Aviation Sri Lanka
site discusses the principles of airborne radio navigation, whereby the pilot
can determine the aircraft's position by triangulation using two ADF or VOR
stations, or by polar coordination using VOR bearing and DME distance
information. Upon request, the pilot can relay the aircraft's position and
altitude to the Air Traffic Control (ATC) center by means of the VHF
communication system. Although this method is effective, it is not an optimum
solution in high-traffic areas where the controller must be constantly informed
of the exact position of all aircraft at all times within the controlled
airspace(Though there is no such higher density of traffic in our region). For
this reason, the ATC center uses a ground-based radar surveillance system to
automatically monitor the location of all aircraft within the control area
without cluttering up the radio communication channels. With this information
constantly displayed on the ATC radar scope, the controller is able to make
timely decisions regarding handing over aircraft to the approach or departure
control center, vectoring aircraft to avoid collision courses, maintaining safe
altitude separation between aircraft, and locating and directing aircraft that
The ground-based ATC radar system consists of a primary surveillance radar (PSR) and secondary surveillance radar (SSR). The PSR locates and tracks aircraft within the control area by transmitting a beam of energy which is reflected from the aircraft and returned to the PSR antenna. The SSR transmits interrogation signals to the airborne radio beacon Transponder. Upon receiving the interrogation, the Transponder sends a coded reply signal back to the SSR system. Data received from the PSR and SSR are used in conjunction to develop the total air traffic situation display on the controller's radar scope. This enables the controller to identify Transponder-equipped aircraft in addition to determining the range and direction of all aircraft within the control area.
PRINCIPLES OF ATC RADAR SURVEILLANCE SYSTEM OPERATION
are two types of radar systems installed each ATC ground station. The first,
called the Primary Surveillance Radar, operates on the principle of sending a
narrow beam of energy, which is reflected from the aircraft under surveillance,
and measuring its distance by noting the time lapse between the radar pulse
transmission and its received echo. The second, called the Secondary
Surveillance Radar, operates on the coded reply sent from the airborne radio
beacon Transponder in response to an interrogation sent from the ground station.
The Radar station at Piduruthalagala(Sri Lanka), the PSR and SSR antennas are
co-located and scan synchronized, and both radars are used in conjunction to
develop the total air traffic situation display on a single CRT radar scope,
called the Plan Position Indicator (PPI). The Radar station at Katunayaka(Sri
Lanka) is a PSR type and rotate at a speed of 15 revolution per minute while
Radar at Piduruthalagala at a speed of 12 rev. per minutes. The reason for this
difference in revolution speed is that, as aircrafts approaches close to the
airport the sky gets congested and needs constant updates of the positions of
the air movements.
ATC PSR/SSR System
PSR sends out radio waves in a very narrow beam. The ground antenna is made to
rotate so that the position of the narrow beam of energy can be directed. When
the directed beam strikes an object or target, some of it is reflected back to
the radar antenna. This reflected signal is detected and processed to provide a
display (indicated by a bright "blip') on the ATC PPI, which shows the
location of the target (i.e., aircraft).
Radar antenna ~ Katunayaka
PSR system works well in low traffic areas; however, as the air traffic
increases in a given area, the PPI display becomes cluttered-and specific
targets may become difficult to distinguish from one another. Also, since the
energy of a radiated RF signal is attenuated as the square of the distance it
travels, the resulting weaker radar returns are accompanied by noise which tends
to obscure the displayed target. Targets may also be lost due to ground clutter
from terrain and precipitation unless a Moving Target Indicator (MTI) circuit is
employed to detect and display only moving objects. Finally, the PSR has the
distinct disadvantage In that the operator has no way of knowing the altitude of
the aircraft unless the pilot reports It. All of the problems associated with
the PSR system have been addressed with the introduction of Air Traffic Control
Radio Beacon System (ATCRBS).
ATCRBS incorporates the use of the Secondary Surveillance Radar in conjunction
with the airborne radio beacon Transponder. The SSR was developed from the
military Identification-Friend-or-Foe (IFF) system, in which an airborne radio
beacon Transponder responds to ground radar interrogations on one frequency by
transmitting, coded replies on another frequency. The coded replies, displayed
as short lines on the PPI, allow the controllers to identify the various targets
by having each one send back a different coded reply.
desired code can be manually selected by the pilot on the Transponder control
head in Mode W operation, or automatically set by an encoding altimeter or
altitude digitizer for reporting the Aircraft's altitude in Mode "C"
operation. Since the reply signal from the airborne Transponder is stronger than
the reflected PSR signal, it will reinforce the "pip" on the PPI to
provide positive aircraft identification.
the ATC radar ground station, received radar video and antenna azimuth
Information signals are relayed from the radar site to the air traffic control
center, where the signals are processed and displayed on plan position
indicators. Since radar coverage of each site includes a large area in high
traffic density areas, several controllers are assigned to various segments of
the area covered. Each controller's segment of the area is displayed on his
respective PPI. (You cannot observe such situation in Sri Lanka due to limited
number of air movements prevail in compared to other countries).
PPI presents the operator with a map like view of the space surrounding the area
covered by the ATC radar antenna. Four dots appear on the PPI; one at the
center, and one of each of the three 1 0-mile points out to the edge of the
radar scope. These dots rotate, in synchronize with the rotation of the radar
antenna, to display concentric circles that indicate range.
incoming radar video signals are applied to a decoder control before being
displayed. By adjusting the decoder to pass only a selected code, Transponders
operating on the controller's code will appear as a short arc (blip) on the PPI,
and as a bright arc when transmitting a special position identification pulse.
Replies from Transponders not transmitting the selected code will be filtered
out. "Skin-paint" echoes detected by the primary surveillance radar
will be displayed for all aircraft. An illustration of a typical PPI display
format is shown in the figure below.
PRINCIPLES OF RADIO BEACON TRANSPONDER OPERATION
previously mentioned, the ATC radio beacon system incorporates the use of the
ground based SSR and the airborne radio beacon Transponder to determine the
range and direction of aircraft responding to SSR interrogations. The following
section will discuss the operation of the airborne Transponder in regard to
receiving these interrogation signals and generating a coded reply signal to be
transmitted back to the SSR ground station.
airborne Transponder transmits a reply signal on a frequency of 1,090 MHz in
response to the SSR interrogation which is transmitted on a frequency of 1,030
MHz. Currently, there are two types of SSR interrogations, Mode "K and Mode
"C", that may be transmitted by the ATCRBS ground station. The signal
characteristics of the Mode A and Mode C interrogations are shown in the figure.
SSR Interrogation Modes
A interrogations are sent to request the specified aircraft identification code.
Mode C Is used to request altitude reporting with identification. Mode B is
occasionally used in place of Mode A in some countries and Mode D in presently
not in use. Each interrogation mode is distinct from the other and is
characterized by the spacing between the P3 pulse and the P1 pulse. Regardless
of the interrogation mode, all three pulses are 0.8 microsecond wide.
purpose of the P2 pulse is to allow the Transponder to determine whether the
interrogation was received from the main beam or a side lobe of the SSR
radiation pattern, as shown in the following figure. A reply to a side-lobe
interrogation would give the controller an erroneous indication of the
aircraft's Position. For this reason, Side-Lobe Suppression (SLS) is used to
inhibit the Transponder's reply in response to a side-lobe interrogation.
pattern of SSR Interrogation Signal
three-pulse SLS interrogation method uses a directional radar antenna that
transmits a pair of pulses referred to as PI and P3 pulses. As previously
mentioned, the time spacing between these pulses determines the mode of
operation. Two microseconds after the P1 pulse is transmitted from the
directorial antenna, the second pulse, P2, is transmitted from an
omnidirectional antenna. The P2 pulse is used as a reference pulse for SLS
determination. The signal strength of the omni-directional P2 pulse is just
sufficient to provide coverage over the area that side-lobe propagation presents
interrogation is detected by the airborne Transponder SLS circuitry by comparing
the amplitude of the P2 pulse in relation to the PI pulse. When the
omnidirectional P2 pulse is equal to or greater than the directional P1 pulse,
no reply will be generated. Identification of the side-lobe interrogation is
established before the P3 pulse is received. Therefore, the Transponder will be
inhibited for a period lasting 35 microseconds, regardless of the interrogation
mode. A valid main-lobe interrogation is recognized when the PI pulse is at
least 9dB larger than the P2 pulse, as shown in the following figure.
Detection and Reply Suppression
Transponder Reply Signals
signals are generated by the Transponder when an interrogation signal is
determined to be valid. The coded reply signal is composed of a series of pulses
transmitted on a carrier of 1,090 plus or minus 3 MHz. In Mode A operation, the
number of pulses generated in a reply signal is determined by setting the four
octal (0 to 7) digit code switches on the Transponder control head to the
assigned identification code. Certain switch positions are reserved for special
applications to cause the activation of an aural alert signal at the
controller's console: Code 7700 indicates an
emergency condition, code 7600 is for reporting a communication radio failure,
and code 7500 indicates that a hijack is in process.
The code selector switches provide the Transponder with the capability to send
any one of a possible 4,096 identification codes.
The Transponder replies to Mode C interrogations by generating pulses in the reply signal that corresponds with the aircraft's altitude. The received altitude information is then displayed directly on the controller's PPI. This information is not selected by the code switches on the control head, but is obtained directly from an encoding altimeter or altitude digitizer. These devices commonly use an optical encoder which is driven from an aneroid mechanism that is sensitive to variations in altitude. The encoder outputs a 1 0-bit parallel data code to the Transponder for the generation of Mode C replies.
coded reply signal consists of various arrangements of code pulses within the
boundaries formed by the two framing pulses, F1 and F2. Regardless of the mode
of operation, these framing pulses are always present in the coded reply signal
and are spaced 20.3 microseconds apart.
The reply code is divided into four pulse groups labeled A, B, C, and D. Each group contains three pulses that are assigned subscripts that indicate the binary weight of each. The first digit (1) consists of the Al pulse (=I), the second digit (3) consists of the 131 + B2 pulses (=3), the third digit (2) consists of the C2 pulse (=2), and the fourth digit (4) consists only of the D4 pulse (=4). The assigned reply code-0000 would cause no pulses to appear between the framing pulses, and code 7777 would result in all 12 pulses to be present between FI and F2.
Special Position Identification Pulse (SPIP), initiated upon request of the
controller, is generated by momentarily depressing the IDENT button located on
the Transponder control head. The SPIP causes a special effect on the
controller's PPI that aids in determining the aircraft's position. This pulse
occurs 4.35 microseconds after the last framing pulse (F2) and is transmitted
with each Mode A reply for 15 to 20 seconds after releasing the IDENT button.
is an acronym for Radio Detection and Ranging. The term "radio" refers
to the use of electromagnetic waves with wavelengths in the so-called radio wave
portion of the spectrum, which covers a wide range from 104 km to 1
cm. Radar systems typically use wavelengths on the order of 10 cm,
corresponding to frequencies of about 3 GHz. The detection and
ranging part of the acronym is accomplished by timing the delay between
transmission of a pulse of radio energy and its subsequent
= pulse width. PW has units of time and is commonly expressed in ms.
PW is the duration
of the pulse. RT = rest time. RT is the interval between pulses. It is measured
in ms. PRT = pulse
repetition time. PRT has units of time and is commonly expressed in ms. PRT
is the interval between the start of one pulse and the start of another. PRT is
also equal to the sum, PRT = PW+RT. PRF = pulse repetition frequency. PRF has
units of time-1 and is commonly expressed in Hz (1 Hz = 1/s) or as
pulses per second (pps). PRF is the number of pulses transmitted per second and
is equal to the inverse of PRT. RF = radio frequency. RF has units of time-1
or Hz and is commonly expressed in GHz or MHz. RF is the frequency of the
carrier wave which is being modulated to form the pulse train.
practical radar system requires seven basic components.
The transmitter creates the radio wave to be sent and modulates it to form the
pulse train. The transmitter must also amplify the signal to a high
power level to provide adequate range. The source of the carrier wave
could be a Klystron, Traveling Wave Tube (TWT) or Magnetron. Each has
its own characteristics and limitations.
> (S/N) NEP,
Pr is the power of the return signal. Since this is a significant
quantity in determining radar system performance, it is given a unique
designation, Smin, and is called the Minimum Signal for Detection.
= (S/N) NEP
Smin, expressed in Watts, is usually a small number, it has proven
useful to define the decibel equivalent, MDS, which stands for Minimum
= 10 Log (Smin/1 mW)
using decibels, the quantity inside the brackets of the logarithm must be a
number without units. I the definition of MDS, this number is the fraction Smin
/1 mW. As a reminder, we use the special notation dBm for the units of
MDS, where the "m" stands for 1 mW. This is shorthand
for decibels referenced to 1 mW, which is sometimes written as dB//1mW.
common receiver features are:
= PW / PRF
synchronizer coordinates the timing for range determination.
It regulates that rate at which pulses are sent (i.e. sets PRF) and resets the
timing clock for range determination for each pulse. Signals from the
synchronizer are sent simultaneously to the transmitter, which sends a new
pulse, and to the display, which resets the return sweep.
This is a switch which alternately connects the transmitter or receiver to the
antenna. Its purpose is to protect the receiver from the high power output of
the transmitter. During the transmission of an outgoing pulse, the duplexer will
be aligned to the transmitter for the duration of the pulse, PW. After the pulse
has been sent, the duplexer will align the antenna to the receiver. When the
next pulse is sent, the duplexer will shift back to the transmitter. A duplexer
is not required if the transmitted power is low.
The antenna takes the radar pulse from the transmitter and puts it into the air.
Furthermore, the antenna must focus the energy into a well-defined beam which
increases the power and permits a determination of the direction of the target.
The antenna must keep track of its own orientation which can be accomplished by
a synchro-transmitter. There are also antenna systems which do not physically
move but are steered electronically (in these cases, the orientation of the
radar beam is already known a priori).
wavelength, l=c/f = 0.33 m.
Display. The display unit may take a variety of
forms but in general is designed to present the received information to an
operator. The most basic display type is called an A-scan (amplitude vs. Time
delay). The vertical axis is the strength of the return and the horizontal axis
is the time delay, or range. The A-scan provides no information about the
direction of the target.
of the parameters of the basic pulsed radar system will affect the performance
in some way. Here we find specific examples and quantify this dependence where
duration of the pulse and the length of the target along the radial direction determine
the duration of the returned pulse. In most cases the length of the return
is usually very similar to the transmitted pulse. In the display unit, the pulse
(in time) will be converted into a pulse in distance. The range of values from
the leading edge to the trailing edge will create some uncertainty in the range
to the target. Taken at face value, the ability to accurately measure
range is determined by the pulse width.
= c PW/2
create a perfectly formed pulse with a vertical leading edge would require an
infinite bandwidth. In fact you may equate the bandwidth, b,
of the transmitter to the
minimum pulse width, PW by:
this insight, it is quite reasonable to say that the range can be determined no
more accurately than cPW/2 or equivalently
fact, high resolution radar is often referred to as wide-band radar which you
now see as equivalent statements. One term is referring to the time domain and
the other the frequency domain. The duration of the pulse also affects the
minimum range at which the radar system can detect. The outgoing pulse must
physically clear the antenna before the return can be processed. Since this
lasts for a time interval equal to the pulse width, PW, the minimum displayed
range is then:
= c PW/2
the pulse width while maintaining the other parameters the same will also affect
the duty cycle and therefore the average power. For many systems, it is
desirable to keep the average power fixed. Then the PRF must be simultaneously
changed with PW in order to keep the product PW x PRF the same. For example, if
the pulse width is reduced by a factor of ½ in order to improve the resolution,
then the PRF is usually doubled.
Repetition Frequency (PRF)
frequency of pulse transmission affects the maximum range that can be displayed.
Recall that the synchronizer resets the timing clock as each new pulse is
transmitted. Returns from distant targets that do no reach the receiver
until after the next pulse has been sent will not be displayed correctly.
Since the timing clock has been reset, they will be displayed as if the
range where less than actual. If this were possible, then the range
information would be considered ambiguous. An operator would not know
whether the range were the actual range or some greater value.
maximum actual range that can be detected and displayed without ambiguity, or the
maximum unambiguous range, is just the range corresponding to a time
interval equal to the pulse repetition time, PRT. Therefore, the maximum
= c PRT/2 = c/(2PRF)
= t PRF
may rearrange this equation to make a requirement on the dwell time for a
= Nmin /PRF
it is easy to see that high pulse repetition rates require smaller dwell times.
For a continuous circular scan, for example, the dwell time is related to the
rotation rate and the beam-width.
q = beam-width [degrees] W
= rotation rate [degrees/sec] which will give the dwell time in seconds. These
relationships can be combined, giving the following equation from which
the maximum scan rate may be determined for a minimum number of pulses per scan:
WMAX = q PRF/N
the frequency of the radio carrier wave will also have some affect on how
the radar beam propagates. At the low frequency extremes, radar beams will
refract in the atmosphere and can be caught in "ducts" which result in
long ranges. At the high extreme, the radar beam will behave much
like visible light and travel in very straight lines. Very high
frequency radar beams will suffer high
Maximum Range Equation
radar receiver can detect a target if the return is of sufficient strength. Let
us designate the minimum return signal that can be detected as Smin,
which should have units of Watts, W. The size and ability of a target to
reflect radar energy can be
summarized into a single term, s,
known as the radar cross-section, which has units of
absolutely all of the incident radar energy on the target were reflected equally
in all directions, then the radar cross section would be equal to the target's
area as seen by the transmitter. In practice, some energy is absorbed and
the reflected energy is not distributed equally in all directions.
Therefore, the radar cross-section is quite difficult to estimate and is
normally determined by measurement.
= Pt G 1/4pR2
terms in this equation have been grouped to illustrate the sequence from
transmission to collection. Here is the sequence in detail:
= r Gdir
radar energy spreads out uniformly in all directions. The power per unit area
must therefore decrease as the area increases. Since the energy is spread out
over the surface of a sphere the factor of 1/4pR2 accounts for the
radar energy is collected by the surface of the target and reflected. The radar
cross section s accounts for both of these processes.
reflected energy spreads out just like the transmitted energy.
receiving antenna collects the energy proportional to its effective area, known
as the antenna's aperture, Ae. This also includes losses in the
reception process until the signal reaches the receiver. Hence the subscript
"e" for "effective." The effective aperture is related to
the physical aperture, A, by the same efficiency term used in power gain, given
the symbol r. So that
= r A
Perhaps the most important feature of this equation is the fourth-root dependence. The practical implication of this is that one must greatly increase the output power to get a modest increase in performance. For example, in order to double the range, the transmitted power would have to be increased 16 times. You should also note that the minimum power level for detection, Smin, depends on the noise level. In practice, this quantity constantly be varied in order to achieve the perfect balance between high sensitivity which is susceptible to noise and low sensitivity which may limit the radar's ability to detect targets.