Appendix 1 to ETSO-2C204a – Addition to RTCA Document DO-229E

This Appendix describes the modifications and additions to RTCA DO-229E that are required for compliance with this ETSO.

This Appendix adds a new Section 1.8.3, on cybersecurity and GNSS spoofing mitigation, to RTCA document DO‑229E, and corrects a long-standing mistake in the Section 2.4 environmental requirements tables. The new section provides information for cybersecurity and GNSS spoofing mitigation to make RTCA document DO‑229E consistent with the new RTCA MOPS template and RTCA document DO-253D, Minimum Operational Performance Standards for GPS Local Area Augmentation System Airborne Equipment, dated July 2017.

1.8.3 Cybersecurity and GNSS Spoofing Mitigation.

This section contains information to address intentional interference with the GNSS. Spoofing is caused by RF waveforms that mimic true signals in some ways, but deny, degrade, disrupt, or deceive the operation of a receiver when they are processed. Spoofing may be unintentional, such as effects from the signals of a GNSS repeater, or may be intentional and even malicious. There are two classes of spoofing:

Measurement spoofing introduces RF waveforms that cause the target receiver to produce incorrect measurements of the time of arrival or the frequency of arrival, or their rates of change;

Data spoofing introduces incorrect digital data to the target receiver for its use in the processing of signals and the calculation of positioning, navigation and timing (PNT).

Either class of spoofing can cause a range of effects: from incorrect outputs of PNT to receiver malfunctions. The onset of effects can be instantaneous or delayed, and the effects can continue even after the spoofing has ended. Improperly used or improperly installed GNSS re-radiators act like spoofers. Re-radiators, replay and GNSS emulator devices can present misleading information to GNSS equipment and/or could cause lasting effects.

Equipment manufacturers should implement measures to mitigate the processing of erroneous data. Cross‑checks of GNSS sensor data against independent position sources and/or other detection monitors using GNSS signal metrics or data checks can be implemented in the antenna, receiver, and/or through integration with other systems at the aircraft level. Data validity checks to recognise and reject measurement and data spoofing should be implemented in the receiver. Additional guidance and best practices related to GPS equipment can be found in the U.S. Department of Homeland Security document ‘Improving the Operation and Development of Global Positioning System (GPS) Equipment Used by Critical Infrastructure’[21] and GLOBAL POSITIONING SYSTEMS DIRECTORATE SYSTEMS ENGINEERING & INTEGRATION: INTERFACE SPECIFICATION, IS-GPS-200, Navstar GPS Space Segment/Navigation User Interfaces, Revision H, IRN‑IS‑200H‑003, 28 July 2016.

Aircraft equipment information vulnerabilities (such as cybersecurity risks) have been present for digital systems since the development of the personal computer (PC) in the late 1970s and even longer for RF systems, and the advent of internet connectivity has substantially increased those risks. Typically, access to navigation receivers has been controlled such that they are considered to be vulnerable only to RF signals and OEM and/or aircraft operator controlled processes for maintenance and update. In some cases, aircraft GNSS receivers may be field-loadable by approved personnel, requiring physical access and a physical interface to the ground receivers. However, it is expected that not all aircraft in the future will rely on such physical isolation for the security of avionics. Internet and Wi-Fi connectivity have become popular as means for aircraft or equipment manufacturers to update the software of installed avionics, to update databases, or provide an alternate means of communicating with the flight crew or cabin (e.g. in-flight entertainment, weather, etc.).

In most countries, the State provides oversight of safety-of-flight systems (sometimes referred to as ‘authorised services’) which provide information to aircraft, such as ILS, VOR, GNSS and DME, to name a few. However, the State typically does not provide oversight of ‘non-trusted’ connectivity such as the internet, Wi-Fi, or manufacturer-supplied equipment interfaces which permit the input of externally supplied data into aircraft systems. A manufacturer may expose aircraft information vulnerabilities through the design of the equipment, or the equipment may become vulnerable as a result of being connected to a common interface. Therefore, it is important for manufacturers to consider aircraft information security risk mitigation strategies in their equipment design, particularly when the equipment is responsible for an interface between the aircraft and aircraft-external systems.

Apart from any specific aircraft-information-security-related performance requirements that are contained in the MOPS, it is recommended that manufacturers should consider a layered approach to aircraft information security risk mitigation that includes both technical (e.g. software, signal filtering) and physical strategies. From a technical perspective, for example, this could include signal spoofing detection capabilities or more stringent, multi-factored authentication techniques such as passwords, PINs, and digital certificates. And finally, but just as important, manufacturers should consider supply chain risk management; for example, if a manufacturer outsources the development of software code, are the contractor and its staff properly vetted?

Civil aviation authorities (CAAs) have a regulatory interest when an applicant’s design makes use of a non‑trusted connection through which the installation can potentially introduce aircraft information security vulnerabilities. This requires the applicant to address not only the information security vulnerabilities and mitigation techniques for the new installation, but to also consider how vulnerabilities could propagate to existing downstream systems. Therefore, manufacturers are recommended to reference their equipment aircraft information security review and mitigation strategies in the installation manual of the equipment so that the applicant can consider them in meeting the regulatory requirements of the installation.

Table 2-14 to Table 2-20

The tables incorrectly reference and label RTCA document DO-160 Sections 16.5.1.2 and 16.6.1.2 regarding ‘2.1.1.7 Acquisition Time’ and ‘2.1.1.9 Reacquisition Time’. Change the table references as follows:

The MOPS initial acquisition time requirement (2.1.1.7) applies to both AC and DC equipment under abnormal operating conditions (DO-160E Sections 16.5.2 and 16.6.2) and the satellite reacquisition time requirement (2.1.1.9) applies to both AC and DC equipment under normal operating conditions (DO‑160E Sections 16.5.1 and 16.6.1).

[Amdt ETSO/16]