Home' RTCA Documents for Review : C2 Link Systems MASPS_Draft Contents Appendix B
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remote pilot and air traffic controllers (see 14 CFR, Part 91, §91.183 ). All IFR aircraft
are required by the FAA airspace rules to maintain two-way voice communications with
ATC (ref. 14 CFR Part 91 §91.126-131, and §91.135) while operating in controlled
airspace (i.e., Classes A, B, C, D, and E)33. Consequently, all UAS flights conducted under
Part 91 rules and in controlled airspace require the ability to communicate via voice with
air traffic controllers.
Current FAA ATC Ground Voice Communications Services
The FAA maintains a complex system of networks and VHF voice radio stations across
the entire U.S. (including the U.S. Territories) that support FAA Air Traffic Controllers
conducting two-way communications in all controlled airspace. There are a few small
airports in remote areas that do not have ATC communications at ground level , but they
are managed via procedural separation techniques. The voice communications service is
provided with very high reliability using multiple radios and redundant c onnectivity. The
FAA (ground) voice communications service is considered safety-critical, so the service
must be at least dual redundant and have a minimum availability of .99999 (see NAS-RD -
Voice Communications Latency
The FAA has established a requirement for voice communication latency in their NAS
Enterprise Architecture of 250 milliseconds (ms) average, 300 ms 95%, and 350 ms not to
exceed for the ground infrastructure (see NAS-RD-2013). The FAA conducted two
studies in 2003 to develop these requirements (see DOT/FAA/CT-TN04/02  and
DOT/FAA/CT/TN03/04 ). These studies assumed a 40 ms delay between the FAA VHF
radios and the crew on the aircraft. These studies confirmed that a significant increase in
blocked transmissions and subsequently repeated transmissions result when the latency
reaches the 390 ms threshold. Consequently, the UAS must meet the latency requirements
for manned aircraft for successful integration into the NAS.
Future FAA Voice Communications Services
The FAA is in the process of upgrading the ATC voice system to a more modern, Voice
over Internet Protocol (VOIP) based network through a program called the NAS Voice
System (NVS). This new system may eventually include the ability to connect the
controller and pilot over ground/ground links through a networked connection that would
allow ATC and other pilots to hear the remote pilot/controller conversations (party line).
However, based on the current budget and program schedule this capability will not be
widely deployed until 2025 at the earliest. Further, the FAA has no funding to install this
capability at towers or small radar approach control facilities. Unless the FAA receives
additional funding and direction to upgrade all voice systems for ATC communications in
controlled airspace, voice switch modifications would not solve the UAS voice latency
challenge in all airspaces. The FAA Interim Voice Switch Replacement (IVSR) system
also can provide direct connection capability to UAS. However, this ability is not currently
a funded requirement for the IVSR program and has not been assessed by the FAA for
feasibility or cost.
Possible Architecture Alternatives
The current known alternatives for meeting the voice latency requirement to maintain two -
way voice communications with ATC have challenges for supporting the integration of
UAS into the airspace; these alternatives/methods are:
33 See APPENDIX B Figure B-2 for a description of the Airspace Classes.
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