There are several defense systems used to counter drones, depending on the threat level and specific scenario. Some of the most common systems include:

1. Radio Frequency (RF) Jamming: This technique involves the use of signal jammers to disrupt the communication between the drone and its operator, causing the drone to lose control and either crash or return to its starting point.

2. GPS Jamming or Spoofing: These methods target the GPS system on the drone, either by blocking GPS signals or sending false data to confuse the drone's navigation system, leading it to veer off course or hover in place until its battery is depleted.

3. Drone Detection Systems: These systems use radar, acoustic sensors, or optical sensors to detect the presence of drones and monitor their location and flight path.

4. Anti-Drone Lasers and Microwave Energy Weapons: Some defense organizations, like the military, have developed high-energy lasers and microwave systems that can destroy or disable drones, even the small ones, from a distance.

5. Drone Interception: These solutions involve launching another drone (called an interceptor drone) to physically disable or capture the unauthorized drone, usually by firing a net or using other entanglement devices.

6. Electronic Countermeasures and Hacking: Some approaches involve hacking into the drone's control systems, either to take control of the drone or disable it altogether.

Different countries and organizations may utilize one or several of these defense systems to protect their airspace and key infrastructure from unauthorized or hostile drone activity.