Radar and Navigation Systems

Radar and Navigation Systems

By: Najeeb Haider Zaidi

Navigation

• Navigation is the process of directing the movements of a vehicle successfully from one point to another. The vehicle may be spacecraft, aircraft, ship, submarine or a car.

Navigational Techniques

Navigational Techniques

Dead Reckoning Piloting Celestial Navigation Electronic Navigation

Position and Direction on Earth Surface

• Position• Direction• Speed• Distance

The Earth

• Poles• North and South• East and West• Great Circle• Small Circle• Equator• Meridians

Positioning on Earth

• Latitude• Longitude

EquatorPrime Meridian

Positioning on Earth

F

CE

C

F

G

Figure 1 Figure 2

H

Course and Bearing

• Direction

• Bearing

• True Bearing

• True Course

The Compass

• The navigational Compass is an instrument that gives the necessary datum line from which the course and bearing can be measured.

• There are three kinds of Compass.– Magnetic Compass– Gyro Compass– Gyro-Magnetic Compass

Magnetic Compass

• Earth behaves like a magnet with its magnetic south towards the North pole and magnetic north towards the south pole.

• So if a magnet is suspended or pivoted, its north would be attracted towards the magnetic south of the earth or towards the north pole. And vice versa.

Gyro Compass

• The spinning free wheel defends the change in the direction of its spinning axis, this property of the wheel is used to point the true north.

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Gyro-Magnetic Compass

• It uses the magnetic compass to seek the north and then uses gyro to stabilize it.

• When correct variations and deviations are set on compass console, it transmits the true north.

Use of Electromagnetic Radiations for Navigations

• Radio Devices were started more than a century ago.

• Before the invention of the radio waves, ships used classical methods of piloting, and dead reckoning by using the magnetic compass, or classical sounding techniques.

• But there were many problems with those techniques.

Direction Finding

• There are two methods of Determining the bearing line from a shore based or ship based transmitter.– Measuring the direction of propagation of the

received waves, which have followed the great circle between the transmitter and receiver.

– Listening with a normal receiver to a transmitter at fixed position radiating different signals in different directions.

Radio Direction Finders

• The use of wireless receiver and special loop antennas for direction finding had been developed by Marconi Communications by 1916.

• The secret chains of DF stations were in use tracking Surface Crafts, Submarines.

• But the development was kept secret until after the end of the war. German U-67 showing DF antenna.

Courtesy Robert Derencin (www.uboat.net)

Radio Direction Finders

• A direction finder consists of a ship board receiver and aerial system used together to determine the direction of the incoming radio wave.

• The angle between the direction and the known direction (eg. True North) is determined and the position of the transmitting station is known, it is possible to draw a line on which the ship is situated on the chart.

• The 2nd position line can be found a bearing from a 2nd stations. The point of intersection of the two lines may be assumed to be the position of the ship.

Advantages of Direction Finder

• Bearings of any transmitting station can be taken.

• Direction finders can be used for costal navigation almost everywhere in the world owing to the many special radio beacons on or near the coast.

• The Radio Direction Finders are not expensive as compared to other navigational aids.

Working Principle 1

• If a changing number of magnetic lines of forces pass through the surface enclosed by the windings of a coil, voltage is induced in the windings.

• The alternating current in a transmitting aerial also sets up lines of magnetic force, which are propagated in the speed of light.

Working Principle 2

• In order to use this rediation to determine the radio bearings from a transmitter, a loop aerial is used instead of coil

• The diameter of the loop is about 1 meter, there are number of windings inside the circular tube, so this aerial can be considered as a coil.

Working Principle 3

• When this type of aerial is located in the radiation field of the transmitter, the changing number of magnetic lines, induce alternating voltage in the coil.

• The induced voltage would be at its peak at the position as shown in the figure, and would be the minimum if aerial is rotated to 90° horizontally, since at that position no lines of force would be passing through the coil.

Working Principle 3

• Diagram below presents the planar view of the antenna. Spinning about the axis O, the circle over here shows the rotation path of the antenna.

B

C

Oα G ED

Direction Finding

• In order to find out the direction of the transmitter, the aerial is rotated and two reading are taken,

• At maximum signal strength the aerial is parallel with the transmitter and at zero signal strength the aerial is perpendicular to the transmitter.

Problem with the System

• During one cycle of the DF there are two maximum positions and one minimum position.

Solution of the Problem

• The problem can be solved easily by employing a simple aerial vertically at the center of the loop. This aerial receives equally from all directions, hence forming a circular pattern.



The Whole System

1). Receiver

2). Loudspeaker

3). Loop antenna

4). Handle

Bellini Tosi System

Principal Errors

• Errors Caused by surroundings:– Transmitters induce the alternating voltage not only in

the aerial but in its surroundings as well, in the hull, the mast of the ship etc.

– In all these conductors small alternating current will be induced, which in turn give rise to the radio waves. The phenomenon is known as reflection.

– These waves arrives at the DF aerial from different directions with different strengths and delays generating noise.

– This results in continuous generation of tone even when the aerial is perpendicular to the transmitter.

Principal Errors

• Night Effect– It is caused by the deviation of the magnetic lines of

force in the ionosphere from the horizontal, although they remain perpendicular to the direction of propagation.

– If the observer is unaware of this he will turn the search coil to a position in which reception is zero and will coincide that the transmitter is located deviated to its true direction.

– Bearings are unreliable during the night effect, which is predominant within one hour of sunrise and sunset.

Factors Effecting the Accuracy of Radio Bearings

• Presence and absence of night effect.• Distance to stations from where bearings

are taken (An error of 1° means a position error of 0.17 nautical mile for every 10 nautical miles distance from the beacon).

• Angle of intersection of bearing lines.• Quality of DF apparatus.• Accuracy with which the bearings are

taken.

Radars

Brief Historical Survey


• Pulse radar was developed from basic ideas originated by physicists who were investigating the ionosphere, particularly with a view to establish the height from which the radio waves appeared to be reflected to earth.

• Their technique was to cause a transmitter to emit a short burst of radiation, and by using a suitable aerial to direct the radiation vertically.

• The signals that returned to earth were picked up on another aerial, and the time between the emission and reception was measured, to calculate the effective height of the layer.


• Bats use a similar technique, to enable them to fly about in enclosed spaces. They emit supersonic cries and listen for the time elapsing before they hear the echo from any object in their path.

• Man has also used a sonic form of radar when, for example lost on fog at sea of a cliff bound shore, he shouts and waits for the echo from the cliffs.

• By using megaphone to increase the directivity of his voice he can gain a rough indication of the direction and the distance of the coast.


• Around 1920’s British American scientists discovered that Radio waves are reflected by objects in their path.

• As early as 1922 Marconi forecast that reflections could be obtained from targets, such as ships or aircrafts so establishing their position.


• The urge towards a practical application of the discovery came in 1930’s when a mean of protecting Britain against bomb attack was being sought.

• So, a British Scientist R.A. Watson-Watt was given the facilities to design experimental equipment on the Essex coast.

• The equipment proved a success far beyond the expectations of the designers.

• The chain of stations round the Britain was installed by 1939, which made it capable of giving warnings of the approaching aircrafts out to range in excess of 100 miles.

• To conceal the revolutionary nature of the equipment, it was first called RDF.


• The second world war revealed the immense possibilities of radar, and enabled great efforts to be expanded upon it , leading to rapid developments and improvements.

• The early radar sets utilized a wavelength of about 7 meters but to design a radar sets which could give good performance against small targets at close range such as submarine periscopes, or buoys, it was necessary to use a much shorter wavelength of around 10cms.


• Invention of multicavity magnetron permitted to development of shorter wavelength radar.

• Since 1945 the development of radar has progressed, and diverged continuously, resulting initially in naval radar sets designed for specific purposes such as – Navigation: Low Power, High Definition– Target Indication: Medium Power, Accurate Bearing– Air Warning: High Power, Long Range.

• Present day, radars are general purpose devices, there are radars for civil purposes, like treasure hunting, minesweeping, car collision warnings, etc. But their primary use is still air surveillance.

Echo Principle of the Pulsed Radar

• The range is obtained on the Echo-Ranging Principle. Seamen have long been familiar with this principle. In ships approaching cliffs in fog, it was used in the following way.– ‘A short blast was sounded on the siren and

simultaneously a stop-watch was started. After an interval the echo was heard and time noted. Since the speed of sound waves is known(335m/s) the range of the cliffs could be estimated.’

• Radar works on the same principle but uses radio waves instead.

Basic Radar Groups

Radar development along various lines resulted in certain groups

Primary Radar

• Radar Sends a signal, if it strikes a suitable target, it is reflected back to the receiver. The receiver portion of the signal, or echo, will be very small compared with the transmitted signal.

• Measurement of the time taken for the transmission Radio wave to reach the contact and reflected back enables the range if contact to be measured.

Primary Radar

• Two possible methods are used.– Pulsed Radar: A short pulse is transmitted and the

radar waits for a possible echo to be returned before transmitting the next pulse.

• Used in Surveillance Radars.

– FM CW Radar: Frequency of the Continuous transmission is altering at a known rate, and therefore comparison of outgoing and incoming frequencies give a measure of time and hence range.

• Used in Missile Radar Systems.

Secondary Radar

• In Secondary radar, the signal transmitted is used to trigger a transmission from the contact. Thus the received signal is not an echo but a transmitted signal from the contact. Like;– An Airport radar can trigger responses from aircraft

waiting to land.– Aircraft transmission can trigger responses from fixed

transmissions whose positions are accurately known, in order to aid the aircraft for its position determination.

– An Aircraft can be challenged by a warship’s secondary radar at a considerable range. A friendly aircraft should provide a known response. IFF

Basic Principle of Radar

Transmitter

Receiver

Target Detection

Range to Target

Echo Signal

Transmitted Signal

Target

Radar Range

• A pulse of microwave energy is transmitted and the range R of the target can be determined by measuring the out-and-back time, TR

• With the range in Kilometers or in nautical miles, and T in microseconds, R becomes,

2RcTR

)(081.0)(

)(15.0)(

sTnmiR

sTKmR

R

R

Maximum Unambiguous Range

• Once a signal is radiated into space by a radar, sufficient time (TP) must elapse to allow all echo signals to return to the radar before the next pulse is transmitted.

• The rate at which the pulses are transmitted, therefore, is determined by the longest range at which the targets are expected. This range is categorized as Maximum Unambiguous range. Given by;

p

Pun f

ccTR

22

Range Resolution

• The range resolution of a radar is specified in terms of minimum distance between targets that allows the targets to be seen as independent objects. It can be determined by the Pulse Duration(). By using the following formula;

2Re

cR s

Radar Range Equation

• Echoed power from the Target that the radar Receives is given by;

Where;– PR= Received Power– PT=Transmitted Pulse Power– G=Gain of Transmitted and Received Antennae (Same antenna)– R=Range of target– σ= Radar Cross Section of target-the value of RCS depends on

a number of factors, including; size and orientation of target, material of target, frequency of transmitted signal.

= Wavelength of transmitted Microwave Signal

43

22

64 R

GPP TR

Class Exercise

• The following data apply to a 3-cm pulsed radar system:

– Pulse Period=100μsec– Pulse Duration=0.8μsec– Average Pulse Power=300W– Common transmit/receive antenna with gain of 18dB

• Determine1. The maximum unambiguous range of the radar2. The range resolution3. The pulse power received from a target having a rcs of 40

sqm located at the maximum range4. The required receiver sensitivity with the Fade Margin of 10dB

Solution

• Maximum Unambiguous Range:

• Range Resolution:

• Pulse Power Received:

• Receiver Sensitivity:

Receiver Sensitivity= PR-FM=-153.7dB-10=-163.7dBW

KmcT

R Pun 15

2

100103

2

8

metersc

R s 1202

8.0103

2

8

Re

dBWWKR

GPP TR 7.153103.4

)15()14.3(64

)40()03.0()10(300

6416

43

228.1

43

22

Radar Block Diagram

WaveformGenerator

PowerAmplifier

Duplexer

LNA IFamplifier

MatchedFilter

DemodulatorVideo

amplifierThresholdDecision

LocalOsc.

Pulsemodulator

Output

Antenna

Applications of Radar

Air Traffic Control (ATC)

• Radars are employed throughout the world for the purpose of safely controlling air traffic en-route and in the vicinity of airports.

• Aircrafts and ground vehicular traffic is monitored by High resolution radars.

• Radars has been used with GCA (Ground Control Approach) systems to guide aircraft safe landing in bad weather.

• In addition to that Microwave Landing system and ATC radar beacon systems are largely based in large part on radar Technology.

Aircraft Navigation

• Weather avoidance radar used on aircraft to outline regions of precipitation to the pilot is a classical form of radar.

• Radar is also used for terrain avoidance and terrain following.

• The FM/CW or pulse radars are used as radio altimeter and Doppler navigators.

Ship Safety

• Radars are used as warning systems of potential collision with other ships.

• For the detection of navigation buoys, especially in poor visibility

• High resolution radars are used for the surveillance of harbors.

• Automatic Detection and tracking equipments employ radars systems as well mostly for collision avoidance.

Space

• Space vehicles use radars as an important part of their design.

• Ground based tracking and detection of Satellites.

• Satellite borne radars are used for remote sensing.

Remote Sensing

• Radars has been used as remote sensors of weather.

• Geographical explorations.

• Remote sensing for the Measurement and mapping of sea condition, water resources, agriculture, forestry conditions, geological formations and environmental pollution.

Law Enforcement

• Speed measurements of cars.

• Intruder detection.

• UWB radars are used to see through the walls.

Military

• Surveillance, Navigation, Control and guidance of weapons.

• Air defense.

• The largest use of radar is in this sector.

Documents

Radar and Navigation Systems