In this lecture, we will be studying the Tracking of the targets with the help of tracking radar. Tracking means follow the path of the target using the radar.
There are following parameters of the targets are important:
- Range of the target
- The velocity of the target
- The azimuth angle of the target
There are different types of techniques of tracking the targets such as sequential lobbing, conical scanning, mono pulse tracking is being explained in this lecture.
What is Tracking Radar?
A Tracking Radar system measures the different coordinates of a target such as range, elevation angle, azimuth angle, and doppler shift frequency to determine the target path and to predict its future position.
In general, it is the method by which angle tracking accomplished may be called the tracking radar.
One of the most useful features of radar is the ability of a radar set to continuously predict the next location of the target from the information received from the target and to align itself to continuously point at the predicted location. When this is occurring, the radar is said to be tracking radar.
Servo Mechanism of Tracking Radar
One of the most basic tracking systems is the servo tracking system, as shown in the figure below.
Here, the radar antenna is initially trained on the target after which it automatically remains pointed at the target as it follows its motion. Furthermore, the system provides continuous position information to the operator and possibly to a fire control system. The antenna is rotated by a motor which provides a negative position feedback signal to a controller. This system is known as a servo mechanism.
Sometimes single antenna is not favorable for both search and tracking the target at the same time. So in this condition, a separate search radar is employed to provide the information related to the position of the target to the tracking system. When a separate search radar is employed for giving the information of the target to the tracking radar system is called acquisition radar.
Types of Tracking Radar
There are many types of tracking radar, which is being used to track the targets. They are as follows:
- Single Target Tracker (STT)
- Track While Scan (TWS)
- Automatic Detection And Track (ADT)
- Phased Array Tracking
1. Single Target Tracker (STT)
A single target tracker is used where continuous tracking of a single target with a higher data rate is required. Generally, this type of tracking is used in controlling the missile movements as it is carried out by a missile guidance radar.
In this type of tracker, a closed-loop servomechanism is used to keep the angle coordinates error very small. If the errors are less then there will be more accurate.
2.Track While Scan (TWS)
TWS radar scans their beam over relatively large areas. It rapidly scans in the angular sector to keep track of the targets, maybe move one target at a time. The radar computer still measures returned power as a function of beam location to provide tracking but the large scanning area enables the radar to still see the target even if the track has beam broken or lost. However, this large scan makes the TWS highly vulnerable to ECM jamming.
3.Automatic Detection and Track
This type of technique is used in air traffic control radar. This technique is employed with the air surveillance radar, which has 7.5 to 15 RPM. It does provide updating data according to the rotation of the antenna. It can track an enormous number of targets simultaneously may be few hundred. This antenna position is not controlled by closed-loop, it works in the open-loop system.
4.Phased Array Radar Tracking
In this technique, multiple targets are tracked on a time-sharing basis by the computer. Large numbers of tracks can be scanned rapidly in this technique because of electronically steered phased array method is used here. the antenna beam is scanned electronically make to switch in from one angular direction to another in a small-time such as a few microseconds.
Block Diagram of Tracking Radar
Most tracking radars use angular information as the basis for tracking operations. For better accuracy, it is important that radar concentrates on one target at a time. Range gating/Doppler filtering can be used for that purpose. Time and frequency control for range and doppler gating is done in range and doppler trackers respectively. The angular error signal for the desired target to be tracked is developed in the error demodulator block which is also controlled by range/doppler gate generation block and then fed back to the steerable antenna in a closed-loop for tracking.
What is Sequential Lobbing?
A single beam is switched between two angular positions to obtain an angle measurement. This is called sequential Lobbing, lobe switching, or sequential switching.
Here, the direction of the antenna beam is rapidly switched between two positions. The echo signal from the target will fluctuate at the switching rates unless the target reaches exactly between two directions. The error signal obtained from a target not located on the switching axis is shown in the figure. The difference between the amplitude of these two echo signals is given the error signal.
The error signal may be defined as the angular displacement of the target from the switching axis. These tracking error signals are applied to the servomechanism unit, which attempts to position the antenna beam on the target.
When the target is located at the reference’s direction then the angular error is zero. Thus sequential lobbing with a two-position beam is used for tracking the target in only one place. The sign of the difference determines the direction of the antenna to move in order to align the switching axis with the direction of the target.
Four switching positions are required to obtain angle measurement in the orthogonal coordinate. Thus a two-dimensional sequentially lobbing radar consists of a cluster of four fed horns illuminating a single reflector antenna, arranged so that the right-left, up-down sectors are covered by successive antenna positions. A cluster of five feed horns might also be used, with a control feed used for transmission and four outer feeds used for reception on a sequential basis.
In the conical scanning, the squinted beam is scanned rapidly and continuously on a circular path around the axis. The angle between the axis of rotation and the axis of the antenna beam s called the squint angle. If a target is present within a squint angle then the echo signal from the target will be amplitude modulated at a frequency equal to the rotation frequency of the beam also called conical scan frequency.
The amplitude of the modulation depends on the angular distance between the target direction and the rotation axis. The location of the target in two angles coordinates determines the phase of the conical scan modulation is extracted from the echo signal and applied to a servo control system which positions the antenna on the target both azimuth and elevation.
Two servo motors are required one for azimuth and another one for elevation. The conical scan modulation becomes zero and thus the target is tracked accurately by conical scanning, both azimuth, and elevation.
The antenna beam s squinted and scanned either mechanically by offsetting the feed and rotating it or electronically with the help of phase shifters. Electronic scans are very fast than mechanical scans. Typical scan rates are 30 to 40 scans/sec.
Block Diagram of Conical Scanning
A block diagram of a conical scan tracking radar system is shown in the figure below. The antenna is mounted such that it can be positioned both in elevation with the help of respective motors. The scan antenna beam generates two signals, the azimuth detector and the other for the elevation detector, which is 90 degrees out of phase.
The receiver is a conventional superheterodyne except for features related to the conical scan tracking radar. The error signal is extracted in the video after the second detector. The error signal is compared with the elevation and azimuth references signal, in the angle detectors, which are phase-sensitive detection.
The phase-sensitive detector is a non-linear device in which the input signal is mixed with a reference signal. This angle detection produces a DC voltage, which is proportional to the error, and the sign is an indication of the direction of the error.
The angle error detector output is amplified and drives the antenna elevation and azimuth servomotors. The angular position of the target may be determined from the elevation and azimuth of the antenna axis.
Conical scan systems require a minimum amount of hardware and therefore are commonly used on inexpensive, mobile systems such as AAA or mobile SAM sites.
They suffer the serious disadvantage of not being able to see a target outside their narrow scan patterns. This means that not only is a second radar is required to help it find that target but also the tracked aircraft can easily escape it is successful in breaking track since the conical san radar cannot see the target except in the track mode.
Mono Pulse Tracking
There are two disadvantages connected with the use of sequential lobbing and conical scanning which is as follows:
- The motion of the antenna is more complex in lobe switching and conical scanning.
- In conical scanning, a minimum of four pulses is required. the difficulty here is that if the cross-section of the target, the tracking accuracy may be degraded.
The effect of the fluctuating echo can be sufficiently serious is some application severely limits the accuracy of those tracking radar which requires many pulses to be processed in extracting the error signal.
The above problems can be overcome by using single-pulse only. There are several methods by which angle error information might be obtained with a single pulse.
The tracking techniques which derive angle error information on the basis of a single pulse is known as mono pulse tracking or simultaneous lobbing more than one antenna beam is used simultaneously in their method whereas in conical scanning or sequential lobbing, which uses one antenna beam on a time-shared basis.
There are several methods by which a mono pulse angle measurement can be made. The most popular method is the amplitude-comparison mono pulse, which compares the amplitude of the signals simultaneously received in multiple squinted beams to determine the angle.
Amplitude comparison Mono Pulse tracking Radar
In an amplitude mono pulse system, four feeds are used with one paraboloid reflector. There are four horn antennas displaced about the central focus of the reflector. The transmitter feeds the horns simultaneously so that a sum signal is transmitted. The echo signal is received by a receiver duplexer using a hybrid ring to provide the following three signals:
- Sum signals (A + B + C + D)
- Azimuth error signal = (A+ C) – (B + D)
- Elevation error signal = ( A + B) – ( C + D)
Block Diagram of Amplitude comparison Mono Pulse tracking Radar
A block diagram of the amplitude comparison mono pulse tracking radar for a single angular coordinate is as shown in the figure below.
The receiver has three input channels consisting of three mixers, a common local oscillator, three IF amplifiers, and three detectors. The elevation and azimuth error signals are used to drive a servo amplifier and a motor in order to position the antenna in the direction of the target.
The o/p of sum channel is used to provide the data generally obtained from a radar receiver so that it can be used for applications like automatic control of the firing weapon.
The advantage of this technique is that it obtains with one pulse ( online 4 pulses in conical scanning) all the information regarding the target and is able to locate the target in less time compared to other methods. Also, the mono pulse technique is not subject to error due to the variation in the target cross-section.
The disadvantage is that it requires two extra Rx channels and more complex duplexer feeding arrangements, which makes the system bulky and more expensive.
Phase comparison Mono Pulse tracking Radar
The angular error may also be determined by comparing the difference in the phase between the signals received from the two separate antennas. The antennas used in phase comparison mono pulse tracking are not offset from the axis as used in amplitude comparison trackers. It is essential that the amplitude of the target echo signal is the same from each antenna, but the phases are different.
Tracking radar, which operates with phase information, might be called simultaneous phase comparison radar, or phase comparison mono pulse radar. The measurement of the angle of arrival by comparison of the phase relationship in the signals from the separated antenna of radio interferon meer has been widely used by radio astronomers for precise measurement of the position of stars.
Tracking radar which operates with phase information is similar to an active interferometer.
An additional antenna and receiving channel are necessary in order to track two orthogonal coordinates. In the phase comparison radar, four antennas are required, one of these antennae for transmitter only, while the other three for the receiver.
This technique has not been as widely used as the other techniques. There are two main drawbacks to why this might be so:
- The side lobes levels, which result higher than the single antenna
- The phase comparison radar does not usually make efficient use of the total available antenna patterns.