Track While Scan (TWS) modes: Track-while-scan means that the radar does its normal right-to-left, left-to-right scanning while it is actively tracking a couple of targets. TWS uses either high or medium PRF's. In TWS mode the radar beam covers an area much smaller than the maximal 120 degrees, but this way the target updates are much quicker. In takes around 2 seconds for the radar to complete a full scan. The arc and number of bars covered by TWS scan can be set to different settings: 'wide' (60 degrees with 2 bars), 'medium' (30 degress with 4 bars) and 'narrow' (15 degrees with 6 bars). See diagram below.
TWS mode scan options
Since the radar normally can cover a 120 degree arc in front of the jet, it can be manually slewed in TWS mode, which means that the actually scanned area (15, 30 or 60 degrees) can be placed anywhere within the 120 degree limits.
The AN/APG-70 stores tracking information of up to 10 targets in TWS mode. This track record serves as a defense against loosing radar contact - if a target disappears from the screen, the system extrapolates from its track records and tries to predicts where the target should be next and then tries to re-acquire the target at the predicted spot. Additionally TWS provides speed and heading information to targets. Screen symbology of speed and heading information are small 'vector sticks' to point from the dot representing the target. The vector stick points to the heading of the target (the top of the screen meaning a heading of north) while the length of the stick illustrates the speed of the target. This is very useful for the aircrew to get a quick overall impression of the tactical situation. In TWS mode it is possible to put the designation cursor over a target to get important data of it (altitude, range, closure rate, heading, aspect angle, true airspeed) without locking up the target - this way triggering the target's RWR systems can be avoided.
There is a sub-mode in TWS which is called High Data Rate TWS (HDTWS). This mode halves the 2 second time needed for a full scan to 1 second at the price of halving the number of bars scanned. In HDTWS mode the radar scans either a 30 degree arc with 2 bars (called 'high data' TWS) or a 15 degree arec with 3 bars (called 'three-bar HD' TWS). Slewing is also possible in both HDTWS submodes.
The AN/APG-70 provides another sub-mode within TWS, other than HDTWS sub-modes. This sub-mode is designed to help target sorting, that is to make a difference between two or more targets that are flying very close to each other.
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