CHAPTER 13
RADAR NAVIGATION
PRINCIPLES OF RADAR OPERATION
1300. Introduction bearings, the top of the scope represents the direction of
the ship s head. In this unstabilized presentation, the ori-
Radar determines distance to an object by measuring entation changes as the ship changes heading. In the
the time required for a radio signal to travel from a transmit- stabilized north-upward presentation, gyro north is al-
ter to an object and return. Since most radars use directional ways at the top of the scope.
antennae, they can also determine an object s bearing.
However, a radar s bearing measurement will be less accu- 1303. The Radar Beam
rate than its distance measurement. Understanding this
concept is crucial to ensuring the optimal employment of The pulses of energy comprising the radar beam would
the radar for safe navigation. form a single lobe-shaped pattern of radiation if emitted in
free space. Figure 1303a. shows this free space radiation
1301. Signal Characteristics pattern, including the undesirable minor lobes or side lobes
associated with practical antenna design.
In most marine navigation applications, the radar sig- Although the radiated energy is concentrated into a
nal is pulse modulated. Signals are generated by a timing relatively narrow main beam by the antenna, there is no
circuit so that energy leaves the antenna in very short puls- clearly defined envelope of the energy radiated. The ener-
es. When transmitting, the antenna is connected to the gy is concentrated along the axis of the beam. With the
transmitter but not the receiver. As soon as the pulse leaves, rapid decrease in the amount of radiated energy in direc-
an electronic switch disconnects the antenna from the trans- tions away from this axis, practical power limits may be
mitter and connects it to the receiver. Another pulse is not used to define the dimensions of the radar beam.
transmitted until after the preceding one has had time to A radar beam s horizontal and vertical beam widths are
travel to the most distant target within range and return. referenced to arbitrarily selected power limits. The most
Since the interval between pulses is long compared with the common convention defines beam width as the angular
length of a pulse, strong signals can be provided with low width between half power points. The half power point cor-
average power. The duration or length of a single pulse is responds to a drop in 3 decibels from the maximum beam
called pulse length, pulse duration, or pulse width. This strength.
pulse emission sequence repeats a great many times, per- The definition of the decibel shows this halving of
haps 1,000 per second. This rate defines the pulse power at a decrease in 3 dB from maximum power. A deci-
repetition rate (PRR). The returned pulses are displayed bel is simply the logarithm of the ratio of a final power level
on an indicator screen. to a reference power level:
P1
1302. The Display
dB= 10 log ------
P0
The most common type of radar display used in the
where P1 is the final power level, and P0 is a reference pow-
Navy is the plan position indicator (PPI). On a PPI, the
er level. When calculating the dB drop for a 50% reduction
sweep starts at the center of the display and moves outward
in power level, the equation becomes:
along a radial line rotating in synchronization with the an-
tenna. A detection is indicated by a brightening of the
dB = 10 log(.5)
display screen at the bearing and range of the return. Be-
dB = 3 dB
cause of a luminescent tube face coating, the glow
The radiation diagram shown in Figure 1303b depicts
continues after the trace rotates past the target. Figure 1302
relative values of power in the same plane existing at the
shows this presentation.
same distances from the antenna or the origin of the radar
On a PPI, a target s actual range is proportional to its
beam. Maximum power is in the direction of the axis of
echo s distance from the scope s center. A moveable
the beam. Power values diminish rapidly in directions
cursor helps to measure ranges and bearings. In the
away from the axis. The beam width is taken as the angle
heading-upward presentation, which indicates relative
207
208 RADAR NAVIGATION
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.
1304. Diffraction And Attenuation
Diffraction is the bending of a wave as it passes an ob-
struction. Because of diffraction there is some illumination
of the region behind an obstruction or target by the radar
beam. Diffraction effects are greater at the lower frequen-
cies. Thus, the radar beam of a lower frequency radar tends
to illuminate more of the shadow region behind an obstruc-
tion than the beam of a radar of higher frequency or shorter
wavelength.
Attenuation is the scattering and absorption of the en-
ergy 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 trans-
mitted 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 re-
flecting properties of the targets.
1305. Refraction
If the radar waves traveled in straight lines, the dis-
tance to the radar horizon would be dependent only on the
power output of the transmitter and the height of the anten-
na. In other words, the distance to the radar horizon would
be the same as that of the geometrical horizon for the anten-
na height. However, atmospheric density gradients bend
radar rays as they travel to and from a target. This bending
is called refraction.
The following formula, where h is the height of the an-
tenna in feet, gives the distance to the radar horizon in
nautical miles:
Figure 1302. Plan Position Indicator (PPI) display.
between the half-power points.
d = 1.22 h .
The beam width depends upon the frequency or wave-
length of the transmitted energy, antenna design, and the
The distance to the radar horizon does not limit the dis-
dimensions of the antenna.
tance from which echoes may be received from targets. As-
For a given antenna size (antenna aperture), narrower
RADAR NAVIGATION 209
Figure1303a. Freespace radiation pattern.
Figure1303b. Radiation diagram.
Figure1303c. Direct and indirect waves.
suming that adequate power is transmitted, echoes may be the first has traveled an equal distance in the opposite
received from targets beyond the radar horizon if their reflect- direction. At B, the transmitted pulse has continued on
ing surfaces extend above it. Note that the distance to the radar beyond the second target, and the two echoes are re-
horizon is the distance at which the radar rays pass tangent to turning toward the transmitter. The distance between
the surface of the earth. leading edges of the two echoes is twice the distance
between targets. The correct distance will be shown on
1306. Factors Affecting Radar Interpretation the scope, which is calibrated to show half the distance
traveled out and back. At C the targets are closer to-
Radar s value as a navigational aid depends on the nav- gether and the pulse length has been increased. The
igator s understanding its characteristics and limitations. two echoes merge, and on the scope they will appear as
Whether measuring the range to a single reflective object or a single, large target. At D the pulse length has been de-
trying to discern a shoreline lost amid severe clutter, knowl- creased, and the two echoes appear separated. The
edge of the characteristics of the individual radar used are ability of a radar to separate targets close together on
crucial. Some of the factors to be considered in interpreta- the same bearing is called resolution in range. It is re-
tion are discussed below: lated primarily to pulse length. The minimum distance
between targets that can be distinguished as separate is
" Resolution in Range. In part A of Figure 1306a, a half the pulse length. This (half the pulse length) is the
transmitted pulse has arrived at the second of two tar- apparent depth or thickness of a target presenting a flat
gets of insufficient size or density to absorb or reflect perpendicular surface to the radar beam. Thus, several
all of the energy of the pulse. While the pulse has trav- ships close together may appear as an island. Echoes
eled from the first to the second target, the echo from from a number of small boats, piles, breakers, or even
210 RADAR NAVIGATION
large ships close to the shore may blend with echoes ferent in appearance. A metal surface reflects radio
from the shore, resulting in an incorrect indication of waves more strongly than a wooden surface. A surface
the position and shape of the shoreline. perpendicular to the beam returns a stronger echo than
a non perpendicular one. For this reason, a gently slop-
" Resolution in Bearing. Echoes from two or more tar- ing beach may not be visible. A vessel encountered
gets close together at the same range may merge to form broadside returns a stronger echo than one heading di-
a single, wider echo. The ability to separate targets is rectly toward or away.
called resolution in bearing. Bearing resolution is a
function of two variables: beam width and range be- " Frequency. As frequency increases, reflections occur
tween targets. A narrower beam and a shorter distance from smaller targets.
between objects both increase bearing resolution.
Atmospheric noise, sea return, and precipitation com-
" Height of Antenna and Target. If the radar horizon is plicate radar interpretation by producing clutter. Clutter is
between the transmitting vessel and the target, the low- usually strongest near the vessel. Strong echoes can some-
er part of the target will not be visible. A large vessel times be detected by reducing receiver gain to eliminate
may appear as a small craft, or a shoreline may appear weaker signals. By watching the repeater during several ro-
at some distance inland. tations of the antenna, the operator can discriminate
between clutter and a target even when the signal strengths
" Reflecting Quality and Aspect of Target. Echoes from clutter and the target are equal. At each rotation, the
from several targets of the same size may be quite dif-
Figure 1306a. Resolution in range.
RADAR NAVIGATION 211
signals from targets will remain relatively stationary on the cause it lies too low in the water.
display while those caused by clutter will appear at differ- Coral atolls and long chains of islands may produce
ent locations. long lines of echoes when the radar beam is directed per-
Another major problem lies in determining which fea- pendicular to the line of the islands. This indication is
tures in the vicinity of the shoreline are actually represented especially true when the islands are closely spaced. The rea-
by echoes shown on the repeater. Particularly in cases where son is that the spreading resulting from the width of the
a low lying shore is being scanned, there may be considerable radar beam causes the echoes to blend into continuous
uncertainty. lines. When the chain of islands is viewed lengthwise, or
A related problem is that certain features on the shore obliquely, however, each island may produce a separate re-
will not return echoes because they are blocked from the ra- turn. Surf breaking on a reef around an atoll produces a
dar beam by other physical features or obstructions. This ragged, variable line of echoes.
factor in turn causes the chart like image painted on the One or two rocks projecting above the surface of the
scope to differ from the chart of the area. water, or waves breaking over a reef, may appear on the
If the navigator is to be able to interpret the presentation PPI. When an object is submerged entirely and the sea is
on his radarscope, he must understand the characteristics of smooth over it, no indication is seen on the PPI.
radar propagation, the capabilities of his radar set, the reflect- If the land rises in a gradual, regular manner from the
ing properties of different types of radar targets, and the shoreline, no part of the terrain produces an echo that is
ability to analyze his chart to determine which charted fea- stronger than the echo from any other part. As a result, a
tures are most likely to reflect the transmitted pulses or to be general haze of echoes appears on the PPI, and it is difficult
blocked. Experience gained during clear weather comparison to ascertain the range to any particular part of the land.
between radar and visual images is invaluable. Blotchy signals are returned from hilly ground, because
Land masses are generally recognizable because of the the crest of each hill returns a good echo although the valley
steady brilliance of the relatively large areas painted on the beyond is in a shadow. If high receiver gain is used, the pat-
PPI. Also, land should be at positions expected from the ship s tern may become solid except for the very deep shadows.
navigational position. Although land masses are readily recog- Low islands ordinarily produce small echoes. When
nizable, the primary problem is the identification of specific thick palm trees or other foliage grow on the island, strong
land features. Identification of specific features can be quite echoes often are produced because the horizontal surface of
difficult because of various factors, including distortion result- the water around the island forms a sort of corner reflector
ing from beam width and pulse length, and uncertainty as to with the vertical surfaces of the trees. As a result, wooded
just which charted features are reflecting the echoes. islands give good echoes and can be detected at a much
Sand spits and smooth, clear beaches normally do not greater range than barren islands.
appear on the PPI at ranges beyond 1 or 2 miles because these Sizable land masses may be missing from the radar dis-
targets have almost no area that can reflect energy back to the play because of certain features being blocked from the radar
radar. Ranges determined from these targets are not reliable. beam by other features. A shoreline which is continuous on
If waves are breaking over a sandbar, echoes may be returned the PPI display when the ship is at one position, may not be
from the surf. Waves may, however, break well out from the continuous when the ship is at another position and scanning
actual shoreline, so that ranging on the surf may be the same shoreline. The radar beam may be blocked from a
misleading. segment of this shoreline by an obstruction such as a prom-
Mud flats and marshes normally reflect radar pulses ontory. An indentation in the shoreline, such as a cove or bay,
only a little better than a sand spit. The weak echoes received appearing on the PPI when the ship is at one position, may
at low tide disappear at high tide. Mangroves and other thick not appear when the ship is at another position nearby. Thus,
growth may produce a strong echo. Areas that are indicated radar shadow alone can cause considerable differences be-
as swamps on a chart, therefore, may return either strong or tween the PPI display and the chart presentation. This effect
weak echoes, depending on the density and size of the vege- in conjunction with beam width and pulse length distortion
tation growing in the area. of the PPI display can cause even greater differences.
When sand dunes are covered with vegetation and are The returns of objects close to shore may merge with
well back from a low, smooth beach, the apparent shoreline the shoreline image on the PPI, because of distortion effects
determined by radar appears as the line of the dunes rather of horizontal beam width and pulse length. Target images
than the true shoreline. Under some conditions, sand dunes on the PPI always are distorted angularly by an amount
may return strong echo signals because the combination of equal to the effective horizontal beam width. Also, the tar-
the vertical surface of the vegetation and the horizontal get images always are distorted radially by an amount at
beach may form a sort of corner reflector. least equal to one-half the pulse length (164 yards per mi-
Lagoons and inland lakes usually appear as blank areas crosecond of pulse length).
on a PPI because the smooth water surface returns no ener- Figure 1306b illustrates the effects of ship s position,
gy to the radar antenna. In some instances, the sandbar or beam width, and pulse length on the radar shoreline. Be-
reef surrounding the lagoon may not appear on the PPI be- cause of beam width distortion, a straight, or nearly
Figure 1306b. Effects of ship s position, beam width, and pulse length on radar shoreline.
212
RADAR NAVIGATION
RADAR NAVIGATION 213
Figure 1306c. Distortion effects of radar shadow, beam width, and pulse length.
straight, shoreline often appears crescent-shaped on the 1307. Recognition Of Unwanted Echoes
PPI. This effect is greater with the wider beam widths. Note
that this distortion increases as the angle between the beam The navigator must be able to recognize various abnor-
axis and the shoreline decreases. mal echoes and effects on the radarscope so as not to be
Figure 1306c illustrates the distortion effects of radar confused by their presence.
shadow, beam width, and pulse length. View A shows the Indirect or false echoes are caused by reflection of the
actual shape of the shoreline and the land behind it. Note the main lobe of the radar beam off ship s structures such as
steel tower on the low sand beach and the two ships at an- stacks and kingposts. When such reflection does occur, the
chor close to shore. The heavy line in view B represents the echo will return from a legitimate radar contact to the an-
shoreline on the PPI. The dotted lines represent the actual tenna by the same indirect path. Consequently, the echo
position and shape of all targets. Note in particular: will appear on the PPI at the bearing of the reflecting sur-
face. As shown in Figure 1307a, the indirect echo will
1. The low sand beach is not detected by the radar. appear on the PPI at the same range as the direct echo re-
2. The tower on the low beach is detected, but it looks like a ceived, assuming that the additional distance by the indirect
ship in a cove. At closer range the land would be detected path is negligible.
and the cove-shaped area would begin to fill in; then the
tower could not be seen without reducing the receiver gain. Characteristics by which indirect echoes may be recog-
3. The radar shadow behind both mountains. Distortion nized are summarized as follows:
owing to radar shadows is responsible for more confu-
sion than any other cause. The small island does not 1. The indirect echoes will usually occur in shadow
appear because it is in the radar shadow. sectors.
4. The spreading of the land in bearing caused by beam 2. They are received on substantially constant bear-
width distortion. Look at the upper shore of the peninsu- ings, although the true bearing of the radar contact
la. The shoreline distortion is greater to the west because may change appreciably.
the angle between the radar beam and the shore is small- 3. They appear at the same ranges as the correspond-
er as the beam seeks out the more westerly shore. ing direct echoes.
5. Ship No. 1 appears as a small peninsula. Her return has 4. When plotted, their movements are usually
merged with the land because of the beam width abnormal.
distortion. 5. Their shapes may indicate that they are not direct
6. Ship No. 2 also merges with the shoreline and forms a echoes.
bump. This bump is caused by pulse length and beam
width distortion. Reducing receiver gain might cause Side-lobe effects are readily recognized in that they
the ship to separate from land, provided the ship is not produce a series of echoes (Figure 1307b) on each side of
too close to the shore. The Fast Time Constant (FTC) the main lobe echo at the same range as the latter. Semicir-
control could also be used to attempt to separate the ship cles, or even complete circles, may be produced. Because of
from land. the low energy of the side-lobes, these effects will normally
214 RADAR NAVIGATION
Figure 1307a. Indirect echo.
occur only at the shorter ranges. The effects may be mini- Electronic interference effects, such as may occur
mized or eliminated, through use of the gain and anti-clutter when near another radar operating in the same frequency
controls. Slotted wave guide antennas have largely elimi- band as that of the observer s ship, is usually seen on the
nated the side-lobe problem. PPI as a large number of bright dots either scattered at ran-
Multiple echoes may occur when a strong echo is re- dom or in the form of dotted lines extending from the center
ceived from another ship at close range. A second or third to the edge of the PPI.
or more echoes may be observed on the radarscope at dou- Interference effects are greater at the longer radar
ble, triple, or other multiples of the actual range of the radar range scale settings. The interference effects can be distin-
contact (Figure 1307c). guished easily from normal echoes because they do not
Second-trace echoes (multiple-trace echoes) are ech- appear in the same places on successive rotations of the
oes received from a contact at an actual range greater than antenna.
the radar range setting. If an echo from a distant target is re- Stacks, masts, samson posts, and other structures, may
ceived after the following pulse has been transmitted, the cause a reduction in the intensity of the radar beam beyond these
echo will appear on the radarscope at the correct bearing but obstructions, especially if they are close to the radar antenna. If
not at the true range. Second-trace echoes are unusual, ex- the angle at the antenna subtended by the obstruction is more
cept under abnormal atmospheric conditions, or conditions than a few degrees, the reduction of the intensity of the radar
under which super-refraction is present. Second-trace ech- beam beyond the obstruction may produce a blind sector. Less
oes may be recognized through changes in their positions reduction in the intensity of the beam beyond the obstructions
on the radarscope in changing the pulse repetition rate may produce shadow sectors. Within a shadow sector, small tar-
(PRR); their hazy, streaky, or distorted shape; and the errat- gets at close range may not be detected, while larger targets at
ic movements on plotting. much greater ranges will appear.
As illustrated in Figure 1307d, a target return is detected Spoking appears on the PPI as a number of spokes or
on a true bearing of 090° at a distance of 7.5 miles. On chang- radial lines. Spoking is easily distinguished from interfer-
ing the PRR from 2,000 to 1,800 pulses per second, the same ence effects because the lines are straight on all range-scale
target is detected on a bearing of 090° at a distance of 3 miles settings, and are lines rather than a series of dots.
(Figure 1307e). The change in the position of the return indi- The spokes may appear all around the PPI, or they may be
cates that the return is a second-trace echo. The actual confined to a sector. If spoking is confined to a narrow sector,
distance of the target is the distance as indicated on the PPI the effect can be distinguished from a Ramark signal of similar
plus half the distance the radar wave travels between pulses. appearance through observation of the steady relative bearing
RADAR NAVIGATION 215
Figure 1307b. Side-lobe effects. Figure 1307c. Multiple echoes.
Figure 1307d. Second-trace echo on 12-mile range scale. Figure 1307e. Position of second-trace echo on 12-mile
range scale after changing PRR.
Figure 1307f
of the spoke in a situation where the bearing of the Ramark sig- the same instant as the transmission of the pulse, is most ap-
nal should change. Spoking indicates a need for maintenance or parent when in narrow rivers or close to shore.
adjustment. The echo from an overhead power cable appears on the
The PPI display may appear as normal sectors alternat- PPI as a single echo always at right angles to the line of the
ing with dark sectors. This is usually due to the automatic cable. If this phenomenon is not recognized, the echo can be
frequency control being out of adjustment. wrongly identified as the echo from a ship on a steady bear-
The appearance of serrated range rings indicates a need ing. Avoiding action results in the echo remaining on a
for maintenance. constant bearing and moving to the same side of the channel
After the radar set has been turned on, the display may as the ship altering course. This phenomenon is particularly
not spread immediately to the whole of the PPI because of apparent for the power cable spanning the Straits of Messina.
static electricity inside the CRT. Usually, the static electric-
ity effect, which produces a distorted PPI display, lasts no 1308. Aids To Radar Navigation
longer than a few minutes.
Hour-glass effect appears as either a constriction or ex- Radar navigation aids help identify radar targets and in-
pansion of the display near the center of the PPI. The crease echo signal strength from otherwise poor radar targets.
expansion effect is similar in appearance to the expanded Buoys are particularly poor radar targets. Weak, fluc-
center display. This effect, which can be caused by a non- tuating echoes received from these targets are easily lost in
linear time base or the sweep not starting on the indicator at the sea clutter. To aid in the detection of these targets, radar
216 RADAR NAVIGATION
Figure 1308a. Corner reflectors.
reflectors, designated corner reflectors, may be used. These classes of these beacons: racons, which provide both bear-
reflectors may be mounted on the tops of buoys. Additional- ing and range information to the target, and ramarks which
ly, the body of the buoy may be shaped as a reflector. provide bearing information only. However, if the ramark
Each corner reflector, shown in Figure 1308a, consists installation is detected as an echo on the radarscope, the
of three mutually perpendicular flat metal surfaces. A radar range will be available also.
wave striking any of the metal surfaces or plates will be re- A racon is a radar transponder which emits a character-
flected back in the direction of its source. Maximum energy istic signal when triggered by a ship s radar. The signal may
will be reflected back to the antenna if the axis of the radar be emitted on the same frequency as that of the triggering
beam makes equal angles with all the metal surfaces. Fre- radar, in which case it is superimposed on the ship s radar
quently, corner reflectors are assembled in clusters to display automatically. The signal may be emitted on a sep-
maximize the reflected signal. arate frequency, in which case to receive the signal the
Although radar reflectors are used to obtain stronger ship s radar receiver must be tuned to the beacon frequency,
echoes from radar targets, other means are required for more or a special receiver must be used. In either case, the PPI
positive identification of radar targets. Radar beacons are will be blank except for the beacon signal. However, the
transmitters operating in the marine radar frequency band, only racons in service are in band beacons which transmit
which produce distinctive indications on the radarscopes of in one of the marine radar bands, usually only the 3-centi-
ships within range of these beacons. There are two general meter band.
Figure 1308b. Coded racon signal. Figure 1308c. Ramark signal appearing as a broken radial
line.
RADAR NAVIGATION 217
The racon signal appears on the PPI as a radial line is used so that the PPI can be inspected without any clutter
originating at a point just beyond the position of the radar introduced by the ramark signal on the scope. The ramark
beacon, or as a Morse code signal (Figure 1308b) displayed signal as it appears on the PPI is a radial line from the cen-
radially from just beyond the beacon. ter. The radial line may be a continuous narrow line, a
A ramark is a radar beacon which transmits either con- broken line (Figure 1308c), a series of dots, or a series of
tinuously or at intervals. The latter method of transmission dots and dashes.
RADAR PILOTING
1309. Introduction always a good practice in all navigation scenarios, its im-
portance increases tremendously when piloting using only
When navigating in restricted waters, a mariner most radar. Assuming proper operation of the fathometer,
often relies on visual piloting to provide the accuracy re- soundings give the navigator invaluable information on
quired to ensure ship safety. Visual piloting, however, the reliability of his fixes. When a disparity exists between
requires clear weather; often, mariners must navigate the charted depth at the fix and the recorded sounding, the
through fog. When conditions render visual piloting impos- navigator should assume that the disparity has been
sible and a vessel is not equipped with DGPS, radar caused by fix inaccuracy. This is especially true if the
navigation provides a method of fixing a vessel s position fathometer shows the ship heading into water shallower
with sufficient accuracy to allow safe passage. See Chapter than that anticipated. When there is a consistent disparity
8 for a detailed discussion of integrating radar into a pilot- between charted and fathometer sounding data, the navi-
ing procedure. gator should assume that he does not know the ship s
position with sufficient accuracy to proceed safely. The
1310. Fixing Position By Two Or More Simultaneous ship should be slowed or stopped until the navigator is
Ranges confident that he can continue his passage safely.
The most accurate radar fixes result from measuring 1311. Fixing Position By A Range And Bearing To One
and plotting ranges to two or more objects. Measure objects Object
directly ahead or astern first; measure objects closest to the
beam last. This procedure is the opposite to that recom- Visual piloting requires bearings from at least two ob-
mended for taking visual bearings, where objects closest to jects; radar, with its ability to determine both bearing and
the beam are measured first; however, both recommenda- range from one object, allows the navigator to obtain a fix
tions rest on the same principle. When measuring objects to where only a single navigation aid is available. An example
determine a line of position, measure first those which have of using radar in this fashion occurs in approaching a harbor
the greatest rate of change in the quantity being measured; whose entrance is marked with a single, prominent light
measure last those which have the least rate of change in such as Chesapeake Light at the entrance of the Chesapeake
that quantity. This minimizes measurement time delay er- Bay. Well beyond the range of any land-based visual navi-
rors. Since the range of those objects directly ahead or gation aid, and beyond the visual range of the light itself, a
astern of the ship changes more rapidly than those objects shipboard radar can detect the light and provide bearings
located abeam, measure objects ahead or astern first. and ranges for the ship s piloting party.
Record the ranges to the navigation aids used and lay This methodology is limited by the inherent inaccuracy
the resulting range arcs down on the chart. Theoretically, associated with radar bearings; typically, a radar bearing is
these lines of position should intersect at a point coincident accurate to within 5° of the true bearing. Therefore, the nav-
with the ship s position at the time of the fix. However, the igator must carefully evaluate the resulting position,
inherent inaccuracy of the radar coupled with the relatively checking it particularly with the sounding obtained from
large scale of most piloting charts usually precludes such a the bottom sounder. If a visual bearing is available from the
point fix. In this case, the navigator must carefully interpret object, use that bearing instead of the radar bearing when
the resulting fix. Check the echo sounder with the charted laying down the fix. This illustrates the basic concept dis-
depth where the fix lies. If both soundings consistently cor- cussed above: radar ranges are inherently more accurate
relate, that is an indication that the fixes are accurate. If than radar bearings.
there is disparity in the sounding data, then that is an indi- Prior to using this single object method, the navigator
cation that either the radar ranges were inaccurate or that must ensure that he has correctly identified the object from
the piloting party has misplotted them. which the bearing and range are to be taken. Using only one
This practice of checking sounding data with each fix navigation aid for both lines of position can lead to disaster
cannot be overemphasized. Though verifying soundings is if the navigation aid is not properly identified.
218 RADAR NAVIGATION
1312. Fixing Position With Tangent Bearings And A chart when approaching restricted waters to obtain an approx-
Range imate ship s position for evaluating radar targets to use for
range measurements. Speed is the advantage of this method, as
This method combines bearings tangent to an object the plotter can lay bearings down more quickly than ranges on
with a range measurement from some point on that object. the chart. Unless no more accurate method is available, do not
The object must be large enough to provide sufficient bear- use this method while piloting in restricted waters.
ing spread between the tangent bearings; often an island is
used. Identify some prominent feature of the object that is 1314. Fischer Plotting
displayed on both the chart and the radar display. Take a
range measurement from that feature and plot it on the In Fischer plotting, the navigator adjusts the scale of
chart. Then determine the tangent bearings to the island and the radar to match the scale of the chart in use. He then
plot them on the chart. overlays the PPI screen with a clear surface such as Plexi-
glas and traces the shape of land and location of navigation
1313. Fixing Position By Bearings To Two Or More aids from the radar scope onto the Plexiglas. He then trans-
Objects fers the surface from the radar scope to the chart. He
matches the chart s features with the features on the radar
The inherent inaccuracy of radar bearings discussed by adjusting the tracings on the Plexiglas to match the
above makes this method less accurate than fixing position by charts features. Once obtaining the best fit, he marks the
radar range. Use this method to plot a position quickly on the ship s position as the center of the Plexiglas cover.
RASTER RADARS
1315. Basic Description In order to produce a sufficiently high resolution, some
raster radars require over 1 million pixels per screen com-
Conventional PPI-display radars use a Cathode Ray bined with an update rate of 60 scans per second.
Tube (CRT) to direct an electron beam at a screen coated Completing the processing for such a large number of pixel
with phosphorus. The phosphorus glows when illuminated elements requires sophisticated, expensive circuitry. One
by an electron beam. Internal circuitry forms the beam such way to lower cost is to slow down the required processing
that a sweep is indicated on the face of the PPI. This speed. This speed can be lowered to approximately 30
sweep is timed to coincide with the sweep of the radar s an- frames per second before the picture develops a noticeable
tenna. A return echo is added to the sweep signal so that the flicker.
screen is more brightly illuminated at a point corresponding Further cost reduction can be gained by using an inter-
to the bearing and range of the target that returned the echo. laced display. An interlaced display does not draw the
The raster radar also employs a cathode ray tube; how- entire picture in one pass. On the first pass, it draws every
ever, the end of the tube upon which the picture is formed other line; it draws the remaining lines on the second pass.
is rectangular, not circular as in the PPI display. The raster This type of display reduces the number of screens that
radar does not produce its picture from a circular sweep; it have to be drawn per unit time by a factor of two; however,
utilizes a liner scan in which the picture is drawn, line by if the two pictures are misaligned, the picture will appear to
line, horizontally across the screen. As the sweep moves jitter.
across the screen, the electron beam from the CRT illumi- Raster radars represent the future of radar technology,
nates the pixels on the screen. A pixel is the smallest area and they will be utilized in the integrated bridge systems
of phosphorus that can be excited to form a picture element. discussed in Chapter 14.
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