Glossary of Abbreviations, Acronyms, Definitions, and Symbols

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Definitions and Symbols. The use of italics within a definition denotes another definition contained within this section.

Actual Glidepath Alignment or Actual Glidepath Angle. The straight line arithmetic mean of all deviations around the on-path position derived in ILS zone 2.

Actual Course (Alignment). The straight line arithmetic mean of all deviations around the on-course position derived from the area in which alignment was taken.

Actual Navigation Performance (ANP). Sometimes called Estimated Position Error (EPE) or “Q” factor, is an onboard computation of the estimated 95% Navigation System Error using knowledge of the real world navigation environment, i.e., number of satellites tracked, number/geometry of ground facilities, and statistical error models of the various navigation sources. ANP is continuously compared to RNP, and the crew is alerted if ANP exceeds RNP.

AFIS Corrected Error Trace. A graphical presentation of deviation about the mean of all points measured in ILS Zone 2 for glidepaths and zones 2 and 3 for localizers.

Automatic Gain Control (AGC). A process of electronically regulating the gain in the amplification stages of a receiver so that the output signal tends to remain constant though the incoming signal may vary in strength.

AGC Current or Voltage. A current or voltage responding to the action of the AGC circuit that may be interpreted in terms of signal intensity.

Air Traffic Control Radar Beacon System (ATCRBS). The general term of the ultimate in functional capability afforded by several automation systems. Each differs in functional capabilities and equipment. ARTS IA, ARTS II, ARTS III, and ARTS IIIA (see AIM).

Airway/Federal Airway. A control area or portion thereof established in the form of a corridor, the centerline of which is defined by navigational aids (refer to FAR Part 71, AIM).

Alignment. Coincidence of a positional or directional element with its nominal reference.

Alignment, Azimuth. The azimuth or actual magnetic bearing of a course.

Alignment, Elevation. The actual angle above a horizontal plan originating at a specific point of a course used for altitude guidance.

Alignment Error. The angular or linear displacement of a positional or directional element from its normal reference.

Alignment Error, Azimuth. The difference in degrees between the position of a selected course and the correct magnetic azimuth for this course.Note: The error is positive when the course is clockwise from the correct azimuth.

Alignment Error, Elevation. The difference in degrees between the measured angle of the course and the correct angle for the course.Note: The error is positive when the course is above the correct angle.

ALTITUDES:

a. Absolute Altitude. The altitude of the aircraft above the surface it is flying (AC 00-6A). It may be read on a radio/radar altimeter.

b. Calibrated Altitude. Indicated altitude corrected for static pressure error, installation error, and instrument error.

c. Indicated Altitude. The altitude as shown by an altimeter on a pressure or barometric altimeter. It is altitude as shown uncorrected for instrument error and uncompensated for variation from standard atmospheric conditions (AIM).

d. Pressure Altitude. Altitude read on the altimeter when the instrument is adjusted to indicate height above the standard datum plane (29.92″ Hg.)(AC 61-27 latest revision).

e. True Altitude. The calibrated altitude corrected for nonstandard atmospheric conditions. It is the actual height above mean sea level (AC 61-27 and AFM 51-37).

Ampere. A unit of electric current such as would be given with an electromotive force of one volt through a wire having a resistance of one OHM. See Symbols. See Crosspointer.

Amplitude (Peak). The maximum instantaneous value of a varying voltage or current measured as either a positive or negative value.

Anomalous Propagation. Weather phenomena resulting in a layer in the atmosphere capable of reflecting or refracting electromagnetic waves either toward or away from the surface of the earth.

Angle Voltage. The alignment points of the azimuth and elevation electronic cursors are expressed in angle voltage or dial divisions.

Antenna. A device used to radiate or receive electromagnetic signals.

Antenna Reflector. That portion of a directional array, frequently indirectly excited, which reduces the field intensity behind the array and increases it in the forward direction.

Approach Azimuth. Equipment which provides lateral guidance to aircraft in the approach and runway regions. This equipment may radiate the Approach Azimuth function or the High Rate Approach Azimuth function along with appropriate basic and auxiliary data.

Approach Elevation. The equipment which provides vertical guidance in the approach region. This equipment radiates the Approach Elevation function.

Approach Reference Datum (ARD). A point at a specified height located vertically above the intersection of the runway centerline and the threshold.

Area Navigation (RNAV). A method of navigation that permits aircraft operations on any desired course within the coverage of station referenced navigation signals or within the limits of self-contained system capability (AIM).

Area VOT. A facility designed for use on the ground or in the air. It may be located to provide the test signal to one or more airports.Attenuation. The reduction in the strength of a signal, expressed in decibels (dB).

Average Course Signal. The course determined by drawing the mean of the maximum course deviations due to roughness and scalloping.

Azimuth. A direction at a reference point expressed as the angle in the horizontal plane between a reference line and the line joining the reference point to another point, usually measured clockwise from the reference line.

Auxiliary Data. Data, transmitted in addition to basic data, that provide facilities maintenance equipment siting information for use in refining airborne position calculations and other supplementary information.

Barometric Vertical Navigation (BARO VNAV). A navigation system which presents computed vertical guidance to the pilot. The computer-resolved Glidepath Angle (GPA) is based on barometric altitude, and is either computed as a geometric angle between two waypoints, or an angle from a single waypoint.

Baseline Extension (Loran-C). The extension of the baseline beyond the master or secondary station. Navigation in this region may be inaccurate due to geometrical considerations resulting in ambiguous position solutions.

Basic Data. Data transmitted by the facilities maintenance equipment that are associated directly with the operation of the landing guidance system.

Bearing. The horizontal direction to or from any point usually measured clockwise from true north or some other reference point (see Non-Directional Beacon)(AIM).

Bends. Slow excursions of the course.

Bits per second (BPS). Refers to digital data transfer rate, usually by modem or direct cable.

Blind Speed. The rate of departure or closing of a target relative to the radar antenna at which cancellation of the primary target by MTI circuits in the radar equipment causes a reduction or complete loss of signal (AIM).Blind Zones (Blind Spots). Areas from which radio transmissions and/or radar echoes cannot be received.

Broadband. Nonautomated signal processing.Capture Effect. A system in which coverage is achieved by the use of two independent radiation field patterns spaced on separate carrier frequencies.Control Electronic Unit (CEU). Mobile MLS computer transmitter and monitoring system.Change/Reversal in Slope of the Glidepath. A long term (1,500 ft or more) change in the direction of the on-path position as determined by the graphic averaging of the short term (roughness, high frequency scalloping) deviations as represented by the differential/corrected error trace.

Checkpoint. A geographical point on the surface of the earth whose location can be determined by reference to a map or chart.Circular Polarization (CP). An electromagnetic wave for which the electronic and/or the magnetic field vector at a point describes a circle.Note: Circular Polarization reduces or eliminates echoes from precipitation.Clearance. The preponderance of the modulation signal appropriate to the area on one side of the reference line or point to which the receiver is positioned, over the modulation signal appropriate to the area on the other side of the reference line.

Clearance Guidance Sector. The volume of airspace, inside the coverage sector, within which the azimuth guidance information provided is not proportional to the angular displacement of the aircraft but is a constant fly-left or fly-right indication of the direction relative to the approach course the aircraft should proceed in order to enter the proportional guidance sector.Close-in Courses. That portion of a course or radial which lies within 10 miles of the station.

Code Train. A series of pulses of similar characteristics and specific spacing. Applicable to the group of pulses transmitted by a transponder each time it replies to an interrogator.Common Digitizer Data Reduction Program (CD). A computer data recording of raw narrowband radar data (minimal filtering ability is provided).Cone of Ambiguity. Airspace over a VOR or TACAN station, conical in shape, in which the To/From ambiguity indicator is changing positions.Control Motion Noise (CMN). Those fluctuations in the guidance which affect aircraft attitude, control surface, column motion, and wheel motion during coupled flight but do not cause aircraft displacement from the desired course or glidepath.

Cooperating Aircraft. Aircraft which cooperate by flying courses required to fulfill specific portions of the flight inspection and which meet the requirements for a small aircraft.Cosecant-Squared Beam. A radar beam pattern designed to give approximately uniform signal intensity for echoes received from distant and nearby objects. The beam intensity varies as the square of the cosecant of the elevation angle.

Crosspointer (Deflection Indicator Current (ICAO)). An output current proportional to: ILS– Difference in depth of modulation measured in microamperes. VOR/VORTAC/TAC — The difference in phase of two transmitted signals measured in degrees of two audio navigation components for a given displacement from a navigation aid.

Course Coincidence. The measured divergence of the designated radials of two adjacent facilities in the airway structure. (ICAO Document 8071).Course Displacement. The difference between the actual course alignment and the correct course alignment. (ICAO Document 8071).Course Error. The difference between the course as determined by the navigational equipment and the actual measured course to the facility. This error is computed as a plus or minus value, using the actual measured course to the facility as a reference.Course Line Computer. Airborne equipment which accepts bearing and distance information from receivers in an aircraft, processes it, and presents navigational information enabling flight on courses other than directly to or from the ground navigation aid being used. (Used in Area Navigation–RNAV.)

Course Roughness. Rapid irregular excursions of the course usually caused by irregular terrain, obstructions, trees, power lines, etc.Course Scalloping. Rhythmic excursions of the electromagnetic course or path.Course Width (Course Sensitivity). The angular deviation required to produce a full-scale course deviation indication of the airborne navigation instrument.Coverage. The designated volume of airspace within which a signal-in-space of specified characteristics is to be radiated.Cycle Skip. The receiver uses the incorrect cycle of the 100 kHz carrier of the Loran-C signal, for time measurements. Normally the third cycle of a given carrier pulse is used for time measurements. Each cycle slip will result in a 10-microsecond error in time measurement and a corresponding error in navigation.

Dedicated TRIAD. Three specific Loran-C stations from one CHAIN. Dedicated TRIAD selection is utilized to ensure that receiver positioning is determined only by these stations.Designed Procedural Azimuth. The azimuth determined by the procedure specialist that defines the desired position of a course or bearing.DF Course (Steer). The indicated magnetic direction of an aircraft to the DF station and the direction the aircraft must steer to reach the station.DF Fix. The geographical location of an aircraft obtained by the direction finder.

Difference in Depth of Modulation (DDM). The percentage modulation of the larger signal minus the percentage modulation of the smaller signal.Discrepancy. Any facility operating parameter which is not within the given tolerance values (prescribed in the U.S. Standard Flight Inspection Manual) as determined by flight inspection measurements.Displaced Threshold. A threshold located on the runway at a point other than the designated beginning of the runway (AIM).Distance Measuring Equipment (DME). Electronic equipment used to measure, in nautical miles, the slant range of the aircraft from the navigation aid. (AIM)

Distance Measuring Equipment/Precision (DME/P). The range function associated with the MLS. It is a precision distance measuring equipment providing accurate range (20 to 40 ft at a 2-sigma probability).DME Electronic Unit (DEU). Mobile MLS transmitter and monitoring system.Doppler VOR (DVOR). VOR using the Doppler frequency shift principle.Dual-Frequency Glidepath System. An ILS glidepath in which coverage is achieved by the use of two independent radiation field patterns spaced on separate carrier frequencies within the particular glidepath channel, e.g., Capture Effect Glidepath.

Dual-Frequency Localizer System. A localizer system in which coverage is achieved by the use of two independent radiation frequencies within the particular localizer VHF channel.Envelope to Cycle Discrepancy (ECD). The discrepancy between the desired and actual zero phase crossing at the end of the third cycle of the Loran-C 100 kHz carrier pulse.Essential Data. Essential data words are Basic Data Words 1, 2, 3, 4, and 6; and Auxiliary Data Words A1, A2, and A3.

Expanded Service Volume (ESV). That additional volume of airspace beyond the service volume requested by the FAA’s Air Traffic Service or procedure specialist and approved by frequency management of the Airway Facilities Division and flight inspection for operational use./F. Flight Management System (FMS) and Electronic Flight Instrument System (EFIS) equipped aircraft with /E capability having a “Special Aircraft and Aircrew Authorization” issued by the FAA.(Flight Facility). Any ground placed electronic equipment used to assist pilots in air navigation, landing approaches, or to direct air traffic movements. Flight facilities include NAVAID’s, communications, and traffic control facilities.

Feed Horn. Radar antenna focal point. Also reference point in antenna elevation measurements.Final Approach Segment. This is the segment in which alignment and descent for landing are accomplished. The final approach segment considered for obstacle clearance begins at the final approach fix or point and ends at the runway or missed approach point, whichever is encountered last. A visual portion within the final approach segment may be included.

Fixed Map. A background map on the radar display produced by one of the following methods: (1) Engraved marks on an overlay illuminated by edge lighting (2) Engraved fluorescent marks on an overlay illuminated by means of ultraviolet light. (3) Projected on the display by means of film and a projector mounted above and in front of the scope. (4) Electronically mixed into the display as generated by a “mapper” unit

Flag (Flag Alarm). A warning device in certain airborne navigation equipment and flight instruments indicating: (1) instruments are inoperative or otherwise not operating satisfactorily, or (2) signal strength or quality of the received signal falls below acceptable values. (AIM)Flag Alarm Current. The d.c. current flowing in the Flag Alarm Circuit, usually measured in microamperes, which indicates certain characteristics of the modulation of the received signal.

Flight Inspection (Flight Check). Inflight investigation and evaluation of air navigation aids and instrument flight procedures to ascertain or verify that they meet established tolerances and provide safe operations for intended use.Note: Flight checked describes the procedure to accomplish the function of flight inspection. The two terms are interchangeable.

Flight Inspection Standard Service Volume (FISSV) is the volume of airspace defined by the lower limit of a facility Standard Service Volume. On “T” class facilities, this FISSV is 25 nm and 1,000 ft (2,000 ft in designated mountainous areas) above site elevation or intervening terrain. On “L” and “H” class facilities, the distance extends to 40 nm, and the altitudes are the same as for the “T” class. The FISSV is used to determine the performance status of VOR/TAC/DME facilities.Fly-By Waypoint. A waypoint that requires the use of turn anticipation to avoid overshoot of the next flight segment.Fly-Over Waypoint. A waypoint that precludes any turn until the waypoint is overflown.Geometric Dilution of Precision (GDOP). A factor used to express navigational error at a position fix caused by divergence of the hyperbolic lines of position as the aircraft’s receiver distance from the baseline increases. The larger the GDOP, the larger the standard deviation of position errors.Glidepath. See: ILS Glidepath.

Glidepath Angle. The angle between the downward extended straight line extension of the ILS glidepath and the horizontal.Glidepath Structure. Characteristics of a glidepath including bends, scalloping, roughness, and width.Glide Slope. A facility which provides vertical guidance for aircraft during approach and landing.Glide Slope Intercept Altitude. The true altitude (MSL) proposed or published in approved let-down procedures at which the aircraft intercepts the glidepath and begins descent. (FAA Order 1000.15 latest revision)Graphical Average Path. The average path described by a line drawn through the mean of all crosspointer deviations. This will usually be a curved line which follows long-term trends (1,500 ft or greater) and averages shorter term deviations.Ground Point of Intercept (GPI). A point in the vertical plan on the runway centerline at which it is assumed that the downward straight line extension of the glide path intercepts the runway approach surface baseline. (FAA Order 8260.3 latest revision)Group Repetition Interval (GRI). The time interval (microseconds divided by 10) between one group of 100 kHz carrier pulses and the next, from any transmitter within a Loran-C CHAIN. All stations in a specific CHAIN use the same GRI.Hertz (Hz). A unit of frequency of electromagnetic waves which is equivalent to one cycle per second. See Symbols in this chapter.

Kilohertz (kHz). A frequency of 1000 cycles per second.

Megahertz (MHz). A frequency of one million cycles per second.Gigahertz (GHz). A frequency of one billion cycles per second.Hole (Null). An area of signal strength below that required to perform the necessary function or furnish the required information, which is completely surrounded by stronger signal areas of sufficient strength to perform required functions.ILS–Back-Course Sector. The course sector which is the appropriate reciprocal of the front course sector.ILS–Commissioned Angle–Glide Slope. The glidepath angle calculated by a qualified procedure specialist which meets obstruction criteria (FAA Order 8260.3 latest revision). This nominal angle may be increased to meet additional criteria, i.e., engineering, noise abatement, site deficiencies, etc.

ILS–Commissioned Width–Localizer. The nominal width of a localizer. In practice the width is computed by using the criteria prescribed in Section 217 of FAA Order 8200.1 (latest revision).

ILS–Course Sector. A sector in a horizontal plane containing the course line and limited by the loci of points nearest to the course line at which 150uA is found.ILS–Differential Corrected Trace. The trace on the recording which is the algebraic sum of the Radio Telemetering Theodolite (RTT) crosspointer (DDM) and the aircraft receiver crosspointer (DDM) and which is produced by the differential amplifier within the airborne Theodolite Recording System.

ILS–Downward Straight Line Extension. The mean location of the ILS Glidepath in zone 2.ILS–Facility Reliability. The probability that an ILS ground installation radiates signals within the specified tolerances.ILS–Front Course Sector. The course sector which is situated on the same side of the localizer as the runway.ILS–Glidepath. The locus of points in the vertical plane (containing the runway centerline) at which the DDM is zero, which of all such loci is the closest to the horizontal plane.Note: Offset ILS’s do not contain the runway centerline.

ILS–Glidepath Sector. The sector in the vertical plane containing the ILS glidepath at which 150uA occurs.Note: The ILS glidepath sector is located in the vertical plane containing the localizer on-course signal and is divided by the radiated glidepath called upper sector and lower sector, referring respectively to the sectors above and below the path.ILS–Glidepath Sector Width (Normal Approach Envelope). The width of a sector in the vertical plane containing the glidepath and limited by the loci of points above and below the path at which reading of 150uA is obtained.

ILS–Half Course Sector. The sector, in a horizontal plan containing the course line and limited by the loci of points nearest the course line at which 75uA occurs.ILS–Localizer Back Course Zone 1. The distance from the coverage limit to 4 miles from the localizer antenna.ILS–Localizer Back Course Zone 2. From 4 miles from the localizer antenna to 1 mile from the localizer antenna.ILS–Localizer Back Course Zone 3. One mile from the localizer antenna to 3,000 ft from the localizer antenna.ILS–Localizer Clearance Sector 1. From 0° to 10° each side of the center of the localizer on-course.ILS–Localizer Clearance Sector 2. From 10° to 35° each side of the center of the localizer on-course.

ILS–Localizer Clearance Sector 3. From 35° to 90° each side of the center of the localizer on-course.ILS–Localizer Course Sector Width. The sum of the angular distances either side of the center of the course required to achieve full scale (150uA) crosspointer deflection.ILS–Lowest Coverage Altitude (LCA). That altitude which is final approach fix altitude, glidepath intercept altitude, or 500 ft above all obstructions, whichever is higher. For a back course used solely to provide missed approach guidance, it shall be a procedural altitude within 10 nm of the antenna, 1,500 ft above the antenna, or 500 ft above all obstructions, whichever is highest.

ILS–Performance Category I. An ILS which provides acceptable guidance information from the coverage limits of the ILS to the point at which the localizer course line intersects the glidepath at a height of 100 ft or less above the horizontal plane containing the runway threshold.ILS–Performance Category II. An ILS which provides acceptable guidance information from the coverage limits of the ILS to the point at which the localizer course line intersects the glidepath at a point above the runway threshold.ILS–Performance Category III. An ILS, which, with the aid of ancillary equipment where necessary, provides guidance information from the coverage limit of the facility to, and along, the surface of the runway.

ILS–Point “A”. An imaginary point on the glidepath/localizer on-course measured along the runway centerline extended, in the approach direction, 4 nautical miles from the runway threshold.NOTE: For back Course and installations sited to project a course substantially forward of threshold as in Figure 217-1B(2), this point is 4 nm from the antenna.

ILS–Point “B”. An imaginary point on the glidepath/localizer on-course measured along the runway centerline extended, in the approach direction, 3,500 ft from the runway threshold.NOTE: For back course as in Figure 217-1B(3), this point is 1 nm from the antenna. For installations sited to project a course substantially forward of threshold as in Figure 217-lB(2), this point is 1 nm from the threshold.ILS–Point “C”. A point through which the downward extended straight portion of the glidepath (at the commissioned angle) passes at a height of 100 ft above the horizontal plane containing the runway threshold.Note: Localizer only, Back Course, LDA’s and SDF only facilities, Point C is the missed approach point. Point “C” may be prior to point “B” on a restricted facility.

ILS Point “D”. A point 12 ft above the runway centerline and 3,000 ft from the runway threshold in the direction of the localizer.ILS Point “E”. A point 12 ft above the runway centerline and 2,000 ft from the stop end of the runway in the direction of the runway threshold.ILS Point “T”. A point at specified height located vertically above the intersection of the runway centerline and the runway threshold through which the downward extended straight line portion of the ILS glidepath passes.ILS Reference Datum. Same as ILS Point “T”.

ILS–Zone 1. The distance from the coverage limit of the localizer/glidepath to Point “A” (four miles from the runway threshold).ILS–Zone 2. The distance from Point “A” to Point “B”.ILS–Zone 3. CAT I – The distance from Point “B” to Point “C” for evaluations of Category I ILS.CAT II and III – The distance from Point “B” to the runway threshold for evaluations of Category II and III facilities.Note: Localizer Only Back Course, LDA, and SDF facilities will have no Zone 3 if Point “C” occurs prior to Point “B.” Structure tolerance remains defined by Points “A” to “B.”ILS–Zone 4. The distance from runway threshold to Point “D”.

ILS–Zone 5. The distance from Point “D” to Point “E”.Initial Approach Segment. In the initial approach, the aircraft has departed the en route phase of flight, and is maneuvering to enter an intermediate segment. This is the segment between the initial approach fix/waypoint and the intermediate fix/waypoint or the point where the aircraft is established on the intermediate course or final approach course.

Intermediate Approach Segment. This is the segment which blends the initial approach segment into the final approach segment. It is the segment in which aircraft configuration, speed, and positioning adjustments are made for entry into the final approach segment. The intermediate segment begins at the intermediate fix (IF) or point, and ends at the final approach fix (FAF).In-Phase. Applied to the condition that exists when two signals of the same frequency pass through their maximum and minimum values of like polarity at the same time.

Integrity. That quality which relates to the trust which can be placed in the correctness of the information supplied by the facility.Integrators. Received target enhancement process used in primary radar receivers.Interrogator. The ground-based surveillance radar transmitter-receiver which normally scans in synchronism with a primary radar, transmitting discrete radio signals which repetitiously request all transponders, on the mode being used, to reply. The replies are displayed on the radar scope. Also applied to the airborne element of the TACAN/DME system. (AIM)

Investigator-in-Charge (IIC). Person responsible for on-site aircraft investigation procedure.

Joint Acceptance Inspection (JAI). Inspection at culmination of facility installation and preparation. System is technically ready for commissioning after successful JAI.Joint Use. For this document, refer to radar sites used by both the FAA and military.

Line-of-Position (LOP). LOP is a hyperbolically curved line defined by successive but constant time difference measurements using the signals from two Loran-C transmitters. Two LOP’s from two station pairs define the location of a receiver and establish a position fix.Local Area Monitor (LAM). A stationary receiver designed to monitor and record Loran-C signals and time difference (TD) data. TD information obtained by this unit is used for calculating receiver TD calibration values.

Localizer Type Directional Aid (LDA). A facility of comparable utility and accuracy to a LOC, but which is not part of a full ILS and may not be aligned with the runway. (FAA Order 8260.3, latest revision)Localizer (LOC). The component of an ILS which provides lateral guidance with respect to the runway centerline. (FAA Order 8260.3, latest revision).

Localizer Zones. See ILS-Zones or ILS-Localizer Back Course ZonesLock-On. The condition during which usable signals are being received by the airborne equipment and presentation of steady azimuth and/or distance information starts.Loran-C CHAIN. Loran-C stations are grouped into sets of stations called CHAINs. Each CHAIN consists of a master station and two or more secondary stations that repeat transmissions over a specific period of time (see GRI).

Loran Signal Evaluation System (LSES). The LSES is a Loran-C receiver and a time difference data device used to evaluate approach sites. The device determines if usable signals are present and establishes the time difference relationship with the local area monitor.Loran-C Time Difference (TD). The elapsed time, in microseconds, between the arrival of two signals.Lowest Coverage Altitude (LCA). See ILS-Lowest Coverage Altitude (LCA).

Maximum Authorized Altitude (MAA). A published altitude representing the maximum usable altitude or flight level for an airspace structure or route segment. It is the highest altitude on a Federal airway, Jet route, area navigation low or high route, or other direct route for which an MEA is designated in FAR Part 95, at which adequate reception of navigation and signals is assured.Maximum Error. The maximum amplitude of course alignment from zero, either in the clockwise or counterclockwise direction.Mean Course Error (MCE). The mean value of azimuth or elevation error along the approach course or specified glidepath.Microampere(s). (Microamps)–One millionth of an ampere (amp). In practice, seen on a pilot’s omnibearing selector (OBS), oscillograph recordings, and/or flight inspection meters, as a deviation of the aircraft’s position in relation to a localizer on-course (zero DDM) signal or glidepath on-path (zero DDM) signal, e.g., “5 microamperes (µA) right” (localizer); 75µA low” (glidepath). See Crosspointer and Symbols in this section.Microwave Landing System (MLS). The international standard microwave landing system.Milliampere (mA). One one-thousandth of an ampere.

Minimum Crossing Altitude (MCA). The lowest altitude at certain fixes at which an aircraft must cross when proceeding in the direction of a higher minimum en route IFR altitude (MEA). (AIM)(See Minimum En Route IFR Altitude)

Minimum Descent Altitude (MDA). The lowest altitude, expressed in feet above mean sea level, to which descent is authorized on final approach or during circle-to-land maneuvering in execution of a standard instrument approach procedure where no electronic glidepath is provided. (AIM)Minimum En Route IFR Altitude (MEA). The lowest published altitude between radio fixes which assures acceptable navigational signal coverage and meets obstacle clearance requirements between those fixes. The MEA prescribed for a Federal airway or segment thereof, area navigational low or high route, or other direct route applies to the entire width of the airway, segment, or route between the radio fixes defining the airway, segment, or route. (AIM) (FAR Parts 91 and 95).Minimum Glide Path (MGP). The lowest angle of descent along the zero degree azimuth that is consistent with published approach procedures and obstacle clearance criteria.

Minimum Holding Altitude (MHA). The lowest altitude prescribed for a holding pattern which assures navigational signal coverage, communications, and meets obstacle clearance requirements. (AIM)Minimum Obstruction Clearance Altitude (MOCA). The lowest published altitude in effect between radio fixes on VOR airways, off-airway routes, or route segments which meets obstacle clearance requirements for the entire route segment and which assures acceptable navigation signal coverage only within 25 statute miles (22nm) of a VOR. (AIM) (Refer to FAR Parts 91 and 95)

Minimum Radar Range. The shortest distance from the radar at which the aircraft can be clearly identified on each scan of the radar antenna system.Minimum Reception Altitude (MRA). The lowest altitude at which an intersection can be determined. (AIM) (Refer to FAR Part 95)Minimum Safe Altitude Warning (MSAW). A software function of the air traffic ARTS II/III computer that is site specific. MSAW monitors Mode-C equipped aircraft for obstacle separation. It is designed to generate both aural and visual alerts at the air traffic controller’s display when an aircraft is at or predicted to be at an unsafe altitude.

MSAW Approach Path Monitor (APM). An area normally 1 nm wide, either side of final approach course or runway heading. An APM starts at approximately 5 nm (or final approach fix) from the approach end of runway. An altitude value is determined for obstruction clearance for each APM at the beginning and at the end of the APM. These two values provide MSAW protection as an aircraft descends along the approach path towards the runway. Parallel runways utilize the same APM. For a circling only SIAP, the APM starts at 5 nm (or FAF) from the closest landing surface, and terminates 1 – 2 nm from the closest landing surface.MSAW General Terrain Map (GTM). The area within a 55-mile radius of an Airport Surveillance Radar in which Mode-C equipped aircraft are monitored by an MSAW computer software function for obstacle separation.

MSAW Bin. A 2 nm square area within an MSAW General Terrain Map; 4,096 bins make up an MSAW General Terrain Map.MSAW Bin Altitude. An altitude that is determined by the highest obstacle within the MSAW bin, plus 500 ft.

Minimum Vectoring Altitude (MVA). The lowest MSL altitude at which an IFR aircraft will be vectored by a radar controller, except as otherwise authorized for radar approaches, departures and missed approaches. The altitude meets IFR obstacle clearance criteria. It may be lower than the published MEA along an airway or J-route segment. It may be utilized for radar vectoring only upon the controllers’ determination that an adequate radar return is being received from the aircraft being controlled. Charts depicting minimum vectoring altitudes are normally available only to the controllers and not to pilots. (AIM)Missed Approach Point (MAP). A point prescribed in each instrument approach procedure at which a missed approach procedure shall be executed if the required visual reference does not exist. (AIM: See Missed Approach and Segments of an Instrument Approach Procedure.)

Missed Approach Segment. The missed approach segment is initiated at the decision height in precision approaches and at a specified point in non-precision approaches. The missed approach must be simple, specify an altitude, and whenever practical, a clearance limit (end of the missed approach segment). The missed approach altitude specified in the procedure shall be sufficient to permit holding or en route flight.MLS Approach Reference Datum. A point at a specified height located vertically above the intersection of the runway centerline and the threshold.

MLS Auxiliary Data. Data, transmitted in addition to basic data, that provide facilities maintenance equipment siting information for use in refining airborne position calculations and other supplementary information.MLS Basic Data. Data transmitted by the facilities maintenance equipment that are associated directly with the operation of the landing guidance system.MLS Coverage Sector. A volume or airspace within which service is provided by a particular function and in which the signal power density is equal to or greater than the specified minimum.MLS Datum Point. The point on the runway centerline closest to the phase center of the approach elevation antenna.MLS Function. A particular service provided by the MLS; e.g., approach azimuth guidance, approach elevation guidance, or basic data, etc.MLS Mean Course Error. The mean value of the azimuth error along a specified radial of the azimuth function.

MLS Mean Glidepath Error. The mean value of the elevation error along a specified angle of the elevation function.MLS Minimum Glidepath. The lowest angle of descent along the zero-degree azimuth that is consistent with published approach procedures and obstacle clearance criteria.MLS-Point “A”. An imaginary point on the minimum glidepath and commissioned azimuth radial, 4 nautical miles from the runway threshold.

MLS-Point “B”. An imaginary point on the minimum glidepath and commissioned azimuth radial, 3,500 ft from the runway threshold.MLS-Point “C”. A point through which the downward extended straight portion of the glidepath passes at a height of 100 ft above the horizontal plane containing the runway threshold.Note: Azimuth only facilities, Point C is the missed approach point.MLS-Point “D”. A point 12 ft above the runway centerline and 3,000 ft from the runway threshold in the direction of the azimuth station.MLS-Point “E”. A point 12 ft above the runway centerline and 2,000 ft from the stop end of the runway in the direction of the runway threshold.MLS Proportional Guidance Sector. The volume of airspace within which the angular guidance information provided by a function is directly proportional to the angular displacement of the airborne antenna with respect to the zero angle difference.MLS Reference Point. The point at which flight inspection begins to apply facility budget error tolerances. This will normally be either the ARD or MAP.

Mode. The letter or number assigned to a specific pulse spacing of radio signals transmitted or received by ground interrogator or airborne transponder components of the Air Traffic Control Radar Beacon System (ATCRBS). Mode A (military Mode 3), Mode C (altitude reporting), and Mode S (data link) are used in air traffic control. (See transponder, interrogator, radar.) (AIM)ICAO-Mode (SSR) Mode. The letter or number assigned to a specific pulse spacing of the interrogation signals transmitted by an interrogator. There are five modes: A, B, C, D, and M–corresponding to five different interrogation pulse spacings.

Moving Target Detection (MTD). Type of moving target detection system (like MTI) based on digital storage map techniques. Used in newer primary radars.Moving Target Indicator (MTI). Electronic circuitry that permits the radar display presentation of only targets which are in motion. A partial remedy for ground clutter.MTI Reflector. A fixed device with electrical characteristics of a moving target which allows the demonstration of a fixed geographic reference on a MTI display. (Used to align video maps, azimuth reference, etc.)Narrowband Radar Display. Computer generated display of radar signals.

National Flight Data Center (NFDC). A facility in Washington, D.C., established by FAA to operate a central aeronautical information service for the collection, validation, and dissemination of aeronautical data in support of the activities of government, industry, and the aviation community. The information is published in the National Flight Data Digest. (AIM: See National Flight Data Digest.)National Transportation Safety Board (NTSB). Office responsible for aircraft accident investigations.

NAVAID. Any facility used in, available for use in, or designated for use in aid of air navigation, including landing areas, lights, any apparatus or equipment for disseminating weather information, for signaling, for radio direction finding, or for radio or other electronic communication, and any other structure or mechanism having a similar purpose for guiding or controlling flight in the air or the landing or takeoff of aircraft. (Re: Federal Aviation Act of 1958, as amended.) (AIM)

Nondirectional Beacon/Radio Beacon (NDB). An L/MF or UHF radio beacon transmitting nondirectional signals whereby the pilot of an aircraft equipped with direction finding equipment can determine his bearing to or from the radio beacon and “home” on or track to or from the station. When the radio beacon is installed in conjunction with the Instrument Landing System marker, it is normally called Compass Locator. (AIM)

Nonprecision Approach Procedure/Nonprecision Approach. A standard instrument approach procedure in which no electronic glide slope is provided; e.g., VOR, TACAN, NDB, LOC, ASR, LDA, or SDF approaches. (AIM)

Notices to Airmen/Publication. A publication designed primarily as a pilot’s operational manual containing current NOTAM information (see Notices to Airmen – NOTAM) considered essential to the safety of flight, as well as supplement data to other aeronautical publications. (AIM)Notices to Airmen/NOTAM. A notice containing information (not known sufficiently in advance to publicize by other means) concerning the establishment, condition, or change in any component (facility, service, or procedure of, or hazard in the National Airspace System ) the timely knowledge of which is essential to personnel concerned with flight operations. (AIM)

a. NOTAM (D) - A NOTAM given (in addition to local dissemination) distant dissemination via teletype writer beyond the area of responsibility of the Flight Service Station. These NOTAMS will be stored and repeated hourly until canceled.

b. NOTAM (L) - A NOTAM given local dissemination by voice (teletypewriter where applicable), and a wide variety of means such as: TelAutograph, teleprinter, facsimile reproduction, hot line, telecopier, telegraph, and telephone to satisfy local user requirements.

c. FDC NOTAM A notice to airmen, regulatory in nature, transmitted by NFDC and given all-circuit dissemination.

d. ICAO NOTAM. A notice, containing information concerning the establishment, condition, or change in any aeronautical facility, service, procedure, or hazard, the timely knowledge of which is essential to personnel concerned with flight operations. (AIM)Null. That area of an electromagnetic pattern where the signal has been intentionally canceled or unintentionally reduced to an unacceptable level.

Obstacle. An existing object, object of natural growth, or terrain at a fixed geographical location, or which may be expected at a fixed location within a prescribed area, with reference to which vertical clearance is or must be provided during flight operation. (AIM)

Obstacle Clearance. The vertical distance between the lowest authorized flight altitude and a prescribed surface within a specified area. (FAA Order 8260.19, latest revision)

Obstruction. An object which penetrates an imaginary surface described in FAR Part 77. (AIM) (Refer to FAR Part 77).Omnibearing Selector (OBS). An instrument capable of being set to any desired bearing of an omnirange station and which controls a course deviation indicator.

On-Course. The locus of points in the horizontal plane in which a zero or on-course reading is received.

On-Path. Same as on-course but in the vertical plane. See ILS–Glidepath.

Operational Advantage. An improvement which benefits the users of an instrument procedure. Achievement of lower minimums or authorization for a straight-in approach with no derogation of safety are examples of an operational advantage. Many of the options in TERP’s are specified for this purpose. For instance, the flexible final approach course alignment criteria may permit the ALS to be used for reduced visibility credit by selection of the proper optional course. (FAA Order 8260.3, latest revision)Optimum Error Distribution. Best overall facility alignment error distribution to achieve maximum operational benefits (not necessarily a perfect balance of the errors).

Orbit Flight. Flight around a station at predetermined altitude(s) and constant radius.Oscilloscope. An instrument for showing visually, graphic representations of the waveforms encountered in electrical circuits.Out-of-Coverage Indication (OCI). A signal radiated into areas outside the intended coverage sector where required to specifically prevent invalid removal of an airborne warning indication flag in the presence of misleading guidance information.

Out of Tolerance Condition. See Discrepancy.Path Following Error (PFE). The guidance perturbations which the aircraft will follow. It is composed of a path following noise and of the mean course error in the case of azimuth functions or the mean glidepath error in the case of elevation functions.Path Following Noise (PFN). That portion of the guidance signal error which could cause aircraft displacement from the mean course line or mean glidepath as appropriate.Pilot-Controlled Lighting. Airfield lighting systems activated by VHF transmissions from the aircraft.

Planned View Display (PVD). A display presenting computer generated information such as alphanumerics or video mapping.Polarization Error. The error arising from the transmission or reception of a radiation having a polarization other than that intended for the system.Primary Area. The area within a segment in which full obstacle clearance is applied. (FAA Order 8260.3, latest revision)Proportional Guidance Sector. The volume of airspace within which the angular guidance information provided by a function is directly proportional to the angular displacement of the airborne antenna with respect to the zero angle reference.“Q” Factor. See Actual Navigation Performance.

Quadradar. Ground radar equipment named for its four presentations. a. Height Finding b. Airport Surface Detection c. Surveillance d. Precision Approach/R. RNAV and transponder with altitude encoding capability.Radar Bright Display Equipment (RBDE). Equipment at the ARTCC which converts radar video to a bright raster scan (TV type) display.Radar Data Analysis Software (RDAS). A generic term referring to many types of terminal and en route radar data analysis tools. (COMDIG, RARRE, DRAM, etc.)

Radar Plan Position Indicator (RAPPI). Maintenance display used with CD-1 common digitizers.Radar/Radio Detecting and Ranging. A device which, by measuring the time interval between transmission and reception of radio pulses and correlating the angular orientation of the radiated antenna beam or beams in azimuth and/or elevation, provides information on range, azimuth, and/or elevation of objects in the path of the transmitted pulses.a. Primary Radar. A radar system in which a minute portion of a radio pulse transmitted from a site is reflected by an object and then received back at that site for processing and display at an air traffic control facility.

b. Secondary Radar/Radar Beacon/ATCRBS. A radar system in which the object to be detected is fitted with cooperative equipment in the form of a radio receiver/transmitter (transponder). Radar pulses transmitted from the searching transmitter/receiver (interrogator) side are received in the cooperative equipment and used to trigger a distinctive transmission from the transponder.This reply transmission, rather than a reflected signal, is then received back at the transmitter/receiver site for processing and display at an air traffic control facility. (See Transponder, Interrogator.) (AIM)c. ICAO-Radar. A radio detection device which provides information on range, azimuth, and/or elevation of objects.(1) Primary Radar. A radar system which uses reflected radio systems.

(2) Secondary Radar. A radar system wherein a radio signal transmitted from a radar station initiates the transmission of a radio signal from another station.

Radar Resolution – Azimuth. The angle in degrees by which two targets at the same range must be separated in azimuth in order to be distinguished on a radar scope as individual returns.Radar Resolution – Range. The distance by which two targets at the same azimuth must be separated in range in order to be distinguished on a radar scope as individual returns.Radar Route. A flight path or route over which an aircraft is vectored. Navigational guidance and altitude assignments are provided by ATC. (See Flight Path, Route.) (AIM)

Receiver Autonomous Integrity Monitoring (RAIM). A technique whereby a civil GPS receiver/processor determines the integrity of the GPS navigation signals without reference to sensors or non-DoD integrity systems other than the receiver itself. This determination is achieved by a consistency check among redundant pseudorange measurements.Range of Validity. Area around a local area monitor where published Loran-C receiver TD calibration values are valid.Radial. A magnetic bearing extending from a VOR/VORTAC/TACAN navigation facility. (AIM)

Range, Azimuth, Radar, Reinforced Evaluator (RARRE). An IBM 9020 radar diagonistic program which is used to evaluate narrowband radar.Real Time Quality Check (RTQC). Internally generated test target in automated target processing devices (common digitizers, etc.)Receiver Check Point. A specific point designated and published, over which a pilot may check the accuracy of his aircraft equipment, using signals from a specified station.Recorder Event Mark. A galvo mark on a recorder related to a position or time, required for correlation of data in performance analysis.

Reference Radial. A radial, essentially free from terrain and side effects, designated as a reference for measuring certain parameters of facility performance.Reference Voltage (VOR Reference Voltage). A 30 Hz voltage derived in the reference phase channel of the aircraft VOR receiver.Required Navigation Performance (RNP). A statement of the navigation performance accuracy necessary for operation within a defined airspace.

RHO/THETA Position. Coordinate position described by distance and angle.Ring-Around. A display produced on the scope by front, side, or back antenna lobes of the secondary radar system. It appears as a ring around the radar location and may occur when an aircraft transponder replies to ground interrogations while in close proximity to the antenna site.Rotation (Correct Rotation). A condition wherein the transmitted azimuth angle increases in a clockwise direction.Roughness. Rapid irregular excursions of the electromagnetic course or path.Runway Approach Surface Baseline. An imaginary plane down the runway at the height of the runway surface at threshold.Runway Environment. The runway threshold or approved lighting aids or other markings identifiable with the runway. (FAA Order 8260.3)

Runway Point of Intercept. The point where the extended glide slope intercepts the runway centerline on the runway surface.Runway Reference Point. Where VGSI angle of visual approach part intersects runway profile (see RPI)Runway Threshold. The beginning of that portion of the runway usable for landing. (AIM) (When use for flight inspection purposes, displaced threshold(s) or threshold mean the same thing.)Scalloping. See Course Scalloping. (FAA Order 1000.15, latest revision)Search (DME/TACAN). Rapid movement of the distance or bearing indicators during the period in which either is unlocked. (FAA Order 1000.15, latest revision)Secondary Area. The area within a segment in which required Obstruction Clearance (ROC) is reduced as distance from the prescribed course is increased (FAA Order 8260.3, latest revision).

Segment. The basic functional division of an instrument approach procedure. The segment is oriented with respect to the course to be flown. Specific values for determining course alignment, obstacle clearance areas, descent gradients, and obstacle clearance requirements are associated with each segment according to its functional purpose. (FAA Order 8240.3, latest revision)Sensing (Correct Sensing). A condition wherein the ambiguity indicator gives the correct To/From indication.

Sensitivity Time Control (STC). Procedure used to vary receiver sensitivity with range. Gain is reduced as a function of decreasing range, in an attempt to make all radar replies uniform. (Gain would be maximum to maximum range in this event.)

Service Volume/SV. That volume of airspace surrounding a NAVAID within which a signal of usable strength exists and where that signal is not operationally limited by co-channel interference.Note: For VOR/TACAN/DME and ILS, the following definitions are used:

• a. Standard Service Volume (SSV) - That volume of airspace defined by the national standard.
• b. Expanded Service Volume (ESV) - An approved service volume outside of the standard service volume.
• c. Operational Service Volume (OSV) - The airspace available for operational use. It includes the following:
  • (1) The SSV excluding any portion of the SSV which has been restricted.
  • (2) The ESV.Short-Term Excursions. Excursion characteristics of a navigation on-course or on-path signal which includes scalloping, roughness, and other aberrations but excludes bends.

Side Bands. The separated and distinct signals that are radiated whenever a carrier frequency is modulated. In terms of most air navigation facilities, double sidebands are present. This means that frequencies above and below the carrier frequency differing by the amount of the modulating frequencies are present. These sidebands contain intelligence for actuating navigation instruments.

Simplex. Single channel operation usually referred to at those sites using a single channel where dual channel (diplex) operation is available.

Splits. Two or more beacon targets generated from a single target reply. An undesirable condition due to problems in the beacon transmitter, antenna, propagation, aircraft transponder, or processing equipment.

Simplified Directional Facility/SDF. A NAVAID used for nonprecision instrument approaches. The final approach course is similar to that of an ILS localizer.

Slant Range. The line-of-sight distance between two points not at the same elevation.

Stagger. A feature used with primary MTI radar systems to vary the PRF at pre-selected intervals. This moves the inherent blind speed to a less troublesome value.

Standard VOT. A facility intended for use on the ground only (See VHF Omnidirectional test range).

Structure. Excursion characteristics of a navigation on-course or on-path signal which includes bends, scalloping, roughness, and other aberrations.

Structure Below Path. An angular measurement of clearance below path.

Subclutter Visibility. A performance characteristic of the system to detect a moving target in the presence of relatively strong ground clutter.

Symmetry. (ILS)–ICAO: Displacement sensitivity. A ratio between individual width sectors (90 Hz and 150 Hz) expressed in percent.

Systems Performance Analysis Rating (SPAR). A rating based on performance or expected performance. These ratings are related to flight inspection intervals as follows:

SPAR Class 1, 90-day interval; Class 2, 180-day interval; Class 3, 270-day interval.

TACAN Distance Indicator (TDI). A unit of airborne equipment used to indicate distance from a selected facility.

Target of Opportunity. An itinerant aircraft operating within the coverage area of the radar and which meets the requirements for a small aircraft as described in FAA Order 8200.1 (latest revision) Section 215.

Target Return. The return signal transmitted by a beacon-equipped aircraft in reply to the ground facility interrogator. Also, indication shown on a radar display resulting from a primary radar return.

Threshold. See Runway Threshold.

Touchdown Zone (TDZ). The first 3,000 ft of runway beginning at the threshold. (See FAA Order 8260.3, latest revision).

Touchdown Zone Elevation. The highest runway centerline elevation in the touchdown zone.Tracking. Condition of continuous distance or course information.

Transponder. The airborne radar beacon receiver/transmitter portion of the Air Traffic Radar Beacon System (ATCRBS) which automatically receives radio signals from interrogators on the ground, and selectively replies with a specific reply pulse or pulse group only to those interrogations being received on the mode to which it is set to respond. (See Interrogator.) (AIM)

Trend. The general direction or incline of a segment of the glidepath which persists for a distance of 1,500 ft or more along the approach course.

Un-Lock. Condition at which the airborne interrogator (TACAN) discontinues tracking and starts search.

Usable Distance. The maximum distance at a specified altitude at which the facility provides readable identification and reliable bearing or glidepath information under average atmospheric condition.

Variable Voltage (VOR Variable Voltage). A 30 Hz voltage derived in the variable phase channel of the aircraft VOR receiver.

Vertical Angle. An angle measured upward from a horizontal plane.

VHF Omnidirectional test range (VOT). A radio transmitter facility in the terminal area electronic navigation systems, radiating a VHF radio wave modulated by two signals having the same phase relationship at all azimuths. It enables a user to determine the operational status of a VOR receiver. (See Standard VOT and Area VOT.)

Video Map. An electronic displayed map on the radar display that may depict data such as airports, heliports, runway centerline extensions, hospital emergency landing areas, NAVAIDs and fixes, reporting points, airway/route centerlines, boundaries, handoff points, special use tracks, obstructors, prominent geographic features, map alignment indicators, range accuracy marks, and minimum vectoring altitudes (AIM).

Visual Descent Point (VDP). The visual descent point is a defined point on the final approach procedure from which normal descent from the MDA to the runway touchdown point may be commenced, provided visual reference is established. (AIM).

VORTAC. A facility composed of azimuthal information from both VOR and TACAN, plus distance information of TACAN.

VOT–Standard. See Standard VOT.VOT–Area Use. See Area VOT.

VOT Reference Point. A point on or above an airport at which the signal strength of a VOT is established and subsequently checked (applies to both standard and area VOTs).

Waveform. The shape of the wave obtained when instantaneous values of an a.c. quantity are plotted against time in rectangular coordinates.

Waveguide. A hollow pipe usually of rectangular cross section used to transmit or conduct RF energy.

Wavelength. The distance usually expressed in meters traveled by a wave during the timer interval of one complete cycle. Equal to the velocity divided by the frequency.

9960 Hz Voltage. A voltage derived from the VOR 9960 amplitude modulation by the reference channel of the VOR receiver. The 9960 Hz AM is a subcarrier which is frequency modulated by the 30Hz reference. Also referred to the 10 kHz subcarrier.