DGCA ATPL Radio Aids cheat sheet

Frequency and wavelength are inversely related: wavelength (m) = 300 / frequency (MHz); 118 MHz VHF corresponds to about 2.54 m.

VHF and above propagate essentially line-of-sight; radio horizon in nautical miles is approximately 1.23 times the square root of height in feet.

HF (3-30 MHz) uses skywave via ionospheric refraction; usable range depends on time of day, season and sunspot cycle.

Ground wave supports LF/MF transmission over the surface; attenuation rises with frequency and with poor ground conductivity.

Ionospheric D layer absorbs MF/HF by day; E and F layers refract and reflect; F layer height and density peak after sunset.

Reflection, refraction, diffraction, absorption and scattering are the five basic propagation mechanisms; ducting causes anomalous extended VHF range.

Fading is caused by multipath and ionospheric variation; diversity reception and AGC mitigate it.

Radio energy is attenuated by the inverse square law; doubling distance reduces received field strength by about 6 dB.

Civil VHF airband is 118.000-136.975 MHz, channel spacing 8.33 kHz above FL195 in Europe; 25 kHz elsewhere.

VHF is amplitude modulated (AM, double sideband) so that simultaneous carriers can be heard as a heterodyne squeal.

Communication range is line-of-sight; at FL350 a typical VHF range is about 200 NM to a ground station.

Distress frequency 121.5 MHz; emergency and military guard; 243.0 MHz is the military UHF guard.

Readback is required for runway, clearance, hold-short and altitude instructions; standard phraseology per ICAO Annex 10/PANS-ATM.

Squelch mutes receiver output below a set signal threshold; it suppresses noise but can mask weak distant transmissions.

Frequency 122.750 MHz is air-to-air; 123.450 MHz is used for air-to-air and unofficial en-route chat in some regions.

Blocked or stepped-on transmissions are resolved by the controller; the aircraft must not transmit simultaneously on a frequency.

HF band is 3-30 MHz; aeronautical HF uses single sideband (SSB), usually upper sideband, suppressed carrier.

HF provides long-range (oceanic, polar, remote) communication via skywave; range depends on frequency selection relative to ionospheric conditions.

Day/night frequency changes are needed: higher frequencies by day, lower at night, because the F layer height and density change.

SELCAL uses tone-coded selective calling so the crew is alerted without monitoring the channel continuously.

HFDL (HF Data Link) and ALE are data/automatic modes; voice distress on HF is 5680 kHz (and 121.5 for VHF).

Antenna couplers match a short wire or probe antenna to the transmitter across the HF band; tuning is automatic.

Propagation via the F layer can give ranges of several thousand nautical miles with a single hop; skip zone exists between ground-wave and skywave coverage.

HF is susceptible to static, fading and interference; diversity and AGC are used; position reporting is via HF or satellite in remote areas.

NDB transmits in the LF/MF band (190-1750 kHz); ADF receiver gives a relative bearing to the station.

Relative bearing + magnetic heading = magnetic bearing to the station (QDM) when no wind; the tail of the needle points away from the station.

ADF bearing is subject to quadrantal error from aircraft structure and coastal refraction; an ADF sense antenna resolves 180-degree ambiguity.

Magnetic variation must be applied to convert between magnetic and true bearings; ADF indicator shows relative bearing.

Station passage is indicated by a rapid swing of the needle; a dip or momentary loss of signal occurs over the station (cone of silence).

NDB identification is a two- or three-letter Morse code; voice identification may also be transmitted.

Ground DF stations determine aircraft bearing by comparing signals received at spaced antennas; a QDM is the magnetic bearing to the station.

ADF is affected by thunderstorms (static), night effect (skywave) and terrain; accuracy is typically ±5 degrees or better in good conditions.

VOR operates 108.0-117.95 MHz; the 108-112 MHz band is shared with ILS localizer; VOR uses even tenths of a MHz.

Conventional VOR transmits a rotating cardioid pattern (variable phase) and a fixed reference phase; the phase difference equals the magnetic bearing from the station.

The CDI shows the selected radial; TO/FROM flag depends on whether the selected course points toward or away from the station.

VOR accuracy is about ±1 degree at the station, degrading with distance; line-of-sight limits range (roughly 1.23 × √height ft).

Doppler VOR uses a wide-aperture antenna array with electronic rotation to reduce site and terrain errors, especially at high elevation angles.

Cone of confusion overhead the station causes loss of course guidance and a TO/FROM ambiguity; station passage requires a turn to re-intercept.

VOR radials are magnetic bearings from the station; the OBS selects the desired course and the deviation bar centers when on course.

Terminal VOR service volumes are typically 25 NM; low altitude 40 NM; high altitude 100 NM up to FL450.

VOR is subject to scalloping and multipath; a reverse sensing indication occurs when the OBS is set to the reciprocal of the actual course.

DME operates 960-1215 MHz; the aircraft interrogates and the ground transponder replies after a fixed delay; slant range is measured.

DME frequency is paired with VOR/ILS frequency in the same tuning unit; DME gives slant range, not ground range.

Ground range = √(DME² − altitude²); the error is greatest directly overhead and negligible at long range.

DME accuracy is about ±0.25 NM or 0.5% of range; the display typically reads to 0.1 NM.

DME reception is line-of-sight; range is roughly that of VOR, and station passage shows a rapid count down to a small residual range.

The DME transponder has a limited reply capacity and may be overloaded in dense traffic; the aircraft may reduce interrogations.

DME identifiers must match the paired VOR/ILS; a DME frequency can be paired with an ILS localizer for distance to threshold.

DME distance is measured to the ground station, not to the runway threshold, unless the DME is offset (zero-range) at the threshold.

Localizer: 108.10-111.95 MHz (odd tenths), 90 Hz left / 150 Hz right of centerline; full-scale deflection is typically ±2.5 degrees.

Glide path: 328.6-335.4 MHz UHF, 90 Hz above / 150 Hz below the path; nominal 3-degree angle, full-scale about ±0.7 degrees.

Marker beacons: outer 75 MHz at 400 Hz (about 4-7 NM), middle at 1300 Hz (about 3500 ft from threshold), inner at 3000 Hz.

Localizer course is aligned with the runway; the glider path crosses the localizer centerline at the threshold at the published TCH.

ILS categories: CAT I 200 ft DH / 550 m RVR; CAT II 100 ft / 300 m; CAT IIIA 50 ft / 200 m; CAT IIIB 0 ft / 50-200 m.

DME or a marker can substitute for the outer marker; the glide path transmitter is normally offset from the runway centerline.

The localizer is horizontally polarized and the glide path is horizontally polarized; the marker beacons transmit vertically polarized.

False glide paths exist above the true path (multiples of the nominal angle) and must be ignored; the localizer has back-course lobes behind the runway.

Primary radar detects echoes from targets; range = (c × time)/2, so the round trip is used to compute range.

Secondary radar (SSR) interrogates the transponder, which replies with a coded pulse; range and identity are provided.

ATC primary radar uses pulse techniques; range resolution depends on pulse length; azimuth resolution depends on beamwidth.

Weather radar (X or C band) detects precipitation; the display shows reflectivity; the antenna tilt and gain control affect the picture.

Radar shadow and attenuation occur behind heavy precipitation; the radar cannot see through heavy rain to distant targets.

Ground mapping and turbulence detection modes use Doppler or return intensity; the radar beam widens with distance (beamwidth in degrees).

Radar altimeter and primary radar differ: primary radar measures range to a target, the radio altimeter measures height above terrain.

SSR Mode A gives identity, Mode C gives pressure altitude, Mode S gives a unique 24-bit address and data link.

GNSS position is derived from pseudoranges to satellites; four satellites are needed for a 3D fix plus time.

GPS is one constellation; GNSS includes GPS, GLONASS, Galileo and BeiDou; augmentation (SBAS/GBAS/ABAS) improves integrity.

RAIM detects and excludes faulty satellites; RAIM availability depends on satellite geometry and is required for certain approaches.

RNAV is area navigation along any desired path; RNP adds on-board performance monitoring and alerting.

RNP AR (authorization required) approaches use radius-to-fix legs and require special crew/aircraft approval.

The required navigation performance value (e.g., RNP 0.3) is the lateral containment limit in nautical miles, 95% of the time.

WAAS/EGNOS (SBAS) provide vertical guidance (LPV) and improve accuracy; GBAS provides local differential corrections at an airport.

GNSS is subject to ionospheric scintillation, multipath and jamming/spoofing; RAIM prediction must be checked before flight.

The radio altimeter transmits a frequency-modulated continuous wave (FMCW) signal and measures the time delay of the reflected signal.

It measures height above the terrain directly below the aircraft, not above mean sea level; the display is AGL.

Operating range is typically 0-2500 ft (or 0-5000 ft); accuracy is about ±2 ft at low altitude.

The radio altimeter drives the decision height (DH) callout, the autoland flare and the GPWS/EGPWS.

The antenna is a small flush-mounted unit; the beam is narrow and directed downward, so it can be affected by rough terrain or steep bank.

A radio altimeter failure may cause the autopilot to disengage or the DH callout to be lost; check the DH annunciation.

The radio altimeter does not depend on barometric pressure; it is unaffected by QNH setting errors.

Above the operating range the indicator may park or show a flag; the system is used primarily during approach and landing.

SSR transponder replies on 1090 MHz to interrogations on 1030 MHz; Mode A is identity, Mode C pressure altitude, Mode S unique address.

Squawk codes: 7700 emergency, 7600 radio failure, 7500 unlawful interference; codes are four octal digits.

TCAS II issues traffic advisories (TA) and resolution advisories (RA); RAs give vertical escape maneuvers (climb or descend).

TCAS uses Mode S interrogations and replies; the TCAS antenna is on top and bottom of the fuselage.

TCAS RAs must be followed even if they conflict with ATC clearance; the crew must notify ATC of the deviation.

TCAS does not provide separation from terrain; it is inhibited below a set radio altitude (typically 1000 ft) for some RAs.

Mode S provides selective interrogation and data link; the 24-bit address is unique to the aircraft.

Transponder must be on and squawking before takeoff and after landing; ALT mode reports pressure altitude.