RFC 9372

L-Band Digital Aeronautical Communications System (LDACS)

📅 published March 2023, from ietf

Title Block ✨

Okay bestie, let's get the paperwork out of the way first 📋💅 This one is RFC 9372, called L-Band Digital Aeronautical Communications System (LDACS) 🦋. It was written by N. Mäurer, Ed. (German Aerospace Center, DLR), T. Gräupl, Ed. (German Aerospace Center, DLR), and C. Schmitt, Ed. (Research Institute CODE, UniBwM) 🫶. The category is Informational, so it's not a Standards Track spec, and it's dated March 2023 🌸. It doesn't obsolete or update any other RFCs (I checked, no cap 💀).

Abstract ✨

Okay so this doc gives you the overview of LDACS, the L-band Digital Aeronautical Communications System 🛫. It's a secure, scalable, spectrum-efficient terrestrial data link for civil aviation, and it's giving main character energy 👑. LDACS is a scheduled, reliable, multi-application cellular broadband system with IPv6 support 💖. Because flight guidance communication is shifting over to IP-based stuff, high reliability, availability, and security of IP connectivity over LDACS are literally essential 😭. The goal here is to introduce LDACS to the IETF community, raise awareness about related work happening inside and outside the IETF, and get expertise to help shape aviation's move to IP 🧚.

Status of This Memo ✨

Bestie, this is not an Internet Standards Track spec. It's published just for information purposes 🎀.

This document is a product of the Internet Engineering Task Force (IETF) 🌈. It represents IETF community consensus, got public review, and was approved for publication by the Internet Engineering Steering Group (IESG) ✨. Not every IESG-approved doc is a candidate for any level of Internet Standard, so check out Section 2 of RFC 7841 if you want the deets 🙈.

If you want the current status, any errata, or a way to send feedback, go to https://www.rfc-editor.org/info/rfc9372 💜.

Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved 🔒.

This doc is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) as they were on the publication date 📜. Please actually read those, because they spell out your rights and restrictions 🫶. Code Components pulled from this doc must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions, and they come without warranty as described in the Revised BSD License 💅.

Table of Contents 📚

1.  Introduction
2.  Acronyms
3.  Motivation and Use Cases
  3.1.  Voice Communications Today
  3.2.  Data Communications Today
4.  Provenance and Documents
5.  Applicability
  5.1.  Advances beyond the State of the Art
    5.1.1.  Priorities
    5.1.2.  Security
    5.1.3.  High Data Rates
  5.2.  Application
    5.2.1.  Air/Ground Multilink
    5.2.2.  Air/Air Extension for LDACS
    5.2.3.  Flight Guidance
    5.2.4.  Business Communications of Airlines
    5.2.5.  LDACS-Based Navigation
6.  Requirements
7.  Characteristics
  7.1.  LDACS Access Network
  7.2.  Topology
  7.3.  LDACS Protocol Stack
    7.3.1.  LDACS Physical Layer
    7.3.2.  LDACS Data Link Layer
    7.3.3.  LDACS Subnetwork Layer and Protocol Services
  7.4.  LDACS Mobility
  7.5.  LDACS Management Interfaces and Protocols
8.  Reliability and Availability
  8.1.  Below Layer 1
  8.2.  Layers 1 and 2
  8.3.  Beyond Layer 2
9.  Security Considerations
  9.1.  Security in Wireless Digital Aeronautical Communications
  9.2.  Security in Depth
  9.3.  LDACS Security Requirements
  9.4.  LDACS Security Objectives
  9.5.  LDACS Security Functions
  9.6.  LDACS Security Architecture
    9.6.1.  Entities
    9.6.2.  Entity Identification
    9.6.3.  Entity Authentication and Key Establishment
    9.6.4.  Message-In-Transit Confidentiality, Integrity, and
            Authenticity
  9.7.  Considerations on LDACS Security Impact on IPv6 Operational
        Security
10. IANA Considerations
11. Informative References
Appendix A.  Selected Information from DO-350A
Acknowledgements
Authors' Addresses

1. Introduction 🌸

Okay so air traffic management (ATM) is held up by one of its biggest pillars: a communications infrastructure that lets aircraft get controlled efficiently and stay safely separated in every phase of flight ✈️. The current systems are technically mature, but they're struggling 😩. The VHF band is getting super saturated in high-density areas, and analog radio has its limits 📻. So aviation is aiming for a sustainable modernization of its comms infrastructure, built on IP 💖.

This modernization happens in two steps 🪜: (1) moving data links from analog to digital tech, and (2) bringing IPv6-based networking protocols [RFC8200] into aeronautical networks [ICAO2015] ✨.

Step (1) is ATM comms shifting from analog VHF voice [KAMA2010] to digital data communication, which is way more spectrum-efficient 🦋. For terrestrial comms, the Global Air Navigation Plan (GANP) from the International Civil Aviation Organization (ICAO) plans for this transition to happen through the development of LDACS 🧚. Central Europe is the region that suffers most from VHF saturation, so the first LDACS rollout will probably start there and then spread to other crowded zones like the East and West Coasts of the US and parts of Asia [ICAO2018] 🌈.

LDACS enables IPv6-based Air/Ground (A/G) communication tied to aviation safety and regularity of flight [ICAO2015] 🛡️. Passenger communication and similar services are not supported, because only communications related to "safety and regularity of flight" are allowed in protected aviation frequency bands 🙅‍♀️. The big challenge is that no extra frequencies can be made available for terrestrial aeronautical comms, so coexistence mechanisms and procedures had to be developed to let LDACS run interference-free alongside other aeronautical services and systems in that protected band 😭. Since LDACS is for aircraft guidance, high reliability and availability for IP connectivity over LDACS are essential 💅.

LDACS is standardized by ICAO and the European Organization for Civil Aviation Equipment (EUROCAE) 🎀.

This doc gives the IETF community info about the aviation industry moving flight guidance systems from analog to digital 🛫. It also gives context for LDACS relative to related IETF work [LISP-GB-ATN], and it asks for expertise on making reliable IPv6 over LDACS happen for step (1) 🫶. It is not meant to advance LDACS as an IETF Standards Track document 💜.

Step (2) is the worldwide rollout of IPv6-capable digital aeronautical internetworking, called the Aeronautical Telecommunications Network (ATN) / Internet Protocol Suite (IPS), or ATN/IPS for short 🌍. It's specified in ICAO document Doc 9896 [ICAO2015], the Radio Technical Commission for Aeronautics (RTCA) document DO-379 [RTCA2019], the EUROCAE document ED-262 [EURO2019], and the Aeronautical Radio Incorporated (ARINC) document 858 [ARI2021] 📚. LDACS is subject to these regulations because it provides an "access network" (link-layer data link) to the ATN/IPS 🔥.

ICAO picked IPv6 as the basis for ATN/IPS mostly for historical reasons, since an earlier architecture based on ISO/OSI protocols (ATN/OSI) flopped in the marketplace 🙈.

In the context of safety-related comms, LDACS is going to play a major role in future ATM 👑. ATN/IPS data links will provide diversified terrestrial and space-based connectivity in a multilink concept called the Future Communications Infrastructure (FCI) [VIR2021] 🧚. Technically, the FCI will realize airborne and multihomed IPv6 networks connected to a global ground network through at least two independent communication technologies 🦋. More detail is in related documents [LISP-GB-ATN] [RTGWG-ATN-BGP] ✨. That's why ICAO actively asked the IETF for help defining a mobility solution for step (2), which is currently the Locator/ID Separation Protocol (LISP) 💖.

In the context of the Reliable and Available Wireless (RAW) Working Group, they're developing options, like intelligent switching between data links, for reliably delivering content to and from endpoints 🌸. LDACS is part of that kind of concept, so RAW's work is immediately applicable 🫶. Overall, as the aeronautical comms system moves to ATN/IPS and data travels over IPv6, closer cooperation between the aeronautical and IETF communities is desirable 🤝.

LDACS standardization within ICAO started in December 2016 🎀. As of 2022, the ICAO standardization group has produced the final Standards and Recommended Practices (SARPS) document [ICAO2022] that defines LDACS's general characteristics 📘. By the end of 2023, the group plans to have developed an ICAO technical manual, which is ICAO's equivalent of a technical standard 🌈. LDACS standardization isn't finished yet, so this doc is just a snapshot of where things stand right now 📸. The physical characteristics of an LDACS installation (form, fit, and function) will be standardized by EUROCAE 🔧. Generally, the group is open to input from all sources and encourages cooperation between the aeronautical and IETF communities 💜✨.

(Part 1 of 10, more to come bestie! 💖)

2. Acronyms ✨

okay so this part is basically the alphabet soup 🍜 section, and honestly it's the one you'll keep coming back to. here's the full list of terms RAW uses in this doc 💅

A/A:         Air/Air
A/G:         Air/Ground
A2G:         Air-to-Ground
ACARS:       Aircraft Communications Addressing and Reporting System
AC-R:        Access Router
ADS-B:       Automatic Dependent Surveillance - Broadcast
ADS-C:       Automatic Dependent Surveillance - Contract
AeroMACS:    Aeronautical Mobile Airport Communications System
ANSP:        Air Traffic Network Service Provider
AOC:         Aeronautical Operational Control
ARINC:       Aeronautical Radio Incorporated
ARQ:         Automatic Repeat reQuest
AS:          Aircraft Station
ATC:         Air Traffic Control
ATM:         Air Traffic Management
ATN:         Aeronautical Telecommunications Network
ATS:         Air Traffic Service
BCCH:        Broadcast Channel
CCCH:        Common Control Channel
CM:          Context Management
CNS:         Communication Navigation Surveillance
COTS:        Commercial Off-The-Shelf
CPDLC:       Controller-Pilot Data Link Communications
CSP:         Communications Service Provider
DCCH:        Dedicated Control Channel
DCH:         Data Channel
Diffserv:    Differentiated Services
DLL:         Data Link Layer
DLS:         Data Link Service
DME:         Distance Measuring Equipment
DSB-AM:      Double Side-Band Amplitude Modulation
DTLS:        Datagram Transport Layer Security
EUROCAE:     European Organization for Civil Aviation Equipment
FAA:         Federal Aviation Administration
FCI:         Future Communications Infrastructure
FDD:         Frequency Division Duplex
FL:          Forward Link
GANP:        Global Air Navigation Plan
GBAS:        Ground-Based Augmentation System
GNSS:        Global Navigation Satellite System
GS:          Ground-Station
G2A:         Ground-to-Air
HF:          High Frequency
ICAO:        International Civil Aviation Organization
IP:          Internet Protocol
IPS:         Internet Protocol Suite
kbit/s:      kilobit per second
LDACS:       L-band Digital Aeronautical Communications System
LISP:        Locator/ID Separation Protocol
LLC:         Logical Link Control
LME:         LDACS Management Entity
MAC:         Medium Access Control
MF:          Multiframe
NETCONF:     Network Configuration Protocol
OFDM:        Orthogonal Frequency Division Multiplexing
OFDMA:       Orthogonal Frequency Division Multiplexing Access
OSI:         Open Systems Interconnection
PHY:         Physical Layer
QPSK:        Quadrature Phase-Shift Keying
RACH:        Random-Access Channel
RL:          Reverse Link
RTCA:        Radio Technical Commission for Aeronautics
SARPS:       Standards and Recommended Practices
SDR:         Software-Defined Radio
SESAR:       Single European Sky ATM Research
SF:          Super-Frame
SNMP:        Simple Network Management Protocol
SNP:         Subnetwork Protocol
VDLm2:       VHF Data Link mode 2
VHF:         Very High Frequency
VI:          Voice Interface

3. Motivation and Use Cases

bestie, this is the why 🦋 aircraft are constantly connected to Air Traffic Control (ATC) and Aeronautical Operational Control (AOC) services through voice and data systems for the entire flight. ATC is all about flight guidance 🧭. AOC is the airline's business side 💼, meaning the mostly proprietary data swapped between the plane and the airline's operation centers and service partners.

fun fact: the ARINC document 633 was first released in 2007 [ARI2019] to standardize these messages so different companies can actually talk to each other, like between the airline and the fueling or de-icing crews 🧊⛽. then, near the airport and terminal area, the vibe is all about high bandwidth 📶, but en route it's about high reliability, robustness, and range 🌍. voice can use the same equipment as data or totally different gear. anyway, the differences between voice and data are coming up next, and the LDACS use cases are an add-on to the list in [RAW-USE-CASES] and the list of reliable and available wireless technologies in [RAW-TECHNOS] 💖

3.1. Voice Communications Today

voice links are used for Air/Ground (A/G) and Air/Air (A/A) communications 📻. the equipment can live on the ground or in the aircraft, and then either the High Frequency (HF) or VHF band is used. for remote areas, voice can also go over satellite 🛰️.

here's the tea 🍵: all VHF and HF voice runs over open Broadcast Channels (BCCHs) with no authentication, no encryption, and no other protective measures 🙈. well-proven procedures like phraseology or read-backs, done by well-trained people, really help the safety of communications, but they do not replace necessary cryptographical security mechanisms. it's giving "good habits, not a lock" 🔐.

the main voice medium is still the analog VHF Double Side-Band Amplitude Modulation (DSB-AM) technique, backed up by HF single side-band AM and satellite comms for remote and oceanic regions 🌊. DSB-AM has been around since 1948 📼, it works reliably and safely, and the equipment is cheap 💸. that's why VHF DSB-AM is still everywhere, and it'll probably stick around for many more years 👑. the downside is that it creates operational limits and blocks new ATM applications, like flight-centric operation with point-to-point comms between pilots and ATC officers [BOE2019] 🥲.

3.2. Data Communications Today

like voice, data going into the cockpit currently comes from ground-based equipment on HF or VHF bands, or from legacy satellite systems 📡. all of these use narrowband radio channels with throughput in the order of kbit/s 🐢. while the aircraft is on the ground, there are extra options, like the Aeronautical Mobile Airport Communications System (AeroMACS) or public cellular networks, which operate in the Airport (APT) domain and can deliver broadband 🚀 [BOE2019].

now the plot twist 🎬: for regulatory reasons, the networks carrying data about the safety and regularity of flight must be strictly isolated from the ones providing passenger entertainment 🎮. so during flight, the flight crew gets narrowband services while passengers get in-flight broadband. the current HF and VHF data links can't provide broadband now or in the future because there just isn't enough spectrum 📉. that's a real bottleneck for enhanced ATM operations, like trajectory-based operations and 4D trajectory negotiations [BOE2019] 😤.

satellite-based comms are being investigated, and enhanced capabilities that can offer in-flight broadband and support safety and regularity of flight are in development 🛠️. at the same time, the ground-based broadband data link LDACS is being standardized by ICAO and has recently proven itself in flight tests [MAE20211] [BEL2021] ✈️✨. LDACS is scalable, secure, and spectrum-efficient, and it gives real advantages to users and service providers 🧠. it's expected that both satellite systems and LDACS will be deployed to meet future aeronautical communication needs, as envisioned by the ICAO GANP [BOE2019] 🌸.

4. Provenance and Documents

LDACS has already made serious progress in the Single European Sky ATM Research (SESAR) framework, and it's still being developed in the follow-up program SESAR2020 [RIH2018] 🌈. the big goal is to develop, implement, and validate a modern aeronautical data link that can keep evolving with aviation needs over the long term 🚀. an LDACS specification has been produced [GRA2020] and keeps getting updated 📝. transmitter demonstrators were built to test spectrum compatibility with legacy systems in the L-band [SAJ2014], and computer simulations showed that LDACS can fulfill the identified requirements [GRA2011] 💅.

up until now, LDACS standardization has mostly focused on the Physical Layer (PHY) and the Data Link Layer (DLL) 🧩. only recently have higher layers come into the picture. right now there's no "IPv6 over LDACS" specification defined 🙅‍♀️. however, SESAR2020 has started experimenting with IPv6-based LDACS, and ICAO plans to ask the IETF for guidance to develop IPv6 over LDACS 🤝. as of May 2022, LDACS defines 1536-byte user data packets [GRA2020] in which IPv6 traffic shall be encapsulated 📦. on top of that, Robust Header Compression (ROHC) [RFC5795] is being considered at the LDACS Subnetwork Protocol (SNP) layer (cf. Section 7.3.3) 🎀.

the IPv6 architecture for the aeronautical telecommunication network is called the ATN/IPS 🌐. link-layer techs inside the ATN/IPS include LDACS [GRA2020], AeroMACS [KAMA2018], and several SatCOM candidates, and together with the ATN/IPS, they're called the "FCI" 🫶. the FCI will support quality of service, link diversity, and mobility under the umbrella of the "multilink concept" 🦋. that concept is the idea that, depending on link quality, communication can be switched seamlessly from one data link technology to another 🔄. this work is led by the ICAO Communication Panel Working Group (WG-I) 👑.

besides standardization, several industrial LDACS prototypes have been built 🛠️. one set of them was tested in flight trials, which confirmed the theoretical predictions of system performance [GRA2018] [MAE20211] [BEL2021] 🥳.

5. Applicability

LDACS is a multi-application cellular broadband system 📶. it can simultaneously provide several kinds of Air Traffic Services (ATSs), including ATS-B3 and AOC communications services, from deployed Ground-Stations (GSs) 🗼. the physical layer and data link layer are optimized for Controller-Pilot Data Link Communications (CPDLC), but the system also supports digital A/G voice communications 🎙️.

LDACS supports communications in all airspaces (airport, terminal maneuvering area, and en route) and on the airport surface 🛬. the physical LDACS cell coverage is effectively decoupled from the operational coverage a particular service needs, and that's genuinely new in aeronautical communications 🤯. services that need wide-area coverage can be installed across several adjacent LDACS cells, and the handover between those cells is seamless, automatic, and transparent to the user ✨. so the LDACS communications concept helps the aeronautical communication infrastructure support future dynamic airspace management concepts 🌟💜

5.1. Advances beyond the State of the Art ✨

okay so LDACS is bringing some capabilities that current deployed aeronautical comms systems don't have yet 🦋 these were already tested and confirmed in lab or flight trials with available LDACS prototype hardware [BEL2021] [MAE20211]. literally proven, not just vibes 💅

5.1.1. Priorities

LDACS can manage service priorities, and that's a big deal bc it's not available in some of the current data link deployments 🎀 so LDACS guarantees bandwidth availability, low latency, and high continuity of service for safety-critical ATS applications while also fitting in less safety-critical AOC services at the same time 👑

5.1.2. Security

LDACS is a secure data link with built-in security mechanisms 🔐 it enables secure data communications for ATS and AOC services, including secured private communications for aircraft operators and Air Traffic Network Service Providers (ANSPs) 🫶 this includes concepts for key and trust management, Mutual Authentication and Key Establishment (MAKE) protocols, key derivation measures, user and control message-in-transit protection, secure logging, and availability and robustness measures [MAE20182] [MAE2021]. the security glow-up is real 💜

5.1.3. High Data Rates

the user data rate of LDACS is 315 kbit/s to 1428 kbit/s on the Forward Link (FL) for the Ground-to-Air (G2A) connection, and 294 kbit/s to 1390 kbit/s on the Reverse Link (RL) for the Air-to-Ground (A2G) connection, depending on coding and modulation 🚀 that's up to two orders of magnitude greater than what current terrestrial digital aeronautical communications systems, such as the VHF Data Link mode 2 (VDLm2), provide; see [ICAO2019] [GRA2020] the speed is unhinged 🔥

5.2. Application

LDACS will be used by several aeronautical applications, ranging from enhanced communications protocol stacks (multihomed mobile IPv6 networks in the aircraft and potentially ad-hoc networks between aircraft) to broadcast communication applications (Global Navigation Satellite System (GNSS) correction data) and integration with other service domains (using the communications signal for navigation) [MAE20211] 🧚 also, a digital voice service offering better quality and service than current HF and VHF systems is foreseen 🎧

it's expected that LDACS, together with upgraded satellite-based communications systems, will be deployed within the FCI and constitute one of the main components of the multilink concept within the FCI 🌈

both technologies, LDACS and satellite systems, have their own specific benefits and technical capabilities that complement each other 🤝 satellite systems are especially well-suited for large coverage areas with less dense air traffic, e.g., oceanic regions. LDACS is well-suited for dense air traffic areas, e.g., continental areas or hotspots around airports and terminal airspace. in addition, both technologies offer comparable data link capacity; thus, both are well-suited for redundancy, mutual back-up, or load balancing 🦋

technically, the FCI multilink concept will be realized by multihomed mobile IPv6 networks in the aircraft ✈️ the related protocol stack is currently under development by ICAO, within SESAR, and the IETF. currently, two layers of mobility are foreseen 👑 local mobility within the LDACS access network is realized through Proxy Mobile IPv6 (PMIPv6), and global mobility between "multilink" access networks (which need not be LDACS) is implemented on top of LISP [LISP-GB-ATN] [RFC9300] [RFC9301] 💖

5.2.2. Air/Air Extension for LDACS

a potential extension of the multilink concept is its extension to the integration of ad-hoc networks between aircraft 🌸

direct A/A communication between aircraft in terms of ad-hoc data networks is currently considered a research topic, since there is no immediate operational need for it 🙈 although several possible use cases are discussed (Automatic Dependent Surveillance - Broadcast (ADS-B), digital voice, wake vortex warnings, and trajectory negotiation) [BEL2019]. it should also be noted that currently deployed analog VHF voice radios support direct voice communication between aircraft, making a similar use case for digital voice plausible 📻

LDACS A/A is currently not a part of the standardization process and will not be covered within this document 🙅‍♀️ however, it is planned that LDACS A/A will be rolled out after the initial deployment of LDACS A/G and seamlessly integrated in the existing LDACS ground-based system ✨

5.2.3. Flight Guidance

the FCI (and therefore LDACS) is used to provide flight guidance 🧭 this is realized using three applications:

  1. Context Management (CM): The CM application manages the automatic logical connection to the ATC center currently responsible to guide the aircraft. currently, this is done by the air crew manually changing VHF voice frequencies according to the progress of the flight. the CM application automatically sets up equivalent sessions 🎀
  2. Controller-Pilot Data Link Communications (CPDLC): The CPDLC application provides the air crew with the ability to exchange data messages similar to text messages with the currently responsible ATC center. the CPDLC application takes over most of the communication currently performed over VHF voice and enables new services that do not lend themselves to voice communication (i.e., trajectory negotiation) 💬
  3. Automatic Dependent Surveillance - Contract (ADS-C): ADS-C reports the position of the aircraft to the currently active ATC center. Reporting is bound to "contracts", i.e., pre-defined events related to the progress of the flight (i.e., the trajectory). ADS-C and CPDLC are the primary applications used for implementing in-flight trajectory management 📍

CM, CPDLC, and ADS-C are available on legacy data links but are not widely deployed and have limited functionality 🥲

further ATC applications may be ported to use the FCI or LDACS as well 🌟 a notable application is the Ground-Based Augmentation System (GBAS) for secure, automated landings. the GNSS-based GBAS is used to improve the accuracy of GNSS to allow GNSS-based instrument landings 🛬 this is realized by sending GNSS correction data (e.g., compensating ionospheric errors in the GNSS signal) to the aircraft's GNSS receiver via a separate data link. currently, the VHF Data Broadcast (VDB) data link is used. VDB is a narrowband one-way, single-purpose data link without advanced security and is only used to transmit GBAS correction data 😬 these shortcomings show a clear need to replace VDB. a natural candidate to replace it is LDACS, because it is a bidirectional data link, also operates in non-line-of-sight scenarios, offers strong integrated link-layer security, and has a considerably larger operational range than VDB [MAE20211] that's a whole glow-up for landings 🌈

5.2.4. Business Communications of Airlines

in addition to ATSs, AOC services are transmitted over LDACS 💼 AOC is a generic term referring to the business communication of airlines between the airlines and service partners on the ground and their own aircraft in the air. regulatory-wise, this is considered related to safety and regularity of flight; therefore, it may be transmitted over LDACS. AOC communication is considered the main business case for LDACS communications service providers, since modern aircraft generate significant amounts of data (e.g., engine maintenance data) 🔧✨

5.2.5. LDACS-Based Navigation

beyond communications, radio signals can always be used for navigation as well 📡 this fact is used for the LDACS navigation concept 🗺️

for future aeronautical navigation, ICAO recommends the further development of GNSS-based technologies as primary means for navigation. however, due to the large separation between navigational satellites and aircraft, the power of the GNSS signals received by the aircraft is very low. as a result, GNSS disruptions might occasionally occur due to unintentional interference or intentional jamming 😵‍💫 yet, the navigation services must be available with sufficient performance for all phases of flight. therefore, during GNSS outages or blockages, an alternative solution is needed. this is commonly referred to as Alternative Positioning, Navigation, and Timing (APNT) 🛰️

one such APNT solution is based on exploiting the built-in navigation capabilities of LDACS operation 🧠 that is, the normal operation of LDACS for ATC and AOC communications would also directly enable the aircraft to navigate and obtain a reliable timing reference from the LDACS GSs. current cell planning for Europe shows 84 LDACS cells to be sufficient [MOST2018] to cover the continent at a sufficient service level 🇪🇺 if more than three GSs are visible by the aircraft, via knowing the exact positions of these and having a good channel estimation (which LDACS does due to numerous works mapping the L-band channel characteristics [SCHN2018]), it is possible to calculate the position of the aircraft via measuring signal propagation times to each GS. in flight trials in 2019 with one aircraft (and airborne radio inside it) and just four GSs, navigation feasibility was demonstrated within the footprint of all four GSs with a 95th percentile position-domain error of 171.1m [OSE2019] [BEL2021] [MAE20211] 🎯 as such, LDACS can be used independently of GNSS as a navigation alternative. positioning errors will decrease markedly as more GSs are deployed [OSE2019] [BEL2021] [MAE20211] the more GSs, the better the aim 🌸

LDACS navigation has already been demonstrated in practice in two flight measurement campaigns [SHU2013] [BEL2021] [MAE20211] 👑

6. Requirements ✨

okay so the requirements for LDACS are mostly shaped by its job: communications about the safety and regularity of flight 🛫💖

aeronautical comms are lowkey super regulated 💅 Aeronautical data links (for safety and regularity of flight) may only use spectrum licensed to aviation and data links endorsed by ICAO. Nation states can change this locally, but because air transport is a global system, everyone's pretty much expected to stick to these practices 🌍

ATN data links are gonna stay in service for decades ⏳. The VDLm2 data link that's used for digital terrestrial internetworking today was developed in the 1990s (the use of the Open Systems Interconnection (OSI) stack is a total giveaway 💀). VDLm2 is expected to stick around for at least several more decades. so yeah, aero comms for safety stuff is way more like industrial applications than the open Internet 🏭

Internetwork tech is already installed in current aircraft ✈️. Current ATS applications use either the Aircraft Communications Addressing and Reporting System (ACARS) or the OSI stack. The goal of LDACS, as part of the FCI, is to replace the legacy OSI stack and proprietary ACARS internetwork technologies with industry standard IP technology 🎀. It's anticipated that Commercial Off-The-Shelf (COTS) IP technology mostly applies to the ground network. The avionics networks on the aircraft will likely be heavily modified versions of Ethernet or proprietary.

Right now, AOC applications mostly use the same stack (although some applications, like the graphical weather service, may use the commercial passenger network) 🌦️. This creates capacity problems (resulting in excessive amounts of timeouts 😩) since the underlying terrestrial data links don't provide enough bandwidth (i.e., with VDLm2 currently in the order of 10 kbit/s). Using non-aviation-specific data links is considered a security problem 🔒. Ideally, the aeronautical IP internetwork (hence the ATN, over which only communications related to safety and regularity of flight are handled) and the Internet should be completely separated at Layer 3 🙅‍♀️.

The objective of LDACS is to provide a next-generation terrestrial data link designed to support IP addressing and provide much higher bandwidth, so we can avoid the operational problems that are currently being experienced ✨.

So the requirement for LDACS is to provide a terrestrial high-throughput data link for IP internetworking in the aircraft 💖.

To fulfil that, LDACS needs to be interoperable with IP (and IP-based services like Voice-over-IP) at the gateway connecting the LDACS network to other aeronautical ground networks (i.e., the ATN). On the avionics side, in the aircraft, aviation-specific solutions are to be expected 🧚.

Besides these functional requirements, LDACS and its IP stack need to fulfill the requirements defined in RTCA DO-350A/EUROCAE ED-228A [DO350A]. This document defines continuity, availability, and integrity requirements at different scopes for each ATM application (CPDLC, CM, and ADS-C). The scope most relevant to IP over LDACS is the Communications Service Provider (CSP) scope 📡.

Continuity, availability, and integrity requirements are defined in Volume 1 of [DO350A] in Tables 5-14 and 6-13. Appendix A presents the required information 📚.

In a similar vein, requirements to fault management are defined in the same tables 🛠️.

7. Characteristics 🌸

LDACS will become one of several wireless access networks connecting aircraft to the ATN implemented by the FCI 🦋.

The current LDACS design is focused on the specification of Layers 1 and 2. However, for the purpose of this work, only Layer 2 details are discussed here 🤓.

Achieving the stringent continuity, availability, and integrity requirements defined in [DO350A] will require the specification of Layer 3 and above mechanisms (e.g., reliable crossover at the IP layer). Fault management mechanisms are similarly unspecified as of November 2022. Current regulatory documents do not fully specify the above mechanism yet. However, a short overview of the current state shall be given throughout each section here 🧐.

7.1. LDACS Access Network

An LDACS access network contains an Access Router (AC-R) and several GSs, each of them providing one LDACS radio cell 📶.

User-plane interconnection to the ATN is facilitated by the AC-R peering with an A/G Router connected to the ATN 🔗.

The internal control plane of an LDACS access network interconnects the GSs. An LDACS access network is illustrated in Figure 1. Dashes denote the user plane and points denote the control plane 🎀.

wireless                user
link                    plane
  AS-------------GS---------------AC-R---A/G-----ATN
    ..............                 |   Router
       control   .                 |
       plane     .                 |
                 .                 |
                 GS----------------|
                 .                 |
                 .                 |
                 GS----------------+

       Figure 1: LDACS Access Network with Three GSs and One AS

7.2. Topology

LDACS is a cellular point-to-multipoint system 🗼. It assumes a star topology in each cell, where Aircraft Stations (ASs) belonging to aircraft within a certain volume of space (the LDACS cell) are connected to the controlling GS. The LDACS GS is a centralized instance that controls LDACS A/G communications within its cell. The LDACS GS can simultaneously support multiple bidirectional communications to the ASs under its control. LDACS's GSs themselves are connected to each other and the AC-R 💫.

Before using the system, an aircraft has to register with the controlling GS to establish dedicated logical channels for user and control data 📝. Control channels have statically allocated resources, while user channels have dynamically assigned resources according to the current demand. Logical channels exist only between the GS and the AS 💜.

7.3. LDACS Protocol Stack

The protocol stack of LDACS is implemented in the AS and GS 🧩. It consists of the PHY with five major functional blocks above it. Four are placed in the DLL of the AS and GS: Medium Access Control (MAC) layer, Voice Interface (VI), Data Link Service (DLS), and LDACS Management Entity (LME). The fifth entity, the SNP, resides within the subnetwork layer. The LDACS radio is externally connected to a voice unit and radio control unit via the AC-R to the ATN network 🎧.

LDACS is considered an ATN/IPS radio access technology from the view of ICAO's regulatory framework. Hence, the interface between ATN and LDACS must be IPv6-based, as regulatory documents such as ICAO Doc 9896 [ICAO2015] and DO-379 [RTCA2019] clearly foresee that 💅. The translation between the IPv6 layer and SNP layer is currently the subject of ongoing standardization efforts and is not finished yet at the time of writing 🛠️.

Figure 2 shows the protocol stack of LDACS as implemented in the AS and GS. Acronyms used here are introduced throughout the upcoming sections 📖.

                  IPv6                   Network Layer
                    |
Airborne Voice      |
Interface (AVI) /   |               Radio Control Unit (RCU)
Voice Unit (VU)     |
   |                |
   |      +------------------+  +----+
   |      |        SNP       |--|    |   Subnetwork
   |      |                  |  |    |   Layer
   |      +------------------+  |    |
   |                |           | LME|
+-----+   +------------------+  |    |
| VI  |   |        DLS       |  |    |   LLC Layer
+-----+   +------------------+  +----+
   |                |             |
  DCH              DCH         DCCH/CCCH
                   |          RACH/BCCH
                   |             |
+-------------------------------------+
|                  MAC                |   Medium Access
|                                     |   Layer
+-------------------------------------+
                   |
+-------------------------------------+
|                  PHY                |   Physical Layer
+-------------------------------------+
                   |
                   |
                 ((*))
                 FL/RL              radio channels
                                   separated by FDD

          Figure 2: LDACS Protocol Stack in the AS and GS

7.3.1. LDACS Physical Layer

The physical layer provides the means to transfer data over the radio channel 📡. The LDACS GS supports bidirectional links to multiple aircraft under its control. The FL direction at the G2A connection and the RL direction at the A2G connection are separated by Frequency Division Duplex (FDD). FL and RL use a 500 kHz channel each. The GS transmits a continuous stream of Orthogonal Frequency Division Multiplexing Access (OFDM) symbols on the FL. In the RL, different aircraft are separated in time and frequency using Orthogonal Frequency Division Multiple Access (OFDMA). So, aircraft transmit discontinuously on the RL via short radio bursts, sent in precisely defined transmission opportunities allocated by the GS 🌈.

The data link layer provides the necessary protocols to facilitate concurrent and reliable data transfer for multiple users 🫶. The LDACS data link layer is organized in two sub-layers: the medium access sub-layer and the Logical Link Control (LLC) sub-layer. The medium access sub-layer manages the organization of transmission opportunities in slots of time and frequency. The LLC sub-layer provides acknowledged point-to-point logical channels between the aircraft and the GS using an Automatic Repeat reQuest (ARQ) protocol. LDACS also supports unacknowledged point-to-point channels and G2A broadcast transmission 🦄.

7.3.2.1. Medium Access Control (MAC) Services

The MAC time framing service provides the frame structure necessary to realize slot-based time-division multiplex-access on the physical link. It provides the functions for the synchronization of the MAC framing structure and the PHY layer framing. The MAC time framing provides a dedicated time slot for each logical channel ⏰.

The MAC sub-layer offers access to the physical channel to its service users. Channel access is provided through transparent logical channels. The MAC sub-layer maps logical channels onto the appropriate slots and manages the access to these channels. Logical channels are used as the interface between the MAC and LLC sub-layers 💖.

okay so the DLS is the bestie that gives you acknowledged and unacknowledged bidirectional exchange of user data 🫶 (the unacknowledged kind includes broadcast and packet mode voice, obviously 🎀). With the acknowledged one, the sending DLS entity waits for an acknowledgement from the receiver ✨. If nothing comes back within a specified time frame, the sender may automatically try to retransmit its data. But after a certain number of failed retries, it suspends further retransmission attempts and tells its client about the failure 💀. not me getting ghosted by a protocol 😭

The DLS uses the logical channels provided by the MAC: 🦋

  1. A GS announces its existence and access parameters in the Broadcast Channel (BCCH) 📡
  2. The Random-Access Channel (RACH) lets the AS request access to an LDACS cell 🙋‍♀️
  3. In the FL, the Common Control Channel (CCCH) is how the GS grants access to Data Channel (DCH) resources 🎟️
  4. The reverse direction is covered by the RL, where ASs need to request resources before sending. This happens via the Dedicated Control Channel (DCCH) 🔁
  5. User data itself gets communicated in the DCH on the FL and RL 💖

Access to the FL and RL DCH is granted by the scheduling mechanism implemented in the LME discussed below. ⭐

7.3.2.3. Voice Interface (VI) Services 🎤

the VI is giving support for virtual voice circuits 🌸. These circuits can be set up permanently by the GS (for example, to emulate a voice party line 📞) or created on demand. so iconic 👑

7.3.2.4. LDACS Management Entity (LME) Services 🧚

the mobility management service in the LME handles registration and de-registration (cell entry and cell exit) 🚪, scanning RF channels of neighboring cells 📻, and handover between cells 🔄. on top of that, it manages the addressing of aircraft within cells 🛩️ main character energy.

the resource management service handles link maintenance (power, frequency, and time adjustments) ⚡, support for adaptive coding and modulation 🎨, and resource allocation 🍓.

the resource management service accepts resource requests from and for different ASs, and issues resource allocations accordingly 💅. the scheduling algorithm is not specified, and it's a point of possible vendor differentiation, but it is subject to the following requirements:

  1. Resource scheduling must provide channel access according to the priority of the request 🥇
  2. Resource scheduling must support "one-time" requests ☝️
  3. Resource scheduling must support "permanent" requests that reserve a resource until the request is canceled (for example, for digital voice circuits) 🔒

7.3.3. LDACS Subnetwork Layer and Protocol Services 🌐

ok last one for this layer 🫠. the SNP layer directly interacts with IPv6 traffic. Incoming ATN/IPS IPv6 packets get forwarded over LDACS from and to the aircraft ✈️. The final IP addressing structure in an LDACS subnet still needs to be defined, but the current layout consists of five network segments: Air Core Net, Air Management Net, Ground Core Net, Ground Management Net, and Ground Net 🗺️.

any protocols that the ATN/IPS [ICAO2015] defines as mandatory will reach the aircraft. listing them here is out of scope, though 🙈. for more on the technicalities of the above ATN/IPS layer, please refer to [ICAO2015], [RTCA2019], and [ARI2021] 📚.

the DLS provides functions that are required for the transfer of user-plane data and control plane data over the LDACS access network 📦. the security service provides functions for secure user data communication over the LDACS access network 🔐. Note that the SNP security service applies cryptographic measures as configured by the GS. 💜

7.4. LDACS Mobility 🦋

LDACS supports Layer 2 handovers to different LDACS cells 🌈. handovers may be initiated by the aircraft (break-before-make) or by the GS (make-before-break). make-before-break handovers are only supported between GSs connected to each other, and usually GSs operated by the same service provider 🤝.

when a handover between the AS and two interconnected GSs takes place, it can be triggered by the AS or the GS 🎬. once that's done, new security information is exchanged between the AS, GS1, and GS2 before the "old" connection is terminated between the AS and GS1, and a "new" connection is set up between the AS and GS2 🔑. as a last step, accumulated user data at GS1 is forwarded to GS2 via a ground connection before it's sent via GS2 to the AS 📬.

while some information for handover is transmitted in the LDACS DCH, the information remains in the "control plane" part of LDACS and gets exchanged between LMEs in the AS, GS1, and GS2 🧠. as such, local mobility takes place entirely within the LDACS network and utilizes the PMIPv6 protocol [RFC5213] 💫. the use of PMIPv6 is currently not mandated by standardization and may be vendor-specific 🛠️. external handovers between non-connected LDACS access networks or different aeronautical data links are handled by the FCI multilink concept 🌟.

7.5. LDACS Management Interfaces and Protocols 🎀

LDACS management interfaces and protocols are currently not mandated by standardization 🙃. the implementations available right now use SNMP for management and Radius for Authentication, Authorization, and Accounting (AAA) 🖥️. link state (link up, link down) is reported using the ATN/IPS Aircraft Protocol (AIAP), which ICAO WG-I mandates for multilink 📶.

8. Reliability and Availability ✨

8.1. Below Layer 1 🛡️

below Layer 1, aeronautics usually relies on hardware redundancy 🔧. to protect availability of the LDACS link, an aircraft equipped with LDACS has access to two L-band antennae with triple redundant radio systems, as ICAO requires for any safety relevant aeronautical systems 🫶💖

8.2. Layers 1 and 2 ✨

Okay so LDACS was made for the stuff that keeps flights safe and on time ✈️💖, which is why it was designed as a deterministic wireless data link (as far as that's possible, bestie) 🧚.

The team looked at real measurements of the L-band channel, so robustness was baked in from the PHY layer up 💪🌈. Those L-band channel measurements [SCH2016] confirmed that LDACS is super well adapted to its channel 😭✨.

To squeeze the most capacity out of each channel and use the spectrum well 🦋, LDACS is an OFDM-based FDD system that can transmit in FL on the G2A connection and in RL on the A2G connection at the same time. The legacy systems already in the L-band limit both channels to about 500 kHz, so it's a tight space 🙈.

The LDACS physical layer includes guard times for operation at a maximum distance of 200 nautical miles (nm) from the GS 🌸. In real deployments, LDACS can be configured for any range up to that max, so you get to pick your vibe 💅.

The LDACS physical layer supports adaptive coding and modulation for user data. Control data is always sent with the most robust coding and modulation, no exceptions 🔒 (FL: Quadrature Phase-Shift Keying (QPSK), coding rate 1/2; RL: QPSK, coding rate 1/3).

The LDACS medium access layer sits on top of the physical layer and uses a static frame structure so timers stay deterministic ⏱️. As shown in Figures 3 and 4, the framing is based on Super-Frames (SFs) that last 240 ms (milliseconds), which is 2000 OFDM symbols. An OFDM symbol takes 120 microseconds, the sampling time is 1.6 microseconds, and the guard time is 4.8 microseconds 🧁. Figure 3 shows the structure of an SF along with the structure and timings of each part. The FL and RL boundaries line up in time (from the GS's point of view), which gives you deterministic slots for control and DCHs 💜. Initial AS time synchronization, and keeping that sync going, is based on watching the synchronization symbol pairs that show up over and over in the FL stream sent by the controlling GS [GRA2020]. As mentioned before, LDACS data transmission is split into user data (DCH) and control (BCCH and CCCH in FL; RACH and DCCH in RL), with the matching timings in Figure 4 🎀.

   ^
   |     +---------+------------+------------+------------+------------+
   |  FL |  BCCH   |     MF     |     MF     |     MF     |     MF     |
   |     | 6.72 ms |   58.32 ms |   58.32 ms |   58.32 ms |   58.32 ms |
   F     +---------+------------+------------+------------+------------+
   r     <----------------- Super-Frame (SF) - 240 ms ----------------->
   e
   q     +---------+------------+------------+------------+------------+
   u  RL |  RACH   |     MF     |     MF     |     MF     |     MF     |
   e     | 6.72 ms |   58.32 ms |   58.32 ms |   58.32 ms |   58.32 ms |
   n     +---------+------------+------------+------------+------------+
   c     <----------------- Super-Frame (SF) - 240 ms ----------------->
   y
   ------------------------------ Time -------------------------------->
   |

                      Figure 3: SF Structure for LDACS


   ^
   |     +--------------+-----------------+------------------+
   |  FL |     DCH      |     CCCH        |      DCH         |
   |     |   25.92 ms   | 2.16 - 17.28 ms | 15.12 - 30.24 ms |
   F     +--------------+-----------------+------------------+
   r     <-----------  Multiframe (MF) - 58.32 ms ----------->
   e
   q     +---------------+----------------------------------+
   u  RL |    DCCH       |                DCH               |
   e     | 2.8 - 24.4 ms |           33.84 - 55.44 ms       |
   n     +---------------+----------------------------------+
   c     <-----------  Multiframe (MF) - 58.32 ms ---------->
   y
   ----------------------------- Time ---------------------->
   |

                      Figure 4: MF Structure for LDACS

LDACS cell entry happens through an initial control message exchange over the RACH and the BCCH 📡.

Once you're in a cell, LDACS medium access is always under the control of the GS of that radio cell 👑. Any medium access to send user data on a DCH has to be requested with a resource request message that says how many resources you want and what class of service you need 📝. The GS does the resource scheduling based on those requests and hands out resources with resource allocation messages 🫶. These resource request and allocation messages go over the dedicated contention-free control channels (DCCH and CCCH), so nobody's fighting over the slot 🙅‍♀️.

QoS in LDACS medium access is all about giving prioritized access at the bottleneck (the wireless link) 🏃‍♀️💨. Higher-layer QoS requests get signaled to LDACS using Differentiated Services (Diffserv) classes, from CS01 (lowest priority) up to CS07 (highest priority) ⭐.

The GS has full control over resource scheduling, and it can also send forced handover commands for off-loading or channel management 🦋. For example, that happens when the signal quality drops and a better GS is within the AS's reach 📶. With solid resource management of the radio channel's capacity, reliability and robustness measures are anchored in the LME too 💅.

The LDACS control channels also carry keepalive messages whenever they aren't otherwise being used 💓. The control channel framing is deterministic, so a missing keepalive can be detected right away 👀. That info is passed to the multilink protocols for fault management 🔍.

The protocol used to communicate faults is not defined in the LDACS specification 🤷‍♀️. It's assumed that vendors would use industry-standard protocols like the Simple Network Management Protocol or the Network Configuration Protocol (NETCONF) where security permits 🔐.

The LDACS data link layer protocol runs on top of the medium access sub-layer and uses ARQ to give reliable data transmission on the DCH 🛡️. It uses selective repeat ARQ with transparent fragmentation and reassembly to the resource allocation size, which keeps latency and overhead low without giving up reliability 🚀. It makes sure packets arrive in the correct order with no duplicates ✨. If there are transmission errors, it finds the lost fragments using deterministic timers synced to the medium access frame structure and kicks off a retransmission 🔁.

8.3. Beyond Layer 2 🌍

LDACS availability can go up by deploying LDACS infrastructure smartly, and that mostly means adding more terrestrial GSs 🗼. But there are four things to keep in mind, bestie 🫶: (1) aeronautical spectrum is scarce for data link communication (tens of MHz in the L-band in the case of LDACS), (2) adding GSs also boosts the individual bandwidth for aircraft in a cell, because fewer aircraft have to share the spectrum, (3) covering terrestrial ATM worldwide with LDACS is also about cost and reusing spectrum, which means cells can't always get smaller, and (4) the Distance Measuring Equipment (DME) is the main user of the aeronautical L-band, so any LDACS deployment has to plan around DME frequencies 📻.

Aspect (2) is a solid reason to go with smaller cells than the 200 nm max LDACS was developed for, plus it helps with redundancy 💖. But the other three still need to be respected when you do it 🙏. There's early work on LDACS cell planning, like [MOST2018], where the authors concluded that 84 LDACS cells in Europe would be enough to serve European air traffic for the next 20 years 🇪🇺.

The aeronautical community decided not to depend on a single communication system or frequency band, for redundancy reasons 🧯. The plan is to have multiple independent data link technologies on the aircraft (for example, terrestrial and satellite communications), on top of legacy VHF voice 📡🌈.

Here's the tea though 🍵: as of now, no reliability and availability mechanisms that could use the multilink architecture have been specified at Layer 3 and above 😬. Even though LDACS was designed for reliability, the wireless medium makes it really hard to get deterministic properties like a low packet error rate, bounded consecutive losses, and bounded latency 💀. High reliability and availability for IP connectivity over LDACS is highly desirable, but the support has to be adapted to the specific use case 💜.

9. Security Considerations 🔐

The goal of this section is to tell you about the state of security in aeronautical communications, plus the security considerations that apply to all ATN/IPS traffic 🛰️. It also gives an overview of the LDACS link-layer security capabilities 🦋.

9.1. Security in Wireless Digital Aeronautical Communications ⚠️

Aviation needs secure exchanges of data and voice messages to manage air traffic flow safely through airspaces all over the world 🌎. Historically, Communication Navigation Surveillance (CNS) wireless tech came out of the military, and it was built for a threat landscape where the adversaries were assumed to have inferior technological and financial capabilities [STR2016] 🫠. The main communications method for ATC today is still open analog voice broadcast in the aeronautical VHF band 📻. Right now, information security is mostly procedural, relying on well-trained personnel and proven communications procedures 🧑‍✈️. This method has been in service since 1948 🕰️. But the world has changed a LOT since civil aeronautical CNS applications showed up in the 70s 😭.

Civil applications have significantly less spectrum available than military ones 📉. That means several military defense mechanisms, like frequency hopping or pilot symbol scrambling (and therefore a defense-in-depth approach starting at the physical layer), are infeasible for civil systems 🚫. With the rise of cheap Software-Defined Radios (SDRs), the old financial barrier is almost gone, and open source projects such as GNU radio [GNU2021] let a brand-new type of unsophisticated listener, and possible attacker, jump in 👀💀.

Most CNS technology developed in ICAO relies on open standards, so the syntax and semantics of wireless digital aeronautical communications should be expected to be common knowledge for attackers 😱. Digitization and automation of civil aviation keep rising, and the human as control instance is gradually being taken out of the loop 🤖. Autonomous transport drones and single-piloted aircraft show this trend 🛸. Without serious cybersecurity measures, like authenticity and integrity checks of messages in transit on the wireless link or mutual entity authentication, that lack of a control instance can prove disastrous 🚨. So future digital communications will need extra embedded security features to meet modern information security requirements like authentication and integrity 🔑. Those security features need sufficient bandwidth, which is beyond what currently deployed VHF narrowband communications systems can offer 📶. For voice and data communications, there needs to be enough data throughput to support the security functions without degrading performance ⚡. LDACS is a data link technology with enough bandwidth to build in security without losing too much user data throughput 💖✨.

9.2. Security in Depth ✨

Okay so ICAO Doc 9896 [ICAO2015] wants transport layer security for all aeronautical data sent over the ATN/IPS, as described in ARINC 858 [ARI2021] 🔐. It's realized with Datagram Transport Layer Security (DTLS) 1.3 [RFC9147], which is giving very protected energy.

LDACS also has to meet the in-depth security requirements in ARINC 858 for the radio access technologies that carry ATN/IPS data 💅. These requirements mean that LDACS must provide Layer 2 security on top of any higher-layer mechanisms, so it's not just one lock on the door, it's a whole layered situation 🎀. Specifically, ARINC 858 [ARI2021] says that data links within the FCI need to provide:

a secure channel between the airborne radio systems and the peer radio access endpoints on the ground [...] to ensure authentication and integrity of air-ground message exchanges in support of an overall defense-in-depth security strategy.

9.3. LDACS Security Requirements 🔒

Overall, cybersecurity for CNS technology shall protect the following business goals [MAE20181] 🌸:

  1. Safety: The system must sufficiently mitigate attacks that contribute to safety hazards.
  2. Flight regularity: The system must sufficiently mitigate attacks that contribute to delays, diversions, or cancelations of flights.
  3. Protection of business interests: The system must sufficiently mitigate attacks that result in financial loss, reputation damage, disclosure of sensitive proprietary information, or disclosure of personal information.

To analyze assets, derive threats, and create protection scenarios, several threat and risk analyses were performed for LDACS [MAE20181] [MAE20191] 🧚. These results allowed the derivation of security scope and objectives from the requirements and the conducted threat and risk analysis. Note, IPv6 security considerations are briefly discussed in Section 9.7 while a summary of security requirements for link-layer candidates in the ATN/IPS is given in [ARI2021], which states:

Since the communication radios connect to local airborne networks in the aircraft control domain, [...] the airborne radio systems represent the first point of entry for an external threat to the aircraft. Consequently, a secure channel between the airborne radio systems and the peer radio access endpoints on the ground is necessary to ensure authentication and integrity of air-ground message exchanges in support of an overall defense-in-depth security strategy.
⚠️ bestie, the airborne radio is literally the front door for outside threats, so this part is not optional 😭

9.4. LDACS Security Objectives 🎯

Security considerations for LDACS are defined by the official SARPS document from ICAO [ICAO2022] 👑:

  • LDACS shall provide a capability to protect the availability and continuity of the system.
  • LDACS shall provide a capability including cryptographic mechanisms to protect the integrity of messages in transit.
  • LDACS shall provide a capability to ensure the authenticity of messages in transit.
  • LDACS should provide a capability for non-repudiation of origin for messages in transit.
  • LDACS should provide a capability to protect the confidentiality of messages in transit.
  • LDACS shall provide an authentication capability.
  • LDACS shall provide a capability to authorize the permitted actions of users of the system and to deny actions that are not explicitly authorized.
  • If LDACS provides interfaces to multiple domains, LDACS shall provide capability to prevent the propagation of intrusions within LDACS domains and towards external domains.

Work in 2022 includes a change request for these SARPS aims to limit the "non-repudiation of origin of messages in transit" requirement only to the authentication and key establishment messages at the beginning of every session 🙈.

9.5. LDACS Security Functions 🧩

These objectives were used to derive several security functions for LDACS that need to be integrated into the LDACS cybersecurity architecture: Identification, Authentication, Authorization, Confidentiality, System Integrity, Data Integrity, Robustness, Reliability, Availability, and Key and Trust Management 💜. Several works investigated possible measures to implement these security functions [BIL2017] [MAE20181] [MAE20191].

9.6. LDACS Security Architecture 🏰

The requirements lead to an LDACS security model, with different entities for identification, authentication, and authorization, all working together to ensure integrity, authenticity, and confidentiality of data 🫶. A draft of the cybersecurity architecture of LDACS can be found in [ICAO2022] and [MAE20182], and the respective updates can be found in [MAE20191], [MAE20192], [MAE2020], and [MAE2021].

9.6.1. Entities 🌐

A simplified LDACS architectural model requires the following entities: network operators such as the Societe Internationale de Telecommunications Aeronautiques (SITA) [SIT2020] and ARINC [ARI2020]. Both entities provide access to the ground IPS network via an A/G LDACS router. This router is attached to an internal LDACS access network that connects via further AC-Rs to the different LDACS cell ranges, each controlled by a GS (serving one LDACS cell), with several interconnected GSs spanning a local LDACS access network. Via the A/G wireless LDACS data link AS, the aircraft is connected to the ground network. Via the aircraft's VI and network interface, the aircraft's data can be sent via the AS back to the GS, then to the LDACS local access network, AC-Rs, LDACS access network, A/G LDACS router, and finally to the ground IPS network [ICAO2015] ✈️.

9.6.2. Entity Identification 🪪

LDACS needs specific identities for the AS, the GS, and the network operator 💖. The aircraft itself can be identified using the 24-bit ICAO identifier of an aircraft [ICAO2022], the call sign of that aircraft, or the recently founded privacy ICAO address of the Federal Aviation Administration (FAA) program with the same name [FAA2020]. It is conceivable that the LDACS AS will use a combination of aircraft identification, radio component identification, and even operator feature identification to create a unique LDACS AS identification tag. Similar to a 4G's eNodeB-serving network identification tag, a GS could be identified using a similar field. The identification of the network operator is similar to 4G (e.g., E-Plus, AT&T, and TELUS), in the way that the aeronautical network operators are listed (e.g., ARINC [ARI2020] and SITA [SIT2020]) 📡.

9.6.3. Entity Authentication and Key Establishment 🔑

In order to anchor trust within the system, all LDACS entities connected to the ground IPS network will be rooted in an LDACS-specific chain-of-trust and PKI solution, quite similar to AeroMACS's approach [CRO2016] 🌈. These certificates, residing at the entities and incorporated in the LDACS PKI, provide proof of the ownership of their respective public key and include information about the identity of the owner and the digital signature of the entity that has verified the certificate's content. First, all ground infrastructures must mutually authenticate to each other, negotiate and derive keys, and then secure all ground connections. How this process is handled in detail is still an ongoing discussion. However, established methods to secure the user plane by IPsec [RFC4301] and IKEv2 [RFC7296] or the application layer via TLS 1.3 [RFC8446] are conceivable. The LDACS PKI with its chain-of-trust approach, digital certificates, and public entity keys lay the groundwork for this step. In a second step, the AS with the LDACS radio aboard approaches an LDACS cell and performs a cell-attachment procedure with the corresponding GS. This procedure consists of (1) the basic cell entry [GRA2020] and (2) a MAKE procedure [MAE2021] 🦋.

Note that LDACS will foresee multiple security levels. To address the issue of the long service life of LDACS (i.e., possibly greater than 30 years) and the security of current pre-quantum cryptography, these security levels include pre-quantum and post-quantum cryptographic solutions. Limiting security data on the LDACS data link as much as possible to reserve as much space for actual user data transmission is key in the LDACS security architecture. This is also reflected in the underlying cryptography. Pre-quantum solutions will rely on elliptic curves [NIST2013], while post-quantum solutions consider Falcon [SON2021] [MAE2021] or similar lightweight PQC signature schemes and CRYSTALS-KYBER or SABER as key establishment options [AVA2021] [ROY2020] 🔮.

9.6.4. Message-In-Transit Confidentiality, Integrity, and Authenticity 📦

The key material from the previous step can then be used to protect LDACS Layer 2 communications by applying encryption and integrity protection measures on the SNP layer of the LDACS protocol stack. As LDACS transports AOC and ATS data, the integrity of that data is most important, while confidentiality only needs to be applied to AOC data to protect business interests [ICAO2022] 🎀. This possibility of providing low-layered confidentiality and integrity protection ensures a secure delivery of user data over the wireless link. Furthermore, it ensures integrity protection of LDACS control data 🥰.

9.7. Considerations on LDACS Security Impact on IPv6 Operational Security 🧠

In this part, considerations on IPv6 operational security in [RFC9099] and interrelations with the LDACS security additions are compared and evaluated to identify further protection demands. As IPv6 heavily relies on the Neighbor Discovery Protocol (NDP) [RFC4861], integrity and authenticity protection on the link layer, as provided by LDACS, already help mitigate spoofing and redirection attacks. However, to also mitigate the threat of remote DDoS attacks, neighbor solicitation rate-limiting is recommended by [RFC9099]. To prevent the threat of DDoS and DoS attacks in general on the LDACS access network, rate-limiting needs to be performed on each network node in the LDACS access network. One approach is to filter for the total amount of possible LDACS AS-GS traffic per cell (i.e., of up to 1.4 Mbit/s user data per cell and up to the amount of GS per service provider network times 1.4 Mbit/s) 🚦.

10. IANA Considerations 🌷

This document has no IANA actions.

11. Informative References 📚

okay bestie, this is the reference list 📚 every entry here is a receipt for the stuff we've been talking about, so I kept every title, author, DOI, and URL exactly as written 💅

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              Maino, F., and B. Venkatachalapathy, "Ground-Based LISP
              for the Aeronautical Telecommunications Network", Work in
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              draft-haindl-lisp-gb-atn-08>.

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   [MAE20191] Mäurer, N., Gräupl, T., and C. Schmitt, "Evaluation of the
              LDACS Cybersecurity Implementation", IEEE 38th Digital
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              of the LDACS Cybersecurity Architecture: An Updated
              Datalink Security Threat- and Risk Analysis", IEEE
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   [MAE2020]  Mäurer, N., Gräupl, T., Gentsch, C., and C. Schmitt,
              "Comparing Different Diffie-Hellman Key Exchange Flavors
              for LDACS", IEEE/AIAA 39th Digital Avionics Systems
              Conference (DASC), pp. 1-10,
              DOI 10.1109/DASC50938.2020.9256746, October 2020,
              <https://doi.org/10.1109/DASC50938.2020.9256746>.

   [MAE2021]  Mäurer, N., Gräupl, T., Gentsch, C., Guggemos, T.,
              Tiepelt, M., Schmitt, C., and G. Dreo Rodosek, "A Secure
              Cell-Attachment Procedure for LDACS", IEEE European
              Symposium on Security and Privacy Workshops (EuroS&PW),
              pp. 1-10, DOI 10.1109/EuroSPW54576.2021.00019, September
              2021, <https://doi.org/10.1109/EuroSPW54576.2021.00019>.

   [MAE20211] Mäurer, N., Gräupl, T., Bellido-Manganell, M.A., Mielke,
              D.M., Filip-Dhaubhadel, A., Heirich, O., Gerberth, D.,
              Felux, M., Schalk, L.M., Becker, D., Schneckenburger, N.,
              and M. Schnell, "Flight Trial Demonstration of Secure GBAS
              via the L-band Digital Aeronautical Communications System
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              Vol. 36, Issue 4, pp. 8-17, DOI 10.1109/MAES.2021.3052318,
              April 2021, <https://doi.org/10.1109/MAES.2021.3052318>.

   [MOST2018] Mostafa, M., Bellido-Manganell, M.A.., and T. Gräupl,
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              Aeronautical Communications System Under the Constraint of
              Secondary Spectrum Usage", IEEE Transactions on Vehicular
              Technology, Vol. 67, Issue 10, pp. 9721-9733,
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   [NIST2013] National Institute of Standards and Technology (NIST),
              "Digital Signature Standard (DSS)", FIPS PUB 186-4,
              DOI 10.6028/NIST.FIPS.186-4, July 2013,
              <https://doi.org/10.6028/NIST.FIPS.186-4>.

   [OSE2019]  Osechas, O., Narayanan, S., Crespillo, O.G., Zampieri, G.,
              Battista, G., Kumar, R., Schneckenburger, N., Lay, E.,
              Belabbas, B., and M. Meurer, "Feasibility Demonstration of
              Terrestrial RNP with LDACS", 32nd International Technical
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              Navigation, pp. 3254-3265, DOI 10.33012/2019.17119,
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              Thubert, P., Ed., Cavalcanti, D., Vilajosana, X., Schmitt,
              C., and J. Farkas, "Reliable and Available Wireless
              Technologies", Work in Progress, Internet-Draft, draft-
              ietf-raw-technologies-06, 30 November 2022,
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              technologies-06>.

   [RAW-USE-CASES]
              Bernardos, C. J., Ed., Papadopoulos, G. Z., Thubert, P.,
              and F. Theoleyre, "RAW Use-Cases", Work in Progress,
              Internet-Draft, draft-ietf-raw-use-cases-09, 13 March
              2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
              raw-use-cases-08>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC5213]  Gundavelli, S., Ed., Leung, K., Devarapalli, V.,
              Chowdhury, K., and B. Patil, "Proxy Mobile IPv6",
              RFC 5213, DOI 10.17487/RFC5213, August 2008,
              <https://www.rfc-editor.org/info/rfc5213>.

[RFC5795] Sandlund, K., Pelletier, G., and L. Jonsson, "The RObust Header Compression (ROHC) Framework", RFC 5795, DOI 10.17487/RFC5795, March 2010, <https://www.rfc-editor.org/info/rfc5795>.

[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. Kivinen, "Internet Key Exchange Protocol Version 2 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 2014, <https://www.rfc-editor.org/info/rfc7296>.

[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017, <https://www.rfc-editor.org/info/rfc8200>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, <https://www.rfc-editor.org/info/rfc8446>.

[RFC9099] Vyncke, É., Chittimaneni, K., Kaeo, M., and E. Rey, "Operational Security Considerations for IPv6 Networks", RFC 9099, DOI 10.17487/RFC9099, August 2021, <https://www.rfc-editor.org/info/rfc9099>.

[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022, <https://www.rfc-editor.org/info/rfc9147>.

[RFC9300] Farinacci, D., Fuller, V., Meyer, D., Lewis, D., and A. Cabellos, Ed., "The Locator/ID Separation Protocol (LISP)", RFC 9300, DOI 10.17487/RFC9300, October 2022, <https://www.rfc-editor.org/info/rfc9300>.

[RFC9301] Farinacci, D., Maino, F., Fuller, V., and A. Cabellos, Ed., "Locator/ID Separation Protocol (LISP) Control Plane", RFC 9301, DOI 10.17487/RFC9301, October 2022, <https://www.rfc-editor.org/info/rfc9301>.

[RIH2018] Rihacek, C., Haindl, B., Fantappie, P., Pierattelli, S., Gräupl, T., Schnell, M., and N. Fistas, "L-band Digital Aeronautical Communications System (LDACS) activities in SESAR2020", Integrated Communications Navigation and Surveillance Conference (ICNS), pp. 1-8, DOI 10.1109/ICNSURV.2018.8384880, April 2018, <https://doi.org/10.1109/ICNSURV.2018.8384880>.

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[RTCA2019] Radio Technical Commission for Aeronautics (RTCA), "Internet Protocol Suite Profiles", RTCA DO-379, September 2019, <https://standards.globalspec.com/std/14224450/rtca-do-379>.

[RTGWG-ATN-BGP] Templin, F., Ed., Saccone, G., Dawra, G., Lindem, A., and V. Moreno, "A Simple BGP-based Mobile Routing System for the Aeronautical Telecommunications Network", Work in Progress, Internet-Draft, draft-ietf-rtgwg-atn-bgp-19, 7 November 2022, <https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-atn-bgp-19>.

[SAJ2014] Haindl, B., Meser, J., Sajatovic, M., Müller, S., Arthaber, H., Faseth, T., and M. Zaisberger, "LDACS1 conformance and compatibility assessment", IEEE/AIAA 33rd Digital Avionics Systems Conference (DASC), pp. 1-11, DOI 10.1109/DASC.2014.6979447, October 2014, <https://doi.org/10.1109/DASC.2014.6979447>.

[SCH2016] Schneckenburger, N., Jost, T., Shutin, D., Walter, M., Thiasiriphet, T., Schnell, M., and U.C. Fiebig, "Measurement of the l-band air-to-ground channel for positioning applications", IEEE Transactions on Aerospace and Electronic Systems, Vol. 52, Issue 5, pp. 2281-2297, DOI 10.1109/TAES.2016.150451, October 2016, <https://doi.org/10.1109/TAES.2016.150451>.

[SCHN2018] Schneckenburger, N., "A Wide-Band Air-Ground Channel Model", Dissertation, Technischen Universitaet Ilmenau, February 2018.

[SHU2013] Shutin, D., Schneckenburger, N., Walter, M., and M. Schnell, "LDACS1 ranging performance - An analysis of flight measurement results", IEEE 32nd Digital Avionics Systems Conference (DASC), pp. 1-10, DOI 10.1109/DASC.2013.6712567, October 2013, <https://doi.org/10.1109/DASC.2013.6712567>.

[SIT2020] "Societe Internationale de Telecommunica Aéronautique (SITA)", <https://www.sita.aero/>.

[SON2021] Soni, D., Basu, K., Nabeel, M., Aaraj, N., Manzano, M., and R. Karri, "FALCON", Hardware Architectures for Post-Quantum Digital Signature Schemes, pp. 31-41, DOI 10.1007/978-3-030-57682-0_3, 2021, <https://doi.org/10.1007/978-3-030-57682-0_3>.

[STR2016] Strohmeier, M., Schäfer, M., Pinheiro, R., Lenders, V., and I. Martinovic, "On Perception and Reality in Wireless Air Traffic Communication Security", IEEE Transactions on Intelligent Transportation Systems, Vol. 18, Issue 6, pp. 1338-1357, DOI 10.1109/TITS.2016.2612584, October 2016, <https://doi.org/10.1109/TITS.2016.2612584>.

[VIR2021] Virdia, A., Stea, G., and G. Dini, "SAPIENT: Enabling Real-Time Monitoring and Control in the Future Communication Infrastructure of Air Traffic Management", IEEE Transactions on Intelligent Transportation Systems, Vol. 22, Issue 8, pp. 4864-4875, DOI 10.1109/TITS.2020.2983614, August 2021, <https://doi.org/10.1109/TITS.2020.2983614>.

Appendix A. Selected Information from DO-350A ✨

This appendix is the cheat sheet for the continuity, availability, and integrity requirements that apply to LDACS, as defined in [DO350A]. 💜

Here are the terms I'll be throwing around:

  • CPDLC: Controller-Pilot Data Link Communications
  • DT: Delivery Time (nominal) value for RSP
  • ET: Expiration Time value for RCP
  • FH: Flight Hour
  • MA: Monitoring and Alerting criteria
  • OT: Overdue Delivery Time value for RSP
  • RCP: Required Communication Performance
  • RSP: Required Surveillance Performance
  • TT: Transaction Time (nominal) value for RCP

Okay, the first table is for RCP 130 in CPDLC. Ngl, it's giving spreadsheet 😭🎀

          +========================+=============+=============+
          |                        |   RCP 130   |   RCP 130   |
          +========================+=============+=============+
          | Parameter              |      ET     |    TT95%    |
          +------------------------+-------------+-------------+
          | Transaction Time (sec) |     130     |      67     |
          +------------------------+-------------+-------------+
          | Continuity             |    0.999    |     0.95    |
          +------------------------+-------------+-------------+
          | Availability           |    0.989    |    0.989    |
          +------------------------+-------------+-------------+
          | Integrity              | 1E-5 per FH | 1E-5 per FH |
          +------------------------+-------------+-------------+

                 Table 1: CPDLC Requirements for RCP 130
    +========================+=========+=========+=========+=========+
    |                        | RCP 240 | RCP 240 | RCP 400 | RCP 400 |
    +========================+=========+=========+=========+=========+
    | Parameter              |    ET   |  TT95%  |    ET   |  TT95%  |
    +------------------------+---------+---------+---------+---------+
    | Transaction Time (sec) |   240   |   210   |   400   |   350   |
    +------------------------+---------+---------+---------+---------+
    | Continuity             |  0.999  |   0.95  |  0.999  |   0.95  |
    +------------------------+---------+---------+---------+---------+
    | Availability           |  0.989  |  0.989  |  0.989  |  0.989  |
    +------------------------+---------+---------+---------+---------+
    | Integrity              |   1E-5  |   1E-5  |   1E-5  |   1E-5  |
    |                        |  per FH |  per FH |  per FH |  per FH |
    +------------------------+---------+---------+---------+---------+

               Table 2: CPDLC Requirements for RCP 240/400

Now the monitoring and alerting bit for CPDLC. These are the MA rules, and they're non-negotiable vibes 🙈

RCP Monitoring and Alerting Criteria in case of CPDLC:

  • MA-1: The system shall be capable of detecting failures and configuration changes that would cause the communication service to no longer meet the RCP specification for the intended use.
  • MA-2: When the communication service can no longer meet the RCP specification for the intended function, the flight crew and/or the controller shall take appropriate action.

Next up is ADS-C, and it's giving the same energy with different numbers 🌸

   +==============+========+========+========+========+========+=======+
   |              |  RSP   |  RSP   |  RSP   |  RSP   |  RSP   |  RSP  |
   |              |  160   |  160   |  180   |  180   |  400   |  400  |
   +==============+========+========+========+========+========+=======+
   | Parameter    |   OT   | DT95%  |   OT   | DT95%  |   OT   | DT95% |
   +--------------+--------+--------+--------+--------+--------+-------+
   | Transaction  |  160   |   90   |  180   |   90   |  400   |  300  |
   | Time (sec)   |        |        |        |        |        |       |
   +--------------+--------+--------+--------+--------+--------+-------+
   | Continuity   | 0.999  |  0.95  | 0.999  |  0.95  | 0.999  |  0.95 |
   +--------------+--------+--------+--------+--------+--------+-------+
   | Availability | 0.989  | 0.989  | 0.989  | 0.989  | 0.989  | 0.989 |
   +--------------+--------+--------+--------+--------+--------+-------+
   | Integrity    |  1E-5  |  1E-5  |  1E-5  |  1E-5  |  1E-5  |  1E-5 |
   |              | per FH | per FH | per FH | per FH |  per   |  per  |
   |              |        |        |        |        |   FH   |   FH  |
   +--------------+--------+--------+--------+--------+--------+-------+

                        Table 3: ADS-C Requirements

RCP Monitoring and Alerting Criteria:

  • MA-1: The system shall be capable of detecting failures and configuration changes that would cause the ADS-C service to no longer meet the RSP specification for the intended function.
  • MA-2: When the ADS-C service can no longer meet the RSP specification for the intended function, the flight crew and/or the controller shall take appropriate action.

Acknowledgements ✨

okay bestie, the thank-you list is SO long and every single name deserves a shoutout 🫶 First, huge love to everyone who worked on LDACS and the ICAO Project Team Terrestrial (PT-T) 🌸, plus everyone in the RAW Working Group who had the deep, nerdy discussions and gave feedback. 🔥

Then the authors want to give a special thank-you to Klaus-Peter Hauf, Bart Van Den Einden, and Pierluigi Fantappie for commenting on this doc 💖

Next up: the Chair of Network Security gave input, and the Research Institute CODE gave comments and improvements. Iconic teamwork 👑

The colleagues at the Research Institute CODE at the UniBwM also get a big hug 🦋. They work on the AMIUS project, which is funded under the Bavarian Aerospace Program by the Bavarian State Ministry of Economics, Regional Development and Energy with the GA ROB-2-3410.20-04-11-15/HAMI-2109-0015. Thank you for the fruitful chats about aeronautical communications and the security incentives for the target market 💅

Thanks to SBA Research Vienna for the ongoing conversations about security infrastructure in fast-growing markets like the air space, and the possible economic spillovers into the technologies and protocols people already use. Main character research energy 🌈

Thanks to the Aeronautical Communications group at the Institute of Communications and Navigation of the German Aerospace Center (DLR) too 🧚. The authors also want to explicitly thank Miguel Angel Bellido-Manganell and Lukas Marcel Schalk for their thorough feedback. Literally the best reviewers 😭💜

Authors' Addresses

The editors are the three people below 🎀

Nils Mäurer (editor) 🌸

Nils Mäurer (editor)
German Aerospace Center (DLR)
Münchner Strasse 20
82234 Wessling
Germany
Email: Nils.Maeurer@dlr.de

Thomas Gräupl (editor) 🦋

Thomas Gräupl (editor)
German Aerospace Center (DLR)
Münchner Strasse 20
82234 Wessling
Germany
Email: Thomas.Graeupl@dlr.de

Corinna Schmitt (editor) 💖

Corinna Schmitt (editor)
Research Institute CODE, UniBwM
Werner-Heisenberg-Weg 39
85577 Neubiberg
Germany
Email: corinna.schmitt@unibw.de
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