IEEE - Aerospace and Electronic Systems - April 2021 - 16

Flight Trial Demonstration of Secure GBAS via the L-band Digital Aeronautical Communications System (LDACS)
better time synchronization, optimized packet formats,
and more lightweight hash functions.
The latency of LDACS is sufficient to enable time-shifted
broadcast authentication protocols, such as TESLA. However, we have seen that the parameter choice of TESLA is
important here. Up to a delay of 1 s, the latency for TESLAsecured GBAS messages via LDACS remains low enough to
stay within the requirements for GAST-C and GAST-D services. Tweaking the time interval and key delay times might
lead to even shorter latencies than demonstrated in our
experiments.
LDACS offers at least a range of 94 km for GBAS service.
With higher LDACS transmission power, this range could be
drastically increased to 200 NM. However, depending on the
region and the local properties of the ionosphere, GBAS correction data may not necessarily be useful more than 100 NM
away from the reference station [24], [25]. However, the current GBAS service volume of 42 km was criticized as too
small in the past. The argument is that the corrections are not
used beyond the service volume point for landing service but
from the intercept point to the final approach, but for the pilot
to verify that the system is already operational. An increased
range of GBAS services would reduce the pressure and stress
level of the pilots during the final approach, as they can check
availability and operational correctness of the GBAS service
significantly earlier than today.
The transmission of GBAS correction data can be limited
in range or signal quality due to siting problems of VDB, as
VDB requires a line-of-sight connection for expected system
behavior. LDACS offers some improvements here, too. On
large complex airports such as Frankfurt, one VDB transmitter cannot ensure coverage on all runways due to buildings
blocking the signal. Today, those airports work with two
VDB transmitters on the same channel, which halves the
capacity due to alternating slots. A different technical solution not halving the capacity of the data link is very much
desirable. This solution must ensure coverage on all runways
for the rollout guidance up to 12 ft above the ground. With
our demonstration of GBAS via LDACS on the apron, taxiway, and runway, we demonstrated that LDACS can provide
a solution for transmitting GBAS correction data reliably
even under nonline-of-sight conditions.

CONCLUSION
Our experiments demonstrated that multiconstellation,
multifrequency GBAS can be realized efficiently and
securely via LDACS.
Also, LDACS may provide additional benefits to GBAS:
It offers enough data-rate to broadcast cryptographically
secured GBAS data while offering spare capacity for other
ATS or AOC services on the same channel. We have shown
that LDACS can extend the GBAS service range into the
order of 100 km. And we have seen that GBAS via LDACS
16

works in nonline-of-sight scenarios with approximately the
same GBAS availability as measured during flight. We demonstrated the possibility to protect broadcast data with the
broadcast authentication protocol TESLA. Our results
show that TESLA is well suited for securing GBAS data via
LDACS; however, TESLA parameters have to be
chosen carefully.
The advantages of GBAS over LDACS-(1) increased
data rate, (2) increased range, and (3) cyber-security-clearly
indicate that further development of secure GBAS over
LDACS may be a key enabler for the future evolution
of GBAS.

NOMENCLATURE
DLR

German Aerospace Center

EIRP

Equivalent Isotropically Radiated Power

FAS

Final Approach Segment

GAST

GBAS Approach Service Type

GBAS

Ground Based Augmentation System

GNSS

Global Navigation Satellite System

ICAO
ILS

International Civil Aviation Organization
Instrument Landing System

LDACS

L-band Digital Aeronautical
Communications System

MAC

Message Authentication Code

MICONAV

Migration towards Integrated
COM/NAV Avionics

PVT

Position, Velocity and Time

TESLA

Timed Efficient Stream
Loss-tolerant Authentication

UDP

User Datagram Protocol

Internet Protocol

VDB

VHF Data Broadcast

ACKNOWLEDGMENTS
Nicolas Schneckenburger was with the Institute of Communications and Navigation of the German Aerospace
Center (DLR) during the preparation and realization of the
flight trials.

REFERENCES
[1] M. Felux, T. Dautermann, and H. Becker, " GBAS landing
system-precision approach guidance after ILS, " Aircraft
Eng. Aerosp. Technol., vol. 85, pp. 382-388, Aug. 2013.
[2] J. Lee and M. Kim, " Optimized GNSS station selection to

IEEE A&E SYSTEMS MAGAZINE

support long-term monitoring of ionospheric anomalies for
aircraft landing systems, " IEEE Trans. Aerosp. Electron.
Syst., vol. 53, no. 1, pp. 236-246, Feb. 2017.

APRIL 2021

IEEE - Aerospace and Electronic Systems - April 2021

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