SSR and ADS-B each play a critical role in modern air traffic surveillance, offering distinct advantages and facing different cybersecurity challenges. SSR is reliable and widely deployed but remains vulnerable to spoofing because its transponder messages are not authenticated. ADS-B improves overall aircraft tracking at a lower cost, but its broadcast-based design makes it particularly susceptible to spoofing and false data injection. GPS benefits from enhanced security through authentication, augmentation systems, and redundancy, although it remains exposed to threats such as jamming and spoofing. A multi-layered surveillance approach combining SSR, ADS-B, GPS, multilateration, and AI-powered anomaly detection represents the most resilient solution for future air traffic management. Continued research efforts, investment and international collaboration are essential to strengthening cybersecurity and supporting the implementation of secure and interoperable surveillance systems.