Angle tracking – Conical scan, sequential lobing, low angle tracking

What is angle tracking?

All the tracking radars that we discuss here; mono pulse, conical scan radar, sequential lobing employ beams that are slightly off target. Why though, you might wonder? The purpose here is to develop a system that can track an object. In a normal radar, the object is in the beam of the radar and the location and angle can be measured. But to accurately track the object more information is needed to determine which direction the antenna should move in. And we gain this information from the error signal that generates when the beams are slightly off target. This form of tracking is known as angle tracking.

Basically, in a tracking radar, we use a signal to locate the position of the object. Additionally, the positioning of the antenna beam is done by a sort of a feedback mechanism. We use the data that we get from an error signal to position the antenna beam. The tracking radar has a system of motors that change the direction of the beam based on the data it acquires from the error signal.

In this post, we will take a look at two methods through which an error signal is generated.

1. Conical scan
2. Sequential lobing.

There are two more methods of angle tracking that we will cover in subsequent articles.

1. Monopulse tracking
2. Triangulation.

How does angle tracking work?

Angle tracking employs two (or four) different beams at different angles. The angle between them is called the squint angle relative to ‘boresight’. Boresight is the angle at which the two beams’ polar shapes meet.

These two beams need to be positioned such that the boresight is always in the direction of the target. This is done by the amplitudes of the echo signals A and B, they help in finding the distance of the target from boresight.

Let’s look at the two cases of angle tracking.

Case 1: Two beams A & B for one angle co-ordinate.

• The beams that we use here can be simultaneous or switched. Which means that they are time-shared – One beam switches OFF for the other to light up and vice versa.
• This is an older technique. It is used in sequential lobing and conical trackers.

Case 2: Four beams are used to get two angle co-ordinates

• This is a modern technique. And it has higher precision than that of using just two beams.
• Simultaneous lobing trackers. Example: Monopulse tracking.

Conical Scan

The conical scan is another method to find the angular error in angle tracking. This error is then used to control the servo motors to find and predict the accurate location, movement and direction of a moving object. Angular error is the error signal that we get when we compare the amplitudes of the voltage signals that we receive from the two switching beams that we use to track an object.

Working of the conical scan method

The rotating beam of a conical scan traces out the shape of a cone. Hence, it is named as a conical scan. This beam can be achieved by using a rotating feed that is driven by a motor. This motor is kept in a housing or enclosure at the end of the dish. Let’s take a look at the different elements of the beam that we use in a conical scan.

If you are standing at point A and look at the lobe. Assuming that the lobe is visible. Then you would see it tracing the shape of a circle. If you are the centre of this circle constantly then the amplitude of the reflected signal will be constant. However, if you start moving a bit towards the left or right, your position relative to the axis changes. Correspondingly, this causes a change in the amplitude of the reflected signal too. This data from the echo signal is sent to an angle-error detector circuit. The data from this circuit drives the servo motors that control the antenna.

A nutating feed design is given preference over a rear feed design in the conical scan. This is because we need to maintain the plane of polarization. A rear feed design rotates it. This can cause amplitude shifts and hence, is undesirable. Nutating means to spin without affecting the polarisation. However, the nutating feed is more complex than the rear feed. Let’s take a look at the block diagram of the conical scan to understand its working.

Block diagram of conical scan

The reference generator has two outputs that extract elevation and azimuth errors. The echo signal that is received is fed to the receiver from the antenna via two rotary joints (not shown in the block diagram). One rotary joint controls the azimuth movement. And the other controls the elevation movement.

The receiver is a superheterodyne receiver. The error signal is extracted after the second detector. Range gate is used to search and lock the target and continuously track the target.

What is sequential lobing?

• Sequential lobing is also known as lobe switching or sequential switching.
• As we understood above, there is generally a difference between the actual target location and the reference direction of the tracking radar. This difference is known as the angular error.
• In sequential lobing, the position of the antenna beam is switched between two positions. This gives us the direction and magnitude of the angular error.
• The method of switching a single beam between two squinted angular positions to obtain an angle of measurement is called sequential lobing.
• Though it is possible to use a time-shared antenna beam to obtain an angle measurement; it’s simpler. However, it’s not very accurate.
• The polar representation and the rectangular representation of the antenna beams in the two switched positions is shown below.

• The error obtained from the target being  NOT on the boresight is also shown.

• The difference in the amplitude between the voltages that we obtain from the two positions gives the angular displacement of the target from the boresight.
• The direction in which to move the antenna comes from the beam that has the larger signal.

What is low angle tracking?

A radar that tracks low elevation angles illuminates the target via two paths. One is the direct path from radar to target and the other is the reflection from the earth’s surface.

This is the equivalent to the radar illuminating two targets, one above the surface, the other below the surface.

The effect of glint is quite prevalent here, with errors in the measured elevation angle of the target. At small angles, (assuming) over a perfectly reflecting surface, the reflection coefficient from the surface is approximately 1.

Since they are almost in line, the phase is equal to 180 degrees and the magnitude is equal to 1. Thus the amplitudes of the incident signal are equal to the amplitude of the reflected signal.