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Radar and Navigational Aids

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Navigational Aids

By

K. M. Vyas

DIET, Rajkot

Introduction

• Navigation : The art of directing the movements of a craft (object) from one point to another along a desired path is called navigation.

• Aids of navigation : • Compass

• Chronometer

• Sextant

• The Sun, The Moon, The Stars & The Winds

• The Theodolite & Charts (Maps of known world)

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Introduction

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The Compass

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Introduction

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The Chronometer

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Introduction

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The Sextant

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Introduction

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The Theodolite

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Introduction

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• Magellan circumnavigated the Globe in the early sixteenth century with the aid of listed instruments.

• In eighteenth century the Chronometer, a very accurate clock, was produced.

• With the chronometer the navigator was able to determine his longitude by noting the transit time.

• Navigation became science as well as art. • In twentieth century, electronics entered

the field. 7

Introduction

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• Time signals were broadcast by which the Chronometers could be corrected

• Direction finders and other navigational aids which enable the navigator to obtain a fix using entirely electronic aids were developed and came into extensive use

• Our aim is to study about all navigational aids which employ electronics in some way

• To start with a brief account of other methods of navigation 8

Four Methods of Navigation

• Navigation requires the determination of the position of the craft & the direction in which it has to go to reach desired destination

• The currently used methods of navigation may be divided into four classes :

1. Navigation by Pilotage (or Visual Contact)

2. Celestial or Astronomical Navigation

3. Navigation by dead-reckoning

4. Radio Navigation

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Navigation by Pilotage

• In this method, the navigator fixes his position on a map by observing known visible landmarks

• For e.g., in air navigation when the ground is visible the navigator can see the principal features on the ground such as rivers, coastlines, hills etc. and thereby fix his position

• Even at night, light beacons, cities and towns provide information about position of the craft

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• Pilotage navigation requires good visibility

• With aid of air-borne radar it is called as Electronic-Pilotage

• The radar used for this purpose is microwave search radar provided with PPI display on which the terrain is mapped

• The PPI picture has poor resolution compared to human eye because the angular resolution is typically 3°

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• Electronic-Pilotage has the range of 50 to 100kms that is advantageous in poor visibility.

• Can not applicable over sea.

• Both methods of Pilotage depend upon the availability of accurate maps of the terrain.

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Celestial Navigation

• Also called as astronomical navigation is accomplished by measuring the angular position of celestial bodies.

• Almanacs giving the position of celestial bodies at various times measured in terms of GMT

• The navigator measures the elevation of celestial body with a sextant and notes the precise time at which the measurement is made with a chronometer

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• These two measurements are enough to fix the position of the craft on a circle on the face of the globe

• If two such observations are made, the position or fix of the craft can be identified as one of the two points of intersections of the circles

• Sometimes the 3rd observation may have to be made to remove the ambiguity

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• Its advantage is relative independence of external aids

• Its disadvantage is that the visibility should be good enough to take elevation angles of bodies

• This may not be always possible at sea, but in air navigation, with modern aircraft flying at altitudes above 5000 m. visibility is always good

• The accuracy is totally dependent on measured elevation of the body and generally correct to 1 min. of arc

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Navigation by Dead-Reckoning

• In this method, the position of craft at any instant of time is calculated from the previously determined position, the speed of its motion w.r.t. Earth along with the direction of its motion and the time elapsed

• Abbreviated as DR stands for “Deduced Calculation”

• This is the most common and widely used method of navigation

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Coastal Navigation With Dead Reckoning


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• The navigator plots their 9am position, indicated by the triangle, and, using their course and speed, estimates their own position at 9:30am and 10am


• This method requires the direction of motion of the craft and speed of motion

• First requirement may be met by magnetic compass & second by an instrument such as air speed indicator in aircraft and the mechanical log in ships

• DR Navigation would be straight forward if the medium in which the craft travels is stationary

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• In air navigation, wind velocity is generally obtained in the course of flight from weather broadcasts or by communication with ground station

• In long flights over water, modern air operations resort to minimal flight paths i.e. the paths which requires min. flying time

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Radio Navigation

• This method is based on Electromagnetic waves to find the position of the craft

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DECCA navigation system

Operates in LF band.

Between 70 to 120 kHz.

Uses unmodulated continuous waves.

In DECCA navigation system the fix is obtained by measuring the phase difference between the signals of the two stations which is phase locked.

DECCA chain consists of 4 stations, 1 master & 3 slaves.

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The master station is at the centre & three slaves at the corners of a triangle.

This arrangement gives the three sets of hyperbolic position lines, one set corresponding to the master & each slave.

Fix is obtained over a considerable area by the intersection of two hyperbolic lines.

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In DECCA system each transmitter has different frequency so the radiation from each station will differentiate by the receiver.

Generally harmonically related frequencies radiated by each transmitters & phase measurements done at common harmonic frequency which is obtained at the receiver by using multiplying circuits.

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Master Station Slave Common Harmonic

6f (Red) 8f 24f

6f (Purple) 5f 30f

6f (Green) 9f 18f

The slave stations are distinguished by the colours which are used on the charts for the hyperbolic lines which they generate with the master station.

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5 DECCA Receiver

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Normal Transmission & Lane Identification

DECCA NAVIGATION SYSTEM

• Decca navigation is a low-frequency hyperbolic navigation system that compares the phase difference of radio signals emitted by several radio stations.

• This method of navigation was used in aeromagnetic surveying, before the advent of Global Position System, over water and ice where visual flight path recovery methods were not reliable. K

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DECCA NAVIGATION SYSTEM

• Receivers identified which hyperbola they were on and a position could be plotted at the intersection of the hyperbola from different patterns, usually by using the pair with the angle of cut closest to orthogonal as possible.

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PRINCIPLES OF OPERATION

• The Master station provided the 'master' signal which was used by its associated Slave stations to derive signal frequency and timing sequences. Loss of a Master would disable a station, while loss of a Slave would reduce accuracy.

• DECCA operates by measuring the phase differences between continuous signals from a master and slave stations.

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• When two stations transmit at the same PHASE LOCKED FREQUENCY, the difference in phase between the two signals is constant along a hyperbolic path.

• Of course, if two stations transmit on the same frequency, it is practically impossible for the receiver to separate them; so instead of all stations transmitting at the same frequency, each chain was allocated a nominal frequency, 1f, and each station in the chain transmitted at a harmonic of this base frequency, as follows: STATION HARMONIC FREQUENCY

MASTER 6F 84 - 86 kHz

PURPLE SLAVE 5F 70 - 72 kHz

RED SLAVE 8F 112 - 115 kHz

GREEN SLAVE 9F 126 - 129 kHz

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• The interval between two adjacent hyperbolas on which the signals are in phase was called a lane. Since the wavelength of the common frequency was small compared with the distance between the Master and Slave stations there were many possible lines of position for a given phase difference, and so a unique position could not be arrived at by this method.

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DECOMETERS

• The detected phase differences are displayed on phase meters called 'decometers', and the readings may be plotted onto Decca lattice charts, on which the lines of position are numbered in the same units as those shown on the decometers.

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DECOMETERS

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DECCA RECEPTION

The transmissions from the chain are received by a special ship borne receiver, which measures the difference in phase of signals arriving from master and slaves.

Each slave station is fitted with equipment which receives the master signal, converts it to the slave frequency, and uses it to control the drive oscillator of the slave transmitter. Thus a constant phase relationship is maintained. To ensure that this relationship is maintained accurately, a monitoring station checks the transmissions.

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DECCA RECEPTION

MULTIPULSE

Multipulse provided an automatic method of lane and zone identification by using the same phase comparison techniques described above on lower frequency signals.

The nominally continuous wave transmissions were in fact divided into a 20 second cycle, with each station in turn simultaneously transmitting all four Decca frequencies (5f, 6f, 8f and 9f) in a phase-coherent relationship for a brief period of 0.45 seconds each cycle.

This transmission allows the receiver to extract the 1f frequency and so to identify which lane the receiver was in (to a resolution of a zone).

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RANGE AND ACCURACY

During daylight ranges of around 400 nautical miles (740 km) could be obtained, reducing at night to 200 to 250 nautical miles (460 km), depending on propagation conditions.

The accuracy depended on:

- Width of the lanes

- Angle of cut of the hyperbolic lines of position

- Instrumental errors

- Propagation errors (e.g. SKYWAVE)

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RANGE AND ACCURACY

By day these errors could range from a few meters on the baseline up to a nautical mile at the edge of coverage. At night, skywave errors were greater and on receivers without multipulse capabilities it was not unusual for the position to jump a lane, sometimes without the navigator knowing.

Although in the days of differential GPS this range and accuracy may appear poor, in its day the Decca system was one of the few, if not the only, position fixing system available to many mariners. Since the need for an accurate position is less when the vessel is further from land, the reduced accuracy at long ranges was not a great problem.

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APPLICATIONS

A more accurate system named Hi-Fix was developed using signalling in the 1.6 MHz range. It was used for specialised applications such as precision measurements involved with oil-drilling, etc. Other systems were used in the Middle East.

An interesting characteristic discovered on BOAC, later British Airways, test flights to Moscow, was that the carrier switching could not be detected even though the carrier could be received with sufficient strength to provide navigation. Such testing, involving civilian aircraft, is quite common and may well not be in the knowledge of a pilot.

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APPLICATIONS

The 'low frequency' signalling of the Decca system also permitted its use on submarines. One 'enhancement' of the Decca system was to offer the potential of keying the signal, using Morse code, to signal the onset of nuclear war. This was never optioned by the UK government. Messages were clandestinely sent, however, between Decca stations thereby bypassing international telephone calls, especially in non-UK chains.

A long range trans North Atlantic system was in operation from the mid nineteen fifties. It was called DECTRA. It utilised two stations in Newfoundland and two in Scotland. The transmissions used normal "pattern" transmitters of a much higher power than on standard DECCA frequencies. It was intended as an air navigational aid.

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Distance Measuring Equipment(DME)

Both DME & TACAN(TACtical Air Navigation) are secondary radar systems.

DME was developed by civil aviation authorities

Provides only distance information (as a part of TACAN)

Interrogator transmits rf pulses periodically at frequency f1 these are received by the receiver of the transponder, amplified, demodulated & made to trigger the transmitter after a small delay.

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Consists of pulse transmitter & receiver

Interrogator carried in the craft & a pulse receiver transmitter receiver system called as transponder at ground station

The frequency of the receiver is say f2 is transmitted by transponder & receiver of the craft is tuned at f2 which receives the signal measure the delay to obtain the distance from the transponder

Both systems works on this principle but some modifications are introduced to overcome the limitation of the basic system.

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Limitations

Number of aircrafts interrogating the transponder in the same channel

Time based trigger needed to isolate

Requirement of AGC decreases the sensitivity

Solution

Overcome by operating transponder at constant pulse rate independent of number of aircraft & it is called as ‘constant duty cycle operation’

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Doppler Navigation

Self contained system as the Dead-Reckoning system

It employs Doppler Effect to determine the velocity of the craft w.r.t. true or magnetic North

The complete Doppler navigation equipment generally includes a computer which computes navigational data required

Only used in aircrafts

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Doppler Effect

Doppler radar directs a beam of electromagnetic waves towards the earth

Some of the energy re-radiated by the earth towards the aircraft is received & compared

When the aircraft has a component of the velocity in the direction of the beam, the difference frequency called Doppler shift is nearly proportional to the velocity component. This is Doppler Effect

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Beam Configuration

Consider an aircraft flying over the earth, transmitting EM waves in a narrow beam making an angle Ф with the horizontal

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Beam Configuration

If the aircraft is in level flight & the beam is directed in the vertical plane containing the forward velocity V of the craft the component of the velocity in the direction of the beam is V cosФ & the Doppler shift is 2 V cos Ф

𝝀

But this is only one component of the shift is obtain in general three component is needed

Some of the configurations is shown in figure below

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Beam Configuration

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Beam Configuration

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Track Stabilization

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Components of DNS

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Pulsed Doppler system

Pulsed Doppler radar may be one of the two types

Incoherent type

Coherent type

In incoherent operation the phase of radiation will change from pulse to pulse

To obtain Doppler shift the pulses received from two opposite beams, which arrives at same time, are compared

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Incoherent Pulsed Doppler system

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Generally four beam configuration is used

Pulsed magnetron is used as the transmitter & this is switched to the beam pairs(1-3, 2-4) sequentially

A duplexer is used to permit common antenna for transmission & reception

Received signals is applied to a super-heterodyne receiver, the output of this is Doppler frequency signal

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AFC of the local oscillator is necessary at these frequencies therefore a sample of transmitted signal is taken from a directional coupler & applied to the AFC circuit

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Coherent Pulsed Doppler system

Compare to incoherent in Coherent Doppler radar system employs a continuous wave oscillator & a pulsed power amplifier

Relatively low frequency generated by a quartz-crystal oscillator & stepped up by a chain of multipliers using step-recovery diodes or varactors

The local oscillator frequency is generated by heterodyning the oscillations at the transmission frequency with an oscillator at the intermediate frequency

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The output of mixer is centred at the IF

The mixer & IF amplifier are followed by a coherent detector to which the other input is a reference frequency voltage

Reference frequency is obtained by mixing the IF with an offset oscillator output & taking the difference frequency output

By setting the offset oscillator both positive & negative Doppler shifts are obtained

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This system is capable to detect sense of the velocity as well as the vertical velocity

The only disadvantage of the coherent pulsed system is its greater complexity

Another problem occurs at lower altitudes is that the transmitted pulse is received back before the next pulse is transmitted

Solved by setting PRF

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Continuous Wave Doppler Radar

Separate transmitting & receiving antennas are required for preventing the transmitter output from entering the receiver

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The Doppler difference frequency is obtained by direct heterodyning of the transmitted & received signals

This is equivalent to having an IF of zero & is called homodyne reception

The difference signal is amplified in an audio amplifier & applied to frequency tracker

In homodyne operation the sense of the velocity can not be obtained

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Suffers from reflection from nearby objects, turbulent air, precipitation

Generally used fixed antenna

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FM-CW Doppler Radar

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FM-CW Doppler Radar

Uses common antenna for Tx & Rx

The received signal is mixed with sample of transmitted signal in balanced mixer & desired side band is filtered & applied to coherent mixer

The output of filter is mixed with the nth harmonic of the FM oscillator in the coherent mixer output of which will be difference frequency

After amplification the signal is fed to frequency tracker

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FM-CW Doppler Radar

Sometimes uses separate antenna for Tx & Rx

The sense information may be obtained

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Frequency Trackers

Locates the centre of the noise-like Doppler spectrum & gives pure signal of frequency

Various configurations but most of them employ a tracking oscillator

In this the spectrum is compared with local oscillator frequency & error signal is generated

According to the error signal oscillator is driven & correct frequency tuned

Other is two filter tracker

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Frequency Trackers

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Frequency Trackers

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In this arrangement single filter is used but the oscillator frequency is switched by a square wave & takes on alternately two values which are separated by a spectrum width

Oscillator output is mixed with the Doppler signal & pass through low pass filter & envelope detector

The output of the filter is square wave & applied to the phase detector where two signals compared & error signal is generated

Frequency Trackers

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This is integrated & applied to the voltage controlled oscillator to change its mean frequency

The zero error condition indicates the frequency of the Doppler signal

Performance of the all types of frequency trackers are same

Selection depends upon the complexity & signal to noise ratio

Accuracy of DNS

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The overall accuracy depends upon ground speed measurement & heading accuracy

Computational errors if Analog computers are used

0.25% may be achieved if negligible computational error

Ground Controlled Approach System (GCA)

High precision radar system sited near the airport runway.

With the help of this system controller on the ground can bring the aircraft into approach zone & then guide it along the path.

The system consists of two radars one called surveillance radar element (SRE) & other called as precision approach radar (PAR).

SRE is a search radar with a PPI display.

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Surveillance Radar Element As the SRE is not an essential part of the

approach system.

The following data relating to an early version of SRE may however be noted.

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Precision Approach Radar This precision radar has a maximum range of

about 15-20 km & scans the approach zone both in azimuth & elevation.

The precise performance & display details depend to some extent on the manufacturer of the equipment.

Radar has to scan a 20° azimuth sector & a 7° elevation sector to meet the operational requirements.

For the accuracy we have two separate antennas are used one for azimuth & other for elevation scanning.

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Precision Approach Radar By setting the power we can control the angle

of scanning.

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Precision Approach Radar Also the position of the PAR w.r.t runway is

shown in the figure below.

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Precision Approach Radar PAR precision depends upon precise

determination of the beam position.

The PAR uses the single radar transmitter which is connected alternately to the two antennas so two scans are interlaced.

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Precision Approach Radar For large coverage large antenna is used (13 ft

* 1.625 ft).

Two types of PAR used fixed and movable.

For movable PAR antenna should be lighter than the fixed one.

The data obtained by the PAR are displayed on two CRTs one displaying range & elevation angle & other displaying range & azimuth angle.

The accuracy of PAR is such that at a distance of 1 mile it is possible to detect deviations of glide-slope as little as 8m.

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Microwave Landing System(MLS) The later development in landing systems is

the microwave landing system.

Operates in the range of 5031 to 5090 MHz.

Developed to overcome the disadvantages of the ILS particularly in busy airports where ground conditions are unfavourable for the operation of the ILS.

ILS is site sensitive, further it operates in VHF & UHF bands where surrounding terrain plays an important role in shaping the beam.

Also it provides only 40 channels.

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Microwave Landing System(MLS) The MLS can accommodate 200 channels.

Because of small wavelength the antennas are small & they can be designed to relatively free from the effect of the surrounding area.

In MLS horizontal guiding equipment produces a beam narrow in the horizontal plane & wide in vertical plane which is swept rapidly about a vertical axis from side to side.

While vertical guiding equipment produces a beam narrow in the vertical plane & wide in horizontal plane is similarly rapidly scanned about a horizontal axis. This is located at the end of the runway near the touch down point.

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Microwave Landing System(MLS)

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In addition a distance measuring equipment is provided. This is located near the horizontal guiding equipment at the far end of the runway.

This equipment gives the distance of aircraft form touchdown point.

The beam scans at a uniform rate from +40° w.r.t centre line of the runway to -40° & back to +40°.

The first is called as TO scan & second is called as FRO scan.


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The air craft picks up the signals as the beam scans past it.

If each scan takes T secs an aircraft on the centre line of the runway picks up beam twice at T/2 & 3T/2.

The MLS being an all-weather landing system which gives the guidance at the most critical phase of the flight i.e. approach & landing.

The MLS is planned to replace the ILS in stages in course of time.


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Satellite Navigation Systems

Using artificial satellite for navigation

Idea was given by the American scientists in 1957 after the launch of Soviet satellite Sputnik

They carefully measuring the Doppler shift of transmissions of the satellite

Later it was realised that reverse was also true i.e. if the position of the satellite is known the receiver could measure the Doppler shift & determine his position

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Satellite Navigation Systems

First used by the US Navy for the navigation of their missile carrying ships

The Navy Navigation Satellite System (NNSS) also known as “Transit System” came into operation in 1967

Was released for general use in 1977

Suitable for low speed vehicles

Uses six satellite & some spares in polar orbit

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Navstar Global Positioning System(GPS)

Developed by Department of Defence (DoD)

Used by both military & general users with slightly reduced accuracy who equipped with the GPS receiver processes signal broadcast by the satellites

Continuously provides global coverage

The fully operational system consists of 21 satellites, with 3 spares in semi-geosynchronous circular orbits at height of 20200 km with corresponding period of 12 hours

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Navstar Global Positioning System(GPS)

Six orbital plane each inclined 55° to the equatorial plane

All satellites carry highly stable Cesium & Rubidium atomic clocks which is synchronised with GPS time defined by DoD

Continuously broadcast their identity code, ephemeris constants from which their current position can be determined, their health status & almanac constant from which the approx. position of all satellite can be determine

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NGPS

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Principle of operation

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Hyperbolic Navigation Systems


Chapter – 10

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Hyperbolic Navigation Systems

Hyperbolic systems are based on the measurement of the difference in the time of reaching of EM waves from the two or more transmitters to the receiver in the craft.

Four types: LORAN, DECCA, Consol & Omega.

LORAN: Long Range Navigational aid.

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Principle of Hyperbolic Electronic Systems


The name arises from the fact that the locus of points which have a constant value of such a delay is a hyperbola on a plane surface.

In other words when you know the difference in your distance from two objects, you know you are on a curved line defined by that difference.

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In hyperbolic navigation system receiver measures the difference in time of received radio waves & this time difference converted into distance difference by knowing the speed of EM waves.

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LORAN-A


Less accurate system. LORAN-Long Range Navigational Aid Operating in 1750 – 1950 KHz. Used before LORAN-C. OPERATION: System required at least 3 transmitting

stations for each chain & observer used a special Loran receiver.

A chain consist one master & two slave stations.

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LORAN-A


The difference in arrival of two time pulses from a pair of stations are measured & displayed on CRT.

Each fix required two observations and took 5 minutes.

Then readings transposed(plotted) on Loran lattice chart & position could be plotted.

LORAN-A signals were pulsed not continuous transmission.

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LORAN-A


So relatively small transmitter could achieve high peak power levels.

Maximum reliable range for Loran-A was 700 miles by day and 1400 miles at night.

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Signal Characteristics


Each transmission pulse lasted about 40 µsec & reoccurred at regular accurately controlled intervals.

This interval, called the Pulse Repetition Interval (PRI) varied fro each station and lasted between 29,000 and 40,000 µsec

These pulses provided precise index marks for use in time measurements.

The transmission of corresponding master and slave, plus one-half the PRI 10

Signal Characteristics Pulse & additional small time called the

‘coding delay’.

It should be noted that the observer is interested only in measuring the difference between the time of arrival of two pulses, and not the actual time taken for each pulse to reach the receiver.

Therefore, for an absolute synchronization of the receiver time based with transmitter.


Signal Characteristics At all points in the coverage area, the

time interval between a master pulse and the next slave pulse was greater than the interval between a slave pulse and the next master pulse.

That methodology provided a positive method of identifying the signals arriving room each station, even though their actual appearance was similar.



In the measuring process, the time difference was always measured from the master pulse to the slave pulse, and the time delay of one half of the pulse recurrence interval was automatically removed.


LORAN-A Equipment LORAN transmitters have a peak

power of 100KW which fed to quarter-wavelength antenna.

Repetition rates of the pulses are accurately controlled by the crystal clocks.

Master station operates independently & transmits the pulses of required periodicity.


LORAN-A Equipment The slave station also provided with

clock to maintain the repetition rate but the timing of this controlled manually or semi- automatically to maintain fixed delay.

LORAN employs three basic repetition rates 20Hz, 25Hz & 11Hz.

Each basic rate is sub divided into eight frequencies.


LORAN-A Equipment Thus on any single carrier frequency

there can be 3*8 = 24 channels.

There are 4 carrier frequencies so, 24*4 = 96 channels.

The Loran receiver is a conventional super heterodyne receiver with one special feature i.e. the gain of the receiver in two parts of the time base are different & their relative values can be controlled manually.


LORAN-A Equipment

This permits the equalization of the A & B pulses to provide matching.

The bandwidth of the receiver is 40kHz.

Both this bandwidth and the shape of the transmitted pulses are carefully controlled so that the received pulses are of the same shape and in the process of finding the delay between A and B pulses.


LORAN-A Equipment

Thus though the pulse widths are nominally 40µsec, the error in the measurement of delay can be brought down to 1µsec.

As the basic principle of Loran measurement is to measure a delay between A and B pulses.

This is done by Loran indicator.

In the earlier indicators, the time bases were controlled by crystals.


LORAN-A Equipment

This whole process took considerable time so in modern receiver the oscilloscope is still used to bring the pulses into coincidence by a delay control but the reading is obtained from an electronic counter which indicates the time difference in 3 decades.


Range & Precision of standard Loran

As Loran operates in upper MF band, both ground wave and sky wave receptions are possible.

Ground wave reception is operative mainly in the day and is particularly good over the sea.

At night, both ground and sky wave receptions are possible.

For navigation, at least two stations are required for coverage.



The area of coverage is dependent both on the maximum range of the station and the distance between the stations.

The service area depends on the latitude of the region, the season of the year and the time of the day.

The average range for ground waves over the sea in the temperate latitudes is about 600Kms and in equatorial regions about 500Kms.



The range over land is considerably less about a half or third compare to sea.

Sky wave ranges, which are the same over land & sea, may be appreciably higher.

The accuracy of the Loran system is dependent on which time interval measurement can be made & it is also depends on signal strength.



Other factor is accuracy with which ground stations are synchronized.

After achieving these two factors the probable error is 1.5 to 2 µsec in measurement of time interval.

Error may introduced by the sky wave propagation.

As the path taken by the sky wave is longer one via the ionosphere.



Signal received via ionosphere is the delayed signal & the shape of the signal also changes so it is difficult to match.

In practice generally the observer take more readings to restrict the errors up to 3µsec.



Now consider the t is measured interval then Δt is the probable error, than the vehicle may be between two hyperbola of t + Δt & t – Δt.

If the simultaneous measurement of the interval between two station gives delays t1 & t2 uncertainty Δt1 & Δt2.

Now the position of the vehicle will be within a quadrilateral area bounded by the two hyperbola of each transmitter.


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LORAN-C

Operates in the band 90-110 kHz.

It is developed after world war-2.

As the disadvantage of previous Loran was removed i.e. the low coverage area with 2MHz.

With low frequency the ground wave transmission is very much larger & attenuation is nearly same over land & sea.


LORAN-C

As the larger ground wave range permits the grater separation between master & slave transmitters.

Range of Loran-C is several hundred Kilometres.

With the limited bandwidth available for Loran-C, the pulse width has to be very long & this would reduce the accuracy but it is still higher than Loran-A system.


LORAN-C

This is achieved by using caesium atomic clock for generation of carrier signals of the transmitters.

If synchronization is proper than pulse matching is easy & less errors occurred in the transmission.

Compared to Loran-A Loran-C has 0.1µsec.

Peak power is 1 MW.

In Loran-C transmission envelope is used.


LORAN-C

This envelope consists of eight pulses for the slave transmitters & nine pulses for master transmitters.

Ninth pulse transmitted by the master station is used for coding to indicate malfunction in any station.

To measure the time delay peak of the third pulse is used so noise is avoided.

Loran-C has range of 3500km over sea & 2200km over land.


Basics of Landing When visibility is good, whether in the day or at

night, this operation is carried out by visual observation of the ground & landing lights.

The landing is then performed under ‘Visual Flight Rules’ (VFR) conditions.

Usually this is taken to indicate a horizontal visibility of 5 km or more & vertical visibility of 300 m. when these conditions are not satisfied, the landing is under ‘Instrument Flight Rules’ (IFR) conditions.

Special arrangements are provided at airports to enable the aircraft to execute landings under bad visibility.

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Basics of Landing So we have to provide information about its

exact position in relation to desired path & horizontal & vertical positions.

Two types:

ILS (Instrument Landing System) MLS (Microwave Landing System)

Ground Controlled Approach (GCA)

GCA does not required any special navigational equipment only a communication set is needed in the aircraft.

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Instrument Landing System The instrument landing system (ILS) comprises

the units localizers, glide path (or glide slope) & marker beacons.

The localizer defines a vertical equi - signal plane which passes over the centre line of the runway & the glide-slope.

Three marker beacons are also installed at certain specified distances from the end of runway.

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Localizer The localizer operates in the VHF band (108-

110 MHz) & consists of a transmitter with an antenna system.

The radiation from the antenna system has two lobes one with a modulation of 90Hz & other with a modulation of 150Hz.

The two signals are equal hence both the lobes are symmetrical to the runway.

The antenna array by means of which this pattern is obtained consists seven or eight elements.

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Localizer

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http://upload.wikimedia.org/wikipedia/en/7/79/ILS_illustration.jpg

Localizer Loop antenna is used in localizer and this

entire antenna system is placed at the centre line of the runway and about a 300 m from the end of the runway.

Total of 7 or 8 loops divided in formation of 3-1-3 or 3-2-3 the centre loop fed with carrier of 90Hz & 150Hz modulated wave.

While one side loop is fed with side-band of 90Hz & 150Hz.

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Localizer

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Glide-Slope System The principle of operation of the glide-slope

currently in use, called null-type glide-slope is very similar to the localizer.

This system operates in the band 339.3 -335 MHz band employs two antennas having a polar diagram as shown in figure.

Here the larger lobe represents the radiation from the lower antenna & transmit the carrier & smaller lobes represents the radiation from the top antenna & having only side-band frequencies.

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Glide-Slope System

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Glide-Slope System If the aircraft flies along the null, it receives

the signal of lower antenna only & the two modulations are equal, giving an equi-signal course.

The glide-slope equipment & antenna have to be sited away from the runway so that they do not constitute a hazard.

The modifier array is used here for error correction so the pilot can easily make out the correct course.

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Receiving Equipment The receiver is typically a crystal controlled

multi-channel receiver.

Separate receiver are required for the localizer & the glide-slope because they operate in widely different bands.

Receivers having very efficient automatic gain control this keeps the output of the receiver constant when the input varies from 20µV to 100000µV.

So the meter indication is perfect & ensure the correct path finding.

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Receiving Equipment It is important, both in the localizer & the

glide-slope, that the courses are maintained correctly & the modulation levels preserved.

To achieve this we have to detect the signal strength for this dipole antennas are fixed at certain specified points on & off course.

Now the received signal is processed to find the modulation components & monitor the course alignment, width & clearance.

If the certain condition is not satisfied then it enables the alarm circuit.

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Receiving Equipment

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Receiving Equipment The output of receiver is applied to two filters

which separate the 90Hz & 150Hz signals, each of which is rectified by a bridge rectifier.

The outputs of rectifier are connected so as to give the difference between the rectified voltages & this is applied to the indicator coil.

R1 is used to compensate for different losses in the two rectifiers & filters. Voltage across R3 is applied to a coil which operates the ‘flag alarm’.

Thermistor is used for temperature compensation.

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Receiving Equipment The indicator shown in figure consists of a

meter with two centre-zero movements.

Horizontal needle indicates deviation from the glide-path & the vertical needle indicates deviation from the localizer.

FSD of the meter typically set at 150µA.

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Course Sharpness & Width The sharpness & width of the course are

dependent on the relative depths of modulation of the 90Hz & 150Hz signals.

The total signal modulation is defined by the relation : M=(A+B)/C where M is total signal modulation, A & B are the amplitudes of the 150Hz & 90Hz signals respectively & C is the carrier amplitude.

The difference in the depth of modulation (ddm) of the two signals is given by (A-B)/C.

Now the meter indicates when current is pass through the coil. If equi-signal course followed by the aircraft then indication is null.

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Course Sharpness & Width The sharpness of the course is derived on the

basis of ratio of the 90Hz & 150Hz side band signals of both localizer & glide-slope.

This ratio generally measured in dB. This measure is called the ‘clearance’.

In the localizer the clearance at +/- 1.5° off the course is called ‘course sharpness’. K

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Site effects in the ILS The localizer & glide-slope courses are

affected by the nature of the site on which they are installed.

The terrain type introduce error in equi-signal course.

For the type of terrain if we restrict the radiation it would difficult for aircraft to achieve the right path.

The power of radiation & site conditions must taken into account while designing the system so capture effect can be avoided.

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Marker Beacons The ILS employs three marker beacons.

It gives an indication in the aircraft when it passes over them.

All of them operate at 75 MHz & work with an antenna which gives a fan-shaped beam which is typically +/- 40° wide along the approach path & +/- 80° perpendicular to it.

The outer marker(OM) is placed at 7km from the touchdown point of the runway.

The radiation is modulated at 400Hz giving two dashes/sec.

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Marker Beacons The second one called as middle marker (MM)

is placed where the glide path is 200 ft which is generally about 1km from the touch down point.

The modulation is at 1300Hz with one dash every 2/3 sec.

The last inner marker which is not used at all airports is placed where the glide-path is 100 ft above the ground.

It is modulated at 3000Hz & transmits 6 dots /sec.

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Marker Beacons In the aircraft a single receiver tuned to

75MHz is employed.

The output is available as an audio signal & also actuates three lamps one for each marker beacon.

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Radio Direction-Finding

• The earliest method of electronic navigation was by direction-finding i.e. the determination of the direction of arrival of electromagnetic waves at the receiving station

• The waves are omnidirectional so the location of the transmitter is great circle

• Oldest method so it is still used both on ships & aircraft

• If Direction finder is on craft, the determination of bearing of two or more fixed stations will give a ‘fix’

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Radio Direction-Finding

• If the direction finder on ground it will find the bearing of the craft & passes on the information to the craft by a radio communication channel

• Can use any region of the radio spectrum

• Certain frequencies are specifically allotted for navigational purpose in the LF/MF, HF & VHF bands.

• The technical features may differ but basic principles remains same

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Loop Antenna

• Mainly used at Low & Medium frequencies

• Consider a loop antenna of length a & width b vertically mounted so it can be rotate.

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3

Loop Antenna

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4

• Let vertically polarized electromagnetic wave incident on it making an angle θ with the plane of the loop.

Loop Antenna

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• Voltages are induced in the vertical members of the loop but not in horizontal members as the wave is vertically polarized

• Magnitude of the voltage induced in two vertical members is aɛ

• The voltages in the two members will not be in phase can be seen from phasor diagram

• The voltage induced in AB is lags by an angle Ф.

Loop Antenna

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Ф = 2π

λ 1

2 b cos θ

Ф = π

λ b cos θ ------------------ (1)

If the electric field at the centre of the

loop is ɛ 𝑡 = 2 ɛ cos(ω𝑡) Voltage induced in AB & CD

e1 = 2 aɛ cos (ω𝑡 - π

λ b cos θ )

e2 = 2 aɛ cos (ω𝑡 + π

λ b cos θ )

Loop Antenna

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7

OMEGA SYSTEM

OMEGA was the first truly global radio navigation system for aircraft, operated by the United States in cooperation with six partner nations.

Uses continuous waves of very low frequencies in the 10 kHz range.

The only advantage of using very low frequencies is that the coverage is increased, as the waves are propagated between the earth & D-layer of the ionosphere.

The attenuation is less at these frequencies enabling long ranges to be obtained.

Eight stations distributed over the whole world gives global coverage.

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OMEGA SYSTEM

Each station transmits a signal precisely controlled in time & frequency which is derived from a Cesium Atomic Clock.

Three frequencies of 10.2 kHz, 13.6 kHz & 11/3 kHz are transmitted on a 1sec each & the sequence is repeated every 10 seconds.

At any time instant only one station transmit any of given three frequencies while in other time slots it transmits a characteristic frequency of its own indicated by f1, f2 etc.

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OMEGA SYSTEM

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OMEGA SYSTEM

To determine the line of position each Omega station can be paired with any other Omega station.

There are no master & slaves.

All the stations constitute one chain.

The receivers are equipped with “flywheel oscillators” which lock on to the phase of received signal and thus “remember“ the phase of the signal till next transmission at that frequency.

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OMEGA SYSTEM

Thus at any time the phase of received signal at these frequencies is available and the measurement of phase difference between the signals at any of these frequencies from two stations can be maid though they are not transmitting that time.

Omega system has a relatively low accuracy this is due to variations in propagations conditions.

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