SCANNING AND TRACKING TECHNIQUES
A tracking-radar system measures the coordinates of a target and
provides data which may be used to determine the target path and
to predict its future position.
Tracking radars lock on to a target and track it for a certain distance
or for a certain time period.
Target tracking is important in military and Airport Surveillance
Radars (ASRs).
Military applications of radar systems include gun control, missile
guidance, and airspace surveillance linking with spacecraft and/or
satellites.
ASRs play a major role in airports for aircraft traffic control, and they
utilize tracking function as a means of controlling incoming and
departing aircrafts.
The main aim of the tracking radar is to continuously track the target
and find out its location or the course in which it is moving.
Once the radar detects the target in the given volume of space, it will
track the target by estimating the target parameters accurately.
To find the path of the target and its destination where it is going, the
radars determine the coordinates of the moving target continuously.
There are three tracking techniques:
(1) range tracking,
(2) velocity tracking,
(3) Angle tracking
RANGE TRACKING
In many tracking radars such as Approach Surveillance Radar
(ASR), the target is continuously tracked in range as well as
angle.
Range tracking is defined as tracking the moving target
continuously in range. The range of the target is measured by
calculating the round-trip delay of the transmitted pulses.
The position of the moving target keeps on changing with
time, so the range tracking radar should be continuously
updated with the new position of the target so that target is
not missed.
Range tracking is implemented using a split-gate system, in which
two range gates (early and late) are used.
Split-gate system
Range tracking is implemented similar to dual-beam angle
tracking. After measuring the range of the target, the tracking
radar predicts the range of the target on the next pulse.
The range on first pulse is compared with the predicted range on
next pulse using two range windows called the early and late
range gates.
The concept of split-gate tracking is illustrated in Figure
Range error signals at the output of a
split gate
The early gate pulse starts when the radar echo starts and closes
at half of radar echo signal duration.
The late gate pulse starts at the center of the radar echo signal
and ends at the end of the echo signal.
So the echo signal duration and center of the pulse signal should be
sent to the range tracker to start the early and late gates at the start
and center of the expected echo. This process is commonly known as
the designation process.
During the positive voltage of the radar echo signal, the early
gate pulse exists and during the negative voltage of the radar
echo signal, the late gate pulse exists.
The difference signal, obtained from comparing the early gate
pulse and late gate pulse, is sent to an integrator to produce an
error signal.
ANGLE TRACKING
Angle tracking is concerned with generating continuous
measurements of the target’s angular position in the azimuth and
elevation angles. The target tracking is achieved by keeping the
antenna beam’s main axis (or angle indicator) on the target angle.
The main aim of the angle tracking system is to maintain the
bore-sight axis of the antenna beam aligned with the target.
The ability of the radar to find the exact angle of the target
depends on the size of the antenna beam employed.
An error signal will be generated if the antenna beam’s main
axis is not exactly on the target. The deviation of the antenna
beam’s main axis from the target is corrected using the error
signal.
Depending on the sign of the error signal, the antenna beam’s
main axis is moved right or left to align with the moving target.
There are three methods for generating the error signals for
tracking:
Sequential lobing
Conical scan
Monopulse tracking
Sequential Lobing
Sequential lobing is the foremost angular tracking techniques to
determine the angular position of the target.
It is also referred to as lobe switching or sequential switching
since the angular position of target is determined by switching
the lobe positions of the beam.
Sequential lobing technique uses the symmetrical pencil beam
i.e., azimuth and elevation beam widths are equal and the
tracking is performed by continuously switching the pencil
beam between two pre-determined symmetrical positions
about the Antenna’s Line of Sight axis (LOS).
If the target is on the beam axis of left or right of the beam
position, then the signal strength observed in each beam will not
be equal.
So the two beam positions are moved towards the left or right
until the signals in each of the beams are equal in strength.
If the target is in line of sight with the antenna, the signal strength
observed in each beam will be equal.
Conical Scan
Conical scanning technique is another method to determine the
angular position of the target.
It does not use lobes as in sequential switching; it tracks the
target by continuously rotating the antenna at an offset angle
around an axis.
The pencil beam forms a conical shape when it is rotated
around the axis, so it is termed as conical scanning.
The angle error is detected and it generates a correction
voltage that is proportional to the tracking error with a sign
showing its direction.
The error measurement is sent to the servo mechanism which
moves the antenna in the direction of the target. This tracking
signal can be refined to predict the future target motion as well.
Squint angle is defined as the angle between the rotating axis and
the beam axis.
Conical scan frequency is defined as the frequency where the
amplitude of the echo signal is modulated. The beam scan frequency
is denoted as wS (radians per second).
Conical scan beam
Error signal produced when the target is
on the tracking axis for a conical scan
Error signal produced when the target is on the
beam axis of B for conical scan
Velocity Tracking
Doppler information, which can be achieved by a narrow band filter,
is obtained by using Tracking radars similar to CW or pulsed
Doppler radar systems.
This has two advantages:
If the Doppler frequency shift is higher than the clutter, then
the signal-to-noise ratio is increased
It is used to detect a target from a group where all the targets
have same angle and range
The radar which estimates the target’s velocity by using
Doppler effect is called a Doppler radar.
The estimation of target’s velocity using Doppler effect is
performed by sending a microwave signal to the target,
capturing its reflection, and evaluating the change in the
frequency of the returned signal due to the movement of the
target.
The direct and absolutely accurate measurements of the radial
velocity component of a target relative to the radar can be
obtained due to these variations.
There will be a shift in the carrier frequency of the received
signal, when the target is moving relative to the radar and this
effect is called the Doppler effect.
This shift in frequency is the Doppler shift, and it is a measure of
the velocity of the target
Doppler tracking machine
The Doppler shift frequency
where fd is the Doppler-frequency shift, and λ is the radar signal
wavelength
The target radial velocity vr can be measured by a coherent radar
from the Doppler-frequency shift of the received signal. It can also
be obtained by dividing the difference of the two range
measurements by the time between the measurements:
where R1 and R2 are the ranges, and t1 and t2 are the respective
measurement times
