1. Fundamentals of RADAR

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The fundamentals of RADAR.

Basic radar principles and general

characteristics.

Lieutenant JG

SERGIU ERBAN


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Introduction

RADARis a word derived from RAdio Detection And Rangingand appliesto electronic equipment designed for detecting and tracking objects (targets) at

considerable distances.

It is of great practical value to the navigator in the piloting waters, being used

for:

locating navigational aids; performing radar navigation;

tracking other vessels in the vicinity;

avoiding risk of collision.

The basic principle of radar is to determine the range to an object or "target"

by measuring the time required for an extremely short pulse of very high radiofrequency, transmitted as a radio wave, to travel from a reference source (own

ship) to a target and return as a reflected echo.

Such measurements can be converted into lines of position (LOPs)comprised

of circles with radius equal to the distance to the object.

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Since marine radars use directional antennae, they can also determine an

objects bearing. The radar antenna (called the scanner) rotates to scan the

entire surrounding area. Bearings to the target are determined by the orientation

of the antenna at the moment when the reflected echo returns.

However, due to its design, a radarsbearing measurement is less accurate thanits distance measurement. Understanding this concept is crucial to ensure the

optimal employment of the radar for safe navigation.

Once time and bearing are measured, these targets or echoes are calculated and

displayed on the radar display. The radar display provides the operator a birds

eye view of where other targets are relative to own ship.

There are two groups of radio frequencies allocated by international standards

for use by civil marine radar systems:

X-band: wavelength of 3 cm, frequency range of 9300 to 9500 MHz;

S-band: wavelength of 10 cm, frequency range of 2900 to 3100 MHz.

A fundamental requirement of marine radar is that of directional transmissionand reception, which is achieved by producing a narrow horizontal beam.

The radio-frequency energy transmitted by pulse-modulated radars consists of a

series of equally spaced pulses, frequently having durations of about 1

microsecond or less, separated by very short but relatively long periods during

which no energy is transmitted.

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A brief history

1865The Scottish physicist James Clerk Maxwellpresents his Theory of the

Electromagnetic Field (description of the electromagnetic waves and their

propagation). He demonstrated that electric and magnetic fields travel through

space in the form of waves and at the constant speed of light.

1886 The German physicist Heinrich Rudolf Hertz discovered

electromagnetic waves, thus demonstrating the Maxwell theory.

1904 The German engineer Christian Hlsmeyer invents the

telemobilscopefor a traffic monitoring on the water in poor visibility. This isthe first practical radar test.

1922 The American electrical engineers Albert H. Taylor and Leo C.

Youngof the Naval Research Laboratory (USA) locate a wooden ship for the

first time.

1931A ship is equipped with radar. As antennae are used parabolic dishes withhorn radiators.

1930sDuring the WWII, different radar equipments are developed in the USA,

Russia, Germany, France and Japan.

1939 British physicist created the magnetron oscillator which operated at

higher frequencies, that made microwave radar a reality, marking thebeginning of modern radar. 11.06.2014 5


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Basic block diagram of a Radar

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The functional breakdown of a basic pulse-modulated radar system usually

includes six major components. The functions of the components may be

summarized as follows:

1. The power supply furnishes all AC and DC voltages necessary for the

operation of the system components.

2. The modulator produces the synchronizing signals that trigger the

transmitter (magnetron) the required number of pulses per second, at same

frequency and proper length between them. It also triggers the indicatorsweep and coordinates the other associated circuits.

3. The transmittergenerates the radio-frequency energy in the form of short

powerful pulses through the magnetron.

4. The antenna system, that continuously rotates (10 24 rpm), takes the

radio-frequency energy from the transmitter, radiates it in a highlydirectional beam, receives any returning echoes, and passes these echoes to

the receiver.

5. The receiveramplifies the weak radio-frequency pulses (echoes) returned

by a target and reproduces them as video pulses passed to the indicator.

6. The indicator(screen)produces a visual indication of the echo pulses in a

manner that furnishes the desired information.11.06.2014 7

Components


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The type of scannerused by most vessels is the slotted array, an antenna with a

series of slits spaced at suitable intervals and angles from which radio pulses are

transmitted.

The length of the array affects horizontal beamwidth, and thus the radarsability

to determine target bearing. The longer the array, the more accurately the radarcan determine bearing.

Scanner directivity is a measure of the two beamwidths:

horizontal beamwidth;

vertical beamwidth.

The narrower the horizontal beamwidth the sharper the beam. The vertical

beamwidth should be wide; it is typically 20 to 25 degrees. The main reason for awide vertical beamwidth is to ensure the ability to display a target while own ship

is pitching and rolling.


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Radio pulses are emitted from the scanner in a certain direction. When the

pulse strikes an object such as a ship or island some of the energy returns to thescanner. The direction in which the scanner is pointing when the reflection is

received is the direction of the target causing the reflection.

Since radio waves travel at a near-constant speed, the time required for the

reflected echo to return to the scanner is a measure of the range to the target.

How radar determines range

The distance is determined from the running time of the high-frequency

transmitted signal and the propagation c0.

Since the waves travel to a target and back, the round trip time is dividing by

two in order to obtain the time the wave took to reach the target. Therefore thefollowing formula arises for the range:

where: c0speed of light = 3108 m/s, tmeasured running time [s],Rrange [m]

The distances are expressed in kilometers or nautical miles.11.06.2014 10

Basic principles of RADAR


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How radar determines bearing

By measuring the direction in which the antenna is pointing when the echo is

received, azimuth angle from the radar to the object or target can be

determined.

The angular determination of the target is determined by the directivityof the

antenna. Directivity is the ability of the antenna to concentrate the transmitted

energy in a particular direction.

The True Bearing (referenced to true north) of a radar target is the angle

between true north and a line pointed directly at the target. This angle is

measured in the horizontal plane and in a clockwise direction from true north. The bearing angle to the radar target may also be measured in a clockwise

direction from the centerline of own ship and is referred to as the relative

bearing.

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How the radar displays targets

Radar targets are displayed on what is called a Plan Position Indicatoror PPI.

This display is essentially a polar diagram, with the transmitting shipspositionat the center.

Images of target echoes are received and displayed at their relative bearing,

and at their distances from the PPI center. Early model radars displayed targets

and possess few features such as heading marks and range rings. To view the

display, a viewing hood was required to block out extraneous light.

Almost all late model radars use Liquid Crystal Display (LCD) or daylight

bright Cathode Ray Tube (CRT) displays. These types of displays provide

steady, bright, non-fading radar echoes in monochrome or color depending on

model.

Bearing on the PPI scope is indicated around the periphery of the screen. On

ships having a gyro compass the display has a gyro input and the presentationis oriented so that the true north lies under the 000 degrees mark.

As the antenna rotates a thin line sweeps around the center of the screen and

illuminates or "paints" any objects within the range of the radarscope. The

presentation of objects is called a "pip" or "blip".

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Radar range

Atmospheric conditions and target shape, material and aspect slightly affect

radar range. However, radar range is generally calculated as follows.

D is the distance from the scanner to the target horizon. Under normal

atmospheric conditions, this distance is 6% greater than the optical horizon.

This is because radio waves bend or refract slightly by atmospheric change.

The higher the scanner or target is above the surface, the longer the detection

range.

Unusual propagation conditions

Air ducts created by atmospheric conditions can affect radio pulse propagationand thus radar range. When the radio pulse is bent downward, radio pulses can

travel great distances thereby increasing the ranges at which targets can be

detected. This is called super-refraction.

The opposite condition, in which radar waves bend upward and decrease the

range at which targets can be detected, is called sub-refraction.

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Radar Resolution: describes the radarsability to distinctly display two radar

targets which are close to each other.

Radar has two types of resolution: range and bearing.

Bearing resolution is a measure of the capability of the radar to display as

separate targets the echoes received from two targets that are at the same range

and close together. The principal factor affecting bearing resolution is horizontal

beamwidth. The narrower the horizontal beamwidth the better the bearing

resolution.

Basic RADAR terms


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Range resolution is a measure of the capability of the radar to display as

separate pips the echoes received from two targets that are on the same bearing

and are close together. The main factor that affects range resolution is pulse length.

A short pulse length gives better range resolution than a long pulse length.

Practically, a 0.08 microsecond pulse offers the discrimination better than 25

meters.

Generally, it is used a short pulse length on short ranges for better range

resolution, and a long pulse length on long ranges for longer range detection.


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Beamwidth: Beamwidth is the

angular width, horizontal or vertical,

of the path taken by the radar pulse.

Horizontal beamwidth ranges from

0.75 to 5 degrees, and verticalbeamwidth from 20 to 25 degrees.


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Pulse Repetition Rate: Pulse repetition rate is the number of radio pulses

transmitted in one second. It is automatically determined by pulse length and

detecting range. For short ranges, pulsel ength is short and the pulse repetition

rate is high. For long ranges, pulse length is long and the pulse repetition rate is

low.

Minimum detectable range: This is the minimum range at which a target is

detectable by the radar. It is determined by scanner height, vertical beamwidth,

blind sector within the scanner beam and pulse length.

Maximum detectable range and output power: Doubling the output power of

a typical radar raises the maximum detectable range by only 19 percent. In the

reverse case, halving the output power lowers the maximum detectable range by16 percent. While you can increase the maximum detectable range by using a

high output power radar, a better (and more economical) way to do it would be to

mount the scanner as high as possible above the waterline and/or utilize a longer

antenna to increase horizontal beamwidth.


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RADAR = RAdioDetection AndRanging

Basic principle: range determination to an object by measuring thetime required for an extremely short pulse of very high radio

frequency, transmitted as a radio wave, to travel from own ship to a

target and return as a reflected echo.

A radars bearing measurement is less accurate than its distance

measurement. A basic pulse-modulated radar system usually includes six major

components:

power supply

modulator

transmitter

antenna system

receiver

indicator

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Conclusions


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Questions ?

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1. Fundamentals of RADAR

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