## 2 Phased Array Radar (PAR)
### Rocket Tracking System (RTS)
#### 2.1.2 RTS Operation  
To determine the trajectory of a target, the RTS must go through some steps. Initially, one must determine the tracking mode, which can be either radar or transponder. The transponder mode allows greater range and works with different frequencies of transmission and reception, since the signal received by the radar sensor is the \"response\" sent by the transponder installed on the target. On the radar mode (also called skin tracking), the range is smaller and the frequency is the same for both transmission and reception, since the received signal is an echo of the signal transmitted by the radar, and it is reflected by the target. In transponder mode, tests are conducted to check if the target is in  
Figure 2: (**a**) Rocket tracking radar near launch pad. (**b**) Surveillance radar for remote control air traffic. communication with the radar. Tracking is in automatic mode since the moment of launch, with the radar correcting its position without relying on manual intervention. However, if the tracking is on radar mode, it is necessary to know the expected trajectory of the target, called the nominal trajectory (NT), so that the radar operator directs the antenna to certain locals (holding points) of the NT, in order to locate the target in space and start automatic tracking. If, during the flight, an abnormality occurs and the radar loses automatic tracking, there are some alternatives to resume tracking. First, simpler and less accurate, is manual scanning, in which the operator positions the radar antenna near where location was lost or in the next holding point of the NT. This option is less recommended, due to the high speed at which the target (e.g., rocket or missile) changes position. The second alternative is called designation, which is obtained from data from other radar sensors or NT itself. The designation originates from the existing redundancy in the network of radar sensors that integrate an RTS, so that, preferably, more than one radar is tracking the target at the same time. This is possible if each radar sensor operates at a different frequency, thus avoiding interference among them. However, different frequencies must be previously established and properly controlled, including the fidelity of the transmission equipment, in order to ensure that the pre-set values will be accurate and will not interfere with each other, such as the production of harmonics or unwanted noise in each radar sensor transmitter. By ensuring this separation into different frequencies, it is possible to build an RTS network [36], which enables tracking of vehicles over long trajectories, the so-called tracking network. This is possible because the radar sensors of the network are interconnected and strategically placed in different locations, so that each radar tracks the target during part of its trajectory. This could not be done by radars near the platform, due to lack of visibility, either by obstacles in the direct sight line or the curvature of Earth itself. Thus, radar sensors have target visibility only for a limited period of flight and when they are close to losing target location, they inform the next network radar, which resumes tracking. The data obtained by the network allow a more robust performance [37,38] and a more precise estimate of the target position [39]. This happens because radars operate with redundancy, preventing the loss of location by one radar harming the tracking as a whole. Moreover, it enables the comparison of data from multiple radars (during or after tracking) to check the reliability and accuracy of the measurement. Therefore, redundancy in an RTS prevents errors caused by human failures, equipment breakdowns, unexpected behaviors of targets (such as abrupt maneuvers or fragmentation) and undesirable conditions in the propagation environment (such as lack of direct sight line, clutter, noise and interference). Thus, if a radar sensor misses the target location, some other radar's information on the network is used as a backup to locate the target again.