Radio and Radar: Radar

Advanced Radio And Radar

Radar

• RADAR

• RAdio Detection And Ranging.

• As World War II approached, scientists and the military were keen to find a method of detecting aircraft outside the normal range of eyes and ears. They found one, and at first called it Radio Detection Finding (RDF), followed by RADAR.

• Radar works by firing powerful radio waves towards the target, and collecting the reflected energy.

• The radar operators can then find the position of the target in terms of its range, intensity and position.

A signal is transmitted, it bounces off an object and it is later received by some type of receiver

• Radio waves and microwaves are two types of electromagnetic waves. Electromagnetic waves.

• A basic radar system is spilt up into a transmitter, switch, antenna, receiver, data recorder, processor and some sort of output display.

• Everything starts with the transmitter as it transmits a high power pulse to a switch which then directs the pulse to be transmitted out an antenna.

• Just after the antenna is finished transmitting the pulse, the switch switches control to the receiver which allows the antenna to receive echoed signals.

• Once the signals are received the switch then transfers control back to the transmitter to transmit another signal.

• The switch may toggle control between the transmitter and the receiver as much as 1000 times per second.

• Any received signals from the receiver are then sent to a data recorder for storage on a disk or tape.

• Later the data must be processed to be interpreted into something useful which would go on a display.

• There are basically two types of Radar –Primary and secondary.

• Both work on the same principle, however the purpose is different.

Primary Radar

• A Primary Radar transmits high-frequency signals which are reflected at targets.

• The arisen echoes are received and evaluated. This means, unlike secondary radar, no reply is required from the target.

Primary radar systems may be found in ground, air, ship or space platforms and are used in roles such as:

• Surveillance (including weather)

• Early warning

• Navigation

• Ground mapping (from space or aircraft)

• Guidance control

• Target detection and tracking

• Terrain following/avoidance

• Collision avoidance and altitude measurement

• Air Traffic Control

• Radars operate their high-powered radio waves in 2 different modes: pulse-modulated (pulsed) and continuous wave (CW).

• Most radars operate in the Ultra High Frequency (UHF) or Super High Frequency (SHF) bands. The Frequency of operation will depend on the function the radar is to perform,

• A pulse radar is a radar device that emits short and powerful pulses and in the silent period receives the echo signals.

• In contrast to the continuous wave radar the transmitter is turned off before the measurement is finished.

• This method is characterized by a radar pulse modulation with very short transmission pulses

• The speed of radio waves in free space, as we know is 3 x 108 ms-1 (186,000 miles per second).

• So if we measure the elapsed time between the transmission of the pulse and its reception back at the radar, we can use the formula:

• Distance = Speed x Time

• The pulses from a radar are transmitted at a rate which determines the range of the radar, called the pulse repetition frequency or PRF.

• In practice the PRF might range from 250pps for long-range radars to 2000pps for short-range radars.

• For long-range radar, to get a satisfactory return from a pulse, a massive one million watts (megawatt) of radio frequency (RF) power is required.

• This high power is used only during the brief transmission of the pulse.

• The transmitter is then allowed to rest until the next pulse and the receiver meanwhile is listening for an echo.

• Continuous Wave Radar transmit a high-frequency signal continuously.

• The echo signal is received and processed.

• The receiver need not to be mounted at the same place as the transmitter.

• Every firm civil radio transmitter can work as a radar transmitter at the same time, if a remote receiver compares the propagation times of the direct signal with the reflected one.

• Tests are known that the correct location of an airplane can be calculated from the evaluation of the signals by three different television stations.

• Continuous Wave radar sets transmit a high-frequency signal continuously.

• The echo signal is received and processed permanently.

• One has to resolve two problems with this principle:

• Prevent a direct connection of the transmitted energy into the receiver (feedback connection),

• Assign the received echoes to a time system to be able to do run time measurements.

• A direct connection of the transmitted energy into the receiver can be prevented by:

• Spatial separation of the transmitting antenna and the receiving antenna, e.g. the aim is illuminated by a strong transmitter and the receiver is located in the missile flying direction towards the aim;

• Frequency dependent separation by the Doppler-frequency during the measurement of speeds.

• There are two basic types of Continuous Wave radar. These are called Continuous Wave Doppler and frequency-modulated Continuous Wave (FMCW).

• Consider the situation where a radar equipment sends out a pulse of radio waves and then "listens" for an echo.

• If the target is moving towards the Transmitter the reflected waves become bunched up (i.e. they acquire a higher frequency), due to the target’s velocity.

• If the target is moving away, the waves are spread out and their frequency drops slightly.

• Of course, a target rarely approaches the radar installation head-on and in the diagram below, the target’s track (AC) is at an angle to its bearing from the radar (AB).

• The velocity in direction AB is less than the true target speed in direction AC.

• However, by comparing changes in Doppler shift at the radar receiver over a short period, the target’s velocity can be calculated.

• In the case of frequency-modulated Continuous Wave , the transmitters signal is made to vary in frequency in a controlled cyclic manner with respect to time, over a fixed band.

• By measuring the frequency of the returning echo it is possible to calculate the time interval elapsed since that frequency was transmitted, and thus the target’s range.

Secondary Radar

• Radar was born in the due to the pressure of war.

• The need to detect “hostile” aircraft led to a vast investment in intellect and money to develop RADAR.

• Classical Radar (now called Primary Radar) by definition is a non co-operative technology, that is it needs no co-operation from the “Target” being detected.

• Why do we need a different system then?

• It is vital to know the identity of an aircraft displayed on an air traffic controller’s screen, particularly in a military situation.

• A method of identifying aircraft was first used in the second World War and called Identification Friend or Foe (IFF).

• It is still in use today

• As well as seeing “hostile” aircraft it soon became apparent that Radar was a good tool to see “friendly” aircraft and hence control and direct them.

• If the “friendly” aircraft is fitted with a transponder (transmitting responder), then it sends a strong signal back as an “echo”.

• An active also encoded response signal which is returned to the radar set then is generated in the transponder.

• This proved very useful for the military in seeing their own aircraft clearly.

• In this response can be contained much more information, as a primary radar unit is able to acquire (E.g. an Altitude, an identification code or also any technical problems on board such as a radio contact loss ...).

• Secondary Radar relies on a piece of equipment aboard the aircraft known as a 'transponder'.

• The IFF equipment is specifically for military use, but a civilian version does exist, called Secondary Surveillance Radar (SSR).

• Both systems are compatible with the ground based interrogators which use a transmission frequency of 1030 MHz, while aircraft transponders use 1090 MHz.

• The transponder is a radio receiver and transmitter operating on the radar frequency.

• The target aircraft's transponder responds to interrogation by the ground station by transmitting a coded reply signal.

The great advantages of Secondary Surveillance Radar are:

• firstly, because the reply signal is transmitted from the aircraft it is much stronger when received at the ground station, therefore giving the possibility of much greater range and reducing the problems of signal attenuation;

• similarly, the transmitting power required of the ground station for a given range is much reduced, thus providing considerable economy;

• and thirdly, because the signals in each direction are electronically coded the possibility is offered to transmit additional information between the two stations.

• The IFF/SSR systems have been developed so that specific information can be obtained from an aircraft.

• The aircraft is interrogated on 1030 MHz using coded pulses or modes.

• Similarly, the aircraft will respond on 1090 MHz using a standard system of codes.

There are 3 modes in use and they are:

• Mode 1 Military Aircraft Identify

• Mode 2 Military Mission Identify

• Mode 3

A - Common Military/Civilian Aircraft Identify

B - Civil Identify

C - Height Encoded Data

• IFF/SSR system provide ATC authorities with a wealth of information about particular aircraft – far exceeding the amount of information gained by simply using a primary radar.

• The types of information available are:

• Aircraft height (direct from aircraft’s altimeter)

• Direction

• Speed

• Type of aircraft

• The aircraft can also send emergency information such as:

• Loss of radio communications (code 7600)

• Hijack (code 7500)

• SOS (code 7700)

The main advantages of IFF/SSR over primary radar are:

• No clutter problems (i.e. unwanted returns from rain clouds and mountains) since transmitter and receiver operate on different frequencies.

• Increased range with less transmitted power, as the radio waves only have to travel one way.

• More information from each target.

• Ability to use wide bandwidth receivers.

• Focusing radio waves into a beam requires a much more complicated aerial system than a simple straight wire.

• In order to produce a beam of radiation we need to radiate from a shaped area, and not a single wire.

• In theory, this means that for long wavelengths great areas of aerial would be needed.

• To overcome this problem, reflectors are used on the aerial to reflect the radio waves in one direction.

• The situation is very similar to the reflector in a torch or headlight focusing the light into a narrow beam.

• To detect accurate bearings of aircraft the aerial is rotated through 360°, sweeping a narrow beam of radiation in a complete circle (called scanning).

• All reflections can now be plotted around a circle – with the aerial at the centre.

• To obtain vertical information about the aircraft the aerial is moved up and down through 90° – in a type of nodding movement.

• From the reflections received, accurate height and range information can be measured.

• Whenever a single antenna is used for both transmitting and receiving, as in a radar system, an electronic switch must be used!

• Switching systems of this type are called duplexers.

• Switching the antenna between the transmit and receive modes presents one problem; ensuring that maximum use is made of the available energy is another.

• The simplest solution is to use a switch to transfer the antenna connection from the receiver to the transmitter during the transmitted pulse and back to the receiver during the echo pulse.

• No practical mechanical switches are available that can open and close in a few microseconds. Therefore, electronic switches must be used.

• Transmitter. The transmitter creates the radio wave to be sent and modulates it to form the pulse train.

• Receiver. The receiver is sensitive to the range of frequencies being transmitted and provides amplification of the returned signal.

• Power Supply. The power supply provides the electrical power for all the components.

• Synchronizer. The synchronizer coordinates the timing for range determination.

• Display. The display unit may take a variety of forms but in general is designed to present the received information to an operator.

• Duplexer. This is a switch which alternately connects the transmitter or receiver to the antenna. Its purpose is to protect the receiver from the high power output of the transmitter.

• During the transmission of an outgoing pulse, the duplexer will be aligned to the transmitter for the duration of the pulse.

• After the pulse has been sent, the duplexer will align the antenna to the receiver.

• When the next pulse is sent, the duplexer will shift back to the transmitter

• Obtaining a target is only part of the detecting process. The operator needs to "see" the target in visual form.

• For this we use a cathode ray tube (CRT) which works on a similar principle to a television screen.

• As the time interval between pulses is short the screen can be calibrated in miles to match the range of the pulse

• All CRT's have three main elements: an electron gun, a deflection system, and a screen. The electron gun provides an electron beam, which is a highly concentrated stream of electrons.

• The deflection system positions the electron beam on the screen, and the screen displays a small spot of light at the point where the electron beam strikes it.

• There are two possibilities for the deflection of the electron beam:

• the electrostatic controlled deflection and focusing, and

• the electromagnetic controlled deflection and focusing of the electron beam.

There are many factors that prevent efficient operation of a radar system, such as:• Noise – unwanted signals from radio, stars and the

atmosphere.• Interference – unwanted man-made signals such as other

radar transmitters, electrical apparatus and electrical machinery. The correct siting of a radar system can reduce the effects of some of these problems.

• Clutter – unwanted echoes from hills, buildings, sea, clouds, hail, rain and snow. These false echoes will weaken real echoes from targets, but fortunately they can be reduced by electronic techniques.

• Target characteristics – a target’s shape and composition will have an effect on its echo. Metal, for example, is a better reflector of radio waves than wood or plastic – flat surfaces are better reflectors than curves. The USA’s stealth fighters and bombers make use of the different reflecting capabilities of materials and shapes to effectively "hide" from enemy radars.

What does RADAR stand for?

• Radar Detection and Ranging

• Radiation Aircraft Ranging

• Radio Detection and Ranging

• Ranging and Direction Radio

• Radio Detection and Ranging

How many modes are there in IFF/SSR?

• 4

• 1

• 2

• 3

• 3

What does SSR stand for?

• Secondary Surveillance Radar

• Service Surveillance Radio

• Single Side Radio

• Second Sense Radar

• Secondary Surveillance Radar

What does CRT stand for?

• Cathode Radio Tube

• Cathode Ray Tube

• Cathode Radiation Test

• Capacitor Resistor Transistor

• Cathode Ray Tube