CHAPTER 7. TARGET TRACKING
4. TRACK-WHILE-SCAN (TWS)
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 through the covered airspace. There are two types of radar systems capable of TWS operation: conventional and phased array.
a. Conventional track-while-scan threat radars use two separate antennas to generate two separate beams (Figure 7-7). These beams operate at two different
frequencies and are sectored so they overlap the same region of space. This overlap area provides a tracking area for a single target. One beam is sectored in the vertical plane to give range and elevation. The other beam is sectored in the horizontal plane to provide range and azimuth. Each beam scans its sector at a rate of 5 to 50 times per second. This provides a rapid update on target range, azimuth, and elevation.
Figure 7-7. Conventional TWS Radar Beams
(1) The two TWS antennas generate their beams using an electro-mechanical principle. Each antenna provides inputs to its own display and provides angle and range information for all targets in the coverage of the radar.
The display from the elevation beam is calibrated in range and elevation, while the display from the azimuth beam is calibrated in azimuth and range. Operators position a cursor over the returns on these displays using range as the primary parameter. Once a target has been designated for engagement, the radar automatically attempts to keep the tracking axis of the radar beams centered on the target.
(2) Once the target is designated by the operator, the range gate is enabled and tracks the target using a split-gate tracker. The azimuth and elevation tracking loops receive information only from targets inside the range gate. As the beams scan across the target, a burst of pulse returns is received that have an
amplitude envelope corresponding to the beam pattern. The azimuth beam pattern is shown in Figure 7-8.
Figure 7-8. Conventional TWS Azimuth Tracking
(a) The azimuth tracker is typically a split-gate tracker, identical in concept to a split-gate range tracker. However, range delay time is replaced by azimuth scan time. The azimuth tracker uses a left gate and right gate. Each gate integrates its share of the target return to generate a voltage/time value. When the azimuth gate is centered on the target, the areas are equal and the error signal (right gate minus left gate) is zero. The azimuth tracking loop sends signals to the antenna servos to keep the target centered in the scan area.
(b) Elevation tracking is accomplished in the same manner by using an up gate and a down gate. The elevation tracking loop also sends signals to the antenna servos to keep the target centered in the scan area.
(3) Once the target is designated and the radar is automatically keeping the radar return centered in the tracking area, target range, azimuth, and elevation information is sent to a fire control computer. The radar continues to provide information on other targets in the scan area. The fire control computer indicates the firing solution has been achieved for the designated target, and a missile is launched. The radar tracks the target and the missile and provides in-flight corrections to the missile right up to the moment of missile impact. These corrections are based on both target and missile azimuth, range, and elevation
information. Information is passed to the missile from a dedicated antenna on the radar to special antennas on the missile. Commands from the radar to the missile are called uplink guidance commands. Information from the missile back to the radar and fire control computer is called downlink information.
(4) The advantages of a conventional TWS radar include the following: (1) 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 (2) the rapid sector scan rate provides a rapid update on target parameters. The primary disadvantages of a conventional TWS radar include: (1) a large resolution cell due to the wide azimuth and elevation beams, and (2) vulnerability to modulation jamming based on the scan rate of the independent beams.
b. Many modern radars employing a planar or phased array antenna system have a TWS mode. The radar does not really track and scan simultaneously, but rapidly switches between search and track (Figure 7-9).
Figure 7-9. Phased Array TWS
(1) The most common air-to-air radar system uses a planar array antenna.
In the scan mode, the radar antenna generates a pencil beam and uses a raster scan to detect targets in the search area. Targets detected are presented to the pilot on the aircraft's radar display (Figure 7-10).
Figure 7-10. Air-to-Air TWS Display
(a) In the 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. These tracking techniques will be covered later in this chapter. The radar initiates a track file on each detected target that contains all current data on the target and an estimate of future target position.
(b) As the radar switches between track and scan modes, target parameters are updated in the tracking loop (Figure 7-11). The new target information is compared to the predicted information in the measurement data processing cell. If the two sets of data agree within certain limits, target position and information are updated. This process is called gating.
Figure 7-11. Planar/Phased Array TWS Tracking Loop
(c) If the updated target information does not correspond to the predicted values, the information is sent to the correlation processor. The correlation processor attempts to resolve the conflict based on further refinement of target data. If the correlation processor cannot assign the target parameters to an existing track file, a new track file is generated and displayed.
(2) The obvious advantage of a planar/phased array TWS radar is that it can search a large volume of airspace while tracking individual targets. The number of targets that can be tracked is limited by the number of beams the radar can generate. Planar/phased array radars have increased peak and average power when compared to pulse radar systems. Since the radar beam of a planar/phased array radar is electronically controlled and can rapidly change beams and scans, it is resistant to many jamming techniques. The primary disadvantages of a planar/phased array TWS radar include its complexity, cost, and reliance on computer processing.