Tracking Radar .pdf

25. Tracking - 1

Tracking Radar •  •  •  •  •  •  •  •  •  •  •  • 

Introduction Single target tracking: range, Doppler and angle measurement Track-while-scan Angle tracking: sequential lobing, conical scan and monopulse Range tracking Doppler tracking Track initiation Tracking filters: α-β tracker, Kalman filters Tracking errors: target and environment effects Multi-function radar Multi site radar tracking Examples of tracking radars

Introduction Tracking" " Most of the example radars systems considered so far have largely been for the purpose of surveillance - i.e. scanning a given area and detecting targets that are present. " " A tracking radar measures the coordinates of a target and provides data which may be used to determine the target trajectory and to predict its future position. Tracking can be carried out using range, angle or Doppler information. Angle is perhaps the most characteristic feature associated with tracking radar systems.!

Tracking and Prediction for a Phased Array Radar

Gating and Assignment

Track Association

Introduction We can consider two types of tracking. " " (1) Track-while-scan. Here the radar is in the surveillance mode and attempts to form tracks of detected targets (although it is optimised for surveillance only). In this way multiple targets may be tracked" " (2) Continuous trackers. Here the radar is dedicated to the tracking function. A surveillance radar might provide co-ordinates for the tracker so that it doesn t have to search large volumes of space. Often only a single target is tracked.!

Tracking

Tracking

Single Target Tracking The objective of single target tracking is to continuously and " accurately measure a target s position, velocity and acceleration." " There are four functions performed in tracking:! " (1) Measurement i.e. the determination of the parameter set." " (2) Filtering i.e. the processing of successive measurements to " minimise errors due to noise, target scintillation, multipath etc." " (3) Control i.e. the generation of a command signal based " upon the information contained in the filtered measurement." " (4) Response, for example the repositioning of the antenna " beam to the new target position.!

Single Target Tracking Tracking is done in Range, Doppler and Angle.! ! Range: The objective is usually to keep the range gate centered on" the target of interest. A series of range measurements are made. " Position and range rate are calculated. The results are used to predict " the targets future range and adjust the radar accordingly." Doppler: A Doppler filter is formed, for example, by Fourier " transforming a series of radar pulses to estimate target velocity " and rate of change of velocity. This can be used to improve the " position estimate and alter the radar to maintain an optimum track." Angle: This is measured using lobing or monopulse techniques. " The information is used, in conjunction with Doppler, to centre the" antenna beam to follow the target and maintain maximum power " on the target. This is achieved in an angle tracking loop which measures " the angle between the antenna boresight and the line of sight to" the target and attempts to correct for any differences.!

Track-While-Scan " Track-while-scan is a method of combining the search and track " functions and of tracking multiple targets. The radar searches for " targets and whenever a target is detected the radar estimates the " targets range, range-rate, azimuth angle and elevation angle. For " any one detection the estimates are referred to collectively as an " observation. After successive scans the radar accumulates accurate " tracks of valid targets. There are three main components comprising " this process: "

Track-While-Scan

These three components of track-while-scan are:! ! (1) Preprocessing: Targets having the same range, range-rate and angle " after successive scans are combined. Detections are translated on to a " reference co-ordinate system." (2) Correlation: A prediction is made of the next target position based on " previous observations. An example of such a prediction technique is the " Kalman filter. Predictions are compared with measurements. If the prediction " lies within specified limits then the new observation forms part of the track. " When closely spaced detections occur conflicts can arise. These have to " be resolved using sophisticated statistical metrics tailored to the " performance of the radar in its environment." (3) Track creation or deletion: If a new observation is evaluated as not " being part of a track a new track is initiated. A second observation is used " to confirm the track or delete it if there is no or poor correlation. " Similarly if, over the course of a few scans there are no new " detections against a track, then the track is dropped.!

Track Initiation Track initiation" " Consider a mechanically scanned surveillance radar. A target detection" is reported. On the next scan the target is detected once again. A " counter is incremented. If the target is not subsequently detected then " the counter is decremented. If a pre determined number of counts is " reached then a track is declared. Track initiation schemes tend to be " system specific and bespoke. A scheme must also be implemented to" decide whether or not a detection is close enough to the track to be " counted or not. Movement patterns help with this decision."

Angle Tracking: Sequential Lobing In sequential lobing the radar beam is switched " between different positions in order to gain " improved angular information about the position " position 1 position 2 of the target. Consider two beam positions " switched between the left and right of a target. " If the target is in the centre of the two beams then " a signal of equal strength will be observed in each " beam. If the target, on average, is growing in the " left hand beam then the two beam positions are " moved to the left until the signals are of equal " strength in each of the beams. In this way the " target is tracked. " " A common method of generating the necessary beams is to use " four microwave horns feeding a parabolic dish at its focus. " This requires four pulses to be transmitted in sequence (one from " each horn or beam position) to track in azimuth and elevation.!

Angle Tracking: Sequential Lobing normal

- θ

target position

+θ

The angular accuracy can be much higher than the antenna beamwidth."

Angle tracking: Conical Scanning Conical scanning - 1! " An alternative to sequential lobing is to rotate the antenna beam around" the direction of the target on a continuous basis. This is known as " conical scanning. The angle error is detected and used to generate a " correction voltage proportional to the tracking error with a phase to " indicate its direction. The error measurement actuates a servo system " to drive the antenna in the direction of the target. This tracking signal " may be quite sophisticated, particularly if it is attempting to predict " future target motion."

Angle tracking: Conical Scanning Conical scanning - 2" " Conical scanning is achieved by mechanically moving the feed of the " antenna. The feed is typically moved in a circular path. This causes " the antenna beam to move likewise in a circular path. " " If the scanning is receive only, with a fixed transmit beam, the system is " called conical-scan-on-receive-only (COSRO). The equivalent with " sequential lobing is known as lobing-on-receive-only (LORO)."

A.I. radar began as a means of guiding fighters • 

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Night interception of bombers is impossible without radar guidance." By 1941, fixed-beam VHF systems were installed on night fighters on both sides." This was quickly followed by magnetron equipment at S and X-band, using spiral and conical scan for search and track."

A.I. Mk 8

Angle Tracking: Monopulse Monopulse - 1" " Sequential lobing, conical scanning and their derivatives all require " more than one pulse in order to derive their error signals. There is " an implicit assumption that nothing changes between pulses - but " in practice, target fluctuations may mean that this is not valid. Pulse " to pulse target fluctuations cause signal fading that can lead to " missed detections and incorrect deletion of tracks."

Angle Tracking: Monopulse Monopulse - 2! ! As its name implies monopulse is able to derive error signals from a single" pulse, thereby avoiding such errors. There are two types of monopulse " (1) amplitude-comparison and (2) phase-comparison monopulse. Amplitude " comparison tends to be the more common of the two." " In two dimensions generally monopulse employs two feeds to a parabolic " dish, generating two overlapping beams. These are fed to a hybrid coupler " ( magic T ) which produces Sum (S) and Difference (D) patterns." " The radar transmits via the Sum pattern, and processes the received echoes via both Sum and Difference channels."

Angle Tracking: Monopulse Monopulse - 2! ! As its name implies monopulse is able to derive error signals from a single" pulse, thereby avoiding such errors. There are two types of monopulse " (1) amplitude-comparison and (2) phase-comparison monopulse. Amplitude " comparison tends to be the more common of the two." " In two dimensions generally monopulse employs two feeds to a parabolic " dish, generating two overlapping beams. These are fed to a hybrid coupler " ( magic T ) which produces Sum (S) and Difference (D) patterns." " The radar transmits via the Sum pattern, and processes the received echoes via both Sum and Difference channels."

Angle Tracking: Monopulse

Angle Tracking: Monopulse Amplitude-Comparison Monopulse! (a) Overlapping antenna patterns" " " 0 " θ

" " Sum (transmit and Difference (receive only) " receive) (R - L) " + " (b) Sum and Difference patterns! 0

0

θ

θ

Angle Tracking: Monopulse Amplitude comparison - 1! ! This is the simplest form of monopulse. Two antenna beams are set " at an angle to one another and the outputs connected to a hybrid, " which forms sum and difference signals. The sum channel forms a " beam that has a higher signal to noise ratio and is used for detection. " However, the sum beam is wider than the individual beams and is " not used to measure angle. The difference channel produces an " error voltage approximately proportional to the angular deviation " of the target from the boresight. No signal is received when the " target is on boresight. A phase sensitive detector is used to " determine the sign of the error signal so that the servo motors " know which way to drive the antenna back to the target.!

Angle Tracking: Monopulse Amplitude comparison - 2" " Amplitude comparison can be improved by taking the ratio of the sum " and difference channels, giving a quotient independent of signal strength " and linear against the angle error over a wide range of angles." " Overall the improvement in angular accuracy over that of a single beam " equivalent is of the order of ten-fold, depending on the signal to noise " ratio.!

Angle Tracking: Monopulse

feeds

video (range signal)

hybrid Σ

Δ

L.O.

P.S.D.

error signal error voltage +

Transmit using the 'sum' channel -θ

+θ -

Angle Tracking: Monopulse The same scheme can be extended to " both azimuth and elevation :! sum Σ

elevation difference

Δ Σ

Σ Δ

Δ

Σ Δ

azimuth difference

Mecort - RSA

CSIR

Mecort - RSA

CSIR

Angle Tracking: Monopulse Phase comparison monopulse - 1! ! Here, instead of comparing the amplitude of the echoes from two " squinted beams, the angle of arrival from a target is inferred by " comparing the phase of the echoes from two separate beams " (or two antennas).! An echo on boresight will" arrive at the two antennas" at the same time and therefore" the phase difference will be zero." An echo from a target at an " angle q to boresight will arrive " at one antenna later than the " other due to the extra distance " it travels.!

Angle Tracking: Monopulse Phase comparison monopulse - 2

The extra distance that one echo travels with respect to the other is given by:

target

X=dsinθ

Or expressed as a phase difference:

Δψ = 2πd sin θ / λ [rads]

boresight

X = dsinθ

θ θ d

For small error angles sinθ ~ θ

Δψ = 2πd sin θ / λ

Angle Tracking: Monopulse Block diagram of a phase comparison monopulse mixer

duplexer

transmitter

L.O.

IF amplifier

phase detector

angle error range information

IF amplifier

envelope detector

Range Tracking In most tracking radar applications the target is tracked in range. Tracking can be done in range, using early-late gate techniques.! early late gate gate

difference between early and late gates

time

time

A higher echo will be " observed in one of the " gates unless the target " is located in the middle " of them. The difference " signal between the gates " is a measure of the " position of the target and the " centre of the gates.!

Doppler Tracking Tracking radars designed to extract Doppler information, such as CW " or pulsed Doppler radar systems, can also track the Doppler " frequency shift. This can be accomplished using a narrow band " filter. This has two advantages:" " (1) the signal to noise ratio is improved, especially if the Doppler " frequency shift is large compared to that of clutter. " " (2) it can resolve a target from a group which otherwise have the " same angle and range.!