CH 7: Target Tracking

Question 1

Target tracking radar (TTR) def and characteristics

Answer 1

Designed to provide all necessary info to guide a missile or aim a gun to destroy a target. Once a target has been detected, either by a dedicated search radar or by using an acquisition mode, the TTR is designed to provide accurate target range, azimuth, elevation, or velocity info to a fire control computer.

A typical TTR has individual tracking loops to track a target in range, azimuth, elevation or velocity.

Most TTRs use a high frequency to provide narrow antenna beamwidths for accurate target tracking. Most TTRs employ narrow pulse width and high pulse repetition frequencies (PRF) to rapidly update target info.

Question 2

Range tracking def/purpose

Answer 2

Most TTRs employ an automatic range tracking loop either through a split gate or leading-edge automatic range tracking system.

Automatic range tracking serves two functions:

1: provides critical value of target range
2: provides a target acceptance range gate that excludes clutter and interference from other returns

Question 3

Split gate range tracker def

Answer 3

A range gate circuit is simply an electronic switch that is turn on for a period of time after a pulse has been transmitted. The time delay for switch activation corresponds to a specific range. Any target return that appears inside this range gate is automatically tracked.

A split gate tracker employs two range gates. The automatic tracking loop attempts to keep the amount of energy from the target return in the early gate and late gate equal.

Question 4

Leading edge range tracker def

Answer 4

Leading edge range tracking is an electronic protection technique used to defeat range-gate-pull off (RGPO) jamming. The leading edge tracker Boston’s all data from the leading edge of the target return. All RGPO cover pulse jamming tends to lag the target return by some increment of time. By differentiating the entire return with respect to time, the target return can be separated from the jamming pulse.

Employing a split gate tracker electronically positioned at the initial pan, or leading edge, of the returning pulse, the range tracking loop can track the target return an ignore jamming signals. The range tracking loop then uses split-gate tracking logic to determine the magnitude and direction of range tracking errors and reposition the gate.

Question 5

Conical scan def

Answer 5

A conical scan tracking system is a special form of sequential lobing. Sequential lobing implies that the radar antenna beam is sequentially moved between beam positions around the target to develop angle-error data. For a conical scan radar to generate azimuth and elevation tracking data, the beam must be switched between at least four beam positions.

Works to keep the target energy in each scan position equal and keep the target in the central tracking area.

Question 6

Squint angle for conical scan antennas

Answer 6

The radar beam is rotated at a fixed frequency around the target. The angle between the axis of rotation (normally the axis of the antenna) and the axis of the antenna beam is called the squint angle.

Question 7

Advantage and disadvantage of conical scans

Answer 7

Primary advantage: small beamwidth which provides extremely accurate target tracking info.

Disadvantages: narrow beamwidth makes target acquisition difficult and may take a long time to find and initiate track on a target, conical scan radars are vulnerable to inverse gain modulation jamming based on the scanning frequency of the rotating beam, a conical scan radar must analyze many radar return pulses to generate a tracking solution.

Question 8

Track while scan (TWS) def

Types of radars capable of TWS?

Answer 8

TWS is a combined search and tracking mode that sacrifices the continuous target observation capability of the dedicated tracker in return for the ability to monitor a finite sector of airspace. This is accomplished while maintaining tracks on multiple targets moving thru the covered airspace.

Two types of radar systems capable of TWS operation: conventional and phased array.

Question 9

Conventional TWS radars def

Answer 9

Conventional TWS radars use two separate antennas to generate two separate beams. These beams operate at 2 different frequencies and are sectors so they overlap the same region of airspace. This overlap provides a tracking area for a single target. One (elevation) beam is sectored in the vertical plane to give range and elevation. The other (azimuth) beam is sectored in the horizontal plane to provide range and azimuth.

Question 10

Uplink guidance commands def

Downlink information def

Answer 10

Commands from the radar to the missile

Information from the missile back to the radar and fire control computer

Question 11

Advantages and disadvantages of conventional TWS radar

Answer 11

Advantage: TWS radars have the ability to maintain radar contact with all targets in the sector scan area while maintaining target track on a single target, and the rapid sector scan rate provides a rapid update on target parameters

Disadvantages: a large resolution cell due to the wide azimuth and elevation beams and vulnerability to modulation jamming based on the scan rate of the independent beams.

Question 12

Planar or Phased array TWS def

Answer 12

The radar does not really track and scan simultaneously but rapidly switches between search and track.

In scan mode, the radar antenna generates a pencil beam and uses a raster scan to detect targets in the search area.

In track mode, the antenna generates multiple beams to illuminate individual targets. The radar typically uses monopulse or pulse Doppler techniques to update target range, azimuth. Elevation or velocity.

Question 13

Gating def

Answer 13

The process where target parameters are updated in the track loop as the radar switches between track and scan modes. The new target info is compared to the predicted info in the measurement data processing cell. If the two sets of data agree within certain limits, target position and info are updated.

Question 14

Advantages and disadvantages of planar/phased array TWS radars

Answer 14

Advantages: planar/phased array TWS radars can search a large volume of airspace while tracking individual targets. It is also resistant to jamming techniques since it can rapidly change beams and scans.

Disadvantages: complexity, cost, and reliance on computer processing.

Question 15

Lobe-on-receive-only (Loro) definition and advantages

Answer 15

LORO is a mode of radar operation developed as an EP feature for a track-while-scan radar. LORO can be employed by any radar that has the capability to passively track a target. In a LORO mode, the radar transmits a continuous signal from a set of illuminating antennas. This continuous signal hits the target, and the return echo is received by a different set of receiver antennas.

The limited effectiveness of both noise (due to high power levels) and deception jamming (due to continuous signal) techniques is the major advantage of LORO mode.

LORO mode also provides a track-on-jam (TOJ) capability to exploit noise jamming techniques. In a TOJ mode, the receiver antennas passively track any detected noise jamming signals and assume the most intense jamming signal is the target.

Question 16

Monopulse radars def

Answer 16

Monopulse radars are the most complex radar systems. From a single pulse, a monopulse radar can derive all the data needed to update a target’s position. It does this by comparing the relationship of two or more radar beams that are transmitted together from the same antenna but received separately through the Magic T circuits.

By comparing the phase or amplitude of the energy in these returned beams, target azimuth and elevation can be found.

Question 17

Magic T definition

Answer 17

The Magic T is a sophisticated wave guide that can separate multiple signals by their phase relationships. This allows the radar tracking computer to compare the signal amplitude from the reflected pulses in several different ways.

As the reflected energy enters the Magic T, it is separated by phase. The energy in the ‘H’ arm will be in-phase and will exit from ports 1&2. The received energies entering the wave guide in the ‘E’ arm exit at the number 1 port. This energy is exactly 180 degrees out of phase with energy entering the H arm. This ensures there Is no transfer of energy between the E and H plane arms. A typical monopulse radar would have 8 Magic T’s.

Question 18

Monopulse tracking loops equations

See text book 7-14

Answer 18

See text book 7-14

Question 19

Continuous wave (CW) radars def

Answer 19

CW radars emit a continuous beam of RF energy with no interruptions in the transmissions to detect returning echoes. A continuous radar transmission from the antenna requires that a classic CW radar have two antennas, one for transmission and one for reception.

Range determination is impossible. Azimuth and elevation is based on the antenna position when the target is illuminated. The Doppler principle allows a CE radar to track a target in velocity and reject clutter. This is called a CA Doppler Radar.

Question 20

FM CW radar definition

Answer 20

One method of obtaining range info from a CW radar uses frequency modulation (FM). The modulation can be any shape as long as the frequency change is known. The transmitter emits a continuous signalX but the frequency is changed to a known pattern. When the echo returns from the target, it is then compared to the frequency being transmitted. The frequency difference is directly proportional to the range of the target.

Question 21

Advantage of FM CW Doppler Radar

Answer 21

Primary advantage is the ability to combine the clutter rejection features of a simple CW Doppler Radar with the capability to detect range.

The widest application of FM CW Doppler radars is in radar altimeters for aircraft.

Question 22

Moving Target Indicators (MTI) def

Answer 22

A method used for clutter rejection in a pulse radar system is to employ special circuits, or Doppler processing, to identify and reject clutter. These special circuits are added to the receiver section of a pulse radar and are called moving target indicators (MTI).

There are two types of MTIs, noncoherent and coherent.

Question 23

Non-coherent MTI or ‘area’ MTI

Answer 23

Non-coherent MTI radars do not process Doppler frequencies. The returns from one scan are subtracted from the returns from the next scan. All targets that move at least one resolution cell in time between scans are displayed. All stationary objects, Including fixed clutter, are cancelled and not displayed. In this type of MTI, clutter cancellation is based on the size and movement of the return.

Question 24

Coherent MTI definition

Answer 24

Coherent MFI uses the fact that Doppler shifts appear to a pulse radar as phase shifts on the received target pulse. Coherent MTI uses sophisticated circuitry, inflicting stable local oscillators (STALOs) and coherent local oscillators (COHOs) to capture and process these phase shifts. Further processing of these phase shifts yields velocities for each return. Those velocities associated with stationary targets are rejected and only moving targets are displayed.

Coherent MTIs have a major problem called ‘blind speeds.’ Blind speeds occur for all targets Doppler frequencies that are the exact PRF or any multiple of the PRF of the radar signal. When a target is moving at a velocity that produces this Doppler frequency, it’s return is cancelled along with fixed returns.

Question 25

3 techniques to limit ‘blind speeds’

Answer 25

1. Use a PRF stagger so that the blind speed associated with one PRF will be covered by the other PRF
2. Delay line and canceler-which involves delaying each pulse so it can be compared to the next pulse before processing. This method enchanted Doppler frequency comparison and rejects clutter more effectively.
3. Use range gates and Doppler filters. A range gate is simply a switch that opens for a time corresponding to the time a radar return would arrive for a target at a specific range. A target return would then trip this gate and be processed by Doppler filters to find velocity. Fixed targets would grip the range gate and be dominated by the Doppler filters.

Question 26

Pulse Doppler Radar def

Answer 26

Pulse Doppler radars combine the advantage of both pulse and Doppler Radar systems. Because the signal is pulses, the radar can find range, azimuth, elevation and velocity. A pulse Doppler Radar transmits a box or pulse of RF energy at the operating frequency of the radar.

Range determination is accomplished by measuring the time it takes for the reflected pulse to return from the target.

Velocity determination and tracking are accomplished by capturing and qual tidying the Doppler shift of the frequencies of each pulse.

Azimuth and elevation are tracked by employing a conical scan (sequential lobing) or monopulse tracking.

Question 27

Doppler Notch def

Answer 27

To remove the ground clutter and avoid tracking unwanted targets, a pulse Doppler Radar has a filter that is designed to eliminate all targets with a low velocity relative to the radar. The key to breaking track of a pulse Doppler Radar is to place the aircraft in a speed less than the speed gate relative to the radar, commonly referred to as the Doppler notch.

Question 28

Weaknesses of pulse Doppler radars

Answer 28

Velocity blind speeds, range ambiguity, and range eclipsing.

1: Doppler notch

2. Range ambiguity occurs primary with long-range targets when the return comes back to the radar after another pulse has already been transmitted.
3. Range eclipsing occurs when the target return comes back to the radar antenna while I pulse is being transmitted. Since the radar cannot receive while transmitting, the return will not be displayed.

To solve the problems of range ambiguity and eclipsing, a pulse Doppler Radar employs different PRFs and computer logic.