Array radar resource management Alexander Charlish* and Fotios Katsilieris*
3.2 Task management
3.2.1.4 Non-uniform search parameters
In the previous sections, it was assumed that search control parameters are uniform over the entire surveillance region. However, it is common for a naval or ground-based radar to define search sectors. Each sector can have varied resource allocations or required performances, based on the sector’s threat level [4]. Consequently, the revisit interval times and transmit waveforms can be varied per sector. It is possible, but less common, to vary the revisit interval time and transmit waveform in each beam position. This enables the distribution of search energy in angle to be matched to information on the current environment [18,19] or expected threatening trajectories [20]. Matching the search angular energy distribution to the current scenario improves the efficiency of the search resource allocation.
An alternative approach to scheduling search dwells is to apply a Bayesian approach to maintain an ‘undetected target density’. This density describes where undetected targets are expected to be located, given the previously executed search dwells and a model of the expected target dynamics. Such an undetected target density can be represented by particles [21] or as a grid [22,23]. The undetected target density can be used as a basis for scheduling search dwells that maximize the probability of detecting previously undetected targets [24].
3.2.2 Confirmation management
An ESA can execute a rapid ‘look-back’ confirmation dwell to determine whether a search detection was due to the presence of a target or a random false alarm.
Scheduling confirmation dwells has the benefit of enabling rapid track acquisition, as it is not necessary to wait for the next search dwell in order to confirm the presence of a target.
Executing a rapid confirmation dwell has the additional benefit that the radar cross section (RCS) between the initial search dwell and the confirm dwell is correlated. Consequently, the probability of detection in the confirm dwell given that a detection has occurred in the initial search dwell (in this context called
‘alert’) is significantly higher than the original search dwell detection probability [25]. As the beam is directed to the angle of the previous alert, the beam positioning losses can be reduced. In addition, the alert detection enables the detection space for the confirmation to be reduced. Therefore, a lower detection threshold and hence higher false alarm probability can be handled in each cell, which increases the confirm probability of detection.
The waveform used in the confirmation dwell can be matched to the alert generated by the original search dwell. For example, the confirm transmit energy and hence dwell length can be varied based on the measured SNR in the original alert. The desired SNR in the confirmation dwell would also be set higher than the SNR in the alert, so that a good quality measurement is obtained to initialize a track. The confirm waveform may also be more complex than the initial search dwell, for example, when an ambiguous search mode is used then the confirm waveform must be capable of resolving the radar ambiguity.
The benefit of using alert-confirm is illustrated in Figure 3.8(a), which plots the target confirmation probability against the normalized target range. In the figure, the following target confirmation processes are compared:
● 2/2 Confirmation: Two detections from two search dwells are required to confirm the target. Therefore, no alert-confirm is applied.
● Alert-confirm:An alert detection is followed by a confirmation dwell that is identical to the original search dwell.
● Adaptive alert-confirm: An alert detection is followed by an adaptive con- firmation dwell, which adapts the dwell length to give an expected 22 dB SNR on the confirm dwell based on the original alert SNR. The length of the con- firmation dwell is limited to a maximum of ten times the original search dwell.
Figure 3.8(a) shows that using alert-confirm increases the target confirmation probability for a given range or increases the range at which targets are confirmed with a specified probability.
The benefit associated with scheduling confirmation dwells comes with the cost of increasing the time taken to perform the search. Based on the choice of the search dwell probability of false alarm and the dwell length of the confirmation, the expected time per search dwellTsis [9, Section 14.3.1]:
Ts¼taþPFANbtc (3.13)
whereNbis the number of range-Doppler cells,tais the search dwell time decided upon by the search management,tcis the confirmation dwell time andPFA is the probability of false alarm per range-Doppler cell in the search dwell. Here, the number of detected and tracked targets is not taken into account because it is not knowna priori.
Figure 3.8(b) shows the increase in time taken to perform the search, for the alert-confirm and adaptive alert-confirm strategies. When using the 2/2 con- formation strategy, there is no increase in search time because there are no con- firmation dwells. Alert-confirm increases the search time by a constant value irrespective of range, which for these example parameters was 1%. As adaptive alert-confirm adapts the confirmation dwell length, the increase in search time depends on the target range. In this example, the confirm dwell length was limited to a maximum increase of 10% of the original search dwell length.
This analysis assumes that the radar is operating in a noise-limited environ- ment. Confirmation is not as effective in environments with time correlated clutter, as the clutter time correlation prevents the immediate discrimination between clutter and a target.
3.2.3 Track management
An ESA antenna can simultaneously track a large number of targets either using measurements from the search function, which is known as track-while-scan/search (TWS), or by scheduling dedicated radar dwells that are optimized to the target, which is known as active tracking. The track manager must decide for each target
(b)
0 0.2 0.4 0.6 0.8 1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Range (R0) (a)
Target confirmation probability
2/2 Confirmation Alert-confirm Adaptive alert-confirm
0 0.2 0.4 0.6 0.8 1
0 2 4 6 8 10 12
Range (R0)
Increase in search time (%)
Alert-confirm Adaptive alert-confirm
Figure 3.8 Improvement in the track confirmation range due to the confirmation function and corresponding increase in search time. (a) Target confirmation probability. (b) Increase in search time
whether to track using active tracking or TWS. If the target is actively tracked, the track manager must also decide the revisit interval time as well as the transmit waveform to use. For tracking, the revisit interval time is the time between mea- surements of a target. When an active track update is executed, the radar beam is directed towards the estimated position of the target in angle space. A beam posi- tioning power loss occurs when the true target angle is offset from the estimated target angle.