Chapt-13.pdf

CHAPTER 13

RADAR NAVIGATION

PRINCIPLES OF RADAR OPERATION

1300. Introduction

helps to measure ranges and bearings. In the “heading-

upward” presentation, which indicates relative bearings,

the top of the scope represents the direction of the ship’s

head. In this unstabilized presentation, the orientation

changes as the ship changes heading. In the stabilized

“north-upward” presentation, gyro north is always at the

top of the scope.

Radar determines distance to an object by measuring

the time required for a radio signal to travel from a

transmitter to the object and return. Such measurements can

be converted into lines of position (LOP’s) comprised of

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

marine radars use directional antennae, they can also

determine an object’s bearing. However, due to its design,

a radar’s bearing measurement is less accurate than its

distance measurement. Understanding this concept is

crucial to ensuring the optimal employment of the radar for

safe navigation.

1303. The Radar Beam

The pulses of energy comprising the radar beam would

form a single lobe-shaped pattern of radiation if emitted in

free space. Figure 1303a shows this free space radiation

pattern, including the undesirable minor lobes or side lobes

associated with practical antenna design.

Although the radiated energy is concentrated into a

relatively narrow main beam by the antenna, there is no

clearly defined envelope of the energy radiated, although

most of the energy is concentrated along the axis of the

beam. With the rapid decrease in the amount of radiated

energy in directions away from this axis, practical power

limits may be used to define the dimensions of the radar

beam.

A radar beam’s horizontal and vertical beam widths are

referenced to arbitrarily selected power limits. The most

common convention defines beam width as the angular

width between half power points. The half power point

corresponds to a drop in 3 decibels from the maximum

beam strength.

The definition of the decibel shows this halving of

power at a decrease in 3 dB from maximum power. A

decibel is simply the logarithm of the ratio of a final power

level to a reference power level:

1301. Signal Characteristics

In most marine navigation applications, the radar

signal is pulse modulated. Signals are generated by a timing

circuit so that energy leaves the antenna in very short

pulses. When transmitting, the antenna is connected to the

transmitter but not the receiver. As soon as the pulse leaves,

an electronic switch disconnects the antenna from the

transmitter and connects it to the receiver. Another pulse is

not transmitted until after the preceding one has had time to

travel to the most distant target within range and return.

Since the interval between pulses is long compared with the

length of a pulse, strong signals can be provided with low

average power. The duration or length of a single pulse is

called pulse length , pulse duration ,or pulse width . This

pulse emission sequence repeats a great many times,

perhaps 1,000 per second. This rate defines the pulse

repetition rate (PRR) . The returned pulses are displayed

on an indicator screen.

1302. The Display

dB 10

log

P 1

P 0

The radar display is often referred to as the plan

position indicator (PPI) . On a PPI, the sweep appears as a

radial line, centered at the center of the scope and rotating

in synchronization with the antenna. Any returned echo

causes a brightening of the display screen at the bearing and

range of the object. Because of a luminescent coating on the

inside of the tube, the glow continues after the trace rotates

past the target.

On a PPI, a target’s actual range is proportional to its

distance from the center of the scope. A moveable cursor

where P 1 is the final power level, and P 0 is a reference

power level. When calculating the dB drop for a 50%

reduction in power level, the equation becomes:

log

()

– 3 dB

The radiation diagram shown in Figure 1303b depicts

relative values of power in the same plane existing at the

same distances from the antenna or the origin of the radar

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RADAR NAVIGATION

beam. Maximum power is in the direction of the axis of

the beam. Power values diminish rapidly in directions

away from the axis. The beam width is taken as the angle

between the half-power points.

The beam width depends upon the frequency or

wavelength of the transmitted energy, antenna design, and

the dimensions of the antenna. For a given antenna size

(antenna aperture), narrower beam widths result from using

shorter wavelengths. For a given wavelength, narrower

beam widths result from using larger antennas.

With radar waves being propagated in the vicinity of

the surface of the sea, the main lobe of the radar beam is

composed of a number of separate lobes, as opposed to the

single lobe-shaped pattern of radiation as emitted in free

space. This phenomenon is the result of interference be-

tween radar waves directly transmitted, and those waves

which are reflected from the surface of the sea. Radar

waves strike the surface of the sea, and the indirect waves

reflect off the surface of the sea. See Figure 1303c. These

reflected waves either constructively or destructively inter-

fere with the direct waves depending upon the waves’ phase

relationship.

radar tends to illuminate more of the shadow region behind

an obstruction than the beam of a radar of higher frequency

or shorter wavelength.

Attenuation is the scattering and absorption of the

energy in the radar beam as it passes through the

atmosphere. It causes a decrease in echo strength.

Attenuation is greater at the higher frequencies or shorter

wavelengths.

While reflected echoes are much weaker than the

transmitted pulses, the characteristics of their return to the

source are similar to the characteristics of propagation. The

strengths of these echoes are dependent upon the amount of

transmitted energy striking the targets and the size and

reflecting properties of the targets.

1305. Refraction

If the radar waves traveled in straight lines, the

distance to the radar horizon would be dependent only on

the power output of the transmitter and the height of the

antenna. In other words, the distance to the radar horizon

would be the same as that of the geometrical horizon for the

antenna height. However, atmospheric density gradients

bend radar rays as they travel to and from a target. This

bending is called refraction .

The distance to the radar horizon does not limit the dis-

tance from which echoes may be received from targets. As-

suming that adequate power is transmitted, echoes may be

received from targets beyond the radar horizon if their re-

flecting surfaces extend above it. The distance to the radar

horizon is the distance at which the radar rays pass tangent

to the surface of the Earth.

The following formula, where h is the height of the an-

tenna in feet, gives the theoretical distance to the radar

horizon in nautical miles:

Figure 1303a. Freespace radiation pattern.

1.22 h .

Figure 1303b. Radiation diagram.

1306. Factors Affecting Radar Interpretation

Figure 1303c. Direct and indirect waves.

Radar’s value as a navigational aid depends on the

navigator’s understanding its characteristics and

limitations. Whether measuring the range to a single

reflective object or trying to discern a shoreline lost amid

severe clutter, knowledge of the characteristics of the

individual radar used are crucial. Some of the factors to be

considered in interpretation are discussed below:

1304. Diffraction and Attenuation

Diffraction is the bending of a wave as it passes an

obstruction. Because of diffraction there is some illumi-

nation of the region behind an obstruction or target by the

radar beam. Diffraction effects are greater at the lower

frequencies. Thus, the radar beam of a lower frequency

• Resolution in Range. In part A of Figure 1306a, a

transmitted pulse has arrived at the second of two

targets of insufficient size or density to absorb or

reflect all of the energy of the pulse. While the pulse

has traveled from the first to the second target, the echo

from the first has traveled an equal distance in the

RADAR NAVIGATION

189

opposite direction. At B, the transmitted pulse has

continued on beyond the second target, and the two

echoes are returning toward the transmitter. The

distance between leading edges of the two echoes is

twice the distance between targets. The correct

distance will be shown on the scope, which is

calibrated to show half the distance traveled out and

back. At C the targets are closer together and the pulse

length has been increased. The two echoes merge, and

on the scope they will appear as a single, large target.

At D the pulse length has been decreased, and the two

echoes appear separated. The ability of a radar to

separate targets close together on the same bearing is

called resolution in range . It is related primarily to

pulse length. The minimum distance between targets

that can be distinguished as separate is half the pulse

length. This (half the pulse length) is the apparent

depth or thickness of a target presenting a flat perpen-

dicular surface to the radar beam. Thus, several ships

close together may appear as an island. Echoes from a

number of small boats, piles, breakers, or even large

ships close to the shore may blend with echoes from

the shore, resulting in an incorrect indication of the

position and shape of the shoreline.

times be detected by reducing receiver gain to eliminate

weaker signals. By watching the repeater during several ro-

tations of the antenna, the operator can often discriminate

between clutter and a target even when the signal strengths

from clutter and the target are equal. At each rotation, the

signals from targets will remain relatively stationary on the

display while those caused by clutter will appear at differ-

ent locations on each sweep.

Another major problem lies in determining which

features in the vicinity of the shoreline are actually

represented by echoes shown on the repeater. Particularly in

cases where a low lying shore is being scanned, there may be

considerable uncertainty.

A related problem is that certain features on the shore

will not return echoes because they are blocked from the

radar beam by other physical features or obstructions. This

factor in turn causes the chart-like image painted on the

scope to differ from the chart of the area.

If the navigator is to be able to interpret the presentation

on his radarscope, he must understand the characteristics of

radar propagation, the capabilities of his radar set, the

reflecting properties of different types of radar targets, and

the ability to analyze his chart to determine which charted

features are most likely to reflect the transmitted pulses or to

be blocked. Experience gained during clear weather

comparison between radar and visual images is invaluable.

Land masses are generally recognizable because of the

steady brilliance of the relatively large areas painted on the

PPI. Also, land should be at positions expected from the ship’s

navigational position. Although land masses are readily

recognizable, the primary problem is the identification of

specific land features. Identification of specific features can be

quite difficult because of various factors, including distortion

resulting from beam width and pulse length, and uncertainty as

to just which charted features are reflecting the echoes.

Sand spits and smooth, clear beaches normally do not

appear on the PPI at ranges beyond 1 or 2 miles because these

targets have almost no area that can reflect energy back to the

radar. Ranges determined from these targets are not reliable.

If waves are breaking over a sandbar, echoes may be returned

from the surf. Waves may, however, break well out from the

actual shoreline, so that ranging on the surf may be

misleading.

Mud flats and marshes normally reflect radar pulses

only a little better than a sand spit. The weak echoes received

at low tide disappear at high tide. Mangroves and other thick

growth may produce a strong echo. Areas that are indicated

as swamps on a chart, therefore, may return either strong or

weak echoes, depending on the density type, and size of the

vegetation growing in the area.

When sand dunes are covered with vegetation and are

well back from a low, smooth beach, the apparent shoreline

determined by radar appears as the line of the dunes rather

than the true shoreline. Under some conditions, sand dunes

may return strong echo signals because the combination of

the vertical surface of the vegetation and the horizontal

• Resolution in Bearing . Echoes from two or more

targets close together at the same range may merge to

form a single, wider echo. The ability to separate targets

close together at the same range is called resolution in

bearing . Bearing resolution is a function of two

variables: beam width and range to the targets. A

narrower beam and a shorter distance to the objects

both increase bearing resolution.

• Height of Antenna and Target . If the radar horizon is

between the transmitting vessel and the target, the

lower part of the target will not be visible. A large

vessel may appear as a small craft, or a shoreline may

appear at some distance inland.

• Reflecting Quality and Aspect of Target . Echoes

from several targets of the same size may be quite

different in appearance. A metal surface reflects radio

waves more strongly than a wooden surface. A surface

perpendicular to the beam returns a stronger echo than

a non perpendicular one. A vessel seen broadside

returns a stronger echo than one heading directly

toward or away. Some surfaces absorb most radar

energy rather that reflecting it.

• Frequency . As frequency increases, reflections occur

from smaller targets.

Atmospheric noise, sea return, and precipitation com-

plicate radar interpretation by producing clutter . Clutter is

usually strongest near the vessel. Strong echoes can some-

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RADAR NAVIGATION

Figure 1306a. Resolution in range.

beach may form a sort of corner reflector.

Lagoons and inland lakes usually appear as blank areas

on a PPI because the smooth water surface returns no

energy to the radar antenna. In some instances, the sandbar

or reef surrounding the lagoon may not appear on the PPI

because it lies too low in the water.

Coral atolls and long chains of islands may produce

long lines of echoes when the radar beam is directed

perpendicular to the line of the islands. This indication is

especially true when the islands are closely spaced. The

reason is that the spreading resulting from the width of the

radar beam causes the echoes to blend into continuous

lines. When the chain of islands is viewed lengthwise, or

obliquely, however, each island may produce a separate

return. Surf breaking on a reef around an atoll produces a

ragged, variable line of echoes.

One or two rocks projecting above the surface of the

water, or waves breaking over a reef, may appear on the

PPI.

If the land rises in a gradual, regular manner from the

shoreline, no part of the terrain produces an echo that is

stronger than the echo from any other part. As a result, a

general haze of echoes appears on the PPI, and it is difficult

to ascertain the range to any particular part of the land.

Blotchy signals are returned from hilly ground, because

the crest of each hill returns a good echo although the valley

beyond is in a shadow. If high receiver gain is used, the pat-

tern may become solid except for the very deep shadows.

Low islands ordinarily produce small echoes. When

thick palm trees or other foliage grow on the island, strong

echoes often are produced because the horizontal surface of

the water around the island forms a sort of corner reflector

with the vertical surfaces of the trees. As a result, wooded

islands give good echoes and can be detected at a much

RADAR NAVIGATION

191

Figure 1306b. Effects of ship’s position, beam width, and pulse length on radar shoreline.

greater range than barren islands.

Sizable land masses may be missing from the radar dis-

play because of certain features being blocked from the radar

beam by other features. A shoreline which is continuous on

the PPI display when the ship is at one position, may not be

continuous when the ship is at another position and scanning

the same shoreline. The radar beam may be blocked from a

segment of this shoreline by an obstruction such as a prom-

ontory. An indentation in the shoreline, such as a cove or bay,

appearing on the PPI when the ship is at one position, may

not appear when the ship is at another position nearby. Thus,

radar shadow alone can cause considerable differences be-

tween the PPI display and the chart presentation. This effect

in conjunction with beam width and pulse length distortion

of the PPI display can cause even greater differences.

The returns of objects close to shore may merge with

the shoreline image on the PPI, because of distortion effects

of horizontal beam width and pulse length. Target images

on the PPI are distorted angularly by an amount equal to the

effective horizontal beam width. Also, the target images al-

ways are distorted radially by an amount at least equal to

one-half the pulse length (164 yards per microsecond of

pulse length).

Figure 1306b illustrates the effects of ship’s position,

beam width, and pulse length on the radar shoreline. Be-

cause of beam width distortion, a straight, or nearly

straight, shoreline often appears crescent-shaped on the

PPI. This effect is greater with the wider beam widths. Note

that this distortion increases as the angle between the beam

axis and the shoreline decreases.

Figure 1306c illustrates the distortion effects of radar

shadow, beam width, and pulse length. View A shows the

actual shape of the shoreline and the land behind it. Note the

steel tower on the low sand beach and the two ships at an-

chor close to shore. The heavy line in view B represents the

shoreline on the PPI. The dotted lines represent the actual

position and shape of all targets. Note in particular:

1. The low sand beach is not detected by the radar.

2. The tower on the low beach is detected, but it looks like a

ship in a cove. At closer range the land would be detected

and the cove-shaped area would begin to fill in; then the

tower could not be seen without reducing the receiver gain.

3. The radar shadow behind both mountains. Distortion

owing to radar shadows is responsible for more

confusion than any other cause. The small island does

not appear because it is in the radar shadow.

4. The spreading of the land in bearing caused by beam

width distortion. Look at the upper shore of the

peninsula. The shoreline distortion is greater to the west

because the angle between the radar beam and the shore

is smaller as the beam seeks out the more westerly shore.

5. Ship No. 1 appears as a small peninsula. Its return has

merged with the land because of the beam width

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