WG-49 has been reactivated to perform maintenance on the Mode S Transponder MOPS ED-73E and ED-115. It will work joint with RTCA SC-209. The Mode S Secondary Surveillance Radar transponder system is a key element of ATM surveillance. It supports different applications including Mode S Elementary surveillance, Mode S Enhanced Surveillance, ADS-B surveillance applications and anti-collision applications such as ACAS. The interoperability of the Mode S transponder with other systems used on board aircraft or on the ground is crucial to achieve a safe and efficient ATM Operation. During the revision experience from the operational use of Mode S will be integrated and problems detected will be corrected. Furthermore support for new procedures such as Interval Management will be implemented which requires the exchange of information between aircraft. Access to real-time environmental data such as wind, temperature and turbulence will also be realised. Spectrum protection is an additional topic that may require adaptations to the transponder implementations. Finally, preparing the transponder for new functionalities required by ACAS Xu will be an additional task. During this activity, WG-49 will work in close cooperation with WG-51 ADS-B, WG-75 Traffic Collision Avoidance Systems and WG-73 Unmanned Aircraft Systems. Furthermore, WG-49 cooperates with RTCA SC-209. To facilitate this complex cooperation, a Combined Surveillance Committee has been set up.