###### Abstract  
Visibility and communication are the essential pillars for safe flight operations in dense airspaces. Small Unmanned Aerial Vehicles (UAVs) of the order of up to 25 kg are increasingly being used at airports as a cost-effective alternative for maintenance and calibration work. However, the joint operation of manned and unmanned aircraft in busy airspaces poses a major challenge. Due to the small diameter of such UAVs, the established principle of \"see and avoid\" is difficult or even impossible to implement, especially during take-off and landing. For this reason, a certified Mode A/C/S transponder supporting ADS-B was extended with an embedded system and a cellular interface to realize a Multi-Mode-Transceiver (MMT). Integrated into a UAV, the MMT can provide aircraft visibility in the context of traditional manned Air Traffic Management (ATM) and future UAS Traffic Management (UTM) at the same time. This multimodal communication approach was investigated in flight test campaigns with two commercially available UAS that were connected to an experimental UTM with a simulated controlled airspace. The results confirm the safety gain of the multimodal cooperative approach. Furthermore, the collaborative interface with ATC enables the digital transmission of transponder codes, entry clearances and emergency procedures without the need for a voice radio communication. However, the parallel operation of both radio technologies in a confined space requires modifications to the transmission power and alignment of the radio antennas to avoid mutual interference. Furthermore, different reference planes of barometric altitude measurement in manned and unmanned aviation pose additional challenges that need to be addressed. Keywords:XPDR; ADS-B; UAS Traffic Management; collaborative ATC interface; LTE; MQTT +
Footnote †: journal:  
Article Evaluation of a Multi-Mode-Transceiver for Enhanced UAV Visibility and Connectivity in Mixed ATM/UTM Contexts 2022 6 80 10 3390 00 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 0000 000 0000 000 000 000 0000 000 0000 000 000 000 0000  
High visibility and a reliable communication link are essential for safe operations inside and around airports. In addition to the minimum equipment, that consists of a two-way radio and a Secondary Surveillance Radar (SSR) transponder supporting Mode A/C or Mode S [6], \"see and avoid\" is one of the most important principles for safe flights under Visual Flight Rules (VFR). However, due to the relatively small diameter of the unmanned aircraft, maintaining visual contact is difficult if not impossible, especially for arriving and departing traffic [7; 8; 9]. An analysis of the potential hazards of UAVs for runway inspections ([10] p. 284) revealed the following recommendations, among others, to minimize risks of collisions:  
* Control and limit the entry of other UAVs to ensure safe operation within the control zone
* Equip UAVs with a transponder to be visible on radar to the ATC as well to other air traffic participants
* Equip UAVs with collision warning systems  
Today's commercially available Unmanned Aircraft Systems (UASs) in the range of up to 25 kg are usually equipped with geofencing technology to prevent unintentional entries into sensitive airspace. However, this mechanism does not contribute to the visibility or connectivity of the UAV for intended use within controlled airspace. Apart from radio-based traffic reports from flight information services, turbine powered aeroplanes with a maximum certificated take-off mass over 5700 kg or more than 19 passengers are equipped with the Airborne Collision Avoidance System II [11], whereas aircraft from the general aviation domain typically rely on Automatic Dependent Surveillance Broadcast (ADS-B) data for situational awareness, presuming the presence of a suitable transponder. By broadcasting precise position information to surrounding traffic on a permanent basis, evasive maneuvers can be coordinated in the event of dangerous approaches.