# In-flight Wind Field Identification and Prediction of Parafoil Systems
## 1 Introduction  
A parafoil system is a type of unmanned autonomous vehicle, which possesses the advantages of exceptional controllability and gliding performance, and can achieve fixed-point autonomous homing and soft landing compared with traditional parachute systems [1; 2; 3; 4]. Parafoil systems play a significant role and have wide applications in the area of disaster assistance, aerospace, and military fields [5; 6; 7; 8]. Due to their slow airspeed in comparison to conventional fixed-wing aircrafts, the parafoil systems are extremely susceptible to wind fields [9; 10]. In particular, disturbed by both the speed and direction of the ambient wind field, the parafoil system easily deviates from its desired flight path and generates position drifts, even leading to a stall [1; 11; 12]. Accurately obtaining the ambient wind field of the parafoil system can improve the performance of homing trajectory planning and controlling, which is a key to achieve precise autonomous homing of parafoil systems [13; 14; 15]. Researchers have explored a variety of methods to obtain wind field information with certain success. [PERSON] et al. [16] reported on a ground-based mechatronic system consisting of a cup and-vane anemometer coupled to a guided airdrop system through a wireless transceiver, which can provide an improved, real-time estimate of the wind profile. [PERSON] [17] calculated the ambient wind field using the velocity data measured by the velocity sensor and the position information of the parafoil. However, the velocity measurement sensor is hard to install properly in practice because of the flexibility of the parafoil canopy [18]. [PERSON] et al. [19] sampled the wind field by using a ground laser radar system to obtain the ambient wind field data of the landing point, which was sent to the control system to amend the flight. This method can measure the wind data accurately, but its implementation is relatively complicated and expensive, and it is not suitable in areas where ground laser radars are unavailable or not allowed. [PERSON] and [PERSON] [20] provided a technique for obtaining accurate wind estimates in a pre-flight airdrop area using a global positioning system (GPS) dropsonde. However, due to the high uncertainty and variability of wind fields, the estimation results may not satisfy actual airdrop requirements. Besides the direct measurement of the wind field, researchers have also attempted to estimate the wind field using online estimation from its motion and trajectories. [PERSON] [21] proposed enhanced guidance, navigation, and control strategy that combines pre-flight wind information with in-flight identification for maximum resistance against winds and precise landing. [PERSON] and [PERSON] [22] developed a guidance strategy that uses massively parallel Monte Carlo simulation to rank candidate trajectories in terms of robustness to wind uncertainty. [PERSON] and [PERSON] [23] applied wavelet generation to generate a time series model for wind prediction, but the time range is a few days, not a few minutes after the parafoil system launches, which cannot rapidly obtain real-time wind information may result in imprecise landing. [PERSON] et al. [24] presented a system based on small unmanned aerial systems for the identification of wind features, such as gusts and wind shear. [PERSON] and [PERSON] [25] used a quadrotor Unmanned Autonomous Vehicle system for wind speed estimation based primarily on the wind speed triangle and the vector difference between the ground speed and estimated speed. However, none of them could acquire altitude-dependent wind field information in a range of altitude. Despite all the achievements, low-cost methods suitable for estimating the wind field of parafoil systems is still lacking. The traditional wind field acquisition methods mentioned above are complicated and expensive to implement. The state-of-the-art motion-based wind field estimation methods are limited to estimate the wind field of a certain altitude. The wind field, however, varies with the altitude.