The PRF are set at maximum, higher PRF results in more power on target, more power mean more reflection, this mode is specifically for detecting medium and high closure targets, with the cost of not detecting low and no-closure targets at all.
Velocity search mode displays targets on the radar screen by azimuth and velocity instead of by azimuth and range. In velocity search, many new pulses are sent before original pulse coming back.
Thus, radar cannot determine the distance, as a result it can only be used for detection rather than tracking. Main advantage of velocity search mode is very long detection range, could be double of track while scan mode.
The system is set up so as to get quick target information at the expense of accuracy. The RWS mode provides all-aspect (nose-on, tail-on) and all-altitude (look-up, look-down) target detection.
This is the most commonly used mode upon nearing a hostile environment. It is a good balance of wide volume and fairly rapid scan. This mode is used to resolve multiple targets separated by less than the antenna beam width, at long range.
There are three RWS modes, depending on what PRF is used for emitting radar energy. In RWSH mode high PRF is used, while in RWSM mode medium PRF is used.
High PRF’s are better to detect distant contacts with high closure rates with the risk of low or no-closure contact not showing up on the radar screen.
Medium PRF’s are not very good at long ranges since they are subject to clutter when receiving returns from long ranges, but they are useful for detecting medium-range low closure targets or targets which are below the radar water line.
A good mixture of high and medium PRF’s is realised in RWSI mode (interleaved RWS) when the radar emits energy alternating between high and medium PRF’s as it scans through bars.
RWS mode cannot be used to effectively to attack targets, but to get quick information about where the potential targets are. RWS is the most common mode.
Track While Scan（TWS）
With the aid of sophisticated computer systems, multi-function radars are capable of simultaneously tracking many targets. In this case, each target is sampled once (mainly range and angular position) during a dwell interval (scan).
Then, by using smoothing and prediction techniques future samples can be estimated. The mode that allow radar to perform multi-tasking and multi-target tracking are known as Track-WhileScan (TWS) radars.
Once a radar in TWS mode detects a new target it initiates a separate track file for that detection; this ensures that sequential detections from that target are processed together to estimate the target’s future parameters.
Position, velocity, and acceleration comprise the main components of the track file. Typically, at least one other confirmation detection (verify detection) is required before the track file is established.
Unlike single target tracking systems, TWS radars must decide whether each detection belongs to a new target or belongs to a target that has been detected in earlier scans.
And in order to accomplish this task, TWS radar systems utilize correlation and association algorithms. In the correlation process, each new detection is correlated with all previous detections in order to avoid establishing redundant tracks.
Generally, in TWS mode the radar system places a gate around the target position and attempts to track the signal within this gate. The gate dimensions are normally azimuth, elevation, and range.
Because of the uncertainty associated with the exact target position during the initial detections, a gate has to be large enough so that targets do not move appreciably from scan to scan;
more precisely, targets must stay within the gate boundary during successive scans. After the target has been observed for several scans the size of the gate is reduced considerably.
TWS mode uses either high or medium PRF. 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.
It 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 degrees with 4 bars) and ‘narrow’ (15 degrees with 6 bars).
If the pilot marks a single target for tracking, then the radar enters STT mode and begins tracking that specific target.
This tracking uses a 3 degrees mini-raster of radar energy centred on the target with very quick scans thus rapid target updates. Single target track mode often uses monopulse tracking techniques. (missiles launch is made in TWS or STT only).
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