Unmanned aerial vehicles (UAVs) rely heavily on the Global Positioning System (GPS) for navigation and control, making them vulnerable to attacks such as GPS spoofing and jamming, particularly in urban environments. This paper presents a simulation-based study to evaluate the impact of GPS spoofing and jamming attacks on UAV navigation systems. A quadcopter UAV model integrating GPS and inertial measurement unit (IMU) sensors is developed and simulated in a three-dimensional urban environment affected by multipath propagation and non-line-of-sight (NLOS) conditions. Different attack scenarios are analyzed in terms of trajectory deviation, navigation signal degradation, and the ability to maintain stable guidance. The results indicate that GPS spoofing poses a more severe threat than jamming because it can stealthily mislead the UAV's trajectory, whereas GPS-IMU integration using an extended Kalman filter (EKF) significantly improves system stability and reliability. Future research directions, including multi-GNSS integration and artificial intelligence-based detection methods, are proposed to enhance UAV safety in urban environments further.