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Track-While-Scan Concepts
Introduction
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, such as those systems discussed in the previous chapter. 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. 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 multi-target 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) (figure 6-1). 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.
Fundamentals of Automatic Detection and Tracking (ADT)
A functional tracking system must provide the basic target parameters of position and rates of motion. In the systems presented earlier in this text, 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 previously discussed.
A Tracking Algorithm
The following algorithm for solving the track-while-scan problem is based upon the assumption that the radar furnishes target position information once each scan. The scheme can be implemented with software (computer programs) only, or with a combination of special radar circuits (hardware) and software. Existing systems also allow the operator to modify the system tracking parameters.
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 (if no correlation)
(4) Generation of tracking "gates"
(5) Track gate prediction, smoothing (filtering), and positioning
(6) Display and future target position calculation
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. After several scans, the ADT processor contains what amounts to a three-dimensional binary matrix or array represents calls with echo returns with a "I" and those without returns with a "0" (figure 6-2).
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.
Target Acquisition, Tracking, and Turn Detection 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
Range Gate = R 2000 yds. (1828.15 R 120 yds. (109.69 meters)
meters)
Bearing Gate = B 10o (.1745 radians) B 1.5o (.0262 radians)
Elevation Gate = E 10o (.1745 radians) E 1.5o (.0262 radians)
(This is an example only, numbers will vary depending on the actual system.)
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. Although figure 6-3 shows only a very small (120 yards by 1.5o by 1.5o) tracking gate, in actual practice intermediate gate size may be generated until a smooth track is achieved. Note that the track and acquisition gate dimensions given in figure 6-3 are representative, but are only an example and may vary for different systems. 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 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.
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 digital-computer subsystems' memory in rectangular coordinates, and the data are used to perform the various calculations necessary to maintain the track. Figure 6-5 illustrates the data structure of a hypothetical target track file. 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.
Track gate prediction, smoothing, and positioning. As was discussed in the earlier sections on servo-controlled 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.
Smooth position Psn = Ppn + (Pn - Ppn) (6-1)
Smooth velocity Vn = Vn-1 + An-1T + (Pn - Ppn) (6-2)
T
Smooth acceleration An = An-1 + (Pn - Ppn) (6-3)
T2
Predicted Position Ppn + 1 = Psn + VnT + 1AnT2 (6-4)
(future position)
where
Pn is the target's position measured by the radar during scan n.
Psn is the smoother position after scan n.
Vn is the smoothed velocity after scan n.
An is the smoothed acceleration after scan n.
Ppn is the predicted target position for scan n.
T is the scan time (1 sec. in this case).
, , are system constants which determine the system response, damping and gain, etc.
Note: Pn represents the three cartesian coordinates.
Let us now examine equations 6-1 through 6-4 in greater detail and in the process correlate the track-while-scan functions to those of the servo tracking systems discussed earlier.
Equation (6-1)
Psn = Ppn + (Pn - Ppn)
Equation (6-2) is analogous to the rate (velocity) feedback achieved by the tachometer loop in the servo tracking system. In equation (6-2) the target velocity is updated and modified by comparing the observed position with the predicted position and then dividing the position error by the time of the scan to obtain velocity (Pn - Ppn/T.
The velocity is further refined by modifying it by the acceleration An-1T.
Equation (6-3)
An = An-1 + (Pn - Ppn)
Equation (6-3) develops the updated target acceleration to be employed in smoothing velocity on the next scan and in predicting the target position for the next scan. The old acceleration value in modified by the tracking error derived from the term (Pn - Ppn)/T.
Equation (6-4)
Ppn + 1 = Psn + VnT + 1AnT2
2
Equation (6-4) is simply the classic displacement equation derived in physics. It is used to predict the future target position for the next scan. The predicted position for the next scan Ppn + 1 is obtained by modifying the smoothed present position Psn by velocity and acceleration obtained from equations (6-2) and (6-3).
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.
TWS System Operation
Let us now take the logical components of the TWS algorithm and put together the entire tracking algorithm. The algorithm is described by means of an annotated flow diagram, figure 6-8. The reader should follow the flow through at least three radar scans in order to properly understand the system dynamics. For the purpose of this illustration, assume that the system begins with a new detection and that tracking can be initiated on the second scan. (In actual practice, three or more scans are usually necessary to establish a valid track.)
The necessity for digital computer control should now be very evident. It should be observed that the entire algorithm is repeated for every target on every radar scan.
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.
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.
Summary
The central concept underlying any track-while-scan system is that the sensor itself continues to perform its primary function of search (scanning) and data input, while the remainder of the system performs the target tracking function. The sensor simply provides target position data to the computer subsystem where target velocities and position prediction are calculated. In a military application, the major advantage of a TWS system is the elimination of the process of target designation from a search radar to a fire control radar. The tracking information, developed in the TWS system, is used as a direct data input to the computation of a fire control solution. Therefore, as soon as a target is detected, a fire control solution is available without the inherent delay caused by the designation process. The time required from first detection to fire control solution is on the order of seconds for a TWS system, as opposed to tens of seconds or even minutes for a manually designated system employing separate search and fire control sensors.
The focus of this chapter has been to answer the question, "What functions should a TWS system perform in order to combine the search and tracking tasks into one integrated unit?" The functions of (1) target detection, (2) target track correlation and association, (3) track initiation and track file generation, (4) generation of tracking "gates," (5) track gate prediction, smoothing (filtering), and positioning, (6) display and future target position calculations were explained as well as was an explanation of the - filter.
An introduction to employment of Integrated Automatic Detection and Tracking was presented demonstrating the performance of a single data processor with many sensor inputs. The single track output combined the best capabilities of all the sensors employed.
References
Benedict, T.R. "Synthesis of an Optimal Set of Radar Track-While-Scan Smoothing Equations." IRE Transactions on Automatic Control. July 1962, p. 27.
Binias, Guenter. "Automatic Track Initiation with Phased Array Radar." The Record of the IEEE International Radar Conference. New York: April 1975, pp. 423-34.
Cantrell, B. H., G. V. Trunk, F. D. Queen, J. D. Wilson, J. J. Alter. "Automatic Detection and Integrated Tracking System." The Record of IEEE International Radar Conference. New York: April 1975, pp. 391-95.
Commander, Naval Ordnance Systems Command. Gun Fire Control System MK 86 Mod 4. NAVORD OP 3758. Washington, D.C.: GPO.
Hovanessian, S. A. Radar Detection and Tracking Systems. Dedham, Mass.: Artech House, 1978.
Simpson, H. R. "Performance Measures and Optimization Condition for a Third-Order Sample Data System." IRE Transactions on Automatic Control, April 1963, p. 182.
Skolnik, M. I. Introduction to Radar Systems. 2nd ed. New York: McGraw-Hill, 1980, pp. 183-86.
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