Advanced Radioand Radar

Part 6Radar Display

We know a simple aerial will radiate electromagnetic waves

equally in all directions;

it can also receive signals from all directions.

But this is of limited use when trying to determine the direction of a particular reflection.

Introduction

Instead of radiating, the radio wave needs to be concentrated

into a single beam

so that the radar can be made to "look" and "listen"

in one specific direction at a time.

Introduction

Radar AerialsIn order to produce a beam of radiationwe need to radiate from a shaped area,

not a single wire.

To overcome this problemreflectors are usedto modify the aerial’s pattern and reflect the waves in one particular direction.

Radar AerialsIn order to produce a beam of radiationwe need to radiate from a shaped area,

not a single wire.

But there are a numberof different shapes, typesand designs that we can use to produce this singlebeam of radiation.

Radar AerialsWhatever the differentshape, type or design,

there is some form of ‘reflector’ at the centre.

This is similar to the reflectorin a torch or headlight focusing the lightinto a narrow beam.

To detect bearings the aerial is rotated, sweeping a narrow beam in a complete circle.

(called Scanning).

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

Radar Aerials

To obtain vertical information the aerial is moved up and down through 90°.

From the reflections received, accurate range and bearing information

can be measured.

Radar Aerials

Radar AerialsBeam patterns for differing aerial types.

RADIATION PATTERNS EMITTED

Single WireAerial

Yagi ArrayAerial

Parabolic DishAerial

Radar DisplayObtaining a target

is only part of the detecting process.

The operator needs to "see" the target in a visual form.

For this we use a Cathode Ray Tube (CRT) which works on a similar principle

to a television screen.

CRT

Radar Display

As the time interval between the radio pulses is short

the actual CRT screen can be calibrated in miles

to match the range of the pulse.

CRT

12 10 10 12

Radar Display

The instant the pulse is transmitted a spot travels across the CRT screen

moving at a constant speed.

This is known as “Base Velocity".

CRT

Radar Display

If a target is detected a "blip" appears.

Because the screen is calibrated in miles we know the distance (range) to the target.

CRT

Radar Display

This radar,

known as ‘Type A’,

is good for determining the range of an object,

but it cannot determine the object’s bearing,

or its height.

CRT

Radar Display

To get the bearing and height

we must make some alterations

to the CRT display screen,

Starting with the bearing –

CRT

Radar Display

To find the bearing (direction)

of a target

we need to find its Azimuth (bearing measured from North).

CRT

Radar Display

By using a Plan Position Indicator (PPI)

that rotates a beam through 360° the azimuth

(bearing)of any target can be calculated.

CRT

N

W E

S

Radar Display

It is also possible

to display range and bearing

by adding Range Rings

to aid in range finding.

CRT

N

W E

S

Radar Display

The display starts

from the centre of the screen,

and produces a radial trace

moving in time with the rotation of the aerial,

and ‘blipping’ the target contact.

CRT

N

W E

S

Other RangesHaving determined both range and bearing;

by using the Slant Range(distance from the radar to the target).

it is possible to determine object height

CRT

N

W E

S

Ө

Slant Range

DISTANCE

Height

Height = Slant Range x sin Ө

Radar Angle

Other RangesHaving determined both range and bearing;

by using the Slant Range(distance from the radar to the target).

the target’s ground range can also be calculated

Ground Range = Slant Range x cos Ө

Ground RangeDISTANCE

Ө

Slant Range

DISTANCE

HeightRadar Angle

Other RangesUsing the Slant Range

Height = Slant Range x sin Өand

Ground Range = Slant Range x cos Ө

Ө

Slant Range

DISTANCE

HeightRadar Angle

Ground RangeDISTANCE

Other RangesTo pinpoint a target

by both height and bearing requires more than one aerial.

However, there is now a radar systemthat combines both facilities into one aerial,

known as the 3-D.

It works by electronically selecting the various aerial arrays

and passing the informationto the Plan Position Indicator (PPI) display.

What do the initials CRT stand for?

Cathode Radiation Tube

Cathode Ray Tube

Cathode Radio Tube

Capacitor Resistance Translator

Check of Understanding

Check of UnderstandingWhat is the purpose of the reflector

and directors of an aerial?

To allow the aerial to work at many frequencies

To match the aerial to the transmission line

To alter the aerial’selectrical length

To modify the aerial’sradiation pattern

Check of UnderstandingWhat does PPI stand for?

Partial Plan Indicator

Plan Potential Indicator

Partial Position Indicator

Plan Position Indicator

Which of these statements applies to a PPI radar display?

It has a radial trace rotating in time with the radar

It uses a height finding radar

It can only display ranges

It has a circular traceusing radial deflection

Check of Understanding

Bearing Rings

Height Indicators

Range Rings

Sector Ranges

Check of UnderstandingWhat can be added to a PPI

to aid in its operation?

The display intensity is changed

A new set of markers is required

The timebase sweep velocity is changed

The transmitter power is changed

Check of UnderstandingTo change the range display

on a radar screen . . .

What is a slant triangle composed of?

Angle of elevation, height and target velocity

Slant range, target and ground range

Angle of elevation, heightand ground range

Check of Understanding

Slant range, heightand ground range

Check of UnderstandingThe ground range of a target can be calculated by using . . .

Slant range x tan Ө

Slant range x sin Ө

Slant range x height

Slant range x cos Ө

A few more questions.

1. What is meant by the term ‘scanning’?

2. What is meant by the term ‘base velocity’?

3. What is meant by the term ‘azimuth’?

4. True or False ? ‘Type A’, radar cannot determine the object’s bearing, only its height.

5. What do the initials PPI stand for?

6. In the slant range calculations, what does the symbol ‘Ө’ signify?

Check of Understanding

Advanced Radioand Radar

End of Presentation