Drones use various sensors and technologies to detect and avoid obstacles during flight. Some common methods include:

1. **Ultrasonic Sensors**: Ultrasonic sensors emit high-frequency sound waves and measure the time it takes for the sound waves to bounce back after hitting an obstacle. Drones use this information to estimate the distance to nearby objects and avoid collisions.

2. **Infrared Sensors**: Infrared sensors detect the infrared radiation emitted by objects and use it to calculate the distance to obstacles. They are effective for detecting objects in low-light conditions but may have limited range and accuracy.

3. **Lidar (Light Detection and Ranging)**: Lidar sensors emit laser pulses and measure the time it takes for the laser beams to reflect off objects. This data is used to create 3D maps of the drone's surroundings, allowing it to detect obstacles with high accuracy and avoid collisions.

4. **Computer Vision**: Drones may be equipped with cameras and computer vision algorithms that analyze visual data in real time to detect obstacles and identify safe flight paths. This technology can recognize objects, obstacles, and terrain features, enabling autonomous navigation and obstacle avoidance.

5. **GPS and Mapping**: Drones use GPS (Global Positioning System) and mapping data to navigate and plan flight paths. They may incorporate obstacle data from maps or databases to avoid known obstacles and hazards during flight.

6. **Radar**: Some advanced drones are equipped with radar sensors that emit radio waves and detect reflections from objects in their path. Radar can detect obstacles at longer ranges and in various weather conditions, providing additional situational awareness for obstacle avoidance.

7. **Flight Control Systems**: Drone flight control systems integrate sensor data and obstacle detection algorithms to adjust the drone's flight path in real time to avoid collisions. They may automatically slow down, change direction, or stop to prevent accidents when obstacles are detected.

By combining these sensor technologies and algorithms, drones can detect obstacles in their path and navigate safely in complex environments, avoiding collisions with buildings, trees, terrain features, and other objects during flight.