting in homing instead of tracking. Careless heading control and failure to bracket wind corrections makes this error common.

VOR Accuracy

The effectiveness of the VOR depends upon proper use and adjustment of both ground and airborne equipment.

The accuracy of course alignment of the VOR is generally plus or minus 1°. On some VORs, minor course roughness may be observed, evidenced by course needle or brief flag alarm. At a few stations, usually in mountainous terrain, the pilot may occasionally observe a brief course needle oscillation, similar to the indication of “approaching station.” Pilots flying over unfamiliar routes are cautioned to be on the alert for these vagaries, and in particular, to use the TO/ FROM indicator to determine positive station passage.

figure 7-15. Course interception (VOR). Certain propeller revolutions per minute (RPM) settings or helicopter rotor speeds can cause the VOR CDI to fluctuate as much as plus or minus 6°. Slight changes to the RPM setting will normally smooth out this roughness. Pilots are

VOR Operational Errors

urged to check for this modulation phenomenon prior to Typical pilot-induced errors include: reporting a VOR station or aircraft equipment for

1. Careless tuning and identification of station.

unsatisfactory operation.

Failure to check receiver for accuracy/sensitivity.

VOR Receiver Accuracy Check

3. Turning in the wrong direction during an orientation. This VOR system course sensitivity may be checked by notingerror is common until you visualize position rather than the number of degrees of change as you rotate the OBS to

heading.

move the CDI from center to the last dot on either side. The

4. Failure to check the ambiguity (TO/FROM) indicator, course selected should not exceed 10° or 12° either side. particularly during course reversals, resulting in reverse In addition, Title 14 of the Code of Federal Regulations sensing and corrections in the wrong direction.

(14 CFR) part 91 provides for certain VOR equipment accuracy checks, and an appropriate endorsement, within 30 days prior to flight under IFR. To comply with this requirement and to ensure satisfactory operation of the airborne system, use the following means for checking VOR receiver accuracy:

1. VOT or a radiated test signal from an appropriately rated radio repair station.

Certified checkpoints on the airport surface.

Certified airborne checkpoints.

The Federal Aviation Administration (FAA) VOR test facility (VOT) transmits a test signal which provides users a convenient means to determine the operational status and accuracy of a VOR receiver while on the ground where a VOT is located. Locations of VOTs are published in the A/FD. Two means of identification are used. One is a series of dots and the other is a continuous tone. Information concerning an individual test signal can be obtained from the local flight service station (FSS.) The airborne use of VOT is permitted; however, its use is strictly limited to those areas/altitudes specifically authorized in the A/FD or appropriate supplement.

To use the VOT service, tune in the VOT frequency 108.0 MHz on the VOR receiver. With the CDI centered, the OBS should read 0° with the TO/FROM indication showing FROM or the OBS should read 180° with the TO/FROM indication showing TO. Should the VOR receiver operate an RMI, it will indicate 180° on any OBS setting.

A radiated VOT from an appropriately rated radio repair station serves the same purpose as an FAA VOT signal, and the check is made in much the same manner as a VOT with some differences.

The frequency normally approved by the Federal Communications Commission (FCC) is 108.0 MHz; however, repair stations are not permitted to radiate the VOR test signal continuously. The owner or operator of the aircraft must make arrangements with the repair station to have the test signal transmitted. A representative of the repair station must make an entry into the aircraft logbook or other permanent record certifying to the radial accuracy and the date of transmission.

The airborne equipment includes an antenna and a receiver.

The pilot-controllable features of the DME receiver include:

Channel (frequency) selector. Many DMEs are channeled by an associated VHF radio, or there may be a selector switch so you can select which VHF radio is channeling the DME. For the DMEs that have their own frequency selector, you use the frequency of the associated VOR/DME or VORTAC station.

On/Off/Volume switch. The DME identifier will be heard as a Morse code identifier with a tone somewhat higher than that of the associated VOR or LOC. It will be heard once for every three or four times the VOR or LOC identifier is heard. If only one identifier is heard about every 30 seconds, the DME is functional, but the associated VOR or LOC is not.

Mode switch. The mode switch selects between distance (DIST) or distance in NMs, groundspeed and time to station. There may also be one or more HOLD functions which permit the DME to stay channeled to the station that was selected before the switch was placed in the hold position. This is useful when you make an ILS approach at a facility that has no colocated DME, but there is a VOR/DME nearby.

Altitude. Some DMEs correct for slant-range error.

Function of DME

A DME is used for determining the distance from a ground DME transmitter. Compared to other VHF/UHF NAVAIDs, a DME is very accurate. The distance information can be used to determine the aircraft position or flying a track that is a constant distance from the station. This is referred to as a DME arc.

DME Arc

There are many instrument approach procedures (IAPs) that incorporate DME arcs. The procedures and techniques given here for intercepting and maintaining such arcs are applicable to any facility that provides DME information. Such a facility may or may not be colocated with the facility that provides final approach guidance.

DME arc: Flying a track that is a constant distance from the station.

As an example of flying a DME arc, refer to figure 7-16 and follow these steps:

1. Track inbound on the OKT 325° radial, frequently checking the DME mileage readout.
2. A .5 NM lead is satisfactory for groundspeeds of 150 knots or less; start the turn to the arc at 10.5 miles. At higher groundspeeds, use a proportionately greater lead.
3. Continue the turn for approximately 90°. The roll-out heading will be 055° in no-wind conditions.
4. During the last part of the intercepting turn, monitor the DME closely. If the arc is being overshot (more than 1.0 NM), continue through the originally-planned roll-out heading. If the arc is being undershot, roll out of the turn early.

The procedure for intercepting the 10 DME when outbound is basically the same, the lead point being 10 NM minus .5 NM, or 9.5 NM.

When flying a DME arc with wind, it is important that you keep a continuous mental picture of your position relative to the facility. Since the wind-drift correction angle is constantly changing throughout the arc, wind orientation is important. In some cases, wind can be used in returning to the desired track. High airspeeds require more pilot attention because of the higher rate of deviation and correction.

Maintaining the arc is simplified by keeping slightly inside the curve; thus, the arc is turning toward the aircraft and interception may be accomplished by holding a straight course. If you are outside the curve, the arc is “turning away” and a greater correction is required.

To fly the arc using the VOR CDI, center the CDI needle upon completion of the 90° turn to intercept the arc. The aircraft’s heading will be found very near the left or right side (270° or 90° reference points) of the instrument. The readings at that side location on the instrument will give primary heading information while on the arc. Adjust the aircraft heading to compensate for wind and to correct for distance to maintain the correct arc distance. Re-center the CDI and note the new primary heading indicated whenever the CDI gets 2°–4° from center.

Without an RMI, orientation is more difficult since you do not have a direct azimuth reference. However, the procedure can be flown using the OBS and CDI for azimuth information and the DME for arc distance.

Intercepting Lead Radials

A lead radial is the radial at which the turn from the arc to the inbound course is started. When intercepting a radial from a DME arc, the lead will vary with arc radius and groundspeed. For the average general aviation aircraft, flying arcs such as those depicted on most approach charts at speeds of 150 knots or less, the lead will be under 5°. There is no difference between intercepting a radial from an arc and intercepting it from a straight course.

Lead radial: The radial at which the turn from the arc to the inbound course is started.

With an RMI, the rate of bearing movement should be monitored closely while flying the arc. Set the course of the radial to be intercepted as soon as possible and determine the approximate lead. Upon reaching this point, start the intercepting turn. Without an RMI, the technique for radial interception is the same except for azimuth information which is available only from the OBS and CDI.

The technique for intercepting a localizer from a DME arc is similar to intercepting a radial. At the depicted lead radial (LR 070° or LR 084° in figure 7-18), a pilot having a single VOR/LOC receiver should set it to the localizer frequency. If the pilot has dual VOR/LOC receivers, one unit may be used to provide azimuth information and the other set to the localizer frequency. Since these lead radials provide 7° of lead, a half-standard-rate turn should be used until the LOC needle starts to move toward center.

DME/DME fixes (a location based on two DME lines of position from two DME stations) provide a more accurate aircraft location than using a VOR and a DME fix.

DME signals are line-of-sight; the mileage readout is the straight line distance from the aircraft to the DME ground facility and is commonly referred to as slant range distance. Slant range refers to the straight line distance from the aircraft antenna to the ground station, which differs somewhat from the distance from the station to the point on the ground beneath the aircraft. This error is smallest at low altitude and long range. It is greatest when the aircraft is over the ground facility, at which time the DME receiver will display altitude (in NM) above the facility. Slant-range error is negligible if the aircraft is 1 mile or more from the ground facility for each 1,000 feet of altitude above the elevation of the facility.

Area navigation (RNAV) equipment includes VOR/DME, LORAN, GPS, and inertial navigation systems (INS). RNAV equipment is capable of computing the aircraft position, actual track, groundspeed, and then presenting meaningful information to the pilot. This information may be in the form of distance, crosstrack error, and time estimates relative to the selected track or waypoint. In addition, the RNAV equipment installations must be approved for use under IFR. The Pilot’s Operating Handbook/Airplane Flight Manual (POH/AFM) should always be consulted to determine what equipment is installed, the operations that are approved, and the details of how to use the equipment. Some aircraft may have equipment that allows input from more than one RNAV source thereby providing a very accurate and reliable navigation source.

VOR RNAV is based on information generated by the present VORTAC or VOR/DME systems to create a waypoint using an airborne computer. As shown in figure 7-19, the value of side A is the measured DME distance to the VOR/DME. Side B, the distance from the VOR/DME to the waypoint, angle 1 (VOR radial or the bearing from the VORTAC to the waypoint), are values set in the cockpit control. The bearing

2. MODE select switch used to select VOR/DME mode, with:

a.

Angular course width deviation (standard VOR operation); or

b.

Linear crosstrack deviation as standard (±5 NM full scale CDI).

1. RNAV mode, with direct to waypoint with linear crosstrack deviation of ±5 NM.
2. RNAV/APPR (approach mode) with linear deviation of ±1.25 NM as full scale CDI deflection.
3. Waypoint select control. Some units allow the storage of more than one waypoint; this control allows selection of any waypoint in storage.
4. Data input controls. These controls allow user input of waypoint number or ident, VOR or LOC frequency, waypoint radial and distance.

While DME groundspeed readout is accurate only when tracking directly to or from the station in VOR/DME mode, in RNAV mode the DME groundspeed readout is accurate on any track.

Function of VOR/DME RNAV

The advantages of the VOR/DME RNAV system stem from the ability of the airborne computer to locate a waypoint wherever it is convenient, as long as the aircraft is within reception range of both a nearby VOR and DME facility. A series of these waypoints make up an RNAV route. In addition to the published routes, a random RNAV route may be flown under IFR if it is approved by air traffic control (ATC). RNAV DPs and standard terminal arrival routes (STARs) are contained in the DP and STAR booklets.

VOR/DME RNAV approach procedure charts are also available. Note in the VOR/DME RNAV chart excerpt shown in figure 7-21 that the waypoint identification boxes contain the following information: waypoint name, coordinates, frequency, identifier, radial distance (facility to waypoint), and reference facility elevation. The initial approach fix (IAF), final approach fix (FAF), and missed approach point (MAP) are labeled.

To fly either a route or to execute an approach under IFR, the RNAV equipment installed in the aircraft must be approved for the appropriate IFR operations.

In Vertical Nav mode, vertical, as well as horizontal guidance is provided in some installations. A waypoint is selected at a point where the descent begins, and another waypoint is selected where the descent ends. The RNAV equipment computes the rate of descent relative to the groundspeed, and on some installations, displays vertical guidance information on the glide-slope indicator. When using this type of equipment during an instrument approach, the pilot must keep in mind the vertical guidance information provided is not part of the nonprecision approach. Published nonprecision approach altitudes must be observed and complied with, unless otherwise directed by ATC.

To fly to a waypoint using RNAV, observe the following procedure [figure 7-22]:

1. Select the VOR/DME frequency.
2. Select the RNAV mode.
3. Select the radial of the VOR that passes through the waypoint (225°).
4. Select the distance from the DME to the waypoint is selected (12 NM).
5. Check and confirm all inputs, and the CDI needle is centered with the TO indicator showing.
6. Maneuver the aircraft to fly the indicated heading ± wind correction to keep the CDI needle centered.
7. The CDI needle will indicate distance off course of 1 NM per dot; the DME readout will indicate distance (NM) from the waypoint; the groundspeed will read closing speed (knots) to the waypoint; and the time to station (TTS) will read time to the waypoint.

Published approaches have been tested to ensure this is not a 2. Groundspeed by dividing distance covered since last

problem. Descents/approaches to airports distant from the computation by the time since last computation (and

VOR/DME facility may not be possible because during the averaging several of these);

approach, the aircraft may descend below the reception 3. Distance to destination;altitude of the facility at that distance.

The Aeronautical Information Manual (AIM) provides a detailed explanation of how LORAN works. LORAN is a very accurate navigation system if adequate signals are received. There are two types of accuracy that must be addressed in any discussion of LORAN accuracy.

Repeatable accuracy is the accuracy measured when a user notes the LORAN position, moves away from that location, then uses the LORAN to return to that initial LORAN position. Distance from that initial position is the error. Propagation and terrain errors will be essentially the same as when the first position was taken, so those errors are factored out by using the initial position. Typical repeatable accuracy for LORAN can be as good as 0.01 NM, or 60 feet, if the second position is determined during the day and within a short period of time (a few days).

Absolute accuracy refers to the ability to determine present position in space independently, and is most often used by pilots. When the LORAN receiver is turned on and position is determined, absolute accuracy applies. Typical LORAN absolute accuracy will vary from about 0.1 NM to as much as 2.5 NM depending on distance from the station, geometry of the TD LOP crossing angles, terrain and environmental conditions, signal-to-noise ratio (signal strength), and some design choices made by the receiver manufacturer.

LORAN Components

The LORAN receiver incorporates a radio receiver, signal processor, navigation computer, control/display, and antenna. When turned on, the receivers go through an initialization or warm-up period, then inform the user they are ready to be programmed. LORAN receivers vary widely in their appearance, how they are programmed by the user, and how they display navigation information. Therefore, it is necessary to become familiar with the unit, including how to program it, and how to interpret output from it. The LORAN operating manual should be in the aircraft at all times and available to the pilot. IFR-approved LORAN units require that the manual be aboard and that the pilot is familiar with the unit’s functions, before flight.

Function of LORAN

After initialization, you select for the present location waypoint (the airport), and select GO TO in order to determine if the LORAN is functioning properly. Proper operation is indicated by a low distance reading (0 to 0.5 NM). The simplest mode of navigation is referred to as GO TO: you select a waypoint from one of the databases and choose the GO TO mode. Before use in flight, you should verify that the latitude and longitude of the chosen waypoint is correct by reference to another approved information source. An updatable LORAN database that supports the appropriate operations (e.g., en route, terminal, and instrument approaches), is required when operating under IFR.

In addition to displaying bearing, distance, time to the waypoint, and track and speed over the ground, the LORAN receiver may have other features such as flight planning (waypoint sequential storage), emergency location of several nearest airports, vertical navigation capabilities, and more.

LORAN Errors

System Errors

LORAN is subject to interference from many external sources, which can cause distortion of, or interference with LORAN signals. LORAN receiver manufacturers install “notch filters” to reduce or eliminate interference. Proximity to 60 Hz alternating current power lines, static discharge, precipitation static, electrical noise from generators, alternators, strobes, and other onboard electronics may decrease the signal-to-noise ratio to the point where the LORAN receiver’s performance is degraded.

Proper installation of the antenna, good electrical bonding, and an effective static discharge system are the minimum requirements for LORAN receiver operation. Most receivers have internal tests that verify the timing alignment of the receiver clock with the LORAN pulse, and measure and display signal-to-noise ratio. A signal will be activated to alert the pilot if any of the parameters for reliable navigation are exceeded on LORAN sets certified for IFR operations.

LORAN is most accurate when the signal travels over sea water during the day, and least accurate when the signal comes over land and large bodies of fresh water or ice at night; furthermore, the accuracy degrades as distance from the station increases. However, LORAN accuracy is generally better than VOR accuracy.

Operational Errors

Some of the typical pilot-induced errors of LORAN operation are:

1. Use of a nonapproved LORAN receiver for IFR operations. The pilot should check the aircraft’s POH/AFM LORAN supplement to be certain the unit’s functions are well understood (this supplement must be present in the aircraft for approved IFR operations). There should be a copy of FAA Form 337, Major Repair and Alteration, present in the aircraft’s records, showing approval (for use of this model LORAN for IFR operations in this aircraft).

1. Failure to double-check the latitude/longitude values for a waypoint to be used. Whether the waypoint was accessed from the airport, NDB, VOR, or intersection database, the values of latitude and longitude should still be checked against the values in the A/FD or other approved source. If the waypoint data is entered in the user database, its accuracy must be checked before use.
2. Attempting to use LORAN information with degraded signals.

The Department of Defense (DOD) developed and deployed GPS as a space-based positioning, velocity, and time system. The DOD is responsible for the operation the GPS satellite constellation and constantly monitors the satellites to ensure proper operation. The GPS system permits Earth-centered coordinates to be determined and provides aircraft position referenced to the DOD World Geodetic System of 1984 (WGS-84). Satellite navigation systems are unaffected by weather and provide global navigation coverage that fully meets the civil requirements for use as the primary means of navigation in oceanic airspace and certain remote areas. Properly certified GPS equipment may be used as a supplemental means of IFR navigation for domestic en route, terminal operations, and certain IAPs. Navigational values, such as distance and bearing to a waypoint and groundspeed, are computed from the aircraft’s current position (latitude and longitude) and the location of the next waypoint. Course guidance is provided as a linear deviation from the desired track of a Great Circle route between defined waypoints.

GPS may not be approved for IFR use in other countries. Prior to its use, pilots should ensure that GPS is authorized by the appropriate countries.

GPS consists of three distinct functional elements: space, control, and user.

The space element consists of 24 Navstar satellites. This group of satellites is called a constellation. The satellites are in six orbital planes (with four in each plane) at about 11,000

figure 7-23. Instrument landing systems.

The following supplementary elements, though not specific components of the system, may be incorporated to increase safety and utility:

1. Compass locators provide transition from en route NAVAIDs to the ILS system; they assist in holding procedures, tracking the localizer course, identifying the marker beacon sites, and providing a FAF for ADF approaches.
2. DME colocated with the glide-slope transmitter provide positive distance-to-touchdown information or DME associated with another nearby facility (VOR or standalone), if specified in the approach procedure.

ILS approaches are categorized into three different types of approaches, based on the equipment at the airport and the experience level of the pilot. Category I approaches provide for approach height above touchdown of not less than 200 feet. Category II approaches provide for approach to a height above touchdown of not less than 100 feet. Category III approaches provide lower minimums for approaches without a decision height minimum. While pilots must only be instrument rated and the aircraft be equipped with the appropriate airborne equipment to execute Category I approaches, Category II and III approaches require special certification for the pilots, ground equipment, and airborne equipment.

The ILS uses a number of different ground facilities. These facilities may be used as a part of the ILS system, as well as part of another approach. For example, the compass locator may be used with NDB approaches.

The localizer (LOC) ground antenna array is located on the extended centerline of the instrument runway of an airport, remote enough from the opposite (approach) end of the runway to prevent it from being a collision hazard. This unit radiates a field pattern, which develops a course down the centerline of the runway toward the middle markers (MMs) and outer markers (OMs), and a similar course along the runway centerline in the opposite direction. These are called the front and back courses, respectively. The localizer

Localizer (LOC): The portion of the Middle marker (MM): VHF marker ILS that gives left/right guidance beacon used in the ILS. When the information down the centerline of NDB compass locator is colocated the instrument runway for final with an MM, it is shown as LMM on approach. instrument approach charts.

Glide Slope

Glide slope (GS) describes the systems that generate, receive, and indicate the ground facility radiation pattern. The glidepath is the straight, sloped line the aircraft should fly in its descent from where the glide slope intersects the altitude used for approaching the FAF, to the runway touchdown zone. The glide-slope equipment is housed in a building approximately 750 to 1,250 feet down the runway from the approach end of the runway, and between 400 and 600 feet to one side of the centerline.

The course projected by the glide-slope equipment is essentially the same as would be generated by a localizer operating on its side. The glide-slope projection angle is normally adjusted to 2.5° to 3.5° above horizontal, so it intersects the MM at about 200 feet and the OM at about 1,400 feet above the runway elevation. At locations where standard minimum obstruction clearance cannot be obtained with the normal maximum glide-slope angle, the glide-slope equipment is displaced farther from the approach end of the runway if the length of the runway permits; or, the glide-slope angle may be increased up to 4°.

Unlike the localizer, the glide-slope transmitter radiates signals only in the direction of the final approach on the front course. The system provides no vertical guidance for approaches on the back course. The glidepath is normally 1.4° thick. At 10 NM from the point of touchdown, this represents a vertical distance of approximately 1,500 feet, narrowing to a few feet at touchdown.

Glide slope (GS): Part of the ILS that projects a radio beam upward at an angle of approximately 3° from the approach end of an instrument runway to provide vertical guidance for final approach.

Marker Beacons

Two VHF marker beacons, outer and middle, are normally used in the ILS system. A third beacon, the inner, is used where Category II operations are certified. A marker beacon may also be installed to indicate the FAF on the ILS back course.

The OM is located on the localizer front course 4 to 7 miles from the airport to indicate a position at which an aircraft, at the appropriate altitude on the localizer course, will intercept the glidepath. The MM is located approximately 3,500 feet from the landing threshold on the centerline of the localizer front course at a position where the glide-slope centerline is about 200 feet above the touchdown zone elevation. The inner marker (IM), where installed, is located on the front course between the MM and the landing threshold. It indicates the point at which an aircraft is at the decision height on the glidepath during a Category II ILS approach. The back-course marker, where installed, indicates the back-course FAF.

Compass Locator

Compass locators are low-powered NDBs and are received and indicated by the ADF receiver. When used in conjunction with an ILS front course, the compass locator facilities are colocated with the outer and/or MM facilities. The coding identification of the outer locator consists of the first two letters of the three-letter identifier of the associated LOC. For example, the outer locator at Dallas/Love Field (DAL) is identified as “DA.” The middle locator at DAL is identified by the last two letters “AL.”

Approach Lighting Systems (ALS)

Normal approach and letdown on the ILS is divided into two distinct stages: the instrument approach stage using only radio guidance, and the visual stage, when visual contact with the ground runway environment is necessary for accuracy and safety. The most critical period of an instrument approach, particularly during low ceiling/visibility conditions, is the point at which the pilot must decide whether to land or execute a missed approach. As the runway threshold is approached, the visual glidepath will separate into individual lights. At this point, the approach should be continued by reference to the runway touchdown zone markers. The ALS provides lights that will penetrate the atmosphere far enough from touchdown to give directional, distance, and glidepath information for safe visual transition.

Marker beacon: A low-powered transmitter that directs its signal upward in a small, fan-shaped pattern. Used along the flightpath when approaching an airport for landing, marker beacons indicate, both aurally and visually, when the aircraft is directly over the facility.

Visual identification of the ALS by the pilot must be instantaneous, so it is important to know the type of ALS before the approach is started. Check the instrument approach chart and the A/FD for the particular type of lighting facilities at the destination airport before any instrument flight. With reduced visibility, rapid orientation to a strange runway can be difficult, especially during a circling approach to an airport with minimum lighting facilities, or to a large terminal airport located in the midst of distracting city and ground facility lights. Some of the most common ALS systems are shown in figure 7-26.

Runway end identifier lights (REIL) are installed for rapid and positive identification of the approach end of an instrument runway. The system consists of a pair of synchronized flashing lights placed laterally on each side of the runway threshold facing the approach area.

The visual approach slope indicator (VASI) gives visual descent guidance information during the approach to a runway. The standard VASI consists of light bars that project a visual glidepath, which provides safe obstruction clearance within the approach zone. The normal glide-slope angle is 3°; however, the angle may be as high as 4.5° for proper obstacle clearance. On runways served by ILS, the VASI angle normally coincides with the electronic glide-slope angle. Visual left/right course guidance is obtained by alignment with the runway lights. The standard VASI installation consists of either 2-, 3-, 4-, 6-, 12-, or 16-light units arranged in downwind and upwind light bars. Some airports serving long-bodied aircraft have three-bar VASIs which provide two visual glidepaths to the same runway. The first glidepath encountered is the same as provided by the standard VASI. The second glidepath is about 25 percent higher than the first and is designed for the use of pilots of long-bodied aircraft.

The basic principle of VASI is that of color differentiation between red and white. Each light projects a beam having a white segment in the upper part and a red segment in the lower part of the beam. From a position above the glidepath the pilot sees both bars as white. Lowering the aircraft with respect to the glidepath, the color of the upwind bars changes from white to pink to red. When on the proper glidepath, the landing aircraft will overshoot the downwind bars and undershoot the upwind bars. Thus the downwind (closer) bars are seen as white and the upwind bars as red. From a position below the glidepath both light bars are seen as red. Moving up to the glidepath, the color of the downwind bars changes from red to pink to white. When below the glidepath, as indicated by a distinct all-red signal, a safe obstruction clearance might not exist. A standard two-bar VASI is illustrated in figure 7-27.

ILS Airborne Components

Airborne equipment for the ILS system includes receivers for the localizer, glide slope, marker beacons, ADF, DME, and the respective indicator instruments.

The typical VOR receiver is also a localizer receiver with common tuning and indicating equipment. Some receivers have separate function selector switches, but most switch between VOR and LOC automatically by sensing if odd tenths between 108 and 111.95 MHz have been selected. Otherwise, tuning of VOR and localizer frequencies is accomplished with the same knobs and switches, and the CDI indicates “on course” as it does on a VOR radial.

Though some glide-slope receivers are tuned separately, in a typical installation the glide slope is tuned automatically to the proper frequency when the localizer is tuned. Each of the 40 localizer channels in the 108.10 to 111.95 MHz band is paired with a corresponding glide-slope frequency.

When the localizer indicator also includes a glide-slope needle, the instrument is often called a cross-pointer indicator. The crossed horizontal (glide slope) and vertical (localizer) needles are free to move through standard five-dot deflections to indicate position on the localizer course and glidepath.

When the aircraft is on the glidepath, the needle is horizontal, overlying the reference dots. Since the glidepath is much narrower than the localizer course (approximately 1.4° from full up to full down deflection), the needle is very sensitive to displacement of the aircraft from on-path alignment. With the proper rate of descent established upon glide-slope interception, very small corrections keep the aircraft aligned.

The localizer and glide-slope warning flags disappear from view on the indicator when sufficient voltage is received to actuate the needles. The flags show when an unstable signal or receiver malfunction occurs.

The OM is identified by a low-pitched tone, continuous dashes at the rate of two per second, and a purple/blue marker beacon light. The MM is identified by an intermediate tone, alternate dots and dashes at the rate of 95 dot/dash combinations per minute, and an amber marker beacon light. The IM, where installed, is identified by a high-pitched tone, continuous dots at the rate of six per second, and a white marker beacon light. The back-course marker (BCM), where installed, is identified by a high-pitched tone with two dots at a rate of 72 to 75 two-dot combinations per minute, and a white marker beacon light. Marker beacon receiver sensitivity is selectable as high or low on many units. The low-sensitivity position gives the sharpest indication of position and should be used during an approach. The high-sensitivity position provides an earlier warning that the aircraft is approaching the marker beacon site.

The localizer needle indicates, by deflection, whether the aircraft is right or left of the localizer centerline, regardless of the position or heading of the aircraft. Rotating the OBS has no effect on the operation of the localizer needle, although it is useful to rotate the OBS to put the LOC inbound course under the course index. When inbound on the front course, or outbound on the back course, the course indication remains directional. (See figure 7-28, aircraft C, D, and E.)

Unless the aircraft has reverse sensing capability and it is in use, when flying inbound on the back course or outbound on the front course, heading corrections to on-course are made opposite the needle deflection. This is commonly described as “flying away from the needle.” (See figure 7-28, aircraft A and B.) Back course signals should not be used for an approach unless a back course approach procedure is published for that particular runway and the approach is authorized by ATC.

Once you have reached the localizer centerline, maintain the inbound heading until the CDI moves off center. Drift corrections should be small and reduced proportionately as the course narrows. By the time you reach the OM, your drift correction should be established accurately enough on a well-executed approach to permit completion of the approach, with heading corrections no greater then 2°.

The heaviest demand on pilot technique occurs during descent from the OM to the MM, when you maintain the localizer course, adjust pitch attitude to maintain the proper rate of descent, and adjust power to maintain proper airspeed. Simultaneously, the altimeter must be checked and preparation made for visual transition to land or for a missed approach. You can appreciate the need for accurate instrument interpretation and aircraft control within the ILS as a whole, when you notice the relationship between CDI and glidepath needle indications, and aircraft displacement from the localizer and glidepath centerlines.

Deflection of the glide-slope needle indicates the position of the aircraft with respect to the glidepath. When the aircraft is above the glidepath, the needle is deflected downward. When the aircraft is below the glidepath, the needle is deflected upward. [figure 7-29]

ILS Errors

The ILS and its components are subject to certain errors, which are listed below.

Localizer and glide-slope signals are subject to the same type of bounce from hard objects as space waves.

1. Reflection. Surface vehicles and even other aircraft flying below 5,000 feet above ground level (AGL) may disturb the signal for aircraft on the approach.
2. False courses. In addition to the desired course, glide-slope facilities inherently produce additional courses at higher vertical angles. The angle of the lowest of these false courses will occur at approximately 9°–12°. An aircraft flying the LOC/glide-slope course at a constant altitude would observe gyrations of both the glide-slope needle and glide-slope warning flag as the aircraft passed

through the various false courses. Getting established on one of these false courses will result in either confusion (reversed glide-slope needle indications), or result in the need for a very high descent rate. However, if the approach is conducted at the altitudes specified on the appropriate approach chart, these false courses will not be encountered.

Marker Beacons

The very low power and directional antenna of the marker beacon transmitter ensures that the signal will not be received any distance from the transmitter site. Problems with signal reception are usually caused by the airborne receiver not being turned on, or by incorrect receiver sensitivity.

Some marker beacon receivers, to decrease weight and cost, are designed without their own power supply. These units utilize a power source from another radio in the avionics stack, often the ADF. In some aircraft, this requires the ADF to be turned on in order for the marker beacon receiver to function, yet no warning placard is required. Another source of trouble may be the “High/Low/Off” three-position switch, which both activates the receiver and selects receiver sensitivity. Usually, the “test” feature only tests to see if the light bulbs in the marker beacon lights are working. Therefore, in some installations, there is no functional way for the pilot to ascertain the marker beacon receiver is actually on except to fly over a marker beacon transmitter, and see if a signal is received and indicated (e.g., audibly and marker beacon lights).

1. Failure to understand the fundamentals of ILS ground equipment, particularly the differences in course dimensions. Since the VOR receiver is used on the localizer course, the assumption is sometimes made that interception and tracking techniques are identical when tracking localizer courses and VOR radials. Remember that the CDI sensing is sharper and faster on the localizer course.
2. Disorientation during transition to the ILS due to poor planning and reliance on one receiver instead of on all available airborne equipment. Use all the assistance you have available; the single receiver you may be relying on may fail at a busy time.
3. Disorientation on the localizer course, due to the first error noted above.
4. Incorrect localizer interception angles. A large interception angle usually results in overshooting, and possible disorientation. When intercepting, if possible, turn to the localizer course heading immediately upon the first indication of needle movement. An ADF receiver is an excellent aid to orient you during an ILS approach if there is a locator or NDB on the inbound course.
5. Chasing the CDI and glidepath needles, especially when you have not sufficiently studied the approach before the flight.

The SDF provides a final approach course similar to the ILS localizer. The SDF course may or may not be aligned with the runway and the course may be wider than a standard ILS localizer, resulting in less precision. Usable off-course indications are limited to 35° either side of the course centerline. Instrument indications in the area between 35° and 90° from the course centerline are not controlled and should be disregarded.

The SDF antenna may be offset from the runway centerline. Because of this, the angle of convergence between the final approach course and the runway bearing should be determined by reference to the instrument approach chart. This angle is usually not more than 3°. You should note this angle since the approach course originates at the antenna site, and an approach continued beyond the runway threshold would lead the aircraft to the SDF offset position rather than along the runway centerline.

Approach Azimuth Station

The approach azimuth station, unlike ILS, is able to provide approach guidance along any path within its ±40° (to runway alignment) range. [figure 7-30] Therefore, curved and segmented approaches are possible. This facility also provides the data communications capability of the system. This station is normally located about 1,000 feet beyond the stop end of the runway, but beyond this limitation there is considerable latitude in actual station location. A back azimuth station may be operating in conjunction with the approach azimuth station. If so, lateral guidance is available for missed approach and departure navigation.

Elevation Guidance Station

Like the approach azimuth station, the elevation guidance station has considerably more capability than the ILS glide-slope system. Approach glidepath angles are selectable over a wide range up to at least 15°, with coverage to a maximum of 30°. This provides considerable flexibility for developing multipath approaches.

Range Guidance Station

The range guidance station transmits both normal and precision DME (DME/P) signals that function the same as normal DME (DME/N), with some technical and accuracy differences. Accuracy is improved to be consistent with the accuracy provided by the MLS azimuth and elevation stations.

Once airborne, the FMS computer channels the appropriate NAVAIDs and takes radial/distance information, or channels two NAVAIDs, taking the more accurate distance information. FMS then indicates position, track, desired heading, groundspeed and position relative to desired track. Position information from the FMS updates the INS. In more sophisticated aircraft, the FMS provides inputs to the HSI, RMI, glass cockpit navigation displays, head-up display (HUD), autopilot, and autothrottle systems.

Head-up Display (HUD)

Description

The HUD is a display system that provides a projection of navigation and air data (airspeed in relation to approach reference speed, altitude, left/right and up/down glide slope) on a transparent screen between the pilot and the windshield.

Head-up display (HUD): A special type of flight viewing screen that allows the pilot to watch the flight instruments while looking through the windshield of the aircraft for other traffic, the approach lights, or the runway.

Other information may be displayed, including a runway target in relation to the nose of the aircraft. This allows the pilot to see the information necessary to make the approach while also being able to see out the windshield, which diminishes the need to shift between looking at the panel to looking outside. Virtually any information desired can be displayed on the HUD if it is available in the aircraft’s flight computer, and if the display is user-definable.

The radar display system provides the controller with a map-like presentation upon which appear all the radar echoes of aircraft within detection range of the radar facility. By means of electronically-generated range marks and azimuth-indicating devices, the controller can locate each radar target with respect to the radar facility, or can locate one radar target with respect to another.

Another device, a video-mapping unit, generates an actual airway or airport map and presents it on the radar display equipment. Using the video-mapping feature, the air traffic controller not only can view the aircraft targets, but will see these targets in relation to runways, navigation aids, and hazardous ground obstructions in the area. Therefore, radar becomes a navigational aid, as well as the most significant means of traffic separation.

In a display presenting perhaps a dozen or more targets, a primary surveillance radar system cannot identify one specific radar target, and it may have difficulty “seeing” a small target at considerable distance—especially if there is a rain shower or thunderstorm between the radar site and the aircraft. This problem is solved with the Air Traffic Control Radar Beacon System (ATCRBS), sometimes called secondary surveillance radar (SSR), which utilizes a transponder in the aircraft. The ground equipment is an interrogating unit, with the beacon antenna mounted so it rotates with the surveillance antenna. The interrogating unit transmits a coded pulse sequence that actuates the aircraft transponder. The transponder answers the coded sequence by transmitting a preselected coded sequence back to the ground equipment, providing a strong return signal and positive aircraft identification, as well as other special data such as the aircraft’s altitude.

Functions of Radar Navigation

The radar systems used by ATC are air route surveillance radar (ARSR), airport surveillance radar (ASR), precision approach radar (PAR) and airport surface detection equipment (ASDE). Surveillance radars scan through 360°

Air route surveillance radar Airport surveillance radar (ASR): (ARSR): Air route traffic control Approach control radar used to center (ARTCC) radar used primarily detect and display an aircraft’s to detect and display an aircraft’s position in the terminal area. position while en route between terminal areas.

of azimuth and present target information on a radar display located in a tower or center. This information is used independently or in conjunction with other navigational aids in the control of air traffic.

ARSR is a long-range radar system designed primarily to cover large areas and provide a display of aircraft while en route between terminal areas. The ARSR enables air route traffic control center (ARTCC) controllers to provide radar service when the aircraft are within the ARSR coverage. In some instances, ARSR may enable ARTCC to provide terminal radar services similar to but usually more limited than those provided by a radar approach control.

ASR is designed to provide relatively short-range coverage in the general vicinity of an airport and to serve as an expeditious means of handling terminal area traffic through observation of precise aircraft locations on a radarscope. Nonprecision instrument approaches are available at airports that have an approved surveillance radar approach procedure. ASR provides radar vectors to the final approach course, and then azimuth information to the pilot during the approach. Along with range (distance) from the runway, the pilot is advised of MDA, when to begin descent, and when at the MDA. If requested, recommended altitudes will be furnished each mile while on final.

PAR is designed to be used as a landing aid displaying range, azimuth, and elevation information rather than an aid for sequencing and spacing aircraft. PAR equipment may be used as a primary landing aid, or it may be used to monitor other types of approaches. Two antennas are used in the PAR array, one scanning a vertical plane, and the other scanning horizontally. Since the range is limited to 10 miles, azimuth to 20°, and elevation to 7°, only the final approach area is covered. The controller’s scope is divided into two parts. The upper half presents altitude and distance information, and the lower half presents azimuth and distance.

The PAR is one in which a controller provides highly accurate navigational guidance in azimuth and elevation to a pilot. Pilots are given headings to fly, to direct them to, and keep their aircraft aligned with the extended centerline of the landing runway. They are told to anticipate glidepath interception approximately 10 to 30 seconds before it occurs

Precision approach radar (PAR): Airport surface detection A specific type and installation of equipment (ASDE): Radar radar, usually found at military or equipment specifically designed to joint-use airfields. It uses two radar detect all principle features and antennas, one for left/right traffic on the surface of an airport, information and one for glidepath presenting the entire image on the information to provide a precision control tower console. approach.

and when to start descent. The published decision height (DH) will be given only if the pilot requests it. If the aircraft is observed to deviate above or below the glidepath, the pilot is given the relative amount of deviation by use of terms “slightly” or “well” and is expected to adjust the aircraft’s rate of descent/ascent to return to the glidepath. Trend information is also issued with respect to the elevation of the aircraft and may be modified by the terms “rapidly” and “slowly”; e.g., “well above glidepath, coming down rapidly.” Range from touchdown is given at least once each mile. If an aircraft is observed by the controller to proceed outside of specified safety zone limits in azimuth and/or elevation and continue to operate outside these prescribed limits, the pilot will be directed to execute a missed approach or to fly a specified course unless the pilot has the runway environment (runway, approach lights, etc.) in sight. Navigational guidance in azimuth and elevation is provided to the pilot until the aircraft reaches the published decision altitude (DA)/DH. Advisory course and glidepath information is furnished by the controller until the aircraft passes over the landing threshold, at which point the pilot is advised of any deviation from the runway centerline. Radar service is automatically terminated upon completion of the approach.

Radar equipment is specifically designed to detect all principal features on the surface of an airport, including aircraft and vehicular traffic, and to present the entire image on a radar indicator console in the control tower. It is used to augment visual observation by tower personnel of aircraft and/or vehicular movements on runways and taxiways.