## 2 Phased Array Radar (PAR)
### Radiation Pattern Control
#### 2.2.2 Applications  
RTS is usually used at launching and tracking centers with parabolic reflector radars (PRRs) [1; 2; 3; 4; 5; 6], due to their long range and high pointing accuracy [7; 8]. On the other hand, PRRs have some restrictions, such as (a) difficulty in locating the target in space, due to very narrow beam and high target speed after launch, (b) single beam presence, not allowing multiple tracking, like stage-separated vehicles, aircraft-launched missile testing (at the instant of aircraft-missile separation) or accidents with vehicle fragmentation, (c) rotations in antenna azimuth and elevation are made by electromechanical devices (servomechanisms), at low speed and with a greater need for maintenance, (d) breakdown in any antenna device may render the radar sensor inoperative.  
To overcome such restrictions, an alternative is to employ an RTS with PARs, whose sensors are composed of antenna arrays to electronically control the radiation pattern [9; 10]. This generates new functionalities, such as larger scanning areas at critical times (shortly after launch) and the possibility of tracking multiple targets simultaneously. Moreover, without electromechanical devices, there are less conversion errors, fewer losses and lower number of maintenance, which improves calibration, efficiency and reliability. It is also possible to track even if the sensor is deteriorated, in the case of damage to certain elements of the array during tracking.  
Thus, PARs are used for surveillance and tracking from the 1960s up until nowadays, providing attack warnings by enemy missiles and monitoring objects in orbit. As examples, there are Ballistic Missile Early Warning System (BMEWS), Missile Site Radar (MSR), Cobra Dane (AN/FPS-108), PAVE PAWS (AN/FPS-115) and Cobra Judy (AN/SPQ-11), all developed by the U.S. Department of Defense [28; 34].  
Other examples of possible applications of PARs are: allocation of radiation pattern nulls in the direction of arrival of interfering signals, caused by harmonics in commercial transmitters or countermeasures (jammers) [11; 40; 41; 42; 43; 44]; variation of the format of radiation pattern of radar sensors of a RTS, based on time multiplexing between wide beams (larger scanning area) and narrow beams (greater pointing accuracy) [29; 44]; tracking each member of an aircraft formation, even if these members maneuver relatively close to one another, so that the formation does not confuse the defense system [45].  
To illustrate applications of PRRs and PARs, in Figure 5, there are: (a) Bearn radar, with parabolic reflector (PR) in the C band, used for rockets and missiles tracking, at Launching Center of Barreira do Inferno, Brazilian Air Force, in Parnamirim, RN, Brazil [46], (b) Cobra Dane Radar, PA in the L band, used for missile surveillance and subsequently monitoring space at Shemya Base, U.S. Air Force, Shemya Island, AK, USA [47].  
On the other hand, the development of PAs does not mean disuse of PR conventional radars. An interesting option is to make a composition between these two radars, in order to take advantage of both settings. This is the case for the Active Phased Array Radar (APAR), developed by Thales Nederland (Hollandse Signaalapparaten Besloten Vennootschap) to accomplish air defense for German, Canadian and Dutch battleships, in which there is a combination between a PA radar and a long distance conventional surveillance system (SMART-L) [29]. In the RTS case, an association between PARs and PRRs can be performed. The PARs would be responsible for the tracking soon after launching, acting as platform radars, due to their wider and faster beams, for the period in which the rocket speed is high, and the risks of losing the target location are higher. On the other hand, conventional PRRs would take on tracking for longer trajectories, acting as distance radars, since they have high power and narrow beams. This association between PARs and PRRs is shown in Figure 6.