Instrument Landing System (ILS)

The basic navigation aid for the pilots for safe landing because in civil aircrafts there are approx. 200 lives are on stake of death per aircraft, so if any error occurs during landing approach or in landing can cause a hilarious danger with the loss of lives in along with the economical loses of aircraft and airport. In modern aviation industry pilots are trained to take full benefit of ILS facility on the airport as well as Manual Landing after the crash of Air France 447 330-230air bus, and it is made a standard by ICAO for declaring international airport should have the complete facility of ILS. ILS system comprised of the instruments made necessary by the ICAO:

•  Localizer
•  Glide Slope
• Marker beacons
•  Precision Approach Path Indicator (PAPI)

Localizer (LOC):

LOC is an instrument comprised of antenna array placed on the other end of runway which provide guidance in horizontal plane to the aircraft onboard.

PCAA have a frequency band allocated 108MHz to 112MHz which is quite a narrow band, however the frequencies of the localizer are placed on odd decimals. The signal transmitted by the localizer consists of two vertical balloon-shaped patterns that overlap, at the center. They are aligned with the extended centerline of the runway.

§  The right side of this pattern, as seen by an approaching aircraft, is modulated at 150 Hz and is called the "blue" area.

§  The left side of the pattern is modulated at 90 Hz and is called the "yellow" area.

Functioning of LOC is to help in setting up the runway center line guidance to relate aircraft’s virtual center line, resulting aircraft will not deviate from the actual runway center line.

Glideslope:

Glideslope provides vertical guidance to the pilot during the approach. Glide slope consist of a ground-based UHF radio transmitter and antenna system which actually give 2.8 degree angle information of aircraft relative to the runway while landing. Like the localizer, the glide slope signal is also consists of two overlapping beams modulated at 90 Hz and 150 Hz. However, these signals are aligned above each other and are radiated primarily along the approach track. The thickness of the overlap area is 1.4º or .7º above and .7º below the optimum glide slope.

Precision Approach Path Indicator (PAPI):

PAPI are led lights placed on the runway in the form of making markers for visual navigation of aircraft during landing. PAPI’s are usually zero visibility condition aids which can be seen by the pilot through eyes, while landing when uncertainty conditions among the instruments happens. These systems have an effective visual range of at least 3 miles during the day and up to 20 miles at night.

MARKER

Marker serve to identify a particular location in space along an airway or an approach to an instrument runway. This is done by a means of 75 MHz transmitter which transmit a directional signal to be received by aircraft flying overhead.

Marker has three types:

OUTER MARKER

The Outer Marker, which normally identifies the final approach fix (FAF), is situated on the same course/track as the localizer and the runway center-line, four to seven nautical miles^[1] before the runway threshold. It is typically located about 1 NM (2 km) inside the point where the   glideslope intercepts the intermediate altitude and transmits a 400 Hz tone signal on a low-powered (3 watts). The system gives the pilot a visual and aural indication.

MIDDLE MARKER:

A middle marker works on the same principle as an outer marker. It is normally positioned 0.5 to 0.8 nautical miles (1 km) before the runway threshold. When the aircraft is above the middle marker, the receiver's amber middle marker light starts blinking, and a repeating pattern of audible Morse code-like dot-dashes at a frequency of 1,300 Hz in the headset.

Inner Marker:

Similar to the outer and middle markers; located at the beginning (threshold) of the runway on some ILS approach systems (usually Category II and III) having decision heights of less than 200 feet (60 m) AGL. Triggers a flashing white light on the same marker beacon receiver used for the outer and middle markers; also a series of audio tone 'dots' at a frequency of 3,000 Hz in the headset.

RADAR (Radio Detection and Ranging)

Radar is something that is in use all around us, although it is normally invisible. 

• Air traffic controllers radar to track planes both on the ground and in the air, and also to guide planes in for smooth landings.
•  Police use radar to detect the speed of passing motorists. 
• NASA uses radar to map the Earth and other planets, to track satellites and space debris and to help with things like docking and manoeuvring.
• The military uses it to detect the enemy and to guide weapon.

So in this article i would discuss;

• What's RADAR
• Its basic principle
• And Its two types i.e PSR and SSR.

So, What is RADAR? Radar is an acronym for Radio Detecting And Ranging. The name itself suggests that the radars are used to detect the presence of object and determine its range, i.e distance and bearing, using radio frequency waves.

Radars are being used to measure different parameters

1.      Range                    Using Pulse Delay

2.      Velocity                  From Doppler Frequency Shift

3.      Angular Direction     Using Antenna Pointing

4.      Target Size              From magnitude of reflected energy

5.      Target Shape           Analysing reflected signal as a function of direction

6.      Moving Parts            Analysing modulation of the reflected signal

Cost and complexity of radar is dependent upon the number of functions it performs. Radars are used for various applications like Surveillance, imaging, remote sensing, altitude measurement, etc.

BASIC PRINCIPLE OF RADAR

Basic principle governing the functionality of radar is due to the properties of radiated electromagnetic energy.

·         The electromagnetic energy travels through space in a straight line, at a constant speed (approximately the speed of light). The propagation of these waves differs slightly because of atmospheric effects.

·         When the electromagnetic waves strike an electrically conductive surface, a part the energy is reflected back towards the source, rest of the reflected energy gets radiated in different directions.

·         Receipt of reflected energy towards the source is an indication of the obstacle in the direction of propagation.

These basic principles are utilised in Radar to determine distance, and bearings of the target, i.e., a reflecting object.

The block diagram of a primary Radar is shown below:

·         Transmitter

The radar transmitter produces microwave signal, which is typically short duration high-power RF- pulses of energy for a pulsed radar.

·         Duplexer

Duplexer acts as a switch; it switches the antenna between the transmitter and receiver. This obviates the need for separate transmitting and receiving antennas. Duplexer prevents high power energy to go into receiver (high power pulses can damage the receiver) while transmission and prevents reflected signal to be fed to the transmitter during reception.

·         Antenna

The transmitting antenna radiates the transmitting energy to signals in space, in desired directions.

The radiated energy propagates with constant velocity. When it finds the target, the energy is scattered, a part of which is reflected towards the transmitting antenna. The antenna receives the reflected energy and feed it to the duplexer. The duplexer directs this energy towards the receiver.

·         Receiver

The receiver demodulates the received reflected energy and analyses the signal to find target parameters.

·         Display

The receiver sends the output to display, which shows the analysed signal in an easily understandable user friendly manner.

The two general types of radars are 

•         Primary Surveillance Radar (PSR) and
•         Secondary Surveillance Radar (SSR).

Primary surveillance radar (PSR)

Primary radar is what you conventionally think of as radar, a transmitter sends out pulses and tries to detect the reflected pulse off an object, such as an airplane.

A phenomena of signals correlation has been used widely in primary radars, a radio pulse is send through the antenna, which provides the bearing of the aircraft from the ground station. Time taken for the pulse to reach the target and return provides a measure of the distance of the target from the ground station. This measured distance now displayed on the screen or meters of ATC air traffic controllers.

Secondary surveillance radar (SSR)^[1] 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.

International Civil Aviation Organization (ICAO)

The International Civil Aviation Organization (ICAO) is a UN specialized agency, created in 1944 upon the signing of the Convention on International Civil Aviation (Chicago Convention).

Its realy important to have the over view of ICAO because its the only council who adopt standards and recommended practices concerning:

• air navigation,
• its infrastructure,
• flight inspection,
• prevention of unlawful interference,
• And facilitation of border-crossing procedures for international civil aviation.

ICAO basically defines the bridge between different countries, to join or link them together by standardize the procedure for civil aviation globally.

ICAO also defines the protocols for air accident investigation followed by transport safety authorities in countries signatory to the Convention on International Civil Aviation (Chicago Convention).

So only those stuff will be discussed here who is defined and Accepted by ICAO.