NAVIGATIONAL AIDS (NAV-AID)

VOR

VHF Omni-directional radio range (VOR), is a type of short-range Radio navigation system for Aircraft enabling aircraft to determine their position and stay on course by receiving radio signals transmitted by a network of fixed ground Radio beacons, with a receiver unit. It uses radio frequencies in the very high frequency (VHF) band from 108 to 117.95 MHz.

DME

Distance measuring equipment (DME) is a short/medium range navigation system often used in conjunction with VOR system to provide accurate navigation fixes. Distance measuring equipment (DME) is a transponder-based radio navigation technology that measures Slant range distance by timing the propagation delay of VHF or UHF radio signals. The system is based on secondary radar principles.

DME and VOR at EED Civil Aviation Authority Pakistan Karachi

NDBS

       A non-directional (radio) beacon (NDB) is a radio transmitter at a known location, used as an aviation or marine navigational aid. As the name implies, the signal transmitted does not include inborn directional information, in contrast to other navigational aids such as low frequency radio range,VHF Omni-directional range(VOR) and TACAN. NDB signals follow the curvature of the Earth, so they can be received at much greater distances at lower altitudes, a major advantage over VOR. However, NDB signals are also affected more by atmospheric conditions, mountainous terrain, coastal refraction and electrical storms, particularly at long range.

INSTRUMENT LANDING SYSTEM

An instrument landing system (ILS) is a ground-based instrument approach system that provides precision guidance to an Aircraft approaching and landing on a runway, using a combination of radio signals. One transmitter (Glide slope) provides guidance in vertical plane and has a range of approximately 10nm. The second Transmitter (Localizer) guides the aircraft In the horizontal plane and also three marker beacons.

Localizer :

In aviation, a localizer (LOC) provides runway guidance to aircraft as it provides central line on runway at touchdown point.  The localizer transmits in the VHF frequency range,  108 -112 MHz in 0.5 Mhz increments. Note this is the same frequency range as used by VOR system. The localizer antenna is located at the far end of the run way, and transmits two lobes to the left and right of the runway center-line modulated at 90 Hz and 150 Hz respectively.

Glide slope:

Glide slope gives aircraft an angle of 3 degree from the touchdown point for landing. It transmits in UHF frequency band, 328.6 to 335 MHz at 150 KHz spacing. Upper lobes and lower lobes are modulated at  90 Hz and 150 Hz respectively.

Marker beacons :

Three beacons are sited on the extended runway centerline at precise distances. These markers give indication to the pilot that aircraft is approaching towards the runways. The indication is in Morse code as dash (– ) and dot (.).This operates at 75 MHz and radiates 3-4 W power.

Outer marker:

It is located at 4—7 miles away from runway threshold; its Morse code is dash and dash frequency of 400 Hz and illuminates blue light.  It provides the approximate point at which an aircraft on the localizer will intercept the glide slope.

middle marker  :

The middle marker is located at approximately 3500 feet from the runway threshold. Its Morse code is dot and dash modulated at 1300 Hz and corresponding an amber/yellow light is illuminated. The middle marker coincides with the aircraft when aircraft is at 200 feet high.  It is also called the decision height  here captain has to take the decision whether to land or not.

Inner marker:

Runways that are used for low visibility approach and landing have a third inner marker. Its Morse code is dot and dot modulated at 3000Hz on audio system and corresponding white light is illuminated. It is 50 feet away from runway threshold. The marker beacon system is currently being phased out with the introduction of DME and GPS approach.

RADAR :

Radio detection and Ranging is an object detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships,spacecraft,guided missiles, motor vehicles,weather formations, and terrain. The radar dish or antenna transmits pulses of radio waves or microwaves which bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna which is usually located at the same site as the transmitter.

RADAR TYPES:

·       Primary Surveillance Radar (PSR)
·       Secondary Surveillance Radar (SSR)

Primary radar:

Primary surveillance radar (PSR) transmits high-frequency signals which are reflected at targets. The arisen echoes are received and evaluated. This means, unlike secondary radar sets a primary radar set receive its own emitted signals as an echo again.

Secondary radar:

Secondary surveillance radar (SSR) is a radar system used in air traffic control (ATC), that not only detects and measures the position of aircraft i.e. range and bearing, but also requests additional information from the aircraft itself such as its identity and altitude. Unlike primary radar systems that measure only the range and bearing of targets by detecting reflected radio signals, SSR relies on targets equipped with a radar transponder that replies to each interrogation signal by transmitting a response containing encoded data.

The secondary radar used by Civil Aviation Authority is named as SSR RSM 870. It works on two frequencies and is able to identify the aircraft plus it also tells its height. This radar requires active participation of the air craft.

ULTRA HIGH FREQUENCY & VERY HIGH FREQUECY  (UHF - VHF)

VHF :

Very high frequency (VHF) has long been the primary means of communication between aircraft and ground. The system operates in the frequency range 118 MHz to 137 MHz and supports both voice and data communication. VHF is used for various purpose including air traffic control (ATC), approach and departure information, transmission of meteorological information and ground handling of aircraft.

VHF Range and Propagation :

In VHF range (30 MHz to 300 MHz) radio waves usually propagate as direct line-of-sight (LOS) waves. Sky waves propagation still occurs at the bottom end of the VHF range (up to about 50 MHz depending upon solar activity) but the frequencies used for aircraft communication, reflection from ionosphere is exceptionally rare.

DSB Modulation :

Amplitude modulation is used for voice communication as well as several of the VHF data link (VDL) modes. The system uses double side (DSB) modulation and, because this has implications for the bandwidth of modulated signals. The frequency spectrum of an RF carrier wave at 124.575 MHz amplitude modulated by a signal pure sinusoidal tone with a frequency of 1 KHz. Amplitude modulated waveform comprises of:

·       An RF carrier , 124.575 MHz
·       Lower side frequency (LSF), 124.574 MHz
·       Upper side frequency (USF), 124.576 MHz

HIGH FREQUENCY (HF) 

High frequency provides aircraft with an effective means of communication over long distance oceanic and transpolar routes. HF is no longer restricted to voice communication but also for long distance data communication that will augment existing VHF and SATCOM data links.

HF Range and Propagation :

In the HF range (3 MHz to 30MHz) radio waves propagate over long distances due to reflection from the ionized layers in the upper atmosphere. Due to variation in height the intensities of the ionized regions, different frequencies must be used at different time of the day and night and for different paths.

SSB Modulation :

The spectrum used available for aircraft communication at HF is extremely limited. As a result, steps are taken to restrict the bandwidth of transmitted signals, for both voice and data. In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier (SSB-SC) is a refinement of amplitude modulation that more efficiently uses transmitter power and bandwidth. Amplitude modulation produces an output signal that has twice the bandwidth of the original baseband signal. Single-sideband modulation avoids this bandwidth doubling, and the power wasted on a carrier, at the cost of increased device complexity and more difficult tuning at the receiver.