18.2 ILS, ADF & ATC Transponders

18.2 ILS, ADF & ATC Transponders

Quick Answer: ILS = localizer (horizontal, 108.10–111.95 MHz, 90/150 Hz left/right) + glideslope (vertical, ~329.15–335 MHz) + optional marker beacons (75 MHz, tones 400 / 1300 / 3000 Hz). ADF = 190–1750 kHz relative bearing to NDB; night effect = ionospheric multipath. Mode A = squawk; Mode C = + pressure altitude; Mode S = + selective address / collision avoidance. ATCRBS: RX 1030 MHz, TX 1090 MHz, pulse-position modulation.

Key topics 070 (ILS) and 071 (ADF & Transponders) move from enroute navaids to precision approach and surveillance. GROL maintainers support the RF paths that keep aircraft on the centerline and visible to ATC radar.

Instrument Landing System (ILS)

An ILS provides precision approach guidance to a runway. Functionally:

Localizer — frequency and meaning

Element 3 places the localizer beam system used to find runway centerline at 108.10 MHz to 111.95 MHz (VHF, odd-tenth channels in the lower NAV band). On runway approach, the ILS localizer shows deviation left or right of runway center line—not groundspeed, wind, or vertical error.

A required ILS component emphasized on the pool is the localizer: shows aircraft deviation horizontally from center of runway. Altimeters, VHF COM, and DME may appear on the same flight, but they are not the defining horizontal ILS guidance element.

Localizer 90 Hz / 150 Hz patterns

The localizer produces two amplitude-modulated antenna patterns: one pattern with an audio frequency of 90 Hz and one with 150 Hz, one left of the runway centerline and one right of the runway centerline. The airborne receiver compares the depth of 90 Hz vs 150 Hz modulation; equal modulation means on centerline. (The glideslope uses the same 90/150 Hz idea oriented above/below the path—do not reverse which pair is left/right vs up/down on exam day.)

Glideslope — vertical path

The glideslope transmitter operates in the UHF band near 329.15–335.00 MHz (pool distractors use this range; it is the classic glideslope assignment paired to each localizer channel). Selecting the localizer frequency on a modern NAV radio auto-tunes the paired glideslope channel—mirroring the DME pairing concept from Section 18.1.

Element 3 asks for the aircraft ILS glideslope installation antenna type: a folded dipole reception antenna. That is a receive-only, horizontally polarized antenna matched to the UHF glideslope band—not a transmitting phased array and not the ADF loop.

Marker beacons

Marker beacons mark fixed points along the approach. Element 3 states that the outer, middle, and inner marker beacon carrier frequencies are all 75 MHz, but they are 95% tone-modulated at:

As the aircraft passes over a beacon, the receiver produces the distinctive aural tone and panel light. Markers do not provide continuous distance-to-runway readout (that is DME/GPS); they confirm passage over fixed points. Many modern approaches substitute DME or GPS fixes for aging middle/inner markers, but the Element 3 pool still tests the 75 MHz / tone facts.

ILS frequency cheat sheet

Automatic Direction Finder (ADF) & Non-Directional Beacons (NDB)

ADF is the airborne receiver/display that points toward a ground NDB (or, historically, a commercial AM broadcast station). Element 3 frequency range for aircraft ADF equipment: 190 kHz to 1750 kHz (LF–MF). That is kilohertz, not megahertz—common distractors put ADF in the VOR or localizer VHF band.

What ADF provides

ADF indicates the relative bearing from the aircraft’s nose to the selected NDB (or other tuned station). It does not measure slant-range distance (DME does) and does not decode weather radar imagery.

Night effect

Night effect refers to the fact that NDB transmissions can bounce off the Earth’s ionosphere at night and be received from almost any direction. Sky-wave multipath rotates or confuses the apparent bearing—especially around sunrise/sunset. NDBs are not “turned off at dusk,” and dew on the antenna is not the pool definition of night effect.

Propagation literacy (correct ADF statement)

ADF antennas can receive transmissions over the Earth’s horizon (sometimes several hundred miles) because LF–MF ground-wave signals follow the curvature of the Earth. That is the opposite of pure VHF line-of-sight VOR/localizer behavior. Classic ADF installations use a loop (directional null) plus a sense antenna to resolve 180° ambiguity—pool items reject “sense antenna alone” as sufficient for full bearing processing.

Commercial standard broadcast stations in roughly 550–1660 kHz historically could be used as navigational signals within the ADF band; the pool’s “only correct statement” items emphasize ground-wave reach and proper antenna function rather than claiming every frequency except broadcast is an NDB.

ATC transponders (ATCRBS) — Modes A, C, and S

Secondary surveillance radar (ATCRBS—Air Traffic Control Radar Beacon System) interrogates aircraft transponders instead of relying only on primary skin paint.

Frequencies

For an aircraft Mode C transponder in ATCRBS:

• Receive (interrogation uplink): 1030 MHz
• Transmit (reply downlink): 1090 MHz

Memorize the direction: aircraft listens on 1030, talks on 1090. Swapping the numbers is the most common exam trap.

Mode functions

Mode C is the answer when the question asks which mode adds pressure-altitude reporting to the basic identification code. Modes 1 and 2 are military identification modes—not the civil Mode C altitude answer.

Mode S, in addition to duplicating Mode C functions, can also provide mid-air collision avoidance capabilities (the pool’s wording). It is not “primary radar,” lightning detection, or a backup VHF voice radio.

Encoding: pulse position modulation

The type of encoding used in an aircraft’s Mode C transponder transmission to an ATCRBS ground station is pulse position modulation (PPM). Distractors such as differential phase-shift keying, Doppler compression, or “95% AM” belong to other systems.

Altitude is encoded from the aircraft’s encoding altimeter / air data computer in standard pressure-altitude steps; ATC’s ground system converts that report for controller displays. When altitude readout is wrong, techs check the encoder interface and data path, not only the RF power of the 1090 MHz reply.

Transponder service notes

1. Antenna: L-band blade/monopole with a good ground plane (Section 18.3).
2. Suppression: Coordinate with DME/TCAS so simultaneous L-band systems do not desense each other.
3. Self-test / ramp test sets: Verify Mode A code, Mode C altitude, and Mode S address without radiating into open hangars unlawfully.
4. 1090 MHz replies must meet power and timing specs—weak replies look like “radar doesn’t see me.”

Putting approach and surveillance together

On a typical IFR approach day:

1. Enroute — VOR radial + DME slant range (18.1).
2. Transition — vectors or procedure turn onto the localizer.
3. Final — localizer left/right + glideslope up/down; markers/DME for range.
4. Always — Mode C/S altitude and squawk for ATC separation.
5. Backup bearing — ADF/NDB still appears on some approaches and Element 3 items even as GNSS dominates ops.

Exam-day checklist (3-K 070–071)

1. Localizer 108.10–111.95 MHz; shows left/right of centerline; 90/150 Hz AM patterns left/right.
2. Glideslope ~329–335 MHz UHF; aircraft antenna often a folded dipole receive antenna.
3. Markers: all 75 MHz; tones 400 / 1300 / 3000 Hz (outer/middle/inner).
4. ADF 190–1750 kHz; relative bearing; night effect = ionospheric bounce; ground-wave can go beyond horizon.
5. Transponder: RX 1030 / TX 1090 MHz; Mode C adds altitude; Mode S adds selective / collision-avoidance capability; encoding = PPM.

Next section maps aircraft antennas, COM/ELT/altimeter frequencies, and equipment-function identification items (3-K 072–073).