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Radar
Technology Assignment 1
Submitted by:
RUCHI RATHORE (E. NO. 500011123) Course: B. Sc. (Aviation Studies)
Email- [email protected]
1. Explain the working of radar with block diagram in detail.
Distance measurement Transit time
Pulse radar: The round-trip time for the radar pulse to get to the target and return is measured. The distance is
proportional to this time.
Continuous wave (CW) radar
One way to measure the distance to an object is to transmit a short pulse of radio signal
(electromagnetic radiation), and measure the time it takes for the reflection to return. The
distance is one-half the product of the round trip time (because the signal has to travel to the
target and then back to the receiver) and the speed of the signal. Since radio waves travel at
the speed of light (186,000 miles per second or 300,000,000 meters per second), accurate
distance measurement requires high-performance electronics.
In most cases, the receiver does not detect the return while the signal is being transmitted.
Through the use of a device called a duplexer, the radar switches between transmitting and
receiving at a predetermined rate. The minimum range is calculated by measuring the length
of the pulse multiplied by the speed of light, divided by two. In order to detect closer targets
one must use a shorter pulse length.
A similar effect imposes a maximum range as well. If the return from the target comes in
when the next pulse is being sent out, once again the receiver cannot tell the difference. In
order to maximize range, longer times between pulses should be used, referred to as a pulse
repetition time (PRT), or its reciprocal, pulse repetition frequency (PRF).
These two effects tend to be at odds with each other, and it is not easy to combine both good
short range and good long range in a single radar. This is because the short pulses needed
for a good minimum range broadcast have less total energy, making the returns much smaller
and the target harder to detect. This could be offset by using more pulses, but this would
shorten the maximum range again. So each radar uses a particular type of signal. Long-range
radars tend to use long pulses with long delays between them, and short range radars use
smaller pulses with less time between them. This pattern of pulses and pauses is known as
the pulse repetition frequency (or PRF), and is one of the main ways to characterize a radar.
As electronics have improved many radars now can change their PRF thereby changing their
range. The newest radars fire 2 pulses during one cell, one for short range 10 km / 6 miles
and a separate signal for longer ranges 100 km /60 miles.
The distance resolution and the characteristics of the received signal as compared to noise
depends heavily on the shape of the pulse. The pulse is often modulated to achieve better
performance using a technique known as pulse compression.
Distance may also be measured as a function of time. The radar mile is the amount of time it
takes for a radar pulse to travel one nautical mile, reflect off a target, and return to the radar
antenna. Since a nautical mile is defined as exactly 1,852 meters, then dividing this distance
by the speed of light (exactly 299,792,458 meters per second), and then multiplying the result
by 2 (round trip = twice the distance), yields a result of approximately 12.36 microseconds in
duration.
Frequency modulation
Another form of distance measuring radar is based on frequency modulation. Frequency
comparison between two signals is considerably more accurate, even with older electronics,
than timing the signal. By measuring the frequency of the returned signal and comparing that
with the original, the difference can be easily measured.
This technique can be used in continuous wave radar, and is often found in aircraft radar
altimeters. In these systems a "carrier" radar signal is frequency modulated in a predictable
way, typically varying up and down with a sine wave or saw-tooth pattern at audio
frequencies. The signal is then sent out from one antenna and received on another, typically
located on the bottom of the aircraft, and the signal can be continuously compared using a
simple beat frequency modulator that produces an audio frequency tone from the returned
signal and a portion of the transmitted signal.
Since the signal frequency is changing, by the time the signal returns to the aircraft the
broadcast has shifted to some other frequency. The amount of that shift is greater over longer
times, so greater frequency differences mean a longer distance, the exact amount being the
"ramp speed" selected by the electronics. The amount of shift is therefore directly related to
the distance travelled, and can be displayed on an instrument. This signal processing is
similar to that used in speed detecting Doppler radar. Example systems using this approach
are AZUSA, MISTRAM, and UDOP.
A further advantage is that the radar can operate effectively at relatively low frequencies,
comparable to that used by UHF television. This was important in the early development of
this type when high frequency signal generation was difficult or expensive.
A new terrestrial radar uses low-power FM signals that cover a larger frequency range. The
multiple reflections are analyzed mathematically for pattern changes with multiple passes
creating a computerized synthetic image. Doppler effects are not used which allows slow
moving objects to be detected as well as largely eliminating "noise" from the surfaces of
bodies of water. Used primarily for detection of intruders approaching in small boats or
intruders crawling on the ground toward an objective.
Speed measurement
Speed is the change in distance to an object with respect to time. Thus the existing system for
measuring distance, combined with a memory capacity to see where the target last was, is
enough to measure speed. At one time the memory consisted of a user making grease-
pencil marks on the radar screen, and then calculating the speed using a slide rule. Modern
radar systems perform the equivalent operation faster and more accurately using computers.
However, if the transmitter's output is coherent (phase synchronized), there is another effect
that can be used to make almost instant speed measurements (no memory is required),
known as the Doppler effect. Most modern radar systems use this principle in the pulse-
doppler radar system. Return signals from targets are shifted away from this base frequency
via the Doppler effect enabling the calculation of the speed of the object relative to the radar.
The Doppler effect is only able to determine the relative speed of the target along the line of
sight from the radar to the target. Any component of target velocity perpendicular to the line of
sight cannot be determined by using the Doppler effect alone, but it can be determined by
tracking the target's azimuth over time. Additional information of the nature of the Doppler
returns may be found in the radar signal characteristics article.
It is also possible to make a radar without any pulsing, known as a continuous-wave
radar (CW radar), by sending out a very pure signal of a known frequency. CW radar is ideal
for determining the radial component of a target's velocity, but it cannot determine the target's
range. CW radar is typically used by traffic enforcement to measure vehicle speed quickly and
accurately where range is not important.
Other mathematical developments in radar signal processing include time-frequency
analysis (Weyl Heisenberg or wavelet), as well as the chirplet transform which makes use of
the fact that radar returns from moving targets typically "chirp" (change their frequency as a
function of time, as does the sound of a bird or bat).
Reduction of interference effects
Signal processing is employed in radar systems to reduce the radar interference effects.
Signal processing techniques include moving target indication (MTI), pulse doppler, moving
target detection (MTD) processors, correlation with secondary surveillance radar (SSR)
targets, space-time adaptive processing (STAP), and track-before-detect (TBD). Constant
false alarm rate (CFAR) and digital terrain model (DTM) processing are also used in clutter
environment.
Plot and track extraction
Radar video returns on aircraft can be subjected to a plot extraction process whereby
spurious and interfering signals are discarded. A sequence of target returns can be monitored
through a device known as a plot extractor. The non-relevant real time returns can be
removed from the displayed information and a single plot displayed. In some radar systems,
or alternatively in the command and control system to which the radar is connected, a radar
tracker is used to associate the sequence of plots belonging to individual targets and estimate
the targets' headings and speeds.
2. What are the different types of radar, explain in detail?
The preceding paragraphs indicate that radar systems are divided types based on the
designed use. This present the general characteristics of several commonly used radar
systems.
1. Search Radar:
Search radar continuously scans a volume of space and provides initial detection of all targets
within that space. Search radar systems are further divided into specific types, according to
the type of object they are designed to detect. For example: Surface-Search, Air-Search and
Height Finding Search Radars are all types of search radar.
• Surface-Search Radar: A surface search radar system has two primary functions:
1. The detection and determination of accurate ranges and bearings of surface
objects and low-flying aircrafts and
2. The maintenance of a 3600 search pattern for all objects within line-of-sight
distance from the radar antenna.
The maximum range ability of surface search radar is primarily limited by the
radar horizon; therefore, higher frequencies are used to permit maximum
reflection from small, reflecting areas, such as ship masthead structures and the
periscopes of submarines. Narrow pulse widths are used to permit a high degree
of range resolution at short ranges and to achieve greater range accuracies. High
pulse-repetition rates are used to permit a maximum definition of detected
objects. Medium peak power can be used to permit the detection of small objects
at line-of-sight distances. Wide vertical-beam widths permit compensation for the
pitch and roll of own ship and detection of low flying aircraft. Narrow horizontal-
beam widths permit accurate bearing determination and good bearing resolution.
For example, common ship-board surface-search radar has the following design
specifications:
a. Transmitter frequency 5,450-5,825MHz
b. Pulse width 0.25 or 1.3 micro seconds
c. Pulse-repetition rate between 625 and 650 pulses/second
d. Peak power between 190 and 285 kW
e. Vertical beam width between 12 and 16 degrees
f. horizontal beam width 1.5 degrees
Surface-search radar is used to detect the presence of surface craft and low
flying aircraft and to determine their presence. Ship-board surface-search radar
provides this type of information as an input to the weapons system to assist in
the engagement of hostile targets by fire-control radar. It is also used extensively
as a navigational aid in coastal waters and in poor weather conditions.
• Air-Search Radar: Air search radar systems initially detect and determine the position, course and
speed of air targets in a relatively large area. The maximum range of air-search
radar can exceed 300 miles, and the bearing coverage is a complete 3600 circle.
Air-search radar systems are usually divided into two categories, based on the
amount of position information supplied. Radar sets that provide only range and
bearing information are referred to as 2-D radars. Radar sets that supply range,
bearing, and height are called 3-D, or three dimensional radars.
Relatively low transmitter frequencies are used in 2-D search radars to permit
long range transmissions with minimum attenuation. Wide pulse widths and high
peak power are used to aid in detecting small objects at greater distances. Low
pulse-repetition rates are selected to permit greater maximum range. A wide
vertical-beam width is used to ensure detection of objects from the surface to
relatively high altitudes and to compensate for pitch and roll of own ship.
Air-search radar systems are used as early-warning devices because they can
detect approaching enemy aircrafts os missiles at great distances. In hostile
situations, early detection of the enemy is vital to a successful defence againse
attack. Anti-aircraft defences in the form of ship-board guns, missiles, or fighter
planes must be brought to a high degree of readiness in time to repel an attack.
Range and bearing information, provided by air-search radars, is used to initially
position fire-control tracking radar on a target. Another function of the air-search
radar system is guiding combat air patrol (CAP) aircraft to opposition suitable to
intercept an enemy aircraft. In the case of aircraft control, the guidance
information is obtained by the radar operator and passed to the aircraft by either
voice radio or to computer link to the aircraft.
• Height Finding Search Radar: The primary function of a height-finding radar (sometimes reffered to as a three-
coordinate or 3-D radar) is that of computing accurate ranges, bearings, and
altitudes of aircrafts targets detected by air-search radars. Height-finding radar is
also used by the ship’s air controllers to direct CAP aircraft during interception of
air targets. Modern 3-D radar is often used as the primary air-search radar. This
is because of its high accuracy and because the maximum ranges are only
slightly less than those available from 2-D radar.
The range capability of 3-D search radar is limited to some extent by an operating
frequency that is higher than that of 2-D radar. This advantage is partially offset
by higher output power and a beam-width that is narrower in both the vertical and
horizontal planes.
The 3-D radar system transmits several narrow beams to obtain altitude coverage
and, for this reason, compensation for roll and pitch must be provided for ship-
board installations to ensure accurate height information.
Applications of height- finding radars include the following:
a. Obtaining range, bearing, and altitude data on enemy aircraft and missiles to
assist in the control of CAP aircraft.
b. Detecting low-flying aircraft
c. Determining range to distant land masses
d. Tracking aircraft over land
e. Detecting certain weather phenomena
f. Tracking weather balloons
g. Providing precise range, bearing and height information for fast, accurate,
initial positioning of fire control tracking radars.
2. Tracking Radar: Radar that provides continuous positional data on a target is called tracking radar. Most
tracking radar system used by the military are also fired as control radars; the two names
are often used interchangeably.
Fired-control tracking radar systems usually produce a very narrow, circular beam.
Fired-control radar must be directed to the general location of the desired target because
of the narrow-beam pattern. This is called the DESIGNATION phase of equipment
operation. Once in the general vicinity of the target, the radar system switches to the
ACQUISITION phase of operation. During acquisition, the radar system searches a small
volume of space in a pre-arranged pattern until the target is located. When the target is
located the radar system enters the TRACK phase of operation. Using one of several
possible scanning techniques, the radar system automatically follows all targets motions.
The radar system is said to be locked on the target during the track phase. The three
sequential phases of operation are often referred to as MODES and are common to the
target-processing sequence of most fire-control radars.
Typical fire-control radar characteristics include a very high PRF, a very narrow pulse
width, and a very narrow beam-width. There characteristics, while providing extreme
accuracy, limit the range and make initial target detection difficult.
3. Missile-Guidance Radar: A radar system that provides information used to guide a missile to a hostile target is
called GUIDANCE RADAR. Missiles are radar to intercept targets in three basic ways:
a. Beam-rider missiles follow a beam of radar energy that is kept continuously pointed at
the desired target;
b. Homing missiles detect and home in on radar energy reflected from the target; the
reflected energy is provided by a radar transmitter either in the missile or at the
launch point and is detected by a receiver in a missile;
c. Passive homing missiles home in on energy that is radiated by the target. Because
target position must be known at all times, a guidance radar is generally part of, or
associated with, a fire-control tracking radar. In some instances, three radar beams
are required to provide complete guidance for a missile.
The beam-riding missile, for example, must be launched into the beam and then must
ride the beam to the target. Initially, a white beam is radiated by a capture radar to gain
control of the missile. After the missile enters the captured beam, a narrow beam is
radiated by a guidance radar to guide the missile to the target. During both capture and
guidance operations, a tracking radar continues to track the target.
4. Carrier-Controller Approach(CCA) and Ground-Controlled Approach Radar: These radar systems are essentially shipboard and land-based versions of the same type
of radars. Ship-board CCA radar systems are usually much more sophisticated systems
then GCA systems. This is because of the movements of the ship and the more
complicated landing problems. Both systems, however, guide aircraft to safe landing
under conditions approaching zero visibility. By means of radar, aircraft are detected and
observed during the final approach and landing sequence. Guidance information is
supplied to the pilot in the form of verbal radio instructions, or to the automatic pilot in the
form of pulsed control signals.
5. Airborne Radar: This is designed especially to meet the strict space and weight limitations that are
necessary for all airborne equipment. Even so, airborne radar sets develop the same
peak power as ship-board and shore-based sets.
As with ship-board radar, airborne radar sets come in many models and types to serve
many different purposes. Some of the sets are mounted in blisters that form part of the
fuselage; others are mounted in the nose of the aircraft.
In fighter aircraft, the primary mission of a radar is to aid in the search, interception and
destruction of enemy aircraft. This requires that the radar system have a tracking feature.
Airborne radar also has many other purposes. The following are some of the general
classifications: search, intercept and missile control, bombing, navigation and airborne
early warning.
3. Discuss antenna principle, function and parameters in detail.
Principle: When transmitting, a high gain antenna allows more of the transmitted power to be sent in the
direction of the receiver, increasing the received signal strength. When receiving, a high gain
antenna captures more of the signal, again increasing signal strength. Due to reciprocity,
these two effects are equal - an antenna that makes a transmitted signal 100 times stronger
(compared to an isotropic radiator), will also capture 100 times as much energy as the
isotropic antenna when used as a receiving antenna. As a consequence of their directivity,
directional antennas also send less (and receive less) signal from directions other than the
main beam. This property may be used to reduce interference.
Functions of an Antenna;
• An antenna is a device that acts as a transformer to provide a good match between the
feeding line as a local source of power and free space. If the antenna is not matched to
free space, power will be reflected back toward the transmitter, resulting in a loss in
radiated power. The antenna is one of the most critical parts of a radar system. It
performs the following essential functions:
• It transfers the transmitter energy to signals in space with the required distribution and
efficiency. This process is applied in an identical way on reception.
• It ensures that the signal has the required pattern in space. Generally this has to be
sufficiently narrow in azimuth to provide the required azimuth resolution and accuracy.
• It has to provide the required frequency of target position updates. In the case of a
mechanically scanned antenna this equates to the revolution rate. A high revolution rate
can be a significant mechanical problem given that a radar antenna in certain frequency
bands can have a reflector with immense dimensions and can weigh several tons.
• It must measure the pointing direction with a high degree of accuracy.
Parameters: a. Radiation pattern:
The radiation pattern is a graphical depiction of the relative field strength transmitted from or
received by the antenna, and shows sidelobes and backlobes. As antennas radiate in space
often several curves are necessary to describe the antenna. If the radiation of the antenna is
symmetrical about an axis (as is the case in dipole, helical and some parabolic antennas) a
unique graph is sufficient. Each antenna supplier/user has different standards as well as plotting formats. Each format
has its own advantages and disadvantages. Radiation pattern of an antenna can be defined
as the locus of all points where the emitted power per unit surface is the same. The radiated
power per unit surface is proportional to the squared electrical field of the electromagnetic
wave. The radiation pattern is the locus of points with the same electrical field. In this
representation, the reference is usually the best angle of emission. It is also possible to depict
the directive gain of the antenna as a function of the direction. Often the gain is given
in decibels.
The graphs can be drawn using cartesian (rectangular) coordinates or a polar plot. This last
one is useful to measure the beamwidth, which is, by convention, the angle at the -3dB points
around the max gain. The shape of curves can be very different in cartesian or polar
coordinates and with the choice of the limits of the logarithmic scale. The four drawings below
are the radiation patterns of a same half-wave antenna.
b. Efficiency:
"Efficiency" is the ratio of power actually radiated by an antennna to the electrical power it
receives from a transmitter. A dummy load may have an SWR of 1:1 but an efficiency of 0, as
it absorbs all the incident power, producing heat but radiating no RF energy; SWR is no
measure of an antenna's efficiency. Radiation in an antenna is caused by radiation
resistance which cannot be directly measured but is a component of the
total resistance which includes the loss resistance. Loss resistance results in heat generation
rather than radiation, thus reducing efficiency. Mathematically, efficiency is equal to the
radiation resistance divided by total resistance (real part) of the feedpoint impedance.
c. Bandwidth:
IEEE defines bandwidth as "The range of frequencies within which the performance of the
antenna, with respect to some characteristic, conforms to a specified standard." In other
words, bandwidth depends on the overall effectiveness of the antenna through a range of
frequencies, so all of these parameters must be understood to fully characterize the
bandwidth capabilities of an antenna. This definition may serve as a practical definition,
however, in practice, bandwidth is typically determined by measuring a characteristic such as
SWR or radiated power over the frequency range of interest. For example, the SWR
bandwidth is typically determined by measuring the frequency range where the SWR is less
than 2:1.
d. Directivity:
Antenna directivity is the ratio of maximum radiation intensity (power per unit surface)
radiated by the antenna in the maximum direction divided by the intensity radiated by a
hypothetical isotropic antenna radiating the same total power as that antenna. For example, a
hypothetical antenna which had a radiated pattern of a hemisphere (1/2 sphere) would have a
directivity of 2. Directivity is a dimensionless ratio and may be expressed numerically or
in decibels (dB). Directivity is identical to the peak value of the directive gain; these values are
specified without respect to antenna efficiency thus differing from the power gain (or simply
"gain") whose value is reduced by an antenna's efficiency.
4. Discuss radar equation for Search radars, Tracking radars, CW and Pulse Doppler radar. The power Pr returning to the receiving antenna is given by the radar equation:
where
§ Pt = transmitter power
§ Gt = gain of the transmitting antenna
§ Ar = effective aperture (area) of the receiving antenna
§ σ = radar cross section, or scattering coefficient, of the target
§ F = pattern propagation factor
§ Rt = distance from the transmitter to the target
§ Rr = distance from the target to the receiver.
In the common case where the transmitter and the receiver are at the same
location, Rt = Rr and the term Rt² Rr² can be replaced by R4, where R is the range. This
yields:
This shows that the received power declines as the fourth power of the range,
which means that the reflected power from distant targets is very, very small.
The equation above with F = 1 is a simplification for vacuum without interference.
The propagation factor accounts for the effects of multipath and shadowing and
depends on the details of the environment. In a real-world
situation, pathloss effects should also be considered.
5. Explain Radar cross-section (RCS) definition and fundamentals. Definition
Informally, the RCS of an object is the cross-sectional area of a perfectly reflecting sphere
that would produce the same strength reflection as would the object in question. (Bigger sizes
of this imaginary sphere would produce stronger reflections.) Thus, RCS is an abstraction:
The radar cross-sectional area of an object does not necessarily bear a direct relationship
with the physical cross-sectional area of that object but depends upon other factors.
Somewhat less informally, the RCS of a radar target is an effective area that intercepts the
transmitted radar power and then scatters that power isotropically back to the radar receiver.
More precisely, the RCS of a radar target is the hypothetical area required to intercept the
transmitted power density at the target such that if the total intercepted power were re-
radiated isotropically, the power density actually observed at the receiver is produced.This is
a complex statement that can be understood by examining the monostatic (radar transmitter
and receiver co-located) radar equation one term at a time:
where
§ Pt = power transmitted by the radar (watts)
§ Gt = gain of the radar transmit antenna (dimensionless)
§ r = distance from the radar to the target (meters)
§ σ = radar cross section of the target (meters squared)
§ Aeff = effective area of the radar receiving antenna (meters squared)
§ Pr = power received back from the target by the radar (watts)
The term in the radar equation represents the power density (watts per meter
squared) that the radar transmitter produces at the target. This power density is
intercepted by the target with radar cross section σ, which has units of area (meters
squared). Thus, the product has the dimensions of power (watts), and
represents a hypothetical total power intercepted by the radar target. The
second term represents isotropic spreading of this intercepted power from the
target back to the radar receiver. Thus, the product represents the
reflected power density at the radar receiver (again watts per meter squared). The
receiver antenna then collects this power density with effective area Aeff, yielding the
power received by the radar (watts) as given by the radar equation above.
The scattering of incident radar power by a radar target is never isotropic (even for a
spherical target), and the RCS is a hypothetical area. In this light, RCS can be viewed
simply as a correction factor that makes the radar equation "work out right" for the
experimentally observed ratio of Pr / Pt. However, RCS is an extremely valuable
concept because it is a property of the target alone and may be measured or calculated.
Thus, RCS allows the performance of a radar system with a given target to be analysed
independent of the radar and engagement parameters. In general, RCS is a strong
function of the orientation of the radar and target, or, for the bistatic (radar transmitter
and receiver not co-located), a function of the transmitter-target and receiver-target
orientations. A target's RCS depends on its size, reflectivity of its surface, and
the directivity of the radar reflection caused by the target's geometric shape
Fundamentals:
Radar cross section (RCS) is a measure of how detectable an object is with a radar. A
larger RCS indicates that an object is more easily detected.
An object reflects a limited amount of radar energy. A number of different factors determine
how much electromagnetic energy returns to the source such as:
§ material of which the target is made;
§ absolute size of the target;
§ relative size of the target (in relation to
the wavelength of the illuminating radar);
§ the incident angle (angle at which the radar beam hits a
particular portion of target which depends upon shape of target and its orientation to the
radar source);
§ reflected angle (angle at which the reflected beam leaves the part
of the target hit, it depends upon incident angle);
§ strength of the radar emitter;
§ distance between emitter-target-receiver.
While important in detecting targets, strength of emitter and distance are not factors that
affect the calculation of a RCS because the RCS is (approximately) only a property of the
target.
Radar cross section is used to detect planes in a wide variation of ranges. For example,
a stealth aircraft (which is designed to have low detectability) will have design features that
give it a low RCS (such as absorbent paint, smooth surfaces, surfaces specifically angled to
reflect signal somewhere other than towards the source), as opposed to a passenger airliner
that will have a high RCS (bare metal, rounded surfaces effectively guaranteed to reflect
some signal back to the source, lots of bumps like the engines, antennae, etc.). RCS is
integral to the development of radar stealth technology, particularly in applications
involving aircraft and ballistic missiles. RCS data for current military aircraft is most highly
classified.
6. What is ranging and condition of ranging? Radar measurement of range, or distance, is made possible because of the properties of
radiated electromagnetic energy. This energy normally travels through space in a straight line
at a constant speed, and will vary only slightly because of atmospheric and weather
conditions.
Electromagnetic energy travels through air at approximately the speed of light, which is
186,000 statute miles per second. The navy uses nautical miles to calculate distances;
186,000 statute mile = 162,000 nautical miles.
Radar timing is usually expressed in microseconds to relate radar timing to distances
travelled by radar energy. You should know that radiated energy from a radar set travels at
approximately 984 feet per microsecond.
A pulse type radar set transmits a short burst of electromagnetic energy.
Target range is determined by measuring lapsed time while the pulse travels to and returns
from the target. Because to-way travel is involve, a total time of 12.36 microseconds per
nautical mile will elapse between the start of the pulse from the antenna and its return to the
antenna from a target. This 12.36 microsecond time interval is sometimes referred to as a
RADAR MILE, RADAR NAUTICAL MILE or NAUTICAL RADAR MILE. The range in nautical
mile to an object can be found by measuring the elapse time during a round trip of a radar
pulse and dividing the quantity by 12.36.
Equation: range = elapsed time .
12.36 microseconds/nm
Minimum range: As the DUPLEXER alternately switches the antenna between the transmitter and receiver so
that only one antenna need to be used. This switching is necessary because the high power
pulses of the transmitter would destroy the receiver if energy were allowed to enter the
receiver. The timing of this switching action is critical to the operation of the radar system. The
minimum range ability of the radar system is also affected by this timing. The two most
important times in this action are PULSE WIDTH and RECOVERY TIME.
This timing action must be such that during the transmitted pulse (pulse width), only the
transmitter can be connected to the antenna. Immediately after the pulse is transmitted, the
antenna must be reconnected to the receiver.
The leading edge of the transmitted pulse causes the duplexer to align the antenna to the
transmitter. This action is essentially instantaneous. At the end of the transmitted pulse, the
trailing edge of the pulse causes the duplexer to line up the antenna with the receiver;
however, this action is not instantaneous. A small amount of this time elapses at this point
that is referred to as recovery time.
Therefore, the total time in which the receiver is unable to receive the reflected pulse = pulse
width + the recovery time.
Note that any reflected pulses from close targets returning before the receiver is connected to
the antenna will be undetected. The minimum range in yards at which a target can be
detected is determined by :
Minimum range = [ pulse width (m.sec) + recovery time (m.sec) ] x 164 yards
Maximum range: The maximum range of a pulse radar system depends upon CARRIER FREQUENCY,
PEAKPOWER of the transmitted pulse, PULSE-REPETITION FREQUENCY (PRF)
or PULSE-REPETITION RATE (PRR), and RECEIVER SENSITIVITY with PRF as the
primary limiting factor. The peak power of the pulse determines what maximum range the
pulse can travel to a target and still return a usable echo. A usable is the smallest signal
detected by a receiver system that can be processed and presented in an indicator.
The frequency of the RF energy in the pulse radiated by a radar is referred to as the
CARRIER FREQUENCY of the radar system. The carrier frequency is often a limiting factor in
the maximum range capability of a radar system because radio frequency above 3,000 MHz
is rapidly attenuated by the atmosphere. This decreases the usable range of a radio-
frequency energy. Therefore, as the carrier frequency is increased, the transmitted power
must also be increased to cover the same range. Long-range coverage is more easily
achieved at lower frequencies because atmospheric conditions have less effect on low-
frequency energy.
Radar systems radiate each pulse at the carrier frequency during transmit time, wait for
returning echoes during listening or rest time, and then radiate a second pulse. The number
of pulses radiated in one second is called the pulse-repetition frequency (PRF), or the pulse-
repetition rate (PRR). The time between the beginning of one pulse and the start of the next
pulse is called PULSE-REPETITION TIME (PRT) and is equal to the reciprocal of PRF.
7. Discuss the evaluation of PRF classes in detail with their advantages and disadvantages. Pulse repetition frequency (PRF) is the number of pulses per time unit (e.g. Seconds). It is
mostly used within various technical disciplines (e.g. Radar technology) to avoid confusion
with the unit of frequency hertz (Hz) mainly used for waves. Waves are thought of as more or
less pure single frequency phenomena while pulses may be thought of as composed of a
number of pure frequencies. The reciprocal of PRF is called the Pulse Repetition Time (PRT),
Pulse Repetition Interval (PRI), or Inter-Pulse Period (IPP), which is the elapsed time from the
beginning of one pulse to the beginning of the next pulse. Within radar technology PRF is
important since it determines the maximum target range (Rmax) and
maximum Doppler velocity (Vmax) that can be accurately determined by the radar.
ADVANTAGE:
The advantage of pulse frequency modulation is better immunity to noise interference than
PAM.
DISADVANTAGE:
The disadvantage is more complex transmitter and receiver design.
POTENTIAL APPLICATIONS:
It has been proposed that PFM could serve as a suitable retinal prosthesis device. PFM's
ability to operate independently and asynchronously promotes the flow of nutrients through
the chip which is essential for living cells. Cell stimulation would be accomplished through the
output of pulse streams by PFM. A large dynamic range is also practical in the replacement
of photoreceptors. [3] PFM could be used to transmit intelligence signals, such as audio
signals, by demodulating the signal from the receiving end at the transmitting end.
RANGE AMBIGUITY:
A radar system determines range through the time delay between pulse transmission and
reception by the relation:
For accurate range determination a pulse must be transmitted and reflected before the
next pulse is transmitted. This gives rise to the maximum range limit:
The maximum range also defines a range ambiguity for all detected targets.
Because of the periodic nature of pulsed radar systems, it is impossible for a radar
system to determine the difference between targets separated by integer multiples
of the maximum range using a single PRF. More sophisticated radar systems
avoid this problem through the use of multiple PRFs either simultaneously on
different frequencies or on a single frequency with a changing PRT.
8. Discuss various techniques used for tracking. A radar tracker is a component of a radar system, or an associated command and control
(C2)system, that associates consecutive radar observations of the same target into tracks. It
is particularly useful when the radar system is reporting data from several different targets or
when it is necessary to combine the data from several different radars or other sensors.
a. Range Tracking
Range tracking is accomplished in a similar manner to dual-beam angle tracking. Once the
range has been measured, the tracking system attempts to predict the range on the next
pulse. This estimate becomes the reference to which the next measurement will be
compared. The comparison is made by using two range windows called
the early and late range gates.
Figure 8.
Range gates.
The area of the return in each gate is computed by integration. The difference between the
area in the early and late gates is proportional to the error in the range estimate. If the two
areas are equal, the return is centered directly on the range estimate, and there is no error. If
the return has more area in the early gate, the range estimate is too great, and therefore the
range error is positive. In the near vicinity of the range estimate, there will be a linear
relationship between the range error and the difference in the areas.
Figure 9. Range error.
As long as the range estimate is not too far off, the tracking error can be determined and the
target range updated. Again, like dual-beam tracking, the range tracking system can measure
the target range with greater accuracy than the range resolution of the system, Rres, which is
determined by the pulse width and possibly the pulse compression ratio.
b. Track-while-scan (TWS)
In many cases, it would be undesirable to dedicate the entire radar system to tracking
a single target. We have already seen that the servo tracking system maintains the
antenna pointed in the vicinity of the target at all times. Unfortunately, there is no
search capability when tracking using this method. The track-while-scan (TWS)
system maintains the search function, while a computer performs the tracking
functions. The TWS system is capable of automatically tracking many targets
simultaneously. Furthermore, the TWS system can also perform a variety of other
automated functions, such as collision or close CPA warnings.
The TWS system manages targets using gates. We have already seen an example of
gates used in the range tracking system. A TWS system may use range, angle,
Doppler and elevation gates in order to sort out targets from one another. When a
target is first detected, the computer will assign it an acquisition gate, which has fixed
boundaries of range and bearing (angle), and possibly other parameters, depending
on the system. When the radar sweeps by the target again, if the return still falls
within the acquisition gate, the computer will initiate a track on the
target.
figure 10. Tracking and acquisition gates.
By following the history of the target positions, the course and speed of the target can be
found. The combination of range, bearing, course and speed at any one time is known as the
target's solution. It is used to predict where the target will be at the next observation. Once a
solution has been determined, the computer uses a tracking gate about its predicted position.
If the target falls within the predicted tracking gate, the computer will refine its solution and
continue tracking. If the target is not within the tracking gate at the next observation, it will
check to see if the target is within a turning gate which surrounds the tracking gate.
Figure 11. Use of a turning gate
to maintain track on a maneuvering target.
The turning gate encompasses all the area that the target could be in since the last
observation. If the target is within the turning gate, the computer starts over to obtain the new
solution. If the target falls outside of the turning gate, the track will be lost. The system will
continue to predict tracking gates in case the target reappears. Depending on the system, the
operator may be required to drop the track.
The process of assigning observations with established tracks is known as correlation. During
each sweep, the system will attempt to correlate all returns with existing tracks. If the return
cannot be correlated, it is assigned an acquisition gate, and the process begins again. On
some occasions, a new target may fall within an existing tracking gate. The system will
attempt to determine which return is the existing target and which is the new target, but may
fail to do so correctly. It is common for TWS systems to have difficulty when there are many
targets, or when existing tracks cross each other. In the later case, the computer may
exchange the identity of the two crossing tracks. In all these cases if mistaken identity, the
operator must intervene to correct the problem.
Figure 12. Crossing
tracks.
The TWS system uses a track file for each established target that it tracks. The track file
contains all of the observations that are correlated with that particular target. For example, the
range, bearing and time of observation. The track file is given a unique name known as
the track designation. This is usually either a simple number, like "track 25". Depending on
the system, the track file may contain other useful information, such as the classification of
the target, such as "ship" or "aircraft". This information may be used by the computer when
determining the track and turning gates. Finally, the track file also contains the current
solution to the tracks motion. Some systems maintain a history of solutions which can be
useful in determining the pattern of a maneuvering target. For example, if a target alters
course every 15 min, such as a preset "zig" pattern.
c. Phased Array Tracking
We have already seen that a phased array radar system can electronically steer the beam.
But the system can also perform a track-while-scan function. Since the planar array has many
independent elements, they need not all be used to form a single beam. In fact, the great
advantage of the phased array system is its flexibility. The SPY-1 phased array radar has
over 4,000 elements, any number of which may be combined into a single beam. Suppose for
instance, that the array was split into groups of 40 elements each. That would give about 100
independent beams. Granted the beamwidth of the 40 element array would not be as small as
the 4000 element beam, but at shorter range could function more than adequately.
In the phased array radar, some beams could be dedicated to search functions, while other
could perform dedicated tracking functions. Therefore you have the search-while-track
features, but with the added benefit of continuous contact on the targets. Essential to the
SPY-1 capability is a computing system powerful enough to perform all of the necessary
functions to control more than 100 independent tracking beams.
Figure 13. Multiple beams
of phased array radar.
d. Tracking Networks It is a natural extension of the track-while-scan system to create a system which shares
tracking information between users. All that is required is to transmit the contents of the
track file, since it contains all of the observations and the current solution. The sharing of
tracking information has been incorporated extensively into modern combat. There are
now global command and control networks that share this information between users all
over the world.
Tracking networks have adopted a standardized set of symbols for identifying types of
targets.
9. Discuss Tracking Accuracy.
The accuracy can be improved by using a dual-beam system. The two beams are offset in
angle by a small amount to either side. The center between the beams is known as
the boresight axis.
Figure 4. Dual beams.
The two beams can be created by a dual-feed system, where the two parallel beams are fed
into the reflector slightly to one side or the other. When the beam is reflected, the offset in the
feed axis will cause the beam to be reflected off at an angle relative to the boresight.
Figure 5. How offset feed changes
beam.
Now when a dual-beam system scans across a target, the return will be the sum of the two
beams. If one of the beams if inverted (or made out-of-phase), the result will have a well-
defined location of the target, namely where the difference between the beams is zero.
Figure 6. Constructing the dual
beam output.
Since the return strength is changing rapidly to either side, the location of the target can be
determined with great accuracy. For a typical radar beam that is 30 wide, a dual-beam system
could track the target with 0.10accuracy, which is sufficient for weapons delivery.
Another nice feature of the dual-beam system is that the return strength varies nearly linearly
in the vicinity of the target. Therefore it is easy to measure the target location even if the
boresight is not directly on the target. The difference in target location and the boresight will
be linearly proportional to the return strength as long as the target is not too far off center.
Using the maximum strength method, it is not even clear how to determine the correction
direction to reposition the antenna, since the return strength varies equally to either side of
the boresight.
The dual-beam system can also be used for tracking in elevation. In fact, a monopulse
system uses two dual-beam systems, one for elevation and one for azimuth. This requires
four beams which are measured in pairs.
Figure 7. Monopulse radar.
Dual-beam systems are generally used for fire control tracking, where high accuracy is
required. A dual beam system has a limited range because the target is not in the maximum
power portion of the beam. The target is off-axis for either beam.
Some are shown below:
Figure 14. Some standard tracking symbols.
These symbols appear on the common operational picture displays now used for command
and control functions.
10. Discuss various Modulations. 1. Amplitude Modulation
a. Amplitude modulation (AM) is a technique used in electronic communication, most
commonly for transmitting information via a radio carrier wave. AM works by varying
the strength of the transmitted signal in relation to the information being sent. For
example, changes in the signal strength can be used to specify the sounds to be
reproduced by a loudspeaker, or the light intensity of television pixels. (Contrast this
with frequency modulation, also commonly used for sound transmissions, in which
the frequency is varied; and phase modulation, often used in remote controls, in
which the phase is varied)
b. Vary the amplitude of the carrier sine wave
2. Frequency Modulation a. Vary the frequency of the carrier sine wave
b. In telecommunications and signal processing, frequency modulation (FM)
conveys information over a carrier wave by varying its instantaneous frequency. This
is in contrast with amplitude modulation, in which the amplitude of the carrier is varied
while its frequency remains constant.
In analog applications, the difference between the instantaneous and the base
frequency of the carrier is directly proportional to the instantaneous value of the input
signal amplitude. Digital data can be sent by shifting the carrier's frequency among a
set of discrete values, a technique known as frequency-shift keying.
Frequency modulation can be regarded as phase modulation where the carrier phase
modulation is the time integral of the FM modulating signal.
FM is widely used for broadcasting of music and speech, and in two-way
radio systems, in magnetic tape recording systems, and certain video transmission
systems.
In radio systems, frequency modulation with sufficient bandwidth provides an
advantage in cancelling naturally-occurring noise.
Frequency-shift keying (digital FM) is widely used in data and fax modems.
3. Pulse-Amplitude Modulation
a. Vary the amplitude of the pulses
b. Pulse-amplitude modulation, acronym PAM, is a form of signal modulation where
the message information is encoded in the amplitude of a series of signal pulses.
c. Demodulation is performed by detecting the amplitude level of the carrier at every
symbol period.Pulse-amplitude modulation is widely used in baseband transmission
of digital data, with non-baseband applications having been largely replaced by pulse-
code modulation, and, more recently, by pulse-position modulation.
In particular, all telephone modems faster than 300 bit/s use quadrature amplitude
modulation (QAM). (QAM uses a two-dimensional constellation).
4. Pulse-Frequency Modulation a. Vary the Frequency at which the pulses occur
b. Pulse-Frequency Modulation (PFM) is a modulation method for representing
an analog signal using only two levels (1 and 0). It is analogous to pulse-width
modulation (PWM), to which the reader may refer for more detailed information, as
the magnitude of an analog signal is encoded in the duty cycle of a square wave. If a
pulse rate is set to 8000 pulses per second at 0 signal voltage then when the signal
voltage reaches maximum the pulse rate will step up to 9000 but when the negative
maximum voltage is reached the pulse rate will step down to 7000. A stable oscillator
that is frequency modulated is used to create the pulse rate because of this PFM is
not as widely used. Unlike PWM, in which the width of square pulses is varied at
constant frequency, PFM is accomplished using fixed-duration pulses and varying the
repetition rate thereof. In other words, the frequency of the pulse train is varied in
accordance with the instantaneous amplitude of the modulating signal at sampling
intervals. The amplitude and width of the pulses is kept constant. The advantage of
pulse frequency modulation is better immunity to noise interference than PAM. The
disadvantage is more complex transmitter and receiver design.