Basic Radar and Weather Radar Technical Characteristics
WE HAVE ALL heard the expression "blind detection, it should be obvious that it is to
as a bat" and are aware that a bat's blindness your advantage to learn all you can about the
is only relative. Because of his primarily fundamental principles of radar. If you know
nocturnal habits and his making his home in and can apply these principles, you will find
dark caves, the bat has more highly developed that your work is easier; you will be able to
"night vision" than we. You've probably troubleshoot and repair radar and other
experienced the momentary discomfort electronic equipment more quickly and
involved in emerging from a dark theater into accurately.
bright sunlight. No doubt, much the same 5. In this chapter, we begin with a
reaction is experienced by a bat in relation to discussion of basic radar principles. This
bright light, which prompts him to avoid it. discussion explains the way to determine the
His "blindness," then, is more a matter of range and direction of an echo. The discussion
selective sensitivity than organic defect. also introduces you to a basic radar system.
2. The interesting thing about a bat is that Following this discussion is a presentation of
he can flit about at rather high speeds without the functional and technical characteristics of
bumping into things, even though he may be the AN/FPS-77 and AN/TPQ-ll radar sets.
in an environment where his vision is seriously
restricted. Nature has a way of compensating
for the impairment or inefficiency of one 7. Radar Principles
organ of sensory perception by sharpening the 7-1. Radar detects the presence of objects
sensitivity of others. Research has shown that with radio echoes. Radar indicates the
the bat has long since developed a form of direction and range of those objects and
"sonar," which is similar to radar, without the makes it possible for you to recognize some
highly technical electronics upon which radar of their characteristics. It is a means of
depends. He emits almost continuous locating aircraft, flying objects, ships, land
high-pitched shrieks, which strike masses, cities, rivers, lakes, and suspended
obstructions in his path and echo back. The liquid or frozen water particles at short or
bat instantaneously evaluates the "echo data" long distances. Although radar supplies much
and alters his course accordingly. information concerning the character of these
3. The basic notion of radar is similar to objects (targets), it has limitations when it
that of a bat's sonar. The exception is that comes to locating small targets or individual
instead of "high-pitched shrieks," a radar targets within the group.
emits short, intense pulses of radio energy 7-2. For example, if a small group of
that it directs outward in a narrow beam. If a campers or a hunting party is lost in the
pulse intercepts a target within the path of forest, you cannot locate them by radar. On
the beam, some of the energy contained in the other hand, it is possible to locate a small
the pulse is reflected. A small portion of the boat on the high seas if the boat contains a
reflected energy returns to the point of reasonable quantity of metal. Even a floating
transmission, where it is received, amplified, oil drum on the surface of the sea can be
and presented on indicators (scopes). located with search radar sets. In searching for
4. Why is the Air Weather Service small objects with radar equipment, the
interested in radar systems? You will find that degree of the success is largely determined by
radar is the most practical means to detect the skill and experience of the observer.
certain valuable information, such as distant 7-3. Echoes are the sound waves that carry
cumulus cloud formations, for the forecaster. our voices back to us from a reflecting surface
Since AWS is deeply involved in radar storm such as a cliff. Similarly a radar set can receive
~j ~/ INDICATOR
_ L--=-~~:;-::-2~,7~3::.:6~:,(-;;A_RR~D_S - -.- - .
- -- -- -
s~-:JI- - - - - - - - - - -7fI',&
Figure 31. Determination of range.
the faint echo of radio waves that return after the transmitter frequency are used. Unlike
it sends out a powerful radio signal. Radio radio, the radar transmitter and receiver are at
signals behave in a manner similar to light the same location.
waves. We know that light waves are reflected 7-5. The transmitter sends out pulses of
by solid materials and are refracted when they energy at precisely timed intervals, then waits
pass from one medium to another. Radar during a silent period or "listening time"
radio waves are in the frequency range of the between pulses. This silent period is usually
UHF band and above. The higher the only a few thousandths of a second, or
frequency, the more easily the wave is milliseconds as you are more accustomed to
reflected by objects and changing mediums. healing. During the interval between
7-4. Whether it is a sound or a radio signal transmitted pulses, the receiver is working. It
that is transmitted, the original signal must be receives the return signals (echoes) from the
powerful enough to produce an echo. In radar targets that are in the path of the transmitted
equipment, the magnitude of the echo is signal.
determined by the power output of the 7-6. Range and Direction Measurement.
transmitter, the material and size of the Echoes from the targets that are near the
target, the distance to the target, and the receiver are received soon after the
sensitivity of the receiving equipment. Most transmitter is turned off. Echoes from targets
radar sets send out signals (pulses) of that are at a greater distance are received later
tremendous energy. These pulses are only a than those from the nearby objects.
few millionths of a second (microsecond) Therefore, it follows that when you expect
long. To detect the return signal from the echoes from nearby targets the transmitted
target, special antennas and receivers tuned to pulse must have a short duration. The
transmitter must turn off in time for the reached the aircraft 100 microseconds later,
receiver to receive the reflected signal from the beam has moved 1 inch across the face of
the shortest range that is consistent with the the tube. When the reflected pulse returns to
maximum range required from the set. We'll the receiver after 200 microseconds, the
discuss this further later in the text. sweep has moved 2 inches. The trace is
7-7. The time that elapses between the deflected vertically at this point. If the width
transmission of the pulse and the return of of the screen is calibrated in yards or miles,
the echo is used to compute the distance of you can read the distance to the target
the target. Since we know that light and radio directly from the indicator.
waves travel at the speed of 186,000 miles 7-11. Another way to determine the range
(300,000,000 meters) per second, we can of an echo is by superimposing range marks
compute the distance by multiplying one-half on the sweep. A range mark oscillator
of the signal travel time by the speed of light. generates the range marks and causes them to
In actual operation, this problem is solved appear as either dots or spikes along the
automatically and simultaneously by visual sweep. The space between the marks
indicators such as cathode-ray tubes (CRTs). represents a given distance. For example, if
7-8. Light or radar pulses travel 327.36 the distance between range marks represents a
yards (982.08 feet) in one-millionth of a distance of 5 miles and an echo appears three
second (1 microsecond). Since the pulses marks from the start of the sweep, the echo is
travel to the target and return, they travel 15 miles from the transmitter. Range marks
twice the distance between the radar antenna are used by both the AN/FPS-77 and
and the target. Thus, a target 1000 yards from AN/TPQ-ll to determine the echo range.
the antenna returns a pulse echo 7-12. On some sets. such as the
approximately 6 microseconds after the pulse AN/TPQ-ll, a single sweep does not persist
is transmitted. Even though this is an on the CRT long enough to be useful.
extremely short time, the radar set measures Therefore, it is necessary to repeat the pulse
the pulse travel time accurately at target transmission and the sweep. The two
ranges of 5 or 10 yards, or to about 0.30 operations start simultaneously and successive
microsecond. Thus, radar pulses measure echoes from the same target are superimposed
range or distance. on each other. The echoes from all targets
7-9. A receiver at the same location as the within range are shown on the screen at their
transmitter intercepts some of this reflected proper sweep time or range.
energy and produces a visual indication of the 7-13. The direction (bearing) of a target
object on the face of a CRT. Figure 31 shows from the radar system is usually given as an
an aircraft that reflects a radar pulse. The angular direction called azimuth. The azimuth
reflected signal is detected by the antenna, is measured from true north if the radar
and the radar unit converts the time delay to installation is permanent. As shown in figure
indicate a distance of 32,736 yards. A 32, the azimuth of the target is measured by
well-developed cloud produces a similar using the direction the radar antenna is
indication. To employ the time-range pointing.
relationship, the radar system must measure 7-14. The position and shape of the
small units of time very accurately. For an radiating element cause the antenna system to
A-scan indicator as the one in figure 31, the send out more energy in some directions than
horizontal sweep generator makes accurate others. For weather radar, the antenna and
measurement possible. It generates a sawtooth reflector produce a single, narrow beam of RF
voltage that causes the sweep dot to move energy in one direction. This radiation pattern
across the screen at a uniform speed. In figure concentrates maximum energy in the
31, the face of the scope is calibrated to
indicate the range as the electron beam
sweeps across the tube. AZIMUTH
7-10. Assume that a cathode-ray tube uses
a horizontal sweep (A-scan) which produces a
beam whose velocity across the screen is 1
inch per 100 microseconds. Part of the DIRECTIONAL
transmitted pulse and the echo from the ANTENNA
aircraft are applied to the vertical deflection
system. They cause vertical deflections on the
CRT'indicator. When the pulse is transmitted,
a vertical deflection appears at the beginning
of the sweep. When the transmitter pulse has Figure 32. Antenna and azimuth scale.
L ________ _____ ..J ANTENNA
Figure 33. Block diagram of a basic radar set.
direction the antenna is pointing. The 7-17. If the time interval between pulses is
transmitting pattern of an antenna system is too short, a succeeding pulse obscures the
also its receiving pattern. Therefore, the same CRT while echoes are still returning from the
antenna transmits RF energy and receives preceding pulse. If the interval is too long, the
reflected RF energy. The direction the change in the position of the target on the
antenna points is the azimuth of the target CRT is too great, and the system decreases in
and is electrically routed to the direction usefulness. In addition, too few pulses per
indicator. second may allow the image on the CRT to
7-15. Basic Radar System. A basic radar fade between sweeps.
system consists of six units, as shown in figure 7 -18. Transmitter-modula tor. The
33. They are the timer, transmitter- transmitter portion of the
modulator, antenna system, receiver, transmitter-modulator contains the circuits
indicator, and power supply. In some radar that generate the carrier frequency of the
sets they may not be included as six separate radar system. The beam directivity and the
units, but the functions of all six are operational requirements of the set determine
necessary. For example, the timer may be what the carrier frequency should be. The
part of the transmitter, the indicator may be carrier frequency determines the type of RF
part of the receiver, or the power supply may generator that is used. Both the AN/TPQ-ll
be included as part of the transmitter and and AN/FPS-77 use magnetrons as RF
receiver. On the other hand, there may be generators.
more than six units, but the overall operation 7-19. In radar equipment, the magnetron is
of the' system still consists of the same six the only stage of oscillation at the carrier
functions. For example, there may be frequency. The circuits preceding the
separate antennas for the transmitter and magnetron shape the high-voltage pulse that
receiver as in the AN/TPQ-ll. However, most triggers the magnetron and are called the
radar sets, such as the AN/FPS-77, use the modulator section. The modulator determines
same antenna system for both the transmitter the pulse width and pulse amplitude. The
and receiver. The following discussion covers modulator triggers and controls the operation
the functions of each unit in the block of the magnetron. For accurate range
diagram. determination, the output pulse must begin
7-16. Timer. The timer has several very and end sharply. Therefore, the output pulse
important functions. As the name implies, it of the modulator, which triggers the
times or synchronizes the start of each transmitter, must be a square wave with steep
transmitter pulse with the corresponding leading and trailing edges.
sweep on the screen of the indicator. This is 7-20. Antenna system. A radar antenna
necessary for the range indicator to depict system has two functions. It radiates the
accurately the distance to the target. The pulses generated by the transmitter, and it
timer determines the frequency at which receives the reflected pulses for the receiver.
pulses are radiated from the antenna (pulse The most common radar antenna consists of a
repetition frequency, PRF). In other words, half-wave-length dipole antenna with a
the timer is a kind of electronic switch that parabolic reflector. The dipole antenna is very
causes the transmitter to turn on and off at short. At a frequency of 9000 megaHertz
the appropriate time. At the same time, the (MHz), it is only 17'3centimeters (ern) long.
timer starts the sweep on the cathode-ray (2.54 cm is 1 inch.) It is mounted at the end
tube. of the waveguide (transmission line) leading
tube. The rectified video pulses from the
receiver are applied to the grid or cathode of
some types of indicator displays or to the
vertical deflection plates of other displays.
7-25. The A-scan, shown in figure 34,
furnishes an indication of range and
amplitude (intensity). This type of scan
consists of a single horizontal line across the
face of the CRT. The left end of the
-, t t t/ horizontal trace shows part of the radiated
p~lse and is called the main bang. The pulses
(pips) that appear to the right of this pulse
represent the targets and are at a distance
corresponding to their range or distance from
7-26. All targets along the sweep of the
Figure 34. A-scan indicator.
A-scan appear as upward vertical deflections
of the sweep since the video is applied to the
from the magnetron. The parabolic reflector
CRT vertical deflection plates. The amplifier
concentrates the radiated energy into a
has a gain control that the operator uses to
narrow beam that is directed toward a target. vary the amplitude of the echoes. You set the
The reflector is similar to the parabolic gain control so that electronic disturbances
reflector in the headlight of an automobile.
(noise) in the amplifiers appear as innumerable
7-21. When the transmitter signal strikes a small echoes across the whole trace. This
~get, only a part of the energy is reflected. noise is called grass.
Since the reflected signal is nondirectional,
only a small part of the transmitted energy . ~-27. ~he Pp'I-scan (plan position
returns to the reflector. The reflector focuses indicator) ISa maphke presentation of the area
the return on the dipole antenna. The dipole covered by the radar with the antenna
returns the RF energy to the waveguide and location as the center of the presentation as
on to the receiver. Electronic switching tubes shown in figure 35. Targets appear as bright
(duplexers) in the waveguide connect the spots on the scope. You determine the
antenn~ .system to the transmitter during azimuth of a target from the compass rose
transmitting and to the receiver during around the scope. The compass rose is
receiving so that the same antenna is used for calibrated in degrees. You determine the
both transmitting and receiving. range of the targets by calibrated range marks
(which appear as circles) along the sweep as it
7-22. Receiver. All radar receivers are of moves from the center to the edge of the
the superheterodyne type. The mixing CRT.
(heterodyning) action takes place in the 7-28. The PPI-scan is developed by shifting
receiver waveguide. The local oscillator the start of the sweep (time base) to the
output is coupled into the waveguide by a center of the CRT and by apparently rotating
probe or a loop. A crystal detector, circuit
resonant at the IF frequency, is located in the
mixing waveguide. The output signal from the
crystal mixer is applied to the first IF
amplifier circuit. The IF is usually in the
range of 20 to 60 MHz. Several wide-band
stages amplify the IF signals.
7-23. The output signal from the IF
amplifier section is applied to the video
detector. The video detector applies the signal
to the video amplifiers. The video amplifiers
are ~i?e-band, resistance-capacitance-coupled
amplifiers. The output of the video amplifiers
IS a series of rectified pulses, which are
applied to the CRT .
. 7-2~. I!",di~ator. The indicator produces
VISUalndications to depict the position of the
targets, such as showers or clouds. The main
component in the indicator is the cathode-ray Figure 35. PPI presentation.
A MAGNETIC FIELD FORCES THE
BEAM AT A RIGHT ANGLE TO THE
LINES OF FORCE
TIME BASE A-8 IS FOBMED
BY SWEEP CURRENT IN
AS THE COILS ARE REVOLVED
IN SYNCHRONISM WITH THE
ROTATION OF THE ANTENNA
THE TIME BASE LINE SCANS
THE FACE OF THE SCREEN.
Figure 36. Relation of PPI deflection coil position to CRT presentation.
the time base. The rotation is not caused by rotating the deflection coils, as shown in
rotating a single time base sweep but by figure 36, or by rotating their magnetic field.
generating multiple sweeps and slightly The video is applied to the grid or cathode of
changing the position of each succeeding the PPI type indicator. This causes the echoes
sweep. This makes it appear that a single to appear as bright spots on the face of the
sweep is rotating. The sweep is rotated by CRT. The bright spots correspond to the
Figure 37. Radome, AN/FPS-77
A. F,6EDHORN D. YOKE G. PEDESTAL ASSEMBLY
B. FEEDHORN SUPPORT STREET E. ELEVATION DRIV,E MOTOR H. WAVEGUIDE
C. PARABOLIC REFLECTOR F. AZIMUTH D,RIVE MOTOR
Figure 38. Antenna, AN/FPS-77.
Figure 39. Control amplifier, AN/FPS'77.
location of the targets when the direction the assembly (D) is mounted on top of the
antenna points is synchronized with the pedestal and is capable of continuous rotation
position of the CRT sweep. through 360 It carries the antenna dish and
7-29. Power supply. Although the power all the servo components that drive the
supplies for the various components in a radar antenna in elevation and provides a
system are not located in a single unit, they continuous indication of the antenna
are usually represented by a single block in a elevation angle. To permit elevation
block diagram. In most larger radar sets, the movement of the antenna dish, the dish is
receiver, transmitter, timer, and indicator pivoted on the yoke assembly. It is
each has a separate power supply. Each power counterbalanced by an adjustable weight. A
supply is furnished AC power from a central crank and mechanical linkage converts the
source. Since the aquadag voltage of a rotary motion of the elevation motor (E) into
cathode-ray tube is higher than most other oscillatory motion of the antenna dish. A
voltages of a radar system, an additional motor-driven jackscrew determines the angle
high-voltage power supply is required for the that the dish moves through. The jackscrew is
indicator. The transmitter magnetron requires remotely controlled from the console, which
an even higher voltage with a large amount of adjusts the mechanical linkage to provide the
power. Therefore, the modulator, which required scanning arc. An azimuth scale on
supplies the magnetron pulse, must have a the pedestal and an elevation scale on the
high-voltage power supply. yoke assembly permit initial installation and
alinement of the antenna assembly. You level
the antenna during initial installation by
8. Unit Functions means of leveling bolts on the pedestal and
8-1. Both the AN/FPS-77 and AN/TPQ-ll two built-in spirit levels.
radar systems are composed of major units. 8-5. The antenna positioning and
This section gives you an idea of the physical movement is controlled either automatically
appearance of each major unit and presents a or manually. In the automatic mode, the
brief description of the function of each unit azimuth servomotor drives the reflector,
in the system. clockwise, through 360 ° at 5 rpm. In the
8-2. AN/FPS-77. The purpose of this radar manual mode, you can rotate the reflector
set is to detect weather phenomena, such as clockwise or counterclockwise at speeds up to
storms and precipitation, and to display and 8 rpm. In automatic mode, the elevation
record a vertical and horizontal cross section servomotor raises or lowers the reflector from
of the weather. Clouds and other weather _2° to +60° at a minimum rate of 10° per
phenomena reflect radiated RF energy in second. There are three automatic elevation
varying amounts, depending on the sector scans: _2° to +15°, _2° to +30°, and _2°
composition of the phenomena. The radar to +60°. The sector scans are controlled by
processes the return signals and displays them cam-operated switches and are selected by a
on cathode-ray tubes to provide intensity, switch at the main console. In the manual
range, height, and azimuth information about mode, you can move the reflector from _2° to
the weather "target." Keep the basic radar +60° in 4 seconds.
block diagram and the purpose of the set in 8-6. Servoamplifiers are required to
mind as you read the following discussion of generate the power that operates the antenna
the major units, starting with the antenna. motors. The antenna servoamplifiers, as
8-3. Antenna. The antenna is protected shown in figure 39, are housed in a cabinet in
from the weather by a radome, as shown in the shelter located at the base of the antenna
figure 37. The radome consists of fiberglass tower. The azimuth and elevation
sections that are bolted together. The radome servoamplifiers receive two types of signals.
is bolted to the platform that supports the They are either constant signals that are
antenna. The antenna assembly, shown in amplified to drive the antenna motors
figure 38, includes an 8-foot parabolic continuously in the automatic mode or error
reflector (C). The RF pulse from the signals from the manual antenna positioning
magnetron is coupled through a waveguide controls on the console. The manual error
(H) to the J-type feed horn (A). The horn is signals are "nulled" when the antenna reaches
mounted at the focal point of the reflector by within ±0.5° of the position "called for" by
supporting struts (B). the manual controls. The antenna
8-4. The pedestal (G) contains the servo servoamplifiers are associated solely with the
components that operate the azimuth motor antenna drive motors. The range and height
(F) . and provide continuous azimuth indicator (RHI) and PPI deflection yokes are
information for the indicators. A yoke driven by separate followup servosystems.
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A. POWER MONITOR C. FREQUENCY METER E. POWER SUPPLY
B. PERFORMANCE MONITOR D. TRANSMITIER PANEL F. UTILITY AC JACKS
Figure 40. Receiver transmitter (RTM), ANjFPS-77 .
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••• "" .' •
~ lI!f!:;C -
" v .••
•• ,r:- ...
~ <i C;
> ~ .J • :~I
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A, AIR SCOPE D. CONSOLE POWER SUPPLY F, POWER DISTRIBUTION PANel
B, RHI SCOPE E. PPI SCOPE G. REFERENCESIGNAL GENERATOR
Figure 41. Console, ANjFPS-77.