===== TWS =====
Thus far we have considered radar mainly as a surveillance senser that detects target over a region of space.A radar not only recognizes the presence of a target, but it determines the target's location in range and in one or two angle coordinates.As it continues to observe a target over time,the radar can provide the target's tragectory, or track and predict where it will be in the future.Various surface and airborne radars constitute a major sensor input to computerized command and control
systems such as the Marine MACCS and Navy NTDS. These facilities perform multiple functions, including
surface, subsurface, and airspace surveillance; identification; threat evaluation; and weapons control. To
operate in today's environment, command and control systems require near real-time (as it occurs) target
position and velocity data in order to produce fire control quality data for a weapon system. Until recent
times, the interface between the raw data supplied by the sensor and the command and control system was
the human operator; however, this limited the data rate and target handling capacity. Additional personnel
were added as a partial solution to these problems, but even under ideal conditions of training, motivation,
and rest, the human operator can only update a maximum of six tracks about once per two seconds and that
only for a short time.
To supply fire control quality data, the classical solution has been to provide separate precision tracking
sensors. Although the systems have widespread use,
they are limited to tracking at a single elevation and azimuth. Data from these systems were provided to an
analog computer to solve the fire control problem and compute launcher and weapon orders.
==== CONCEPT ====
**Track-while-scan (TWS)****{Limited Sector Scan}**systems are tracking systems for surveillance radars
whose nominal scan time (revisit time) is from 4 to 12 s for aircraft targets. If the
probability of detection (PD) per scan is high, if accurate target location measurements
are made, if the target density is low, and if there are only a few false
alarms, the design of the correlation logic (i.e., associating detections with
tracks) and tracking filter (i.e., filter for smoothing and predicting track positions)
is straightforward. However, in a realistic radar environment these assumptions
are seldom valid, and the design of the automatic tracking system is complicated.With the advent
of the high-speed electronic digital computer, computational capacity exceeded the data-gathering capacity
of the fire control sensors; however, target handling continued to be limited by the non-scanning tracking
methods employed.
The solution to both problems was to interface a high-scan-rate (15-20 rpm) search radar with the electronic
digital computer through a device capable of converting the pattern of echo returns detected by the receiver
into usable binary information. In addition, using the digital computer to direct and compose radar beam
patterns, as described in the next chapter, created a flexible system capable of multimode simultaneous
operation in search, designation, track, and weapon control. This ability to perform such functions as multitarget
tracking, prediction of future target positions, and terrain avoidance for aircraft navigation is
transforming previously fragmented sensor and control capability into a dynamic and powerful sensing,
command and control, and weapon employment system. The combination of **automatic detection and
tracking (ADT)** functions within a search radar is sometimes referred to as **Track-While-Scan (TWS)**. ADT is being retrofitted into older equipment and is included in most new equipment. //The central
concept of TWS is to maintain target tracks in a computer with periodic information updates from a scanning
radar.//This radar rapidly scans a limited angular sector to maintain tracks ,with a moderate data rate, one more than one target within the coverage of antenna.it has been used in the past for air defense radar ,aircraft landing radars,and in some airboune intercept radar to hold multiple targets in track.
TWS radar used to rapidily scan a relatively narrow angular sector, usually in both azimuth and elevation.it combines the search function and the track function.Scanning may be performed with a single narrow beamwidth pencil beam(or a monopulse cluster of beams) that might cover a rectangular sector in a raster fashion.Scanning can also be performed with two orthogonal far beams,one that scan in azimuth and the other in elevation. TWS radar have ben used in airport landing radars,airbone interceptors,and air-defense systems.A difference between a continuous tracker and the TWS radar is that the angle error signal in a continuous tracker is used in a closed-loop servo system to control the pointing of antenna beam.In the TWS radar,however there is no closed-loop positioning of the antenna.its angle output is sent directly to data processor.Another significant difference is that the TWS radar can provide simultaneous tracks on a number of target within its sector of coverage,while the continuous tracker observes only a single target which is why it is sometimes called a single-target tracker(STT).With comparable transmitter and antennas, the energy available to perform tracking is less in a TWS radar than a STT since the TWS store its radiated energy over an angular sector rather than concentrate it in the direction of single target.In airborne-interceptor applications,the TWS radar might be preferred when multiple target have to be maintained in track and the tracking accuracy only has to good enough to launch missiles which contain their own guidance system to home on the target.On the other hand,of highly accurate tracking is needed ,a single target tracker might be preferred.
Limited Sector Scan TWS radar have been used in Precision Approach Radar(PAR) or Ground - Controlled Approach(GCA) system that guide aircraft to a landing.These radars allow a ground controller to direct an aircraft to a safe landing in bad weather by tracking it as its lands.The ground controller communicates to the pilot directions to change his or her heading up,down,right,or left.In the control of aircraft landing,fan beams have been used which are electromagnetically scanned over a narrow sector at a rate of twice per second.The azimuth sector that is scanned might be 20 degree and the elevation sector 6 degree or 7 degree.Landing radar using limited scan phased array antennas,such as the AN/TPS-19, operate differently then scanning fan beams since they electronically scan a pencil beam over a region 20 degree in azimuth 15 degree in elevation at a rate of twice per second.The AN/TPN-19 used multiple recieve beams to obtain a monopulse angle measurement.Being a phased array, the AN/TPN-19 could simutanously track up to six aircraft at a 20HZ data rate.In the past radars for the control of landing aircraft have beeen mainly used by the military.Civilian pilots prefer to use landing systems in which the control of landing is in the aircraft's cockpit rather than from a voice orginating from the ground.
Track while scan radar have also been used successfully for the control of weapons in surface to air miscile systems for both land and ship based air defense especially by the former Soviet Union.Generally ,TWS System using for fan beams have some limtation compared to system that operate that one or more pencil beams.The fan beam system can see more rain and surface clutter it is more vulnerable to electronic countermeasures and there might be problems with associating multiple targets that appear in the two beams.
Unfortunately, the same name track while scan was also applied in the past to what is now usually called **Automatic detection and tracking (ADT)**.This perform tracking as part of an air surviellance radar. it is found in almost all modern civil air traffic control radar as well as military air surveillance radar. The rate at which observtions are made depends on the time for the antenna to make one rotation(which might vary from a few seconds to as much as 12 seconds).The ADT,therefore has a lower data rate than that of the STT,but its advantage is that it can simultanously track a larger number of targets(which might be many hundred or a few thousands of aircraft).Tracking is done open loop in that the antenna position is not controlled by the processed tracking data as it is in the STT.One method of obtaining tracks with a surveillance radar is to have an operator manually
mark with grgase pencil on the face of the cathode-ray tube the location of the target on each
scan. The simplicity of such a procedure is offset by the poor accuracy of the track. The
accuracy of track can be improved by using a computer to determine the trajectory from
inputs supplied by an operator. A human operator, however, cannot update target tracks at a
rate greater than about once per two seconds.Thus, a single operator cannot handle more
than about six target tracks when the radar has a twelve-second scan rate (5 rpm antenna
rotation rate). Furthermore an operator's effectiveness in detecting new targets decreases
rapidly after about a half hour of operation. The radar operator's traffic handling limitation
and the effects of fatigue can be mitigated by automating the target detection and tracking
process with data processing called automatic detection and tracking (ADT). The availability of
digital data processing technology has made ADT economically feasible. An ADT system
performs the fluctuations of target detection, track initiation, track association, track update,
track smoothing (filtering) and track termination.
The automatic detector part of the ADT quantizes the range into intervals equal to the
range resolution. At each range interval the detector integrates 11 pulses, where n is the number of pulses expected to be returned from a target as tile antennas catis past. The integrated pulses
are compared with a threshold to indicate the presence or absence of a target. An example is
the commonly used moving window detector which examines continuously the last r-1 samples
within each quantized range interval and announces the presence of a target if m out of 11 of
these samples cross a preset threshold.By locating the center of the n pulses, an estimate of the target's
angular direction can be obtained. This is called beam splitting.
If there is but one target present within the radar's coverage, then detections on two scans
are all that is needed to establish a target track and to estimate its velocity. However, there are
usually other targets as well as clutter echoes present, so that three or more detections are
needed to reliably establish a track without the generation of false or spurious tracks.
Although a computer can be programmed to recognize and reject false tracks, too many false
tracks can overload the computer and result in poor information. It is for this same reason of
avoiding computer overload that the radar used with ADT should be designed to exclude
unwanted signals, as from clutter and interference. A good ADT system therefore reqirires radar with a good-MTI and a good CEAE-(constant false alarm rate) receiver. A clutter map,
generated by the radar, is sometimes used to reduce the load on the tracking computer by blanking clutter areas and removing detections associated with large point clutter sources not
rejected by the MTI. Slowly moving echoes that are not of interest can also be removed by the
clutter map. The availability of some distinctive target characteristic, such as its altitude, might
also prove of help when performing track association.Thus, the quality of the ADT will
depend significantly on the ability of the radar to reject unwanted signals.
==== FUNCTIONS OF TWS ====
A functional tracking system must provide the basic target parameters of position and rates of motion. In the
systems presented both position data and velocity data were used to maintain the tracking antenna on the target at all times, thus limiting the system to the traditional single-target tracking situation. In a track-while-scan system, target position must be extracted and velocities calculated for many targets while the radar continues to scan. Obviously, in a system of this type, target data is not continuously available for each target. Since the antenna is continuing to scan, some means of storing and analyzing target data from one update to the next and beyond is necessary. The digital computer is employed to perform this function and thus replaces the tracking servo systems.
Any track-while-scan system must provide for each of the following functions:
(1) Target detection
(2) Target track correlation and association
(3) Target track initiation and track file generation
(4) Generation of tracking "gates"
(5) Track gate prediction, smoothing (filtering), and positioning
(6) Track Termination
=== Target detection ===
Target detection is accomplished by special circuitry in the receiver of radars designed as
TWS systems or by signal-processing equipment in a separate cabinet in retrofit installations. The functions
of this ADT processing equipment is similar and will be addressed here in general terms. It should be
remembered that the ADT processor is not a computer, although it does have some memory capacity. //Its
primary function is that of converting data to a form usable by the separate digital computer that actually
performs the smooth tracking, velocity, acceleration rate generation, and prediction functions of the TWS
system.//
The ADT processor's memory allocates space addressed to each spatial cell that makes up the search volume
of the radar. The dimensions of these cells are often defined by the horizontal and vertical angular resolution
capability of the radar as well as by a distance in range equal to the pulse compression range resolution.In some systems, individual beams in the radar are overlapped such that most targets will occupy more than
one beam position at any radar range. When employed, this procedure (called beam splitting) allows angular
resolution less than the beamwidth of the radar. In addition, this avoids gaps in coverage and
allows examination of hit patterns among several beams as a means of rejecting clutter or false targets. A
radar may require N hits out of M beams at the same range interval, with all hits overlapping in elevation and
azimuth before any further consideration is given to initiating a track. N and M may be operator selectable or
preset during the design process.
Most systems employ further means to avoid generation of tracks on clutter or false returns. These can be
grouped into those associated with conventional radar receiver anti-jamming circuitry and those exclusive to
ADT. It should be obvious that an effective TWS radar must retain conventional features such as **Constant
False-Alarm Rate (CFAR), MTI, and automatic gain-control circuitry**. These will help prevent the
processing of obviously false returns, allowing the ADT system to make more sophisticated decisions
between valid an invalid targets. The ADT processor will generate a time history of all echo returns over
several radar scans. Stationary targets remaining within a given resolution cell over a number of scans result
in its identification as a clutter cell. The collected information on all clutter cells within the radar search
volume is referred to as a clutter map. Returns remaining in clutter cells will be rejected as false targets.
Moving targets will not remain in a given cell long enough to be eliminated. Returns that very rapidly in
strength and centroid location, such as clouds and chaff in high winds, could appear non-stationary and
would cause a track to be established. These returns are then tested for the known velocity characteristics of
valid targets by velocity filters, thus reducing the number of false tracks that would otherwise be generated in
this manner.
One approach is to first quantize the range and sometimes the azimuth angle.The quantization increment in range might be the pulse width and that is angle might be the azimuth beamwidth.At each range-azimuth quantization cell, the pulse received during the time the antenna scans past the target are integrated and a detection decision is made. CFAR generally is incorporated before the decision process inorder to prevent excessive false alarm due to clutter echoes. Pulse Integration is performed in some form of automatic detector,or integrator.
Another approach to automatic detection is the moving window detector which examines continuosly the last n pulses and announces the presence of a target if it at least m out of n of the pulses exceed a present threshold.A by product of the automatic detection decision with a moving window detector or something similar is an angle measurement made by beam splitting. if n pulses expected to be recieved from a target ,beam splitting involves recognizing the begining and end of the n pulses and locating their centre.Angle accuracy depends on how well the begining and end of the tram of n pulse can be determined,as well as the number of pulses available and their signal to noise ratio.The beam splitting decision logic usually has no prior knowledge of targets begining.The logic must be sufficiently sensitive to quickly recognize the increased density region that signifies the start of an echo-signal pulse train , yet it must not be so sensitive it generates false starts due to noise alone.Once a target's beginning is recognized,the device must sense the end of the increased density region .If the decision logic is too sensitive to change,it could cause a single target to split into two. A rough rule of thumb often quoted is that the accuracy of beam splitting is about one-tenth of a beamwidth when the signal to noise ratio is high enough to provide a good probability of detection.
=== Track Correlation and Association ===
Target observation on each radar scan that survives hit pattern recognition and clutter rejection functions is
initially treated as new information prior to comparison with previously held data. Logic rules established in
the digital computer program, which perform correlation and association of video observations with
currently held tracks, are the key to preventing system saturation in high-density environments. Generally
speaking, target observations that fall within the boundary of the tracking gate are said to correlate with that
track. Each observation is compared with all tracks in the computer's memory and may correlate with one
track, several tracks, or no tracks. Conversely, a track gate may correlate with one observation, several
observations, or no observations. Whenever there is other than a one-to-one correlation of observations with
tracking gates, **tracking ambiguity** results.
When a new detection is recieved that is not at the location of a clutter echo stored in the clutter map, an attempt is made to associate it with an existing track.Association with an existing track is aided by establishing for each track a small search window or gate ,within which the detection of target on the next scan of the radar antenna is predicted to appear. The gate should be as small as possible in order to avoid having more than one echo fall within it when the traffic density is high or when two tracks are close to one another. On the other hand, a large gate region is needed if the tracker is to follow target turns or maneuvers. More than one gate size is used to over come this dilemma.Figure 1 shows a small nonmaneuvering gate situated around the predicted position of the target in track. The size of the gate is determined by the estimated errors in the predicted position and the estimated errors in speed and direction of the track. The detection threshold might be lowered in the gate region to increase the probability of detection. When an echo is not found within the maneuvering gate ,the larger region encompassing the maneuvering gate is then searched. The size of the maneuvering gate is determined by the estimate of the maneuvering capability of the target under track.
{{:media:maneuvering_and_nonmaneuvering_gates.jpg?500|}}
One reason the target might , not appear in the nonmaneuvering gate is that its radar cross section might decrease, or fade,so that it is not detected.When this is the case, it is possible for a false track to occur when a noise spike or an echo from another target is found in the maneuvering gate.To avoid the problem caused by a target fade and a false indication appearing in the larger maneuvering gate, the tracks can be divided into two tracks.(This is known as bifurcation of the track). One in the original track with no new detection in the nonmaneuvering gate.The other is a new track based on the signal found in the maneuvering gate. After recieving the target position on the next scan of the radar (or sometimes after two scans), a decision is made as to which of two tracks should be dropped.Tracking is usually done in cartesian coordinates, but the coorelation gates are defined in polar (r,θ) coordinates.
**Resolution of track ambiguity**:Track ambiguity arises when either multiple targets appear within a single
track window or two or more gates overlap on a single target. This occurrence can cause the system to
generate erroneous tracking data and ultimately lose the ability to maintain a meaningful track. If the system
is designed so that an operator initiates the track and monitors its progress, the solution is simply for the
operator to cancel the erroneous track and initiate a new one. For automatic systems, software decision rules
are established that will enable the program to maintain accurate track files, resolve ambiguity, and prevent
saturation by false targets.
Depending upon the specific use, many different sets of rules are employed to resolve this ambiguity, an example of which is outlined below:
1. If several video observations exist within one tracking gate, then the video closest to the center of the gate
is accepted as valid.
2. If several tracking gates overlap one video observation, the video will be associated with the gate with the
closet center.
3. If tow contacts cross paths, then the tracking gates will coast through one another, and re- correlation will
occur when the gates no longer overlap.
4. If one video observation separates into two or more, rule I above is applied until there is clear separation
in velocity and position and a new tracking gate is generated for the uncorrelated video. Rule number 2 is
then applied until the gates no longer overlap.
=== Track initiation and track file generation ===
Concurrent with the generation of the acquisition window, a track
file is generated in order to store the position and gate data for each track. In addition to the basic position
and window data, calculated target velocities and accelerations are also stored in each track file. For ease of
calculation and interchange of information with other systems, all data is converted from polar to rectangular
coordinates by the computer, as described in succeeding chapters. Track files are stored within the digitalcomputer
subsystems' memory in rectangular coordinates, and the data are used to perform the various
calculations necessary to maintain the track. Note that within the computer, position data has been converted from polar coordinates to
rectangular coordinates. Each track file occupies a discrete position in the digital computer's high-speed
memory. As data are needed for computation or new data are to be stored, the portion of memory that is
allocated for the required data will be accessed by the system software (programs). In this manner, tactical
data, in addition to the tracking data, may be stored in the "track" file--for example, ESM data, engagement
status, and IFF information. (It is equally important to track friendly forces as well as hostile forces.) The
generation of the track file begins with the initial storage of position data along with a code to indicate that
an acquisition window has been established.
If target position data is obtained on subsequent scans of the radar, the file is updated with the coordinates,
the velocities and accelerations are computed and stored, and the acquisition window code is canceled. The
acquisition window is then decreased in size relative to that of the tracking window, and the track code is
stored, which indicates an active track file. As the radar continues to scan, each input of data is compared
with the gate positions of active track files until the proper file is found and updated. The techniques of
computer data file sorting and searching are beyond the scope of this text; however, it should be noted that
the search for the proper track file is generally not a sequential one-to-one comparison. This method is much
too slow to be used in a system where speed of operation is one of the primary goals.
In principle, a track can be initiated from the target location information obtained on two successive scans of the radar antenna. In practice, however target information from three or more scans is usually needed to intiate a track.Two scans would be adequate when there is only one or a few aircraft within view, but when the radar has in view a larger number of echoes, one or more additional scan may be needed to prevent false tracks from being initiated. Thus it is more usual to require three or more scan before establishing a track.A clutter map is used to store, the locations of fixed clutter echoes and prevent tracks from being initiated based on a clutter echo combined with a real target detection.Such tracks can eventually be recognized as false and can be dropped , but it takes time and computer capacity to do so when there are a large number of them.Clutter echoes for inclusion in the clutter map are those echoes that do not change their location with time or that change loction too slowly to be targets of interest.
The process of initiating a track in a dense environment of targets and clutter not diminated by the radar can be quite demanding in both computer software and hardware. Initiation of a new track may take more computer time and capability than any other aspect of ADT.Requiring three scans for a civil air-traffic control radar to establish a track is usually not a burden. Waiting three scans for track establishment , however may be an excessively long time for a military air-defense radar that has to direct weapon-control radars to defined against high speed trackers that "pop up" at short range over the horizon.it is possible to quickly aquire the target on the basis of a sinlge scan past the target if the radar can obtain a quick second look. This might be done with a look-back beam directed to the angle of the original detection.The quick look-back can provide confirmation of detection and an estimate of target's related velocity. A phased array radar is well suited for this purpose, but mechanical rotating radar can also be outfitted with a fixed look back beam. Look back might also be accomplished with a 3D radar whose eletronically scanning beam in elevation is returned to the elevation angle of initial detection, before the radar beam entirely scans past the target.
=== Generation of tracking "gates" ===
A "gate" can be defined as a small volume of space composed of many of the cells described previously,
initially centered on the target, which will be monitored on each scan for the presence of target information.
Gate dimension and position information is generated by the general-purpose digital computer and sent to the
ADT processor for application. Action by the clutter map is inhibited inside these gates to avoid generation
of clutter cells by valid tracks.
When a target is initially detected, the algorithm receives only the position data for that initial, instantaneous
target position. The acquisition gate is then generated in the following manner:
**Acquisition Gate Tracking Gate**
The acquisition gate is large in order to allow for target motion during one scan period of the radar. If the
target is within the acquisition gate on the next scan, a smaller tracking gate is generated that is moved to the
new expected target position on subsequent scans. In the event that a video observation does not
appear within the tracking gate on a subsequent scan, ADT will enter some type of turn-detection routine. A
common means of dealing with a turn or linear acceleration of the target is the turn-detection gate. The turn detection gate is larger than the tracking gate and is co-located with it initially, employing separate logic and
algorithms different from the tracking routine. The turn-detection gate size is determined by the maximum
acceleration and turn characteristics of valid tracks. Some systems will maintain one track with the original
tracking gate and one with the turn-detection gate for one or more scan after initial turn detection, allowing
for the possibility of inaccurate observation, clutter, or possible decoys prior to the determination of which is
the valid track position. Only one of these tracks would be displayed to the operator, that one being
determined by a programmed validity routine.
=== Track gate prediction, smoothing(filtering) and position ===
As was discussed in the earlier sections on servocontrolled
tracking systems, (conical scan and monopulse), tracking errors were generated as a result of the
target moving off the antenna axis. It was the task then of the error detectors and servo systems to reposition
the antenna axis onto the target. During the process of repositioning the antenna, the system response motion
was smoothed by employing rate (velocity) and position feedback. Recall that this feedback was
accomplished by electrical and mechanical means within the servo system. Recall also that in general the
system lagged the target, i.e., the target would move the system would respond.
In a track-while-scan system, tracking errors also exist due to target motion. The tracking gate now has
replaced the "tracking antenna," and this gate must be positioned dynamically ont eh target in a manner
similar to that of the "tracking antenna." However, there is no "servo" system to reposition and smooth the
tracking gate's motion. This repositioning and smoothing must be done mathematically within the TWS
algorithm. To this end, smoothing and prediction equations are employed to calculate the changing position
of the tracking window. Instead of the system "lagging" the target the tracking gate is made to "lead" the
target, and smoothing is accomplished by comparing predicted parameters with observed parameters and
making adjustments based upon the errors derived from this comparison.
The classic method of smoothing track data is by the - tracker or - filter described below. This simple filter is
ill-suited to extreme target maneuvers and in most current systems is increased in complexity to what is
called the Kalman filter. Among other things, the Kalman filter is capable of dealing with higher order
derivatives of target motion (i.e., beyond acceleration).
In some systems, complete track file information is retained in the separate command and control system
computer, with the TWS computer retaining only position data, velocity, and acceleration.On the basis of a series of past target detections, the automatic tracker makes a smooth (filtered) estimate of the target's present position and velocity , as well as its predicted position and velocity.One method or accomplishing this is with the
α-β(alpha-beta) tracker.
The role of the **track-smoothing** function is to take the current known state (i.e. position, heading, speed and possibly acceleration) of the target and predict the new state of the target at the time of the most recent radar measurement. It is a computing step to improve the estimate of the tracks position as well as to revise the errors in the former prediction. The latest track prediction is combined with the associated plot to provide a new, improved estimate of the next target state. There are a wide variety of algorithms, of differing complexity and computational load that can be used for this process. These algorithms are realized in special filters.
All of these filters work in two steps:
♦prediction, and
♦correction.
The observed position of the target is recorded for the last two or more scans. Then, the smoothed parameters (often called innovations) are computed for position, velocity and acceleration. Using these values, the predictions for position and velocity are computed.
{{:media:process_of_track_smoothing.png?500|}}
process of track smoothing[(cite:Image2>> title: http://www.radartutorial.eu/10.processing/sp23.en.html ,section:"Track Smoothing"
)]
===Track Termination===
Successive applications of this system of equations serves to minimize the tracking error in a manner similar
to the repositioning of the "tracking antenna" in the servo tracking system. It should be noted at this point
that although the techniques of tracking differ between the two, i.e., servo and TWS, the function and the
concepts exhibit a direct correlation.If the radar does not receive target information on a particular scan , the smoothing and prediction operation can be continued by properly accounting for the missing data. (This is sometimes called coasting).When data from a target is missing for a number of consecutive scan, the track is terminated. Although the criterian to be used for determining when to terminate a track depends on the application,it has been suggested that when three target reports are used to establish a track, five consecutive misses is a suitable criterian for termination.
==== ADVANTAGES AND APPLICATIONS OF TWS ====
**TWS Radar System Advantages**
1)The advantage of TWS compared to a continuous tracker is that multipath target can be tracked. Becuase it shares its energy over region of space, the TWS radar needs to have a larger transmitter to obtain the same detection and tracking capabilities of a STT that dwells continuously on a single target.
2)TWS radar are also more vulnerable to angle jamming than are continuous trackers.
3)TWS radar can use monopulse angle measurements is not made with closed-loop tracking a TWS radar should not experience the wild fluctuations in angle caused by glint when there are not tiple scatterers within the resolution ceil or when there is multipath at low elevation angles.
4)Scan and track on same system.
5)Tracking information used for computation of a fire control solution.
6)Immediate fire control solution.
7)Automated tracking
8)Track prediction
9)Tracks multiple targets simultaneously
10)Tracking info used to compute FC solution
11)Tracks with only a search radar and a computer
**Real World Applications**
**1)AN/SYS-2 IADT(Integrated Automated Detection and Tracking)**
♦used on some missile ships
♦Carriers
♦CG
♦DDG
♦FFG
**2)Mk 92 CAANS - FFG**
**3)AN/SPQ-9 – 5” Gun**
**4)Mk 23 TAS – NATO Sea Sparro**w
**5)AEGIS C&D**
♦holds track files for AEGIS
♦dedicated computer
{{:media:ffg_60_uss_rodney_m_davis_.png?600|}}
FFG 60 – USS Rodney M Davis[(cite:Image3>> title: Lesson 05 - Track While Scan.ppt ,section: “Real World Applications"
)]
{{:media:dd_982_uss_nickelson.png?600|}}
DD 982 – USS Nickelson[(cite:Image3)]
{{:media:ddg_81_uss_w._s._churchill.png?600|}}
DDG 81 – USS W. S. Churchill[(cite:Image3)]
===== ACTIVE TRACKING RADAR =====
Conventional TWS procedures applicable to mechanically scanned radar systems do not fully exploit the
potential of radars employing phased array beam steering in all axes. The ability of these systems to position
beams virtually instantly and to conduct many types of scans simultaneously provides great flexibility in
planning tracking strategies. These radars conduct what amounts to a random search, employing different
pulse widths and PRTs applicable to the situation. When an echo that meets required parameters is observed,
the radar immediately transmits several beams in the vicinity of the target while continuing search in the
remainder of the surveillance area. This results in the immediate establishment of a track without the
relatively long (.5 to 4 seconds) time period waiting for the antenna to return on the next scan, which is
experienced in mechanically scanning radars. This technique, called **active tracking**, results in greatly
decreased reaction time and more efficient use of the system.
A typical tracking radar has a pencil beam to receive echoes from a single target
and track the target in angle, range, and/or doppler. Its resolution cell—defined
by its antenna beamwidth, transmitter pulse length, and/or doppler bandwidth—
is usually small compared with that of a search radar and is used to exclude undesired
echoes or signals from other targets, clutter, and countermeasures. Electronic
beam-scanning phased array tracking radars may track multiple targets by
sequentially dwelling upon and measuring each target while excluding other echo
or signal sources.
Because of its narrow beamwidth, typically from a fraction of 1° to 1 or 2°, a
tracking radar usually depends upon information from a search radar or other
source of target location to acquire the target, i.e., to place its beam on or in the
vicinity of the target before initiating a track. Scanning of the beam within a limited
angle sector may be needed to fully acquire the target within its beam and
center the range-tracking gates on the echo pulse prior to locking on the target or
closing the tracking loops.
The primary output of a tracking radar is the target location determined from
the pointing angles of the beam and position of its range-tracking gates. The angle
location is the data obtained from synchros or encoders on the antenna tracking
axes shafts (or data from a beam-positioning computer of an electronic-scan
phased array radar). In some cases, tracking lag is measured by converting
tracking-lag-error voltages from the tracking loops to units of angle. This data is
used to add to or subtract from the angle shaft position data for real-time correction
of tracking lag.
There are a large variety of tracking-radar systems, including some that
achieve simultaneously both surveillance and tracking functions. A widely used
type of tracking radar and the one to be discussed in detail is a
ground-based system consisting of a pencil-beam antenna mounted on a rotatable
platform which is caused by motor drive of its azimuth and elevation position to
follow a target in Figure 6. Errors in pointing direction are determined by sensing
the angle of arrival of the echo wavefront and corrected by positioning the antenna
to keep the target centered in the beam.
{{:media:pencil-beam_antenna.png?600|}}
The principal applications of tracking radar are weapon control and missile-range instrumentation. In both applications a high degree of precision and an accurate
prediction of the future position of the target are generally required. The
earliest use of tracking radar was in gunfire control. The azimuth angle, the elevation
angle, and the range to the target were measured, and from the rate of
change of these parameters the velocity vector of the target was computed and its
future position predicted. This information was used to point the gun in the
proper direction and to set the fuzing time. The tracking radar performs a similar
role in providing guidance information and steering commands for missiles.
In missile-range instrumentation, the tracking-radar output is used to measure
the trajectory of the missile and to predict future position. Tracking radar which
computes the impact point of a missile continuously during flight is also important
for range safety. Missile-range instrumentation radars are normally used with
a beacon to provide a point-source target with high signal-to-noise ratio. Some of
these systems achieve a precision of the order of 0.1 mil in angle and a range
accuracy of 5 yd.
===== INTEGRATED AUTOMATIC DETECTION AND TRACKING (IADT) =====
By extending the concepts described in this chapter and adding additional computer capability, it is possible
to combine the outputs of several radars that are co-located, as on ships or at shore-based command and
control systems such as MACCS. Systems such as the AN/SYS-2 IADT, now entering limited use aboard
missile ships, develop a single track file based on the outputs of several radars. When radars are employed
with different scan rates, a separate external timing reference must be employed, which becomes the scan
rate for the IADT and the synthetic video display. Track updating and smoothing occur as previously
described, except updates are considered relative to narrow sectors of the search volume--typically
a few degrees in width. System software updates tracks a few sectors behind the azimuth position of the
synthetic scan. Observations are accepted from the radar having the oldest unprocessed position data sector
by sector as the synthetic scan passes. This provides the first available position report, no matter which radar
produced it. Thus, position reports are used as they occurred in real time, and no position report is accepted
out of order. IADT reduces loss of data due to individual radar propagation and lobing characteristics while allowing quality weighting of data relative to it source. IADT systems can accept the output of TWS or non-
TWS radars as well as that derived from IFF transponders.When multiple radar view a common volume, there can be improved tracking , the data rate can be greater than any of the radar acting alone ,there is less vulnerability to electronic countermeasures, and less likelihood of having missed detection due to reduced echo-signal strength caused by null in one of the antenna pattern or changes in the target aspect.
**Integrated Tracking From Collocated Radar at a Single Site**:When more than one radar covering approximately the same volume in space, are located in the same vicinity , their individual output can be combined to form a single track.The radar might operate in different frequency bands, have different antenna characteristics , and diferent data rates. There is more than one way to combine the output of multiple radars.A good approach is to combine all the detections from each radar to form a single track and to update the track rather than develop seperate tracks at each radar and either select the best track or combine them is some other manner.The data from the various radars do not arrive at the tracker at a uniform rate.The development of a single track file by the use of the total data available from all radars produce a better track than combining the tracks developed individually at each radar.it reduces the likelihood of a loss of data as might be caused by antenna lobing, target feeding, interference, and clutter since integrated processing permits the favourable weighting of the better data and lesser weighting of poorer data. This method of combining data from multiple radar has been known as either **automatic detection and integrated Tracking (ADIT) or integrated automatic detection and Tracking (IADT)**.
The **Integrated Automated Detection and Tracking (IADT) System AN/SYS-2** is a computer-based radar data processor with automated radar target detection, tracking, and correlation capabilities. The AN/SYS-2 correlates contact data from the 2-D and 3-D air-search radars to provide a single, unduplicated, highly-accurate, surveillance picture output in various operational environments, including clutter and electronic countermeasures. The AN/SYS-2 accomplishes this by taking advantage of the mutually supporting aspects of the 2-D and 3-D radars surveillance volume of coverage and by exploiting their special modes of radar operation. The AN/SYS-2 is designed to enhance significantly the effectiveness of the combat system by reducing reaction time, improving threat assessment, providing earlier warning, and providing a prompt and reliable detection capability in the presence of high target density and electronic countermeasures.
Systems such as the AN/SYS-2, now entering limited use aboard missile ships, develop a single track file based on the outputs of several radars. When radars are employed with different scan rates, a separate external timing reference must be employed, which becomes the scan rate for the IADT and the synthetic video display. Track updating and smoothing occur as previously described, except updates are considered relative to narrow sectors of the search volume -- typically a few degrees in width. System software updates tracks a few sectors behind the azimuth position of the synthetic scan. Observations are accepted from the radar having the oldest unprocessed position data sector by sector as the synthetic scan passes. This provides the first available position report, no matter which radar produced it. Thus, position reports are used as they occurred in real time, and no position report is accepted out of order. IADT reduces loss of data due to individual radar propagation and lobing characteristics while allowing quality weighting of data relative to it source. IADT systems can accept the output of TWS or non-TWS radars as well as that derived from IFF transponders.
The Model 4.0 Combat Direction System software programs for the FFG-7 class Guided Missile Frigate are hosted in either AN/UYK-7 or AN/UYK-43 computers. The software interfaces digitally with other components of the Combat System, including the Mk92 Weapons System, Harpoon, AN/SYS-2 Integrated Automatic Detection and Tracking System, AN/SLQ32 Electronic Warfare System and AN/SQQ-89 Anti-Submarine Warfare System.
===== ALPHA – BETA FILTER =====
An early tracking approach is the so called Alpha-beta tracker. It is a simplified form of observer for estimation, data smoothing and control applications. It is closely related to Kalman filters and to linear state observers used in control theory. Its principal advantage is that it does not require a detailed system model. This tracker assumed fixed Gaussian errors and a constant-speed, non-maneuvering target model to update tracks. On the basis of a series of part target detections, the automatic tracker makes a smoothed (filtered) estimate of the target's position and velocity , as well as its predicted position and velocity.One method for accomplishing this is with the α-β(alpha -beta) tracker that computes the present smoothed target position $ \bar{x_{n}}$ and smoothed velocity $ \bar{\dot{x_{n}}}$ with the following operations.
\begin{equation}
Smoothed position , \bar{x_{n}} = x_{pn} + α( x_{n} - x_{pn})
\end{equation}
\begin{equation}
Smoothed velocity , \bar{\dot{x_{n}}} = \bar{\dot{x_{n-1}}} + \frac{β}{T_{s}}( x_{n} - x_{pn})
\end{equation}
The predicted position on the next scan( the n+1st ) is then,
\begin{equation}
x_{p(n+1)} = \bar{x_{n}} + \bar{\dot{x_{n}}} T_{s}
\end{equation}
where $x_{pn}$ = predicted position of the target on the nth scan, $x_{n}$ = measured position on the nth scan, α = position smoothing parameter, β = velcity smoothing parameter , and $T_{s}$ = time between observations.
if α = β = 0, the tracker uses no current target information, only the smoothed data from prior observations. When α = β = 1, no smoothing of the data is included at all.Thus the closer α and β are to zero, the more important is the track in determining the predicted track. The closer they are to 1, the more important is the currently measured data.if target acceleration is significant , a third equation can be added to describe an α,β,γ tracker , where γ = acceleration smoothing parameter.
Benedict and Border show that if the transient response to a maneuvering target can be modeled by a ramp function , the output noise variance at steady state is minimized in an α - β tracker when $ β = \frac {α^{2}}{2-α}$. it was stated that the analysis does not , and cannot , specify the optimum value of α. The value of α is determined by the bandwidth and will depend on the system application. In selecting α, a compromise usually must be made between good smoothing of the random measurement errors (requiring a narrow bandwidth) and a rapid response to maneuvering target (wide bandwith). Trunk states that an α and a β satisfying the above relation can be chosen so that the tracking filter will follow a specified g turn.
Another criterian for selecting the α - β values is based on the best linear track fitted to the radar data in a least square sense:
\begin{equation}
α = \frac {2(2n-1)}{n(n+1)}
\end{equation}
\begin{equation}
β = \frac {6}{n(n+1)}
\end{equation}
where n is the number of the scan or target obsrevation(n>2). The above equations for α and β are also called the kalman gain components.
The classical α - β tracker is designed to minmise the mean - square error in the smoothed position and velocity. This type of tracker is said to be relatively easy to implement , but it does not handle the maneuvering target. Some means has to be included to detect maneuvers and change the values of α and β accordingly.The two tracking gate described in connection with Fig 1 is one example of how to deal with a large maneuvers. Another example is an adaptive α - β tracker which varies the smoothing parameter to achieve a variable bandwidth that allows the radar to follow target maneuvers. When the target is not maneuvering the adaptive tracking algorithm provides heavy smoothing. if the target maneuvers or makes a turn , the filter bandwidth is widened so as to allow the track filter to follow. As the selection of the values of α and β become more sophesticated and requires knowledge of the statistics fo the measurement errors and the prediction errors, the α - β tracker approaches the Kalman Filter.The selection of (α , β) is a design tradeoff. Small gains make a small correction in the direction of each detection.As a result ,the tracking filter is less sensitive to noise but is more sluggist to respond to maneuvers - deviation from the assumed target model. Conversily, large gains produce more tracking noise but quicker response to maneuvers. These errors are readly calculated as a function of α and β using formula's shown in Table 1.
Characterisation of Tracking errors as a function of Tracking gain α and β
^ Error Source ^ Steady-State Track Error ^ In Position ^In Velocity^
| Radar detection noise(standard deviation - σ) |Standard deviation of filtered tracking state | $σ [\frac {2α^2+β(2-3α)}{α[4-2α-β]}]^1/_2$ |$\frac{σ}{T} [\frac {2β^2}{α[4-2α-β]}]^1/_2$ |
|Radar detection noise(standard deviation - σ) |Standard deviation of predicted tracking state | $σ [\frac {2α^2+αβ+2β)}{α[4-2α-β]}]^1/_2$|$\frac{σ}{T} [\frac {2β^2}{α[4-2α-β]}]^1/_2$|
|Constant maneuver - a(units of g's) |Lag(bias) in filtered track state |$aT^2\frac{(1-α)}{β}$|$aT(\frac{α}{β}-\frac{1}{2})$|
|Constant maneuver - a(units of g's) |Lag(bias) in predicted track state |$\frac{aT^2}{β}$|$aT(\frac{α}{β}+\frac{1}{2})$|