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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