# Intelligent Trajectory Prediction Algorithm for Reentry Glide Target Based on Intention Inference
## 3 Parameter Value Prediction Network
### Association Models between Strategic Places and Trajectory  
#### 4.2.1 Model Setting  
In general, the guidance methods of the reentry glide target can be divided into nominal ballistic guidance and predictor-corrector guidance. The nominal trajectory is usually designed by several optimization algorithms when nominal ballistic guidance is used, so that the target could choose the fastest and safest trajectory to attack the key point. When predictor-corrector guidance is adopted, the guidance trajectory can be obtained based on avoid logic for no-fly zones and guidance logic. Therefore, the analysis of the possible trajectories of targets based on strategic places' distribution and no-fly zone information is of great significance for trajectory prediction.  
According to the analysis in Section 3, the lateral guidance is mainly decided by bank angle. Thus, based on the parameter value prediction network in Section 3, the future  
Figure 3: Effect of Importance map: (**a**) represents no generated key point; (**b**) means one key point is generated by two strategic places; and (**c**) means one key point is generated by all strategic places.  
control parameters \(K_{L}\) and \(K_{D}\) can be obtained. Subsequently, bank angle selects a constants value within \(\left[-\pi/2,\pi/2\right]\) according to a certain step, based on the control parameter model, and several predicted trajectories can be obtained when the setting terminal height \(h_{f}\) is reached. Then, the reach region of target and the association trajectories of each key point can be obtained. The model is shown as Figure 4.  
As shown in the Figure 4, analyzing the key points within the reach region, the relationship between the position of the target, no-fly zones and key points should be discussed. It is assumed that the radius of no-fly zones is \(R_{b}\). The mapping area of no-fly zone is named no-fly zone mapping corridor, which is between the trajectories generated with a constant bank angle and tangent to the no-fly zone circle. If the key point is within the mapping corridor and sheltered by the no-fly zone, the bank angle needs to be adjusted to avoid the no-fly zone. Otherwise, the trajectory can join the target and key point directly keeping the constant bank angle, such as key point a. For the key points located in the mapping corridor, such as key points b and c, it is necessary to obtain the trajectory and position tangent to the no-fly zone circle. It is generally thought that the less maneuvering operations the target performs, the more energy it can save, so the tangent trajectories can be seen as the approximate trajectory. If the lift of the target is sufficient, based on the Dubbins circle, the turning circle radius of the target is \(r_{t}=R_{b}\), and the acceleration of tuning satisfies  
\[a_{t}=K_{L}\rho V^{2}\sin|\beta|=\frac{V^{2}\cos^{2}\theta}{R_{b}} \tag{7}\]  
In general, the flight path angle is a small value; if the target can fly-around the no-fly zones, it satisfies \(\left|\left(R_{b}\rho K_{L}\right)^{-1}\right|\leq 1\).  
After flying-around, the target flies directly to the key point b. If Equation (7) is not satisfied, the minimal radius of flying-around circle will be \(r_{t}=1/\left|\rho K_{L}\right|\). Based on the tangent point position, the new flying-around trajectory is obtained, and the whole trajectory is completed. Through the association model, the simple association trajectory between key points and target can be generated, and the time-to-go can be approximately estimated as a key factor to analyze the intention of the target.  
Figure 4: Association Model between strategic places and trajectories.