Radionavigation Systems

1 Automatic Direction Finder (ADF)

Main purpose: obtain bearing,position,homing.

ADF working alone: provides direction of the aircraft in relation to itself.

ADF working as a network: provides position of the aircraft as the intersection of two bearings.

ADF communicates with NDBs and operates in the MF waveband using a tunable receiver inside the aircraft.

RBI ≡Relative Bearing Indicator. It provides relative bearing in relation to the aircraft heading.

The ADF onboard instrument has got 2needles(indicating NDB1 and NDB2) and one lubble line (indicating HDG).

2 Non-Directional Beacon (NDB)

Ground-based NDB is a transmitter operating in LF and MF waveband. Its coverage has a range of 10-150 NM.

Low-powered NDBs (locators): Used as airfield approach aids.

High-powered NDBs: used for long range navigation on and offairways.

Drawbacks: Failures difficult to detect, constant monitoring of equipment is required. EUROCONTROL is planning to

extinguish them soon and replace them all by VORs.

3 Distance Measurement Equipment (DME)

It is a transponder-based r-nav technology that measures slant range distance by timing the propagation delay of radio signals.

Most of them are associated with a VOR. When pilot codes VOR frequency, DME frequency is automatically coded.

Aircraft interrogates ground station by sending a coded signal (freq A) then ground station send back the signal (freq B). A/c

decodes the signal. Depending on the time it takes to go and come back distance and rel. velocity can be worked out.

Two freq. are used to avoid two a/c interrog. each other. UHF waveband. 1 NM accuracy. Up to 100 interrog. at a time.

Slant error: the close the beacon and the higher the aircraft the less accurate the reading become.

4 VHF Omnidirectional Range (VOR)

VORs operates with two signals: Reference signal (omnidirectionally). Variable signal (circular array of aerials). This way,

user can know with 1 grade of precission his magnetic bearing to the station.

Indicator consists of a moving vertical needle (CDI ≡Course Deviation Indicator) and one TO-FROM indicator. TO indicates

semiplane region that does NOT contain the selected radial. FROM vicerversa.

Uses: homing to station, maintain track along an airway centerline, obtaining a position line (radial). Cone of silence:

VOR beacon does not transmmit usable signal vertically and upwards.

5 Instrumental Landing System (ILS)

It provides accurate guidance both in azimuthal and elevation. Rwys served by ILS are known as Precision Approach

Instrument Rwys. DME automatically tuned when ILS selected. Two main components:

LOC: Indicates the deviation from the optimum lateral approach path. (Located at 300 m from threshold). Glidepath

transmitter: defines the safe descent slope (3 deg) and indicates deviation from the optimum vertical approach path.

Marker beacons: Outer marker: 3-6 NM (blue light). Middle marker: 900-1200 m (amber light). Inner marker: 300-450

m(white light).

Radionavigation Systems

Air Transport Infrastructure