Which technique is NOT typically used to mitigate interference in radar systems?

The technique of fixed frequency usage is not typically employed to mitigate interference in radar systems because it involves operating on a single, constant frequency. This approach can make the radar system more vulnerable to interference, as it cannot change to avoid or reduce the effects of any signal that might be causing disruptions.

In contrast, frequency hopping is a technique where the radar system rapidly changes frequencies according to a predetermined sequence, thus minimizing the risk of interference by avoiding prolonged exposure to disruptive signals. Adaptive filtering involves modifying the system's filter parameters in real-time to suppress unwanted noise or interference based on changing conditions. Spread spectrum techniques spread the radar signals over a wider bandwidth, allowing the system to be more resilient to interference and enhancing its performance in crowded frequency environments.

Utilizing fixed frequency usage limits the flexibility and responsiveness of a radar system to its operating environment, typically leading to increased vulnerability rather than providing mitigation against interference.