Introduction

Course Overview.

This course delivers a complete ATPL 062 Radio Navigation hard-skill syllabus drawn from the CAE Oxford Aviation Academy ground training series. Modern air transport intensity would be impossible without radio and radar systems that allow aircraft to navigate, approach and land without visual reference to terrain. Early visual and celestial techniques have been replaced by terrestrial aids and satellite constellations, yet ICAO still requires pilots to master both the physics of electromagnetic waves and the practical operation of every classical navaid. The material systematically covers properties of radio waves, propagation paths, modulation, antennae, Doppler systems, VDF, ADF, VOR, ILS, MLS, primary and secondary radar, DME, RNAV, EFIS and GNSS so that candidates can solve examination questions and apply the knowledge safely in the cockpit.

Цель курса.

После прохождения курса вы сможете рассчитать частоты, длины волн и зоны покрытия любого навигационного средства, интерпретировать показания ADF, VOR, ILS и GNSS, объяснить влияние ионосферы, помех и геометрии спутников на точность и выполнить процедуры захода на посадку и RNAV маршрутизации в соответствии с требованиями EASA ATPL.

Результаты обучения.

• Знать: частотные диапазоны LF–EHF, законы распространения поверхностной, пространственной и ионосферной волн, принципы модуляции AM/FM/фазовой/импульсной, устройство и поляризацию антенн, работу кардиоиды ADF, фазовое сравнение VOR, формирование лучей ILS и MLS, импульсную технику радаров, режим Mode S SSR, принципы DME, архитектуру RNAV и GNSS.
• Уметь: переводить частоту в длину волны по формуле $\lambda = 300/f$, рассчитывать дальность прямой видимости $Range = 1.23(\sqrt{h_{TX}}+\sqrt{h_{RX}})$, определять QDM/QDR/QTE, интерпретировать CDI и RMI, выбирать оптимальные частоты HF, оценивать ошибки ADF, VOR и GPS, применять RAIM и дифференциальные коррекции.
• Владеть: процедурами homing, tracking, holding и instrument approach по NDB/VOR/ILS, выбором спутников для 4D-фикса, настройкой AWR и SSR, использованием FMC/RNAV для B-RNAV и P-RNAV.

Для кого этот курс.

Курс предназначен для кандидатов на ATPL, студентов авиационных училищ, пилотов, готовящихся к теоретическим экзаменам EASA/ICAO по предмету 062 Radio Navigation, а также инструкторов, которым нужен структурированный самоучитель с полным покрытием всех глав учебника CAE Oxford. Материал рассчитан на тех, кто уже знаком с основами физики и навигации и готов работать с формулами, полярными диаграммами и практическими расчётами.

Курс не подходит абсолютным новичкам без базовых знаний авиации, лицам, ищущим только краткий обзор без детальных расчётов, а также тем, кто ожидает готовые ответы на экзаменационные вопросы без понимания физических принципов.

Module 1. Properties of Radio Waves

Electromagnetic radiation forms the physical foundation of every radio navigation system used in aviation. When an alternating current flows through a wire, the changing magnetic field does not collapse completely before the reversed field begins to establish itself; the result is outward travelling electromagnetic energy consisting of an electric (E) field parallel to the wire and a magnetic (H) field perpendicular to it. The speed of these waves is approximately 300 000 000 m s^-1 or 162 000 nautical miles per second. Understanding this dual-field structure is essential because polarisation, attenuation and antenna orientation all depend on the relative orientation of the E and H vectors. In practice a pilot never measures the fields directly, yet every frequency choice, every aerial design and every propagation path is a direct consequence of the physics described here. Always remember that radio waves and light are the same phenomenon; therefore line-of-sight calculations that work for optical horizons also apply, with a small atmospheric refraction correction, to VHF and higher bands.

Polarisation is defined as the plane of the electric field and is determined by the orientation of the transmitting aerial. A vertical aerial radiates a vertically polarised wave; a horizontal aerial radiates a horizontally polarised wave. Maximum signal strength at the receiver is obtained only when the receiving aerial lies in the same plane as the incoming polarisation. Circular polarisation, produced by helical antennas, rotates the electric field at the radio-wave frequency and can be right-handed or left-handed. Two operational advantages are decisive for aviation: circular polarisation reverses sense on reflection from water droplets, allowing weather-radar clutter to be suppressed, and a simple dipole can receive circularly polarised signals regardless of orientation, which is invaluable for satellite and mobile systems. When installing or selecting antennas always match polarisation; mismatch can reduce received power by more than 20 dB and render an otherwise serviceable signal unusable.

Frequency and wavelength are reciprocal quantities linked by the universal relation $\lambda = c/f$ where c is the speed of light. In aviation units the practical formulae become $f(\text{MHz}) = 300/\lambda(\text{m})$ and $\lambda(\text{m}) = 300/f(\text{MHz})$. Worked examples illustrate the conversion: a wavelength of 3.75 m corresponds to 80 MHz; a frequency of 125 MHz yields 2.4 m; 3.2 cm yields 9.375 GHz. Always convert centimetres or kilometres into metres before applying the simplified formula, otherwise the answer will be wrong by orders of magnitude. Frequency bands used in civil aviation are standardised: LF 30–300 kHz for NDB/ADF, MF 300–3000 kHz for NDB and long-range communications, HF 3–30 MHz for long-range sky-wave communications, VHF 30–300 MHz for short-range voice, VOR, ILS localiser and markers, UHF 300–3000 MHz for ILS glide-path, DME, SSR and GNSS, SHF 3–30 GHz for radio altimeters, airborne weather radar and MLS. Memorising the band edges and the primary aeronautical use of each band is a high-yield examination skill.

Phase comparison is the fundamental measurement technique of VOR, MLS and certain Doppler systems. Two signals of identical frequency are required; one is designated the reference, the other the variable. The phase difference is obtained by locating corresponding zero-crossings and measuring the angular travel of the reference wave before the variable wave reaches the same phase point. The result must always be expressed as a positive angle between 0° and 360°. Mathematically, subtract the instantaneous phase of the variable from that of the reference and add 360° if the difference is negative. Because phase is independent of amplitude, the measurement remains valid even when signal strength fluctuates, provided the signal-to-noise ratio remains adequate. In the cockpit the pilot never sees the raw phase numbers; the airborne receiver converts them into radial, CDI deflection or azimuth guidance. Understanding the underlying comparison, however, allows rapid diagnosis of scalloping, multipath and cone-of-confusion anomalies.

Module 2. Radio Propagation Theory

Attenuation is the progressive loss of signal strength with distance from the transmitter and comprises two distinct mechanisms. Absorption arises from interaction with air molecules, water vapour, dust, vegetation and the Earth’s surface; the loss increases with frequency and becomes severe above approximately 1000 MHz. The inverse-square law follows from spherical spreading of energy: power density is proportional to 1/R², so doubling the range requires a four-fold increase in transmitter power. Static interference generated by weather, industrial activity and geological processes is most troublesome at low frequencies; above VHF it is normally negligible except when the wave has traversed the ionosphere. Fading occurs when two or more paths of different length arrive simultaneously and interfere constructively or destructively. Practical consequence: to extend the usable range of a VHF transmitter from 50 NM to 100 NM the power must be multiplied by four, not merely doubled. Always verify both free-space loss and atmospheric absorption when assessing whether a given power budget will support the required service range.

Four propagation paths are operationally significant: surface wave, space wave, sky wave and satellite. The surface wave, dominant from 20 kHz to about 50 MHz, is diffracted around the curved Earth; range is greater over sea than over land because of higher conductivity, and vertical polarisation is essential. Approximate daytime ranges at 300 kHz are given by $Range_{sea}\approx 3\sqrt{P}$ and $Range_{land}\approx 2\sqrt{P}$ where P is power in watts. The space wave is a combination of direct and ground-reflected rays and is essentially line-of-sight; maximum theoretical range is $Range = 1.23(\sqrt{h_{TX}}+\sqrt{h_{RX}})$ with heights in feet. Sky-wave propagation utilises total internal refraction in the ionospheric E or F layers and provides the long-range HF communications used over oceans. Satellite paths are treated in the GNSS module. Selecting the correct path for a given frequency band is the first step in predicting both coverage and potential interference.

The ionosphere extends from roughly 60 km to the edge of the atmosphere and is stratified into D, E and F layers whose electron densities vary with solar radiation. The D layer forms at sunrise and disappears at sunset, absorbing LF and MF energy by day. The E layer remains present day and night; the F layer splits into F1 and F2 by day. Highest ionisation occurs around 1400 local time; lowest just before sunrise. Critical angle, skip distance and dead space are direct consequences of these density gradients. As frequency increases the critical angle increases, skip distance lengthens and dead space enlarges. Multi-hop sky-wave can achieve ranges of half the Earth’s diameter. For HF communications the maximum usable frequency (MUF) is the highest frequency that still returns at the required range; the optimum working frequency is taken as 0.85 MUF. Night frequencies are typically half the day frequencies. Always choose the lowest frequency that still supports the required range so that attenuation and static remain minimal.

Super-refraction and sub-refraction modify the normal atmospheric refraction of VHF and higher frequencies. Super-refraction occurs when relative humidity decreases with height and temperature falls more slowly than standard, or when warm air flows over a cooler surface; ranges can increase by up to 40 percent and ducting may produce extreme over-the-horizon reception. Sub-refraction, associated with increasing humidity and steeper temperature lapse rates, reduces ranges by up to 20 percent. Both phenomena are most noticeable in high-pressure regimes and in coastal desert areas such as the Mediterranean and Caribbean. Controllers and pilots must treat reported radar or VHF ranges as approximate when anomalous propagation is forecast; ducting can also cause distant stations to interfere on the same frequency. Monitoring surface observations and significant-weather charts for inversion layers is therefore part of pre-flight radio planning.

Module 3. Modulation Techniques

Modulation is the process of impressing information onto a radio-frequency carrier. Five forms are used in aviation: keyed (on–off) modulation for Morse identification of NDBs, amplitude modulation (AM) for voice and many navigation signals, frequency modulation (FM) for high-fidelity broadcasting, phase modulation for MLS and GPS, and pulse modulation for primary and secondary radar. In amplitude modulation the instantaneous amplitude of the carrier is varied in proportion to the audio or data waveform. Heterodyning produces upper and lower sidebands whose combined bandwidth is twice the highest modulating frequency. For a 3 kHz audio tone on a 2182 kHz carrier the occupied bandwidth is 6 kHz. Because the information is duplicated in both sidebands and the residual carrier carries no intelligence, single-sideband suppressed-carrier (SSB) operation doubles the number of available channels, improves the signal-to-noise ratio and reduces transmitter power—advantages that are decisive for HF long-range communications (emission designator J3E).

Frequency modulation varies the instantaneous frequency of the carrier in proportion to the amplitude of the modulating signal while keeping amplitude constant. The frequency deviation is proportional to modulating amplitude; the rate of deviation is proportional to modulating frequency. Broadcast FM occupies 150 kHz; narrow-band FM used for land-mobile services occupies only 8 kHz, still wider than the 6 kHz permitted for aeronautical AM and therefore not yet adopted for civil aviation voice. Phase modulation and its digital variants (binary and differential phase-shift keying) are the backbone of MLS time-referenced scanning beams and of the GPS navigation message. Pulse modulation generates a train of short, high-power bursts whose presence or absence, or whose precise timing, conveys range or identity information. Emission designators (A3E for VHF RTF, J3E for HF RTF, A9W for VOR, A8W for ILS, P0N for DME, etc.) encode the type of modulation, the nature of the modulating signal and the type of information; familiarity with the three-character code allows rapid identification of any emission on a frequency plan.

Module 4. Antennae and Radiation Patterns

Two elementary radiators dominate aviation installations: the half-wave dipole and the Marconi quarter-wave monopole. The dipole is fed at its centre and radiates a toroidal pattern whose polar diagram in the plane perpendicular to the axis is a figure-of-eight. The Marconi aerial is mounted against a conducting ground plane that acts as the missing half of the dipole; its aerodynamic profile makes it the preferred choice for aircraft. Optimum length is one-half or one-quarter of the free-space wavelength, corrected for the lower velocity of electromagnetic energy inside the conductor. For a Marconi aerial at 125 MHz the free-space wavelength is 2.4 m, so the physical length is 0.6 m. Feeders must match the frequency and power level: simple wire at LF/MF, twin-lead at HF/VHF, coaxial cable at UHF, and hollow rectangular waveguide at SHF/EHF. Waveguide internal dimensions are half a wavelength; incorrect dimensions produce severe attenuation or total cut-off.

Directivity is obtained by adding parasitic elements or by using arrays. A reflector 5 percent longer than the driven element and spaced λ/4 behind it suppresses radiation to the rear and increases forward gain. Directors shorter than the driven element focus energy into a narrower main beam but also generate unwanted side lobes. The ILS localiser employs a linear array of 16 or 24 elements with half-wavelength spacing to form two narrow, overlapping lobes symmetrical about the runway centre-line. A loop aerial produces a figure-of-eight pattern with two nulls; addition of a sense aerial converts the pattern into a cardioid with a single, unambiguous null—the classic ADF direction-finding arrangement. Radar antennas historically used parabolic reflectors fed by a waveguide horn; modern systems employ flat-plate phased arrays (slotted waveguides) that produce narrower beams, lower side-lobe levels and require less transmitter power for the same range. The principal advantages of the phased array are therefore higher angular resolution, reduced clutter and improved target discrimination.

Module 5. Doppler Radar Principles

The Doppler principle states that the observed frequency of a wave changes when there is relative motion between source and observer. An approaching source raises the frequency; a receding source lowers it. The magnitude of the shift is proportional to the radial component of relative velocity. Airborne Doppler navigation systems transmit continuous-wave or pulsed energy toward the ground in a Janus array of three or four beams (two forward, one or two aft). Frequency shifts measured on the forward beams are equal in magnitude but opposite in sign to those measured on the aft beams when the aircraft has zero drift. Any lateral drift produces a measurable difference between port and starboard beams; the equipment converts the difference into drift angle and the common-mode shift into ground speed. Early mechanical systems rotated the antenna until port and starboard shifts were equal; modern fixed arrays perform the comparison electronically. Integration of Doppler ground speed and drift with heading yields position, but pure Doppler is vulnerable to heading-reference errors and to signal degradation over water; current installations therefore hybridise Doppler with IRS, VOR/DME or GPS.

Module 6. VHF Direction Finder (VDF)

VHF Direction Finding supplies a pilot with a spoken bearing relative to a ground station using the aircraft’s normal VHF communications radio (118–137 MHz, emission A3E). The pilot transmits a request using the appropriate Q-code: QDM (magnetic heading to station), QDR (magnetic bearing from station), QTE (true bearing from station) or QUJ (true track to station). The ground station replies with the bearing, its class of accuracy (A ±2°, B ±5°, C ±10°, D worse than C) and, if required, the time of observation. Most operational bearings are Class B. The ground antenna is a circular array of vertical elements that resolves the phase (or amplitude) differences among the elements to display a digital read-out of bearing. Range is strictly line-of-sight and is further limited by intervening terrain, transmitter power and anomalous propagation. Accuracy is degraded by multipath from terrain or buildings, aircraft bank, overhead cone, simultaneous transmissions and ducting. Auto-triangulation using several VDF stations is available on the international distress frequency 121.5 MHz. Although rarely used for routine navigation today, VDF remains a valuable emergency tool because it requires no special airborne equipment beyond a serviceable VHF radio.

Module 7. Automatic Direction Finder and NDB

The non-directional beacon transmits vertically polarised energy omnidirectionally in the LF and MF bands (190–1750 kHz). The airborne ADF measures the relative bearing of the incoming wave by means of a fixed crossed-loop aerial whose outputs are combined with a sense aerial to produce a cardioid null that is unambiguous. Rapid switching between two cardioids of opposite sense sharpens the null to the ICAO requirement of ±5°. Relative bearing is displayed on an RBI (fixed card) or an RMI (card driven by the aircraft’s magnetic heading so that the needle points to QDM). Two emission types exist: N0NA1A requires the beat-frequency oscillator to be on for tuning, identification and monitoring; N0NA2A requires the BFO only for tuning. Locator beacons have ranges of 10–25 NM and support instrument approaches; en-route NDBs have ranges of 50 NM or more. Homing is performed by maintaining a relative bearing of 000° (or the reciprocal for outbound); tracking requires an allowance for drift so that the relative bearing equals the drift angle. Holding patterns, instrument approaches and airway centre-line tracking are all standard applications. Designated operational coverage guarantees a 3:1 signal-to-noise ratio by day only; night effect, coastal refraction, mountain effect, quadrantal error, bank angle (dip) and thunderstorm static all degrade accuracy and must be recognised by continuous monitoring of the identification and by cross-checking with other navaids.

Night effect arises because the daytime D-layer absorption disappears after sunset, allowing sky-wave contamination of the surface wave. The resulting phase interference and induction of currents in the horizontal members of the loop cause the needle to hunt and the audio to fade, especially around dawn and dusk and at ranges beyond 70–100 NM. Coastal refraction occurs because radio waves travel faster over sea than over land; the wavefront bends toward the coast, placing the plotted position closer to the shore than the true position. The error is minimal when the signal crosses the coast at 90° and is greatest at acute angles. Mountain effect produces reflections and diffraction that increase at low altitude. Quadrantal error is caused by re-radiation from the airframe and reaches maximum values on relative bearings of 045°, 135°, 225° and 315°. Bank-angle error appears only in turns. Precipitation static and thunderstorm discharges generate noise that can completely mask the desired signal; during Cb activity the ADF needle may point continuously toward the storm. Because most ADF instruments lack a failure flag, continuous identification monitoring and independent cross-checks are mandatory whenever the ADF is the primary navigation source.

Module 8. VHF Omni-directional Range (VOR)

VOR became the ICAO standard short-range navigation aid in 1960. It radiates in the band 108.00–117.95 MHz and provides a continuous magnetic bearing from the station (a radial). The airborne receiver measures the phase difference between a 30 Hz frequency-modulated reference signal that is omnidirectional and a 30 Hz amplitude-modulated variable signal whose phase is proportional to azimuth. The two signals are synchronised so that they are in phase on magnetic north; the measured phase difference is therefore the radial. Conventional VOR produces the rotating limacon by mechanical or electronic means; Doppler VOR achieves the same result with a ring of antennas and is less susceptible to site error. Identification is a three-letter Morse group transmitted at least every 30 seconds. Designated operational coverage protects against co-channel interference; outside DOC the signal may be contaminated by distant co-frequency stations. Cone of confusion exists overhead the station where the horizontal component of the field becomes indeterminate. Accuracy is typically ±1° within DOC, corresponding to a cross-track error of approximately 3.5 NM at 200 NM. When a VOR is co-located with a DME the pair provides an instantaneous rho-theta fix. OBS, CDI and RMI presentations allow the pilot to select a desired radial, observe deviation and fly to or from the station. Scalloping, multipath and FM broadcast interference are the principal operational limitations.

Module 9. Instrument Landing System (ILS)

ILS provides precision approach guidance in both azimuth and elevation. The localiser operates in the band 108.10–111.95 MHz (even decimals) and radiates two overlapping amplitude-modulated lobes (90 Hz and 150 Hz) whose difference in depth of modulation defines the centre-line. The glide-path transmitter operates in the UHF band 329–335 MHz and likewise produces 90 Hz and 150 Hz lobes that define a nominal 3° approach path. Marker beacons (outer, middle and, historically, inner) radiate vertically at 75 MHz with distinctive modulation tones and panel lights (blue, amber, white). Category I systems provide guidance to a decision height of 200 ft; Category II to below 100 ft; Category III to the runway surface. Coverage of the localiser is ±35° to 17 NM and ±10° to 25 NM; glide-path coverage is ±8° to 10 NM. False glide paths exist at multiples of the nominal angle because of multipath from the ground; the pilot must capture the correct path from below. Critical and sensitive areas protect the signals from aircraft and vehicle reflections during low-visibility operations. Back-course approaches reverse the sense of the localiser indications and must be flown with the OBS set to the reciprocal course. Continuous monitoring of the identification and of both localiser and glide-path flags is mandatory; any flag appearance requires an immediate go-around.

Module 10. Microwave Landing System (MLS)

MLS was designed to overcome the siting and multipath limitations of ILS. It operates in the SHF band 5031–5090 MHz with 200 channels and uses time-referenced scanning beams for both azimuth and elevation. A narrow beam sweeps from left to right (or bottom to top); the airborne receiver measures the time interval between the to and fro passages and converts the interval into angular position. Coverage is ±40° in azimuth to 20 NM and 0.9°–20° in elevation. Distance is obtained from a co-located precision DME (DME/P). Back-azimuth guidance is available for missed approaches. MLS supports curved and segmented approaches, dual runways and operations in terrain that would be unsuitable for ILS. Multi-mode receivers can process both ILS and MLS signals, easing the transition. Although operational deployment has been limited by the success of GNSS-based approaches, MLS remains an approved precision-approach system and appears in examination syllabi. The principal advantages are immunity to FM broadcast interference, flexible approach paths and the ability to site the antennas in constrained environments.

Module 11. Radar Principles and Ground Radar

Primary radar determines range by measuring the time delay of an echo returned from a target and determines azimuth by the orientation of a narrow beam. Maximum unambiguous range is set by the pulse-repetition frequency: $R_{max}=c/(2\times PRF)$. Angular resolution improves with narrower beamwidth; range resolution improves with shorter pulse width. Peak power, antenna gain and receiver sensitivity together set the detection range. Continuous-wave radar eliminates the minimum-range restriction of pulsed systems and can extract Doppler velocity, but requires separate transmit and receive antennas. Ground surveillance radars include aerodrome surface-movement radar (ASMR) for taxiways, aerodrome surveillance approach radar for terminal airspace, and long-range area surveillance radar. Moving-target indication (MTI) filters out stationary clutter. Slotted-waveguide antennas reduce side lobes and concentrate energy into the main beam. Weather clutter is minimised by frequency selection and circular polarisation. Controllers use these displays to provide radar vectors, separation and surveillance; pilots must understand that primary radar does not identify the aircraft—identification requires secondary surveillance radar or visual acquisition.

Module 12. Airborne Weather Radar

Airborne weather radar operates at approximately 9375 MHz (X-band) and detects precipitation by the strength of the backscattered echo. Liquid water droplets return stronger signals than ice crystals of the same size; therefore the radar primarily displays rain and wet hail. Colour displays map echo intensity to green–yellow–red (or blue–amber–red) sequences; the steepest colour gradient indicates the most severe turbulence. The cosecant-squared beam illuminates a large ground area for mapping; the pencil beam is used for weather. Tilt control aims the beam relative to the horizon; a 2.5° up-tilt with a 5° beam places the lower edge of the beam at the aircraft altitude, revealing clouds at or above flight level. ISO-ECHO contour circuits highlight the cores of cells. Operation on the ground is permitted only with the beam tilted fully upward and with personnel and equipment clear of the antenna. Snow and dry hail produce weak returns and may be invisible. Shadowing behind strong cells can conceal further weather; always interpret the display in conjunction with visual cues and ATC reports. Stabilisation in pitch and roll keeps the beam correctly oriented during manoeuvres.

Module 13. Secondary Surveillance Radar (SSR)

SSR interrogates airborne transponders on 1030 MHz and receives replies on 1090 MHz. Mode A supplies a 12-bit identity code (4096 possibilities); Mode C supplies pressure altitude referenced to 1013.25 hPa in 100 ft increments. Framing pulses bracket the data pulses; the SPI pulse is added when the pilot presses IDENT. Mode S adds a unique 24-bit aircraft address, selective interrogation, and a data-link capability that supports TCAS, ADS-B and downlink of aircraft parameters. Special codes include 7500 (unlawful interference), 7600 (radio failure) and 7700 (emergency). Because the system does not rely on reflected energy it is free of weather clutter and has far greater range than primary radar for the same power. Garbling occurs when two aircraft reply simultaneously within the same range bin; Mode S selective addressing largely eliminates the problem. Altitude reported to the controller is accurate to ±50 ft under normal conditions. Continuous monitoring of the transponder reply light and correct code selection are basic airmanship requirements.

Module 14. Distance Measuring Equipment (DME)

DME provides slant-range distance by measuring the round-trip time of pulse pairs exchanged between an airborne interrogator and a ground transponder. The aircraft transmits on a frequency in the 960–1215 MHz band; the ground station replies 63 MHz higher or lower after a fixed 50 µs delay. Each aircraft jittered its pulse-repetition frequency so that it can recognise its own replies among the many responses. In search mode the interrogator emits 150 pulse pairs per second; once lock is achieved the rate drops to 25–30 ppps. Maximum capacity of a ground beacon is approximately 100 aircraft; beyond that saturation occurs and some aircraft are denied service. Slant range equals plan range only when the aircraft is at long range or low altitude; overhead the station the indicated range is the aircraft height in nautical miles. Co-location with a VOR or ILS localiser is indicated by identical or paired Morse identifications. DME is the ranging component of VORTAC, ILS/DME and MLS and is an essential input to RNAV computers. Always subtract the 50 µs ground delay when converting measured time to range, and remember that indicated distance is never less than the geometric height above the station.

Module 15. Area Navigation (RNAV) and FMS

RNAV allows an aircraft to fly any desired path within the coverage of ground- or space-based navaids or within the limits of self-contained systems. Basic RNAV (B-RNAV) requires a track-keeping accuracy of ±5 NM on 95 percent of occasions; Precision RNAV (P-RNAV) tightens the requirement to ±1 NM. Two-dimensional RNAV computers use VOR/DME or twin DME to compute range and bearing to a phantom waypoint; three- and four-dimensional systems add barometric or geometric altitude and time. The Flight Management Computer integrates IRS, GNSS, DME/DME and VOR/DME positions through a Kalman filter, producing a continuous best-estimate of position, track, ground speed and wind. The pilot enters the route via the CDU using ICAO identifiers, latitude/longitude or place-bearing-distance waypoints; the FMC then generates lateral and vertical guidance for the autopilot and flight-director. Performance and navigation databases supply optimum speeds, SIDs, STARs and holding patterns. Cross-track deviation on the EHSI is scaled at 5 NM full-scale in en-route mode and 1 NM or less in approach mode. Continuous comparison of FMC position with raw data and with the independent IRS is required; large discrepancies demand reversion to conventional navigation.

Module 16. Electronic Flight Information System (EFIS)

EFIS replaces electromechanical instruments with colour CRT or LCD displays driven by symbol generators. The Electronic Horizontal Situation Indicator (EHSI) can present full or expanded VOR/ILS, MAP, PLAN and approach modes. Weather-radar returns, TCAS traffic, wind vector, track line and selected heading are superimposed on the navigation display. Colour coding is standardised: magenta for active route, white for inactive, cyan for selected navaids, green for active waypoints, yellow for caution, red for warning. Failure of a selected VOR or ILS removes the deviation bar and may display an amber FAIL flag. The MAP mode shows the aircraft symbol, planned track, waypoints, navaids and airports within the selected range; PLAN mode presents a north-up view of the route for verification. Inputs to the EHSI include the FMC, IRS, radio navigation receivers, air-data computer and weather radar. Correct mode selection and continuous cross-check with raw data remain pilot responsibilities even when the display appears complete and authoritative.

Module 17. Global Navigation Satellite System (GNSS)

NAVSTAR GPS comprises a space segment of 24 operational satellites in six orbital planes inclined at 55°, a control segment of a master station, monitoring stations and ground antennas, and a user segment of receivers. Each satellite transmits on L1 (1575.42 MHz) and L2 (1227.6 MHz) using unique pseudo-random noise codes (C/A for civil users, P(Y) for authorised users). The receiver measures the time of arrival of at least four satellite signals, converts the times into pseudo-ranges, and solves for three-dimensional position and receiver-clock error. The WGS-84 ellipsoid is the reference datum. Principal errors are ionospheric delay (mitigated by dual-frequency measurements or a broadcast model), tropospheric delay, satellite ephemeris and clock errors, multipath and receiver noise. Dilution of precision (PDOP, HDOP, VDOP) quantifies the geometric contribution to error; the optimum constellation is one satellite overhead and three near the horizon spaced 120° apart. Selective availability was discontinued in 2000. Accuracy of the Standard Positioning Service is approximately 13 m horizontal and 22 m vertical (95 percent). Receiver Autonomous Integrity Monitoring (RAIM) requires a fifth satellite to detect a faulty measurement; six satellites are recommended for continuous availability. Aircraft manoeuvres that shadow a satellite degrade the solution until another satellite is acquired.

Differential GNSS improves both accuracy and integrity. Ground-based augmentation systems (GBAS/LAAS) place a reference receiver at a surveyed aerodrome site, compute range corrections, and broadcast them to approaching aircraft via a VHF data link; the same corrections also support a pseudolite ranging source. Satellite-based augmentation systems (SBAS) such as EGNOS, WAAS, MSAS and GAGAN use a network of reference stations to generate wide-area corrections that are uplinked to geostationary satellites and re-broadcast on a GPS-like signal. SBAS can support non-precision and Category I approaches; GBAS has demonstrated Category III performance. Combined GPS/GLONASS receivers convert between the WGS-84 and PZ-90 datums and increase the number of visible satellites, improving geometry and RAIM availability. Vertical guidance for approaches is referenced to barometric altitude; the geometric height above the ellipsoid is not used for decision altitude. When GNSS is used as the primary means of navigation an alternate approved system must remain available and the pilot must be prepared to revert if integrity monitoring is lost.

Module 18. Integrated Operation and Examination Technique