navaids

Navigational Aids, often known as navaids or nav-aids, are used in all forms of transport but most prominently in Aviation and marine applications. It is a broad category encompassing radio beacons, light emitting sources, radar, GPS, in fact any device that provides information, on your present. location or on a desired destination relative to a given point

Homer – radio direction finding (DF) station equipped to provide a mobile stations with heading to steer (with no wind) to reach the station

using radio transmission from the mobile station

Navigational Aids, often known as navaids or nav-aids, are used in all forms of transport but most prominently in Aviation and marine applications It is a broad category encompassing radio beacons, light emitting sources, radar, GPS, in fact any device that provides information, on your present location or on a desired destination relative to a given point

Homer – radio direction finding (DF) station equipped to provide a mobile stations with heading to steer (with no wind) to reach the station using radio transmission from the mobile station

NDB – Non Directional Beacon

An L/MF or UHF radio beaco transmitting nondirectional signals whereby the pilot of an aircraft equipped with direction finding equipment can determine his bearing to or from the radio beacon and "home" on or track to or from the station

NDB navigation consists of two parts — the automatic direction finder (or ADF) equipment on the aircraft that detects an NDB's signal, and the NDB transmitter

DF - direction finder

Direction finding (DF) refers to the establishment of the direction from which a received signal was transmitted. This can refer to radio or other forms of wireles communication. By combining the direction information from two or more suitably spaced receivers.

DME – Distance Measuring Equipment

Distance measuring equipment (DME) is a transponder-based radio navigation technology that measures distance by timing the propagation delay of VHF or UHF radio signals

Aircraft use DME to determine their distance from a land-based transponder by sending and receiving pulse pairs - two pulses of fixed duration and separation

The ground stations are typically co-located with VORs

The aircraft interrogates the ground transponder with a series of pulse-pairs (interrogations) and, after a precise time delay (typically 50 microseconds the ground station replies with an identical sequence of reply pulse-pairs. The DME receiver in the aircraft searches for pulse-pairs (X-mode= 12 microsecond spacing) with the correct time interval between them, which is determined by each individual aircraft's particular interrogation pattern. The aircraft interrogator locks on to the DME ground station once it understands that the particular pulse sequence is the interrogation sequence it sent out originally. Once the receiver is locked on it has a narrower window in which to look for the echoes and can retain lock

A radio pulse takes around 12.36 microseconds to travel 1 nautical mile (1,852 m) to and from; this is also referred to as a radar-mile. The time difference between interrogation and reply 1 nautical mile (1,852 m) minus the 50 microsecond ground transponder delay is measured by the interrogator's timing circuitry and translated into a distance measurement (slant range), stated in nautical miles, and then displayed on the cockpit DME display

VOR - short for VHF omnidirectional radio range, is a type of radio navigation system for aircraft. A VOR ground station broadcasts a VHF radio composite signal

including the station's identifier, voice (if equipped), and navigation signal. The identifier is morse code. The voice signal is usually station name, in-flight recorded

advisories, or live flight service broadcasts. The navigation signal allows the airborne receiving equipment to determine a magnetic bearing from the station to the aircraft (direction from the VOR station in relation to the Earth's magnetic North at the time of installation). VOR stations in areas of magnetic compass unreliability are oriented with respect to True North. This line of position is called the "radial" from the VOR. The intersection of two radials from different VOR stations on a chart provides the position of the aircraft

VOR/DME refers to combined radio navigation station for aircraft, which consists of two radio beacons, placed together, a VHF omnidirectional range(VOR) and distance measuring equipment (DME). VOR produces an angle between the station and the receiver in the aircraft, while DME does the same for range. Together, they provide the two measurements needed to produce a navigational "fix" using a chart.

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 and, in many cases, high-intensity lighting arrays to enable a safe landing during instrument meteorological conditions (IMC), such as low ceilings or reduced visibility due to fog, rain, or blowing snow

Principle of operation

An ILS consists of two independent sub-systems, one providing lateral guidance (localizer), the other vertical guidance (glideslope or glide path) to aircraft approaching a runway. Aircraft guidance is provided by the ILS receivers in the aircraft by performing a modulation depth comparison

The emission patterns of the localizer and glideslope signals. Note that the glideslope beams are partly formed by the reflection of the glideslope aerial in the ground plane

A localizer antenna array is normally located beyond the departure end of the runway and generally consists of several pairs of directional antennas

Two signals are transmitted on one out of 40 ILS channels between the carrier frequency range 108.10 MHz and 111.95 MHz (with the 100 kHz digit always odd, so 108.10, 108.15, 108.30, and so on are LOC frequencies but 108.20, 108.25, 108.40, and so on are not). One is modulated at 90 Hz the other at 150 Hz and these are transmitted from separate but co-located antennas. Each antenna transmits a narrow beam, one slightly to the left of the runway centerline, the other to the right

The localizer receiver on the aircraft measures the Difference in the Depth of Modulation (DDM) of the 90 Hz and 150 Hz signals. The differencebetween the two signals varies depending on the position of the approaching aircraft from the centerlinIf there is a predominance of either 90 Hz or 150 Hz modulation, the aircraft is off the centerline. In the cockpit, the needle on the horizontal situation indicator (HSI, the instrument part of the ILS), or course deviation indicator (CDI), will show that the aircraft needs to fly left or right to correct the error to fly down the center of the runway. If the DDM is zero, the aircraft is on the centerline of the localizer coinciding with the physical runway center line

A glideslope or glidepath (GP) antenna array is sited to one side of the runway touchdown zone. The GP signal is transmitted on a carrier frequency between 329.15 and 335 MHz using a technique similar to that of the localizer. The centerline of the glideslope signal is arranged to define a glideslope of approximately 3° above horizontal (ground level). The beam is 1.4° deep; 0.7° below the glideslope centerline and 0.7° above the glideslope centerline

These signals are displayed on an indicator in the instrument panel. This instrument is generally called the omni-bearing indicator or nav indicator

The pilot controls the aircraft so that the indications on the instrument (i.e., the course deviation indicator) remain centered on the display. This ensures the aircraft is following the ILS centreline (i.e., it provides lateral guidance). Vertical guidance, shown on the instrument by the glideslope indicator, aids the pilot in reaching the runway at the proper touchdown point

Identification

In addition to the previously mentioned navigational signals, the localizer provides for ILS facility identification by periodically transmitting a 1,020 Hz Morse code identification signal. For example, the ILS for runway 04R at John F. Kennedy International Airport transmits IJFK to identify itself, while runway 04L is known as IHIQ. This lets users know the facility is operating normally and that they are tuned to the correct ILS. The glideslope transmits no identification signal, so ILS equipment relies on the localizer for identification

Marker beacons

On most installations, marker beacons operating at a carrier frequency of 75 MHz are provided. When the transmission from a marker beacon is received it activates an indicator on the pilot's instrument panel and the tone of the beacon is audible to the pilot. The distance from the runway at which this indication should be received is promulgated in the documentation for that approach, together with the height at which the aircraft should be if correctly established on the ILS. This provides a check on the correct function of the glideslope. In modern ILS installations, a DME is installed co-located with the ILS, to augment or replace marker beacons. A DME continuously displays the aircraft's distance to the runway

Outer marker

Blue outer marker

The outer marker should be located 7.2 km (3.9 nmi) from the threshold except that, where this distance is not practicable, the outer marker may b located between 6.5 and 11.1 km (3.5 and 6 nmi) from the threshold. The modulation is repeated Morse-style dashes of a 400 Hz tone. The cockpit indicator is a blue lamp that flashes in unison with the received audio code. The purpose of this beacon is to provide height, distance and equipment functioning checks to aircraft on intermediate and final approach

Middle marker

Amber middle marker

The middle marker should be located so as to indicate, in low visibility conditions, the missed approach point, and the point that visual contact with the runway is imminent, ideally at a distance of approximately 3,500 ft (1,100 m) from the threshold. It is modulated with a 1.3 kHz tone as alternating Morse-style dots and dashes at the rate of two per second. The cockpit indicator is an amber lamp that flashes in unison with the received audio code. Middle markers are no longer required in the United States so many of them are being decommissioned

Inner marker

White inner marker

The inner marker, when installed, shall be located so as to indicate in low visibility conditions the imminence of arrival at the runway threshold

This is typically the position of an aircraft on the ILS as it reaches Category II minima. Ideally at a distance of approximately 1,000 ft (300 m) from the threshold. The modulation is Morse-style dots at 3 kHz. The cockpit indicator is a white lamp that flashes in unison with the received audio code

DME

Distance measuring equipment (DME) provides pilots with a slant range measurement of distance to the runway in nautical miles. DMEs are augmenting or replacing markers in many installations. The DME provides more accurate and continuous monitoring of correct progress on the ILS glideslope to the pilot, and does not require an installation outside the airport boundary. When used in conjunction with an ILS, the DME is often sited midway between the reciprocal runway thresholds with the internal delay modified so that one unit can provide distance information to either runway threshold. On approaches where a DME is specified in lieu of marker beacons, the aircraft must have at least one operating DME unit to begin the approach, and a DME Required restriction will be noted on the Instrument Approach Procedure.

Approach lighting

Some installations include medium or high intensity approach light systems. Most often, these are at larger airports. The approach lighting system (abbreviated ALS) assists the pilot in transitioning from instrument to visual flight, and to align the aircraft visually with the runway centerline. A many non-towered airports, the intensity of the lighting system can be adjusted by the pilot, for example the pilot can click their microphone 7 times to turn on the lights, then 5 times to turn them to medium intensity

Use of the Instrument Landing System

At controlled airports, air traffic control will direct aircraft to the localizer via assigned headings, making sure aircraft do not get too close to each other (maintain separation), but also avoiding delay as much as possible. Several aircraft can be on the ILS at the same time, several miles apart

An aircraft that has come within two and a half degrees of the localizer course (half scale deflection shown by the course deviation indicator) is said to be established on the approach. Typically, an aircraft will be established by at least two miles prior to the final approach fix (glideslope intercept) at the specified altitude

Aircraft deviation from the optimal path is indicated to the flight crew by means of display dial (a carry over from when an analog meter movement would indicate deviation from the course line via voltages sent from the ILS receiver).

The output from the ILS receiver goes both to the display system (head-down display and head-up display, if installed) and can also go to the Flight

Control Computer. An aircraft landing procedure can be either coupled, where the Flight Control Computer directly flies the aircraft and the flight crew monitor the operation; or uncoupled (manual) where the flight crew fly the aircraft uses the HUD and manually control the aircraft to minimize the deviation from flight path to the runway centreline.

Decision altitude/height

Once established on an approach, the autoland system or pilot will follow the ILS and descend along the glideslope, until the Decision Altitude is reached (for a typical Category I ILS, this altitude is 200 feet above the runway). At this point, the pilot must have the runway or its approach lights in sight to continue the approach.

If neither can be seen, the approach must be aborted and a missed approach procedure will be performed. This is where the aircraft will climb back to a predetermined altitude and position. From there the pilot will either try the same approach again, try a different approach or divert to another airport

Aborting the approach (as well as the ATC instruction to do so) is called executing a missed approach

Frequency list

Localizer and glideslope carrier frequencies are paired so that only one selection is required to tune both receivers. The frequency pairings conform to the following table

Runway Visual Range (RVR) is a term used in aviation meteorology to define the distance over which a pilot of an aircraft on the centreline of the runway can see the runway surface markings delineating the runway or identifying its centre line. RVR is normally expressed in feet or metersRVR is used as one of the main criteria for minima on instrument approaches, as in most cases a pilot must obtain visual reference of the runway to land an aircraft. The maximum RVR reading is 2,000 metres or 6,000 feet above which it is not significant and thus does not need to be reported. RVRs are provided in METARs and are transmitted by air traffic controllers to aircraft making approaches to allow pilots to assess whether it is prudent and legal to make an approach

Radio Test

When it is necessary for an aircraft station to send signals for testing or adjustment which are liable to interfere with the working of a neighboring aeronautical station, the consent of the station shall obtained before such signals are sent. Such transmissions shall be kept to a minimum

When it is necessary for a station in the aeronautical mobile service to make test signals, either for the adjustment of a transmitter before making a call or for the adjustment of a receiver, such signals shell not continue for more then 10 seconds. This transmission shall be composed of spoken numerals (ONE, TWO, THREE, etc.) in radiotelegraphy, followed by the radio call sign of the station transmitting the test signals. Such transmission shall be kept for a minimum

Radio test transmission will comprise of

a. The identification of the station being called.

b. The aircraft identification.

c. The words : "RADIO CHECK

d. The frequency being used.

The reply to a test transmission should be as follows

a. The identification of the aircraft.

b. The identification of the aeronautical station replying.

c. Information regarding the readability of the aircraft transmission.

The test transmission and the reply thereto should be recorded at the aeronautical station