Citation: Schelle, A.; Völk, F.;
Schwarz, R.T.; Knopp, A.; Stütz, P.
Evaluation of a Multi-Mode-
Transceiver for Enhanced UAV
Visibility and Connectivity in Mixed
ATM/UTM Contexts. Drones 2022, 6,
80. https://doi.org/10.3390/
drones6040080
Academic Editors: Diego
González-Aguilera, George
Nikolakopoulos and Pablo
Rodríguez-Gonzálvez
Received: 28 February 2022
Accepted: 18 March 2022
Published: 22 March 2022
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Article
Evaluation of a Multi-Mode-Transceiver for Enhanced UAV
Visibility and Connectivity in Mixed ATM/UTM Contexts
Alexander Schelle
1,
* , Florian Völk
2
, Robert T. Schwarz
2
, Andreas Knopp
2
and Peter Stütz
1
1
Institute of Flight Systems, Universität der Bundeswehr München, 85579 Neubiberg, Germany;
peter.stuetz@unibw.de
2
Institute of Information Technology, Universität der Bundeswehr München, 85579 Neubiberg, Germany;
florian.voelk@unibw.de (F.V.); robert.schwarz@unibw.de (R.T.S.); andreas.knopp@unibw.de (A.K.)
* Correspondence: alexander.schelle@unibw.de
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 al-
titude 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
1. Introduction
Unmanned Aerial Vehicles (UAVs) refer to a wide range of devices, ranging from
lightweight multicopter to autonomous fixed-wing aircraft of several tons. By far the largest
number of UAVs are used for recreational non-commercial purposes. They are operated
at altitudes below
500 ft
, also known as Very Low-Level Airspace (VLL) and share this
altitude band with other manned airspace users, such as general aviation (e.g., emergency
procedures training), Helicopter Emergency Medical Services (HEMS) as well as police
and military aircraft ([
1
] p. 5). Busy airspaces (e.g., around airports) are usually set up as
controlled airspace (Class D CTR), where Air Traffic Control (ATC) regulates entry and
exit and coordinates aircraft movements. UAVs are typically prohibited from entering or
ascending in these airspaces. In recent years, small unmanned platforms in the range of up
to
25 kg
have also been used within controlled airspaces as a cost-effective alternative for
routine maintenance work. This includes inspecting the runway for damage [
2
], scaring
away birds [
3
], improving weather conditions [
4
] or calibrating radio navigation aids, such
as the Instrument Landing System [5].
Drones 2022, 6, 80. https://doi.org/10.3390/drones6040080 https://www.mdpi.com/journal/drones
