## 5 Conclusions  
It is becoming apparent that mobile radio networks will be an essential component in future UTM systems. At the same time, it must be assumed that for the foreseeable future the transponder method in combination with ADS-B will be the primary technology for collision avoidance in manned aviation. The presented multimodal approach provides extended UAV visibility in both ATM and UTM contexts and enables a safe transition from one to another by utilizing the prevalent communication method. However, the simultaneous operation of both radio standards (ADS-B and LTE) in a confined space is associated with certain technical pitfalls that must be taken into account in the system design. This work provided recommendations how to operate ADS-B and LTE on-board UAVs. The field trial shows that two UAVs can be operated autonomously in a controlled airspace with collision avoidance methods utilizing cooperative traffic information data. For safe mixed manned/unmanned operation with certified and non-certified equipment for altimetry, care must be taken to use a common altitude reference plane.  
Conceptualization, A.S., F.V., A.K. and P.S.; methodology, A.S. and F.V.; software, A.S. and F.V.; validation, A.S. and F.V.; formal analysis, A.S. and F.V.; investigation, A.S. and F.V.; resources, A.K. and P.S.; data curation, A.S.; writing--original draft preparation, A.S. and F.V.; writing--A.S., F.V. and R.T.S.; visualization, A.S. and F.V.; supervision, A.S., R.T.S, P.S. and A.K.; project administration, A.S.; funding acquisition, P.S., A.K. and R.T.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Federal Ministry for Economic Affairs and Climate Action on the basis of a decision by the German Bundestag. The work presented is part of the Federal Aviation Research Programme (LuFo) project _MasterULAS_, contract No. 20V1703G. The views expressed herein can in no way be taken to reflect the official opinion of the ministry. Furthermore, this publication was supported financially by Universitat der Bundeswehr Munchen.  
Not applicable.  
Data Availability Statement: Data available on request.  
The authors declare no conflict of interest.  
Appendix A Calculation of Altitude Based on Atmospheric Pressure, Temperature and Pressure at Sea Level  
The pressure reduction formula according to [39]:  
\[p=p_{0}\cdot\left(\frac{T}{T_{0}}\right)^{-\frac{R}{R_{d}T}} \tag{1}\]  
with:\[p = \text{air pressure at specific altitude in hPa}\] \[p_{0} = \text{air pressure at sea level }(1013.25\,\text{hPa})\] \[T = \text{temperature at specific altitude in Kelvin}\] \[T_{0} = \text{temperature at sea level in Kelvin}\] \[g = \text{earth standard gravity }(9.806\,65\,\text{m}/\text{s}^{2})\] \[R_{d} = \text{specific gas constant of dry air }(287)\,\text{kg}/\text{K})\] \[\gamma = \text{temperature gradient in dry air }(-0.0065\,\text{K}/\text{m})\]  
Since temperature varies linearly in dry air with respect to altitude \(h\) for up to 11 km:  
\[T_{0}=T-\gamma\cdot h\] (A2)  
Equation (A1) can be rewritten as:  
\[p=p_{0}\cdot\left(1+\frac{\gamma\cdot h}{T-\gamma\cdot h}\right)^{-\frac{R}{R_ {d}\gamma}}\] (A3)  
Solved for \(h\), this leads to the hypsometric formula:  
\[h=\frac{\left(\left(\frac{p_{0}}{p}\right)^{\frac{1}{5550}}-1\right)\cdot \left(T+273.15\,\text{K}\right)}{0.0065\,\frac{\text{K}}{\text{m}}}\] (A4)